Mechanisms and Sources of Mantle Metasomatism: Major and Trace Element Compositions of Peridotite...

JOURNAL OF PETROLOGY VOLUME 43 NUMBER 12 PAGES 2219–2259 2002

Mechanisms and Sources of MantleMetasomatism: Major and Trace ElementCompositions of Peridotite Xenoliths fromSpitsbergen in the Context of NumericalModelling

DMITRI A. IONOV1,2∗, JEAN-LOUIS BODINIER3,SAMUEL B. MUKASA4 AND ALBERTO ZANETTI5

1DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, UNIVERSITE LIBRE DE BRUXELLES, CP160/02,

B-1050 BRUSSELS, BELGIUM2DEPARTMENT OF GEOLOGY, NIIGATA UNIVERSITY, 2-8050 IKARASHI-NINOCHYO, NIIGATA, 950-2181, JAPAN3LABORATOIRE DE TECTONOPHYSIQUE (UMR 5568 CNRS), ISTEEM, UNIVERSITE MONTPELLIER 2, CASE 049,

34095 MONTPELLIER CEDEX 05, FRANCE4DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF MICHIGAN, ANN ARBOR, MI 48109-1063, USA5CNR–ISTITUTO DI GEOSCIENCE E GEORISORSE (IGG), SEZIONE DI PAVIA, 27100 PAVIA, ITALY

RECEIVED OCTOBER 2, 2000; REVISED TYPESCRIPT ACCEPTED MAY 22, 2002

Mineral and whole-rock chemical data for peridotite xenoliths in medium are strongest near its source (e.g. a vein) whereas elementbasaltic lavas on Spitsbergen are examined to reassess mechanisms patterns farther in the metasomatic ‘column’ are increasingly con-of melt–fluid interaction with peridotites and their relative role versus trolled by fractionation mechanisms.melt composition in mantle metasomatism. The enrichment patternsin the xenoliths on primitive mantle-normalized diagrams rangefrom Th–La–Ce ‘inflections’ in weakly metasomatized samples(normally without amphibole) to a continuous increase in abundances KEY WORDS: Spitsbergen; lithospheric mantle; metasomatism; trace ele-from Ho to Ce typical for amphibole-bearing xenoliths. Numerical ments; theoretical modellingmodelling of interaction between depleted peridotites and enrichedmelts indicates that these patterns do not result from simple mixingof the two end-members but can be explained by chromatographicfractionation during reactive porous melt flow, which produces a

INTRODUCTIONvariety of enrichment patterns in a single event. Many metasomatizedPeridotite xenoliths brought to the surface by alkalixenoliths have negative high field strength element and Pb anomaliesbasaltic magmas on the island of Spitsbergen representand Sr spikes relative to rare earth elements of similar compatibility,continental upper mantle located at present in the vicinityand highly fractionated Nb/Ta and Zr/Hf. Although amphiboleof oceanic spreading centres in the North Atlantic andprecipitation can produce Nb–Ta anomalies, some of these featuresArctic basins. Much of the earlier work on these xenolithscannot be attributed to percolation-related fractionation alone andfocused on carbonates and related interstitial phaseshave to be a signature of the initial melt (possibly carbonate rich).

In general, chemical and mineralogical fingerprints of a metasomatic believed to have formed as a result of interaction between

∗To whom correspondence should be addressed at present address:Laboratoire de Tectonophysique, ISTEEM, CNRS & Universite Mont-pellier 2, case 049, 34095 Montpellier cedex 05, France. Telephone:+33-467143602. Fax: +33-467143603. E-mail: [email protected] Oxford University Press 2002

JOURNAL OF PETROLOGY VOLUME 43 NUMBER 12 DECEMBER 2002

mantle rocks and carbonate-rich melts during en- Bodinier et al., 1990; Johnson et al., 1996; Mukasa &Wilshire, 1997; Yaxley et al., 1998; Ionov et al., 1999a).trainment and transportation to the surface by the host

magmas (Amundsen, 1987; Genshaft & Ilupin, 1987; It has become apparent that the enrichment phenomenagenerally referred to as mantle metasomatism may beIonov et al., 1993b, 1996). The interstitial carbonates,

which are rare in mantle xenoliths elsewhere, attracted implemented through a large number of processes (e.g.batch crystallization, melt fractionation in conduits andparticular attention as direct evidence for carbonate-

rich melts in the mantle. In particular, trace element chambers, melt or fluid percolation). These mechanismscan be identified and constrained based on trace elementcompositions of acid-leached interstitial material fromdistributions in natural samples compared with results ofseveral xenoliths (Ionov et al., 1993b) and in situ analysestheoretical modelling.of carbonates and associated interstitial clinopyroxene

Another widely debated topic of mantle geochemistryand silicate glass (Ionov et al., 1996; Ionov, 1998) wereis the provenance (sources) of melts and fluids responsibleused to infer geochemical signatures of mantle meta-for metasomatism in the continental mantle. It has beensomatism by carbonate-rich fluids.suggested that the metasomatic media may originatePrevious studies have also addressed the petrographywithin the lithospheric mantle, convecting asthenosphericof the xenoliths and provided a limited amount of datamantle, deep-mantle plumes, or subducted oceanic litho-on the compositions of minerals apparently unrelated tosphere (Beccaluva et al., 2001; Churikova et al., 2001;the late-stage interstitial material (Furnes et al., 1986;Downes, 2001 and references therein). Furthermore,Ionov et al., 1996). Analyses of coarse clinopyroxenemantle metasomatism may involve a variety of mediagrains by proton probe (Ionov et al., 1996) indicated(silicate and carbonate melts, water- and CO2-rich fluids)metasomatic enrichments in Sr. Furthermore, laser-that are derived from different sources. Particular at-ablation microprobe (LAM) inductively coupled plasmatention has recently been given to metasomatism bymass spectrometry (ICP-MS) analyses in three samplescarbonate-rich melts, which seem to have a strong en-showed that abundances and elemental ratios in therichment potential and impose specific trace elementcoarse clinopyroxene are distinct from those in clino-signatures (e.g. Green & Wallace, 1988; Yaxley et al.,pyroxene from interstitial carbonate- and glass-bearing1991; Dautria et al., 1992; Hauri et al., 1993; Ionov et al.,pockets (Ionov, 1998). Some xenoliths were found to1993b; Rudnick et al., 1993; Downes, 2001).contain texturally equilibrated amphibole, apatite and

Overall, mantle metasomatism has been attributed tophlogopite (Ionov et al., 1993b, 1996). Altogether, thea variety of: (1) processes, (2) sources, and (3) melt–fluidearlier work has yielded unequivocal petrographic andcompositions. Which factors dominate in creating specificchemical evidence for modal and cryptic metasomatismmetasomatic signatures in the continental lithosphericthat took place before the formation of the silicate glassmantle remains widely debated. For example, trace ele-and carbonate-bearing material. However, the data arement patterns earlier seen as evidence for metasomatismnot sufficient to unravel the nature and evolution of theby carbonate-rich melts have been recently interpretedmantle beneath Spitsbergen.as source signatures or the result of fractionation processesIn this work we establish major and trace element(Bedini et al., 1997; Blundy & Dalton, 2000; Laurora etsignatures of the peridotite mantle beneath Spitsbergenal., 2001). Addressing these complex issues necessitatesthrough analysis of whole-rock xenoliths and of mineralscomprehensive geochemical investigation of a rep-that largely control the incompatible trace element in-resentative sample series, which has been undertaken inventory in the mantle. These data, together with theor-this study, and in the accompanying paper that addressesetical modelling, are used to understand the sequence ofSr–Nd–Pb isotope composition of the same sample seriesupper-mantle processes recorded in the xenolith suite.(Ionov et al., 2002).The major goal of this study is to further constrain

mechanisms of mantle metasomatism and define thenature and sources of the metasomatic components re-sponsible for enrichment of the lithospheric mantle in

GEOLOGICAL SETTINGincompatible elements. We focus on characterizing meta-somatic and other events that pre-date those related to Spitsbergen is the largest island of the Svalbard ar-

chipelago off Norway, which is the subaerially exposedthe late Cenozoic volcanic activity that brought thexenoliths to the surface. corner of the Barents–Kara shelf between the North

Atlantic and Arctic (Eurasian) oceanic basins (Fig. 1a).Petrographic and chemical studies of mantle rocksworldwide have found a variety of mineralogical as- Tectonic reconstructions indicate that Svalbard was con-

tiguous with Greenland until the northward progressionsociations and trace element enrichment patterns thatappear to result from interaction of depleted peridotites of the North Atlantic opening produced Late Cretaceous–

Neogene separation of Eurasia and North Americawith magmas and fluids (e.g. Frey & Green, 1974; Men-zies & Hawkesworth, 1987; McDonough & Frey, 1989; (Blythe & Kleinspehn, 1998). At present, Spitsbergen is

2220

IONOV et al. MECHANISMS AND SOURCES OF MANTLE METASOMATISM

SAMPLE PREPARATION ANDANALYTICAL PROCEDURESSamples for this study were selected from a larger col-lection (Yevdokimov, 2000) to represent major peridotiterock types and modal variations. We have also tried toselect fresh xenoliths that are large enough to preparerepresentative whole-rock samples and provide materialfor mineral separation. The samples are listed in Table1, along with a summary of the petrography, modalcompositions, geothermometry and other information onthe analyses performed.

The xenoliths were cut using a diamond saw; centralparts containing no host basalt or saw marks were crushedin a bench-top jaw crusher, which was carefully cleanedafter each sample to avoid cross-contamination. Aliquotsof the crushed samples were ground in an agate mortarto produce whole-rock powders. Another aliquot wassieved, size-fractioned and magnetically separated to yieldsub-fractions enriched in clinopyroxene and amphibole.Ultrapure mineral separates were handpicked from thesefor isotope analyses (Ionov et al., 2002). Several hand-picked grains ([0·2–0·3 mm) from each sample wereput on mounts for in situ analyses. The handpickedclinopyroxene grains are larger than clinopyroxene frominterstitial carbonate- and glass-bearing material (Fig.2a); the latter are therefore absent from the grain mounts.Samples of host basalts were sawn off from lava attachedto the xenoliths. Basalt chips free of saw marks, weath-ering products and xenocryst material were leached in 6M HCl and ground to powder in an agate mortar.

Major elements in bulk rocks were determined by X-ray fluorescence (XRF) spectrometry at Niigata Uni-versity using low-dilution fused beads [see Takazawa etal. (2000) for analytical details] and at Michigan StateUniversity following standard protocols. Major-elementFig. 1. A sketch map of the North Atlantic (a) and Svalbard Archipelago

(b) showing major tectonic features and location of the recent volcanoes compositions of minerals were determined in thin sectionssampled for xenoliths and basalts [modified from Amundsen et al. with a Cameca Camebax SX-50 electron microprobe(1987)].

(EMP) at Macquarie University, Sydney [for analyticaldetails see Ionov et al. (1996)] and with Camebax SX-100at Universite Blaise Pascale, Clermont-Ferrand, France.Modal compositions were calculated from whole-rocklocated at a distance <200 km from the North Atlanticand mineral major-element analyses using least-squaresmid-ocean ridge (MOR) (Fig. 1b).regression.The xenoliths for this study have been collected in

Trace elements in whole-rock peridotites were de-hawaiitic to nepheline basanitic lavas at three Quaternarytermined by solution ICP-MS at Niigata University;volcanic centres on the western side of Woodfjorden inbasalts were analysed at CODES (Hobart). At bothNW Spitsbergen (Fig. 1b): Sverre, Halvdan and Sigurd.laboratories, the instrument was an HP4500; calibrationTholeiitic lavas, 9–12 Myr old, from nearby areas dowas performed against BHVO-1 using values of Egginsnot contain mantle xenoliths. Relevant data on geology,et al. (1997). As, Rh, In, Tm and Bi were used as internaltectonic setting and geophysical results for this regionstandards. Several peridotites (including duplicates andhave been summarized by Amundsen et al. (1987) andacid-leached rocks) were analysed on a VG PlasmaQuadYevdokimov (2000). Additional information on xenolithsinstrument in Montpellier following the method of Ionovfrom these localities has been provided by Furnes et al.

(1986), Genshaft et al. (1992) and Ionov et al. (1996). et al. (1992). Reference samples BIR-1 and JP-1

2221


Tab

le1

:Sa

mpl

elist

and

info

rmat

ion

sum

mar

yfo

rth

exe

nolith

s

Sam

ple

Loca

tio

nTe

xtu

reo

rM

od

alco

mp

osi

tio

nA

cces

s.E

qu

ilib

rati

on

tem

p.

Mg

#C

r#LA

MW

RS

r–N

d–P

bE

nri

chm

ent

no

.ro

ckty

pe

min

eral

sin

ol

insp

lIC

P-M

SIC

P-M

Sis

oto

pes

typ

e

ol

op

xcp

xsp

lam

ph

Wel

lsC

a-o

px

21-5∗

Hal

vdan

Crs

n.d

.ap

,cb

900

915–

930

0·91

40·

236

cp,

amp

h2

21-6∗

Hal

vdan

Crs

n.d

.ap

,cb

944

925

0·91

20·

221

cp,

amp

h†

2

311-

9H

alvd

anM

os–

Eq

toC

rs69

·618

·58·

52·

01·

4‡97

094

50·

907

0·18

1cp

+cp

2

315-

6H

alvd

anC

rs–P

rb60

·425

·59·

81·

72·

4(c

b)

860

890

0·90

40·

124

cp,

amp

h+

cp,

amp

h1

318

Hal

vdan

Mo

s–P

rb79

·412

·33·

01·

14·

2(c

b)

900

930–

960

0·91

50·

253

cp,

amp

h+

cp,

amp

h2

318-

1H

alvd

anC

rs60

·225

·112

·11·

61·

010

3510

250·

908

0·16

4cp

,am

ph

+cp

2

4-90

-9H

alvd

anC

rsto

Mo

s–E

qn

.d.

(cb

)98

099

00·

897

0·16

2cp

,am

ph

†2

4-25

-90

Hal

vdan

Crs

toM

os–

Eq

75·4

11·3

10·6

2·7

—cb

970

1000

–101

50·

910

0·20

4cp

+cp

1

4-36

-90∗

Hal

vdan

Crs

toM

os–

Eq

74·0

13·2

10·1

2·7

—cb

910

950–

965

0·90

60·

170

cp†

+cp

1

25S

verr

eC

rs–M

os

—cb

870

900

0·89

90·

118

+1

26a

Sve

rre

Crs

–Prb

69·6

26·5

2·9

1·0

—(c

b)

740

1000

/800

0·91

20·

416

cp+

cp—

28b

Sve

rre

Crs

toM

os–

Prb

63·6

27·4

7·5

1·5

—(c

b)

910

940

0·90

50·

133

+1

39-8

6-1

Sve

rre

Crs

toM

os–

Prb

69·5

18·2

10·0

2·3

—(c

b)

—88

70·

906

0·13

5cp

+cp

—

39-8

6-2

Sve

rre

Crs

toM

os–

Prb

68·2

17·3

11·3

3·2

—(c

b)

940

930

0·90

50·

125

cp+

cp1

63-9

0-18

Sve

rre

Crs

toM

os–

Prb

59·4

28·6

10·1

1·8

—(c

b)

840

895–

910

0·90

10·

136

cp+

cp1

63-9

0-30

Sve

rre

Prb

69·5

20·0

8·8

1·7

—89

595

00·

911

0·27

9cp

+cp

—

4-90

-1S

igu

rdam

ph

weh

rlit

ep

hl

0·89

0cp

,p

hl

cp,

ph

lve

in

amp

ham

ph

1-25

-90

Hal

vdan

ho

stb

asal

t+

WR

3-15

-90

Hal

vdan

ho

stb

asal

t+

WR

35S

verr

eh

ost

bas

alt

+W

R

∗See

min

eral

EM

Pan

alys

eso

fIo

no

vet

al.

(199

6).

†See

LAM

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San

alys

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fIo

no

v(1

998)

.‡A

mp

hib

ole

bre

akd

ow

np

rod

uct

s.M

g#,

ato

mic

Mg

/(M

g+

Fe);

Cr#

,ato

mic

Cr/

(Cr+

Al)

;Crs

,co

arse

;Mo

s,m

osa

ic;E

q,e

qu

igra

nu

lar;

Prb

,po

rph

yro

bla

stic

;am

ph

,am

ph

ibo

le;a

p,a

pat

ite;

cb,c

arb

on

ate;

cpx,

clin

op

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xen

e;o

l,o

livin

e;o

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ort

ho

pyr

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ne;

ph

l,p

hlo

go

pit

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l,sp

inel

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R,

wh

ole

rock

;n

.d.,

no

td

eter

min

ed.

Sam

ple

s4-

25-9

0,4-

36-9

0an

d25

hav

eh

igh

est

abu

nd

ance

so

fin

ters

titi

alca

rbo

nat

e(p

rob

ably

0·5–

1%);

oth

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mp

les

con

tain

trac

eam

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of

carb

on

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(cb

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pro

du

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Mo

dal

com

po

siti

on

sw

ere

calc

ula

ted

fro

mw

ho

le-r

ock

and

min

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anal

yses

by

leas

t-sq

uar

esm

eth

od

.Te

mp

erat

ure

esti

mat

esar

eaf

ter

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ls(1

977)

and

the

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op

xm

eth

od

of

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y&

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r(1

990)

.Co

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spec

ific

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defi

ned

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of

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978)

and

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pto

n(1

987)

.

2222


Fig. 2. Photomicrographs of Spitsbergen xenoliths in plane-polarized transmitted light. (a) Relict amphibole (Amph) in a vesicular aggregate ofsilicate glass, fine-grained olivine (ol), clinopyroxene (Cpx) and Cr-spinel (Spl). An apatite (Ap) grain and the large size and lack of alteration inprimary clinopyroxene should be noted. Sample 21-6; field of view is 5 mm. (b) Amphibole mantling spinel in sample 318; field of view is 5mm. (c) Mineralogical variation in composite xenolith 4-90-1. About a quarter of the field of view (4 cm) on the right is amphibole wehrlite,which grades into coarse olivine matrix with clusters of amphibole and phlogopite (Phl). No spinel or orthopyroxene has been found in thissample. (d) Small pockets of vesicular glass–cpx–ol–spl aggregates (dark) in sample 311-9 considered to be breakdown products of pre-existingamphibole; field of view is 10 mm along the long axis.

(Makishima & Nakamura, 1997) were analysed as un- across clinopyroxene and amphibole grains by SIMS(secondary ion mass spectrometry) on Cameca 4f in-knowns for quality control. Trace elements in minerals

were determined by LAM-ICP-MS in grain mounts and struments in Pavia and Montpellier following proceduresreported by Bottazzi et al. (1994).polished rock sections of 200 �m thickness in Sydney and

Pavia. The instrument at Macquarie University is aPerkin–Elmer Sciex ELAN 5100 coupled with a UV(266 nm) laser [see Norman et al. (1996) and Ionov (1998)for details of operating conditions]. The instrument at PETROGRAPHY AND MAJORthe CNR–IGG of Pavia is a double focusing sector field

ELEMENT COMPOSITIONSanalyser (‘Element’ Finnigan Mat) coupled with a Q-Petrographyswitched Nd:YAG laser source (Quantel Brilliant), whose

fundamental emission in the near-IR region (1064 nm) Most of the xenoliths from Halvdan and Sverre aremedium- to coarse-grained spinel lherzolites with low tois converted to 266 nm by two harmonic generators

(Bottazzi et al., 1999). Helium was used as carrier gas moderate modal clinopyroxene (Table 1). Samples 26aand 318 have <3% clinopyroxene and are thereforeand mixed with Ar downstream of the ablation cell. A

BCR2-g reference sample was used as an external stand- classified as harzburgites (Streckeisen, 1976). Many ofthe Halvdan xenoliths contain amphibole (Table 1),ard, with 44Ca as an internal standard for clinopyroxene

and amphibole and 29Si for phlogopite. Precision and whose modal abundance ranges from rare individualgrains to >4%. Amphibole in samples 318 and 315-6accuracy (<10% and <5%, respectively) were assessed

from repeated analyses of SRN NIST 612 reference has textural position, grain size and shape similar tothose of neoblast olivine and pyroxenes. It does notsample. Trace elements were also determined in profiles

2223


appear to replace clinopyroxene or spinel because, in ratio of 1·65 compared with a range of 1·0–1·2 typicalparticular, spinel abundances in amphibole-rich peri- for the spinel lherzolites (Table 2). The composition ofdotites are not noticeably lower than in amphibole-poor Fe-rich sample 39-86-1 is distinct from that of a duplicatesamples (Fig. 2b). A single sample from Sigurd (4-90-1) sample (39-86; Figs 3a and b, and 4a) prepared fromis a composite xenolith of amphibole wehrlite (spinel- the same xenolith (Ionov et al., 1996) indicating rockfree) grading into a coarse-grained olivine-dominated heterogeneity.rock that contains clusters of amphibole and less abundantphlogopite in an olivine matrix (Fig. 2c). Two Halvdanxenoliths (too small for mineral separation) contain apatite

Mineral compositions and temperaturein addition to amphibole (Fig. 2a). Fe–Ni sulphides areestimatescommon accessory minerals. The petrography of theseMajor element compositions of minerals are given inrocks has been presented in more detail by Ionov et al.Table 3. Values of Mg# in olivine (Mg#Ol) are tightly(1996).correlated with Mg# in whole rocks (Fig. 4a) and showSome of the peridotites contain interstitial veins anda general positive trend with the Cr/(Cr + Al) ratios ofpockets of fine-grained material made of silicate glass,spinel (Cr#Spl) (Fig. 4b). Most of the Spitsbergen samplescarbonate and second-generation clinopyroxene, olivineon the latter diagram plot between the tight field of fertileand Cr-spinel, enclosing resorbed relict grains of minerals(Mg# <0·90) xenoliths and highly depleted xenolithsof the host peridotite (Fig. 2a). This late-stage fine-grainedfrom East Greenland and abyssal peridotites, consistentmaterial is not considered in this work, and readers arewith partial melting relationships (Bernstein et al., 1998;referred to Ionov (1998) for more information. SampleHellebrand et al., 2001). Sample 26a, which has the311-9 has pockets of silicate glass with micro-phenocrystshighest Cr#Spl, plots with off-cratonic xenoliths that showand vugs associated with spinel (Fig. 2d); no amphibolea wide range of Cr#Spl at moderately high Mg#Ol (0·90–is present in the thin section available. The shapes and0·915; Fig. 4b). In some xenoliths from Spitsbergen (andtextural position of the glassy pockets resemble those ofSamoa), the Cr#Spl values are higher, at a given Cr2O3amphibole mantling spinel in some other Spitsbergencontent in clinopyroxene, than in other mantle peridotitesxenoliths; similar pockets in samples 21-5 and 21-6 en-(Fig. 4c).close resorbed amphibole relics (Fig. 2a) (Ionov et al.,

The abundances of Ti, Al and Na in clinopyroxene1996). It is most likely that xenolith 311-9 originallyare shown in Fig. 4d–f vs Mg#Ol. Olivine hosts 72–90%contained some amphibole, which broke down shortlyof Mg and Fe in the xenoliths and therefore its Mg#before or during the eruption of the host basalts, asestablished during partial melting is less affected byhas also been inferred for certain types of glass-bearing

pockets in mantle xenoliths worldwide (e.g. Yaxley et al., posterior temperature-dependent sub-solidus inter-1997). We therefore classify sample 311-9 as amphibole- mineral Mg–Fe partitioning (Brey & Kohler, 1990) thanbearing to properly characterize its modal composition the Mg# of pyroxenes. The abundances of Ti and Al inbefore the late-stage melting events. clinopyroxene in the majority of the samples are neg-

atively correlated with the Mg#Ol (Fig. 4d and e). Bycontrast, Na2O contents are nearly the same (1·0–1·5%)

Whole-rock compositions and are higher than for abyssal peridotites and xenolithsfrom Samoa at similar Mg#Ol (Fig. 4f ). ClinopyroxeneWhole-rock major element compositions are given inin 26a has much lower abundances of Na, Al and TiTable 2 and illustrated in Fig. 3. The xenolith suite canthan in the rest of the xenoliths. Amphibole wehrlite 4-generally be characterized as moderately depleted in90-1 plots away from spinel peridotites in Fig. 4 becausebasaltic components (Al, Ca, Na, Ti). Most of the samplesof its low Mg# and high TiO2 in clinopyroxene.plot in the middle of compositional fields of basalt-borne,

The Mg# values of amphibole show a near-linearoff-cratonic peridotite xenoliths and massif peridotitescorrelation with the Mg#Ol. The Cr abundances inworldwide on covariation plots vs MgO and Mg# [molaramphibole are positively correlated with the Cr#Spl (Fig.Mg/(Mg+ Fe) ratio] (Fig. 3; McDonough, 1990). Two5a and b). These and other results indicate that amphibolesamples (including an amphibole-rich peridotite) havein all of the samples is chemically equilibrated with thehigh MgO and low CaO and Al2O3 similar to someother minerals. In comparison, the abundances of Kxenoliths from kimberlites; however, they differ fromand Ti, minor elements strongly concentrated in thetypical cratonic mantle peridotites by higher FeO andamphibole, are poorly correlated (Fig. 5d). Most am-lower Mg# (Fig. 3a and b). A few samples plot abovephiboles have low K2O (<1%), particularly 315-6the MgO–CaO trend on Fig. 3c, consistent with the(0·02%).presence of <1% of Ca-rich interstitial carbonate in-

Equilibration temperature estimates calculated fromdicated by earlier studies (Ionov et al., 1996). Carbonate-poor sample 63-90-30 shows a very high molar Ca/Al the electron microprobe data using methods of Wells

2224


Tab

le2

:W

hole

-roc

kco

mpo

sition

sde

term

ined

byX

RF

(wt

%)

26a

311-

931

5-6∗

318

318-

1∗39

-86-

1∗39

-86-

24-

25-9

063

-90-

3028

b4-

36-9

063

-90-

18B

asal

t

SiO

245

·04

43·5

244

·81

42·7

344

·94

42·9

043

·51

42·1

644

·02

44·2

742

·71

45·0

346

·12

TiO

20·

010·

060·

077

0·06

0·07

40·

064

0·11

0·1

0·12

0·06

0·07

0·08

2·62

Al 2

O3

1·00

2·15

2·83

1·55

2·92

2·42

2·88

2·17

1·57

2·27

2·36

2·65

14·3

4

FeO

7·37

7·90

8·00

7·39

7·64

8·24

7·83

7·43

7·59

8·02

7·69

8·01

10·0

2

Mn

O0·

120·

130·

130·

120·

130·

130·

130·

130·

120·

120·

120·

130·

16

Mg

O43

·90

42·2

740

·89

45·1

540

·78

41·9

041

·42

42·9

942

·54

42·6

043

·19

40·8

39·

23

CaO

0·81

1·95

2·60

1·16

2·74

2·20

2·38

2·18

1·92

1·82

2·28

2·36

8·69

Na 2

On

.d.

n.d

.0·

20n

.d.

0·22

0·19

6n

.d.

n.d

.n

.d.

n.d

.0·

200·

255·

12

K2O

0·02

0·01

0·00

40·

020·

006

0·01

00·

000·

010·

000·

000·

010·

001·

78

P2O

50·

000·

020·

015

0·04

0·03

20·

013

0·01

0·01

0·01

0·00

0·01

0·00

0·73

Cr 2

O3

n.d

.n

.d.

0·36

n.d

.0·

460·

37n

.d.

n.d

.n

.d.

n.d

.n

.d.

n.d

.0·

03

NiO

n.d

.n

.d.

0·27

n.d

.0·

260·

29n

.d.

n.d

.n

.d.

n.d

.n

.d.

n.d

.0·

01

Tota

l98

·27

98·0

110

0·18

98·2

210

0·19

98·7

498

·27

97·1

897

·89

99·1

698

·61

99·3

498

·85

Mg

#0·

914

0·90

50·

901

0·91

60·

905

0·90

10·

904

0·91

20·

909

0·90

40·

909

0·90

10·

621

Ca/

Al a

t1·

091·

221·

241·

011·

271·

231·

121·

361·

651·

081·

311·

20

An

alys

eso

fsa

mp

les

28b

,4-

36-9

0,an

d63

-90-

18ar

efr

om

Ion

ov

etal

.(1

996)

;an

alys

iso

fS

verr

eb

asal

tis

fro

mFu

rnes

etal

.(1

986)

.n

.d.,

no

td

eter

min

ed.

∗Niig

ata

anal

yses

.

2225


Fig. 3. Major element variations in Spitsbergen whole-rock xenoliths. Oxide contents are in weight percent (normalized to 100% total). Α,amphibole-bearing peridotites; Χ, ‘anhydrous’ peridotites. Shown for comparison as grey symbols are xenoliths in basalts from Siberia andMongolia (small open circles), xenoliths in Yakutian kimberlites (crosses) and peridotites from Horoman and Ronda massifs (small rhombs).Continuous grey line outlines typical compositions of xenoliths in basalts; dashed lines contour typical compositions of xenoliths in kimberlitesand massif peridotites. Shaded fields in (c) and (d) show chemical trends calculated for residues from fractional partial melting of fertile spinellherzolites (Niu, 1997). Data sources: Frey et al. (1985), Ionov (1986), Press et al. (1986), Ionov et al. (1993a, 1999b), Boyd et al. (1997), Wiechertet al. (1997), Takazawa et al. (2000) and unpublished data of D. A. Ionov (2000).

(1977) and Brey & Kohler (1990) are listed in Table 1. have roughly estimated equilibration pressures for thelherzolites in this study (11–15 kbar) by projecting theirThe values for the Halvdan xenoliths seem to be generally

higher than those for the Sverre xenoliths, but this Ca-opx T range (890–1025°C) onto a P–T trend cal-culated by applying the same thermometer (Brey &difference may not be meaningful considering the small

number of samples analysed. Our T estimates Kohler, 1990) and relevant barometers to publishedelectron microprobe analyses of garnet-bearing py-(840–1030°C) are much lower than the range of

940–1170°C reported for Spitsbergen spinel lherzolites roxenites from Spitsbergen (Amundsen et al., 1987).Minerals in most samples show no, or only limited,by Amundsen et al. (1987). We failed to obtain T values

>1080°C applying the same thermometer (Sachtleben & chemical zoning. In several xenoliths, pyroxene rims havelower Al and Cr than the cores. Harzburgite 26a hasSeck, 1981) to samples in this study or published data

(Furnes et al., 1986). Projection of our T estimates onto the particularly strong Al–Cr zoning and low Al in pyroxenes(coupled with Cr-rich spinel, Fig. 4b) and a very highgeotherm for northwestern Spitsbergen after Amundsen et

al. (1987) yields unrealistically low pressures of 7–11 kbar. Mg# in clinopyroxene relative to that in olivine andorthopyroxene. Because Fe–Mg partitioning between ol-We conclude that the above geotherm overestimates

temperatures in the uppermost mantle, apparently be- ivine and pyroxenes is temperature dependent, the highMg# in clinopyroxene may be related to the very lowcause of the use of inappropriate thermometers. We

2226


Tab

le3

:E

lect

ron

mic

ropr

obe

conc

entr

atio

nda

tafo

rm

iner

als

(in

wt

%);

data

are

for

min

eral

core

sun

less

othe

rwis

esp

ecifi

ed

4-90

-9(H

alvd

an)

311-

9(H

alvd

an)

315-

6(H

alvd

an)

Op

xc.

Op

xr.

Cp

xc.

Cp

xr.

Ol

Sp

lA

mp

hO

px

c.O

px

r.C

px

c.O

lS

pl

Gla

ssO

px

c.O

px

r.C

px

c.C

px

r.O

lS

pl

Am

ph

av.

of

7av

.o

f2

av.

of

3av

.o

f3

av.

of

2av

.o

f2

av.

of

4av

.o

f3

av.

of

3av

.o

f3

av.

of

2av

.o

f3

av.

of

2av

.o

f4

SiO

255

·07

54·9

152

·15

52·1

140

·71

0·03

42·7

056

·23

55·6

752

·89

41·3

20·

0165

·21

54·1

054

·57

51·7

151

·55

40·2

50·

0442

·99

TiO

20·

040·

060·

160·

190·

071·

300·

050·

010·

190·

080·

390·

070·

070·

290·

240·

051·

31

Al 2

O3

4·10

4·22

5·56

5·72

50·9

214

·34

3·51

3·61

4·98

50·1

018

·92

4·03

3·45

4·77

4·79

54·5

014

·48

Cr 2

O3

0·41

0·39

0·85

0·85

14·6

91·

350·

360·

460·

7716

·48

0·06

0·38

0·24

0·63

0·70

11·4

81·

08

FeO

6·34

6·17

3·07

3·15

10·0

312

·51

4·34

5·71

5·58

2·49

9·23

11·1

61·

826·

166·

262·

582·

549·

3111

·55

4·05

Mn

O0·

150·

150·

080·

080·

140·

050·

120·

080·

080·

160·

040·

140·

110·

080·

040·

150·

05

Mg

O32

·97

32·8

315

·59

15·5

548

·96

19·5

317

·41

34·0

933

·94

15·9

650

·45

19·6

81·

4433

·33

33·6

115

·59

15·4

148

·92

20·3

317

·55

CaO

0·76

0·78

20·3

420

·26

0·06

10·6

10·

630·

6421

·06

5·71

0·47

0·50

22·0

321

·86

0·03

11·1

2

Na 2

O0·

090·

101·

441·

413·

320·

060·

051·

173·

200·

040·

061·

131·

123·

63

K2O

0·80

1·75

0·02

NiO

n.d

.n

.d.

n.d

.n

.d.

0·12

0·12

0·05

0·38

0·27

0·00

n.d

.n

.d.

n.d

.

Tota

l99

·93

99·5

999

·23

99·3

299

·90

97·7

496

·21

100·

8810

0·14

99·6

310

1·55

97·7

898

·53

98·7

398

·85

98·8

298

·25

98·6

697

·93

96·2

7

Mg

#0·

903

0·90

50·

901

0·89

80·

897

0·73

60·

877

0·91

40·

916

0·92

00·

907

0·75

90·

585

0·90

60·

905

0·91

50·

915

0·90

40·

758

0·88

5

4-25

-90

(Hal

vdan

)31

8(H

alvd

an)

318-

1(H

alvd

an)

21-6

(Hal

vdan

)

Op

xc.

Op

xr.

Cp

xc.

Cp

xr.

Ol

c.S

pl

c.O

px

c.O

px

r.C

px

c.C

px

r.O

lS

pl

Am

ph

Op

xc.

Cp

xc.

Ol

Sp

lA

mp

hC

px

Am

ph

av.

of

3av

.o

f3

av.

of

2av

.o

f2

av.

of

2av

.o

f2

av.

of

2av

.o

f2

av.

of

3av

.o

f2

av.

of

3av

.o

f7

SiO

255

·50

55·6

052

·47

52·2

840

·85

0·08

55·5

455

·37

52·1

952

·30

40·4

842

·40

55·7

252

·69

40·6

50·

0742

·99

53·3

143

·43

TiO

20·

060·

050·

230·

270·

050·

120·

020·

020·

130·

130·

050·

760·

080·

290·

131·

400·

130·

73

Al 2

O3

4·03

4·09

4·94

5·40

0·03

48·1

32·

913·

054·

324·

2244

·81

14·1

04·

716·

1451

·12

14·7

53·

9714

·14

Cr 2

O3

0·52

0·50

0·78

0·96

18·4

10·

350·

470·

950·

9222

·60

1·73

0·54

0·95

14·9

11·

410·

741·

43

FeO

5·62

5·62

2·70

2·94

8·97

12·2

65·

315·

362·

292·

238·

3311

·66

3·23

5·76

2·92

9·07

11·4

73·

812·

103·

19

Mn

O0·

120·

100·

040·

100·

080·

130·

150·

080·

060·

100·

020·

160·

050·

130·

000·

040·

060·

05

Mg

O33

·77

33·7

616

·27

15·9

850

·76

19·8

934

·38

34·2

515

·76

15·9

350

·41

19·0

617

·73

33·1

715

·83

50·3

220

·30

17·6

516

·06

17·9

5

CaO

0·81

0·87

20·9

720

·89

0·10

0·58

0·67

21·4

121

·53

0·04

11·1

40·

9019

·79

0·08

10·9

221

·70

11·0

4

Na 2

O0·

080·

091·

231·

300·

070·

081·

241·

233·

200·

121·

442·

671·

063·

32

K2O

0·75

1·71

0·60

NiO

n.d

.n

.d.

n.d

.n

.d.

0·10

0·04

0·46

0·44

0·11

Tota

l10

0·52

100·

6899

·63

100·

1210

0·84

98·8

999

·26

99·4

298

·35

98·5

399

·35

98·1

795

·03

101·

2510

0·15

100·

7298

·40

97·4

899

·12

95·8

6

Mg

#0·

915

0·91

50·

915

0·90

60·

910

0·74

30·

920

0·91

90·

925

0·92

70·

915

0·74

50·

907

0·91

10·

906

0·90

80·

759

0·89

20·

932

0·90

9

2227


Tab

le3

:co

ntin

ued

21-5

(Hal

vdan

)4-

90-1

(Sig

urd

)25

(Sve

rre)

26a

(Sve

rre)

28b

(Sve

rre)

Hal

vdan

Cp

xc.

Sp

lA

mp

hA

mp

hC

px

c.O

lA

mp

hP

hl

Op

xC

px

Ol

Sp

lO

px

c.O

px

r.C

px

c.C

px

r.O

lS

pl

Op

xC

px

av.

of

4av

.o

f3

av.

of

4in

lava

av.

of

3av

.o

f2

av.

of

4av

.o

f3

av.

of

2av

.o

f3

av.

of

3av

.o

f4

av.

of

2av

.o

f2

SiO

253

·09

0·02

43·3

740

·57

50·8

540

·48

41·4

736

·55

55·1

451

·91

40·9

155

·62

56·4

352

·41

53·4

440

·77

0·01

53·8

451

·59

TiO

20·

130·

080·

844·

330·

723·

134·

850·

110·

490·

150·

030·

020·

020·

050·

090·

33

Al 2

O3

4·06

47·0

214

·78

14·4

05·

6413

·99

16·6

33·

435·

8957

·07

2·29

1·49

2·16

1·20

32·6

63·

805·

47

Cr 2

O3

0·77

22·3

51·

710·

081·

401·

381·

170·

280·

6011

·33

0·62

0·21

0·85

0·39

0·05

34·7

40·

320·

87

FeO

2·20

11·5

23·

4611

·93

3·59

10·4

34·

564·

836·

292·

159·

8810

·68

5·62

5·64

1·65

1·65

8·58

15·2

56·

072·

12

Mn

O0·

080·

000·

120·

28n

.d.

n.d

.n

.d.

n.d

.0·

130·

100·

040·

070·

080·

190·

12

Mg

O16

·13

19·2

018

·07

11·7

415

·26

47·3

316

·22

20·0

133

·67

15·1

949

·58

21·4

034

·20

35·0

916

·83

17·4

349

·87

15·3

333

·86

15·3

1

CaO

21·8

011

·50

9·79

20·9

411

·26

0·00

0·56

20·8

00·

800·

2824

·17

24·4

80·

6121

·96

Na 2

O1·

093·

353·

221·

382·

620·

830·

980·

020·

410·

271·

38

K2O

0·60

0·90

1·82

9·13

NiO

n.d

.0·

060·

540·

000·

00n

.d.

n.d

.n

.d.

n.d

.n

.d.

n.d

.n

.d.

n.d

.n

.d.

n.d

.

Tota

l99

·35

100·

1997

·68

97·1

499

·78

98·7

796

·43

93·9

999

·48

98·0

110

0·37

100·

6399

·27

99·2

898

·56

98·9

299

·33

98·0

398

·78

99·1

5

Mg

#0·

929

0·74

80·

903

0·63

70·

883

0·89

00·

864

0·88

10·

905

0·92

60·

899

0·78

10·

916

0·91

70·

948

0·95

00·

912

0·64

20·

909

0·92

8

28b

(Sve

rre)

39-8

6-1

(Sve

rre)

39-8

6-2

(Sve

rre)

63-9

0-18

(Sve

rre)

63-9

0-30

(Sve

rre)

Ol

Sp

lO

px

c.C

px

c.O

lS

pl

Op

xc.

Cp

xc.

Ol

Sp

lO

px

c.O

px

r.C

px

c.C

px

r.O

lS

pl

Op

xc.

Cp

xc.

Ol

Sp

l

av.

of

2av

.o

f3

av.

of

2av

.o

f2

av.

of

3av

.o

f4

av.

of

4av

.o

f3

av.

of

2av

.o

f2

av.

of

2av

.o

f3

av.

of

3

SiO

240

·48

55·4

451

·85

40·9

50·

0556

·07

52·5

840

·88

0·10

54·7

055

·23

51·0

952

·05

40·2

60·

0457

·02

52·3

940

·82

0·02

TiO

20·

050·

050·

240·

080·

080·

370·

070·

100·

070·

450·

290·

060·

150·

490·

08

Al 2

O3

56·6

43·

455·

1955

·76

3·71

5·23

55·2

53·

833·

385·

394·

7353

·77

2·18

4·82

43·3

1

Cr 2

O3

12·9

60·

330·

7912

·98

0·34

0·86

11·7

80·

360·

240·

790·

7112

·57

0·34

1·29

25·0

4

FeO

9·38

10·7

85·

712·

309·

1910

·21

6·06

2·37

9·36

10·2

56·

236·

022·

452·

429·

6310

·82

5·43

2·11

8·77

12·2

5

Mn

O0·

160·

120·

130·

110·

140·

100·

160·

100·

100·

000·

150·

120·

090·

100·

120·

000·

100·

070·

12

Mg

O50

·30

20·4

034

·38

15·8

549

·60

20·3

434

·12

15·6

849

·91

20·6

933

·28

33·5

915

·15

15·6

349

·04

19·8

834

·89

15·5

750

·65

18·3

9

CaO

0·46

21·5

00·

040·

010·

6021

·24

0·06

0·51

0·54

21·5

521

·83

0·05

0·66

21·4

30·

04

Na 2

O0·

071·

370·

000·

071·

220·

050·

051·

231·

150·

041·

34

K2O

NiO

n.d

.n

.d.

0·09

0·04

0·35

0·27

0·11

0·37

0·40

n.d

.n

.d.

n.d

.n

.d.

0·07

0·02

0·42

0·27

Tota

l10

0·32

100·

9510

0·11

99·2

310

0·26

99·8

010

1·32

99·6

410

0·67

98·5

499

·22

99·2

498

·19

98·9

299

·08

97·1

510

0·87

99·5

210

0·80

99·3

7

Mg

#0·

905

0·77

10·

915

0·92

50·

906

0·78

00·

909

0·92

20·

905

0·78

30·

905

0·90

90·

917

0·92

00·

901

0·76

60·

920

0·92

90·

911

0·72

8

Bla

nk

entr

ies,

bel

ow

det

ecti

on

limit

s;n

.d.,

no

td

eter

min

ed;

r,ri

m;

c,co

re.

Mg

#,at

om

icM

g/(

Mg+

Fe);

Cr#

,at

om

icC

r/(C

r+

Al)

.

2228


Fig. 4. Major element concentrations and ratios for minerals from Spitsbergen xenoliths (grain core compositions; symbols as in Fig. 3). Datafor Fe-rich sample 39-86 are from Ionov et al. (1996). Also shown are xenoliths in basalts from Siberia and Mongolia (see Fig. 3 for data sources),Samoa (shaded field; Hauri & Hart, 1994) and East Greenland (open grey circles contoured by dashed line; Bernstein et al., 1998) and abyssalperidotites (grey crosses; Hellebrand et al., 2001).

equilibration temperatures estimated for that sample of the largest orthopyroxene grains from sample 26ahave exsolution lamellae and very high Ca abundances,(740–800°C). The low T values indicate a relatively

shallow depth of origin, compared with the other xeno- indicating incomplete equilibration after cooling from T>1000°C.liths, possibly near the crust–mantle boundary. The cores

2229


Fig. 5. Plots of amphibole compositions: Mg#Amph vs Mg#Ol (a), Cr2O3 in amphibole vs Cr#Spl (b), and TiO2 in amphibole vs Mg# (c) and K2O(d). The regular trends for the major element contents in (a) and (b) indicate chemical equilibration of the amphibole with other minerals. Poorcorrelation in (c) and (d) suggests that the concentrations of K and Ti are controlled by metasomatic processes.

with increasing Mg#Ol (Fig. 6a). In contrast, the abund-TRACE ELEMENT COMPOSITIONSances of highly incompatible elements [Sr, light REE

OF MINERALS (LREE), Nb, Th, U] do not define coherent trends withAverage trace element abundances for the cores of clino- Mg# variations (Fig. 6b and c) and for nearly all thepyroxene, amphibole and mica grains determined by samples are much higher than those expected for residuesLAM-ICP-MS are given in Table 4. Differences between after partial melting (e.g. Johnson et al., 1990). Similarindividual analyses of the same mineral were normally observations were made earlier for many other suites ofwithin analytical precision, except for highly variable Zr mantle peridotites and attributed to metasomatic en-(28–97 ppm), Nb (0·4–1·1 ppm), Hf (1–2·5 ppm) and Ta richment in incompatible elements of residual rocks(<0·02–0·18 ppm) in clinopyroxene 39-86-1, and Rb formed by earlier partial melting events (e.g. Frey &(5·6–10·5 ppm) and Ba (760–1350 ppm) in amphibole Green, 1974; Kempton, 1987; McDonough & Frey,318-1. Sample 39-86-1 is obviously heterogeneous and 1989). Minerals from composite xenolith 4-90-1 andis not considered below. We also use LAM-ICP-MS harzburgite 26a commonly plot off trends defined by theanalyses of minerals in four xenoliths from Ionov (1998). other xenoliths.Several analyses of apatite were published earlier (Ionov Two major types of clinopyroxene can be identified inet al., 1996). the Spitsbergen xenoliths on the basis of their in-

The abundances of moderately incompatible elements, compatible element abundances and ratios (Fig. 6b–f ).such as the heavy rare earth elements (HREE) and Y, Type-1 clinopyroxene is characterized by lower con-are lower in the Spitsbergen clinopyroxenes than for centrations of the middle REE (MREE) and Sr (and

therefore lower MREE/HREE ratios, e.g. Nd/Yb)fertile lherzolites worldwide and decrease systematically

2230


Tab

le4

:A

vera

geL

AM

-IC

P-M

Str

ace

elem

ent

conc

entr

atio

nsof

min

eral

s

Typ

e:2

12

12

21

11

12

vein

vein

vein

21

22

Sam

ple

:21

-526

a31

1-9

315-

631

831

8-1

39-8

6-1

39-8

6-2

4-25

-90

63-9

0-18

63-9

0-30

4-90

-14-

90-1

4-90

-121

-531

5-6

318

318-

1M

iner

al:

Cp

xC

px

Cp

x∗C

px

Cp

xC

px

Cp

x∗C

px

Cp

xC

px∗

Cp

xC

px

Ph

l∗A

mp

h∗

Am

ph

Am

ph∗

Am

ph∗

Am

ph∗

No

.of

anal

yses

:1

24

32

25

32

33

22

31

54

4

Sc

7557

7071

8758

7879

6970

8052

6·6

4153

4957

44Ti

881

174

986

1833

725

1559

1367

2092

1910

2489

3450

4179

2542

818

655

4436

7346

4380

8134

Vn

.d.

171

238

252

231

242

266

271

252

285

297

244

299

356

n.d

.35

032

834

7R

b53

323

·59·

25·

87·

28·

1S

r28

712

·427

018

338

035

319

374

·718

212

765

·232

040

496

762

246

682

098

1Y

12·1

0·92

12·2

14·7

12·4

13·3

17·0

16·4

14·7

19·2

14·4

11·5

0·12

14·5

16·1

23·5

18·4

16·4

Zr

21·3

2·03

16·6

20·1

31·0

16·3

5925

·929

·029

·347

·116

027

·519

521

·220

·635

·819

·4N

b<0

·24

0·10

00·

93<0

·014

0·23

0·77

0·67

0·02

40·

08<0

·02

2·30

110·

816

74·

310·

9228

·268

·9C

s13

·30·

050·

130·

180·

19B

a<0

·30·

040·

170·

130·

210·

290·

110·

041

0·34

0·04

0·22

8935

669

445

1366

842

950

La8·

70·

479·

216

·112

·712

·111

·83·

0321

·05·

241·

3012

·617

·913

·920

·817

·715

·9C

e27

·21·

3226

·711

·629

·828

·919

·64·

4914

·47·

155·

0835

·50·

009

47·8

42·1

15·6

43·2

38·2

Pr

3·95

0·15

3·63

0·71

3·78

3·33

2·13

0·60

1·22

0·78

0·92

4·87

0·00

66·

485·

860·

985·

544·

44N

d16

·50·

8715

·93·

1017

·213

·49·

03·

424·

963·

985·

7722

·028

·423

·94·

223

·817

·2S

m2·

190·

203·

351·

143·

442·

282·

231·

431·

651·

382·

294·

840·

063

5·61

3·42

1·57

4·67

2·89

Eu

0·88

<0·0

31·

160·

541·

230·

880·

800·

600·

700·

680·

931·

832·

111·

410·

761·

741·

09G

d3·

110·

192·

881·

852·

662·

132·

422·

052·

122·

612·

924·

015·

162·

802·

633·

932·

67T

b<0

·02

0·45

0·38

0·39

0·37

0·42

0·41

0·40

0·48

0·49

0·52

0·72

n.d

.0·

560·

590·

45D

y2·

450·

192·

362·

672·

502·

462·

842·

872·

742·

983·

032·

813·

062·

933·

773·

322·

74H

o0·

510·

037

0·47

0·60

0·52

0·53

0·66

0·65

0·56

0·68

0·60

0·48

0·55

0·59

0·87

0·68

0·62

Er

1·60

0·14

1·28

1·72

1·34

1·43

1·83

1·95

1·68

2·00

1·49

1·07

1·33

1·67

2·41

1·92

1·64

Tm

n.d

.n

.d.

0·19

n.d

.n

.d.

n.d

.0·

290·

27n

.d.

0·30

n.d

.n

.d.

0·18

n.d

.0·

400·

290·

23Y

b<1

·30·

191·

241·

741·

321·

391·

831·

901·

651·

901·

311·

021·

021·

882·

471·

771·

59Lu

0·24

0·02

90·

170·

230·

180·

170·

290·

280·

230·

310·

180·

150·

150·

210·

340·

270·

22H

f<0

·66

<0·0

50·

480·

690·

850·

591·

270·

850·

931·

021·

356·

810·

777·

810·

710·

700·

960·

65Ta

0·03

0·12

0·01

70·

008

0·44

6·48

10·3

70·

079

0·46

0·28

Pb

0·81

0·30

0·43

0·51

1·06

0·38

1·62

0·45

1·23

0·56

0·26

0·32

1·24

1·27

2·27

1·97

3·11

1·07

Th

0·90

<0·0

120·

812·

011·

030·

592·

730·

271

4·62

0·51

0·09

40·

470·

0043

0·59

1·26

2·38

1·40

0·63

U<0

·23

0·03

50·

140·

660·

350·

108

0·92

0·09

71·

590·

190·

033

0·07

10·

013

0·06

90·

160·

700·

440·

11S

r/N

d17

·414

·217

·059

·022

·126

·321

·521

·936

·632

·011

·314

·534

·026

110·

434

·557

·2Ti

/Zr

4186

5991

2396

2381

6685

7326

926

9621

035

712

242

0Z

r/H

f34

2936

2846

3031

2935

2436

2530

2937

30N

b/T

a30

·75·

61·

45·

217

·116

·111

·661

·824

5·5

Zr/

Sm

1010

518

97

2618

1821

2133

356

138

7U

/Pb

0·12

0·33

1·29

0·33

0·28

0·57

0·22

1·29

0·34

0·13

0·22

0·05

0·07

0·36

0·14

0·10

Th

/U5·

83·

12·

95·

53·

02·

82·

92·

72·

86·

68·

57·

73·

43·

25·

7

∗An

alys

esat

CN

R-C

SC

C,

Pav

ia.

Bla

nk

entr

ies,

bel

ow

det

ecti

on

limit

;n.d

.,n

ot

det

erm

ined

.Val

ues

init

alic

sin

dic

ate

aver

ages

wit

hsi

gn

ifica

nt

dif

fere

nce

s(w

ella

bo

vean

alyt

ical

pre

cisi

on

)b

etw

een

ind

ivid

ual

anal

yses

.

2231


Fig. 6. Variation plots for abundances of Y, Nd and Sr in clinopyroxene vs Mg#Ol and for abundances and ratios of trace elements inclinopyroxene (symbols as in Fig. 4). Crosses are compositions of clinopyroxene from unmetasomatized fertile lherzolites from the southwesternUSA and Mongolia (Stosch & Lugmair, 1986; Galer & O’Nions, 1989) and unpublished data of D. A. Ionov (1998). PM, primitive mantle(Hofmann, 1988). Melt extraction trends are after Johnson et al. (1990). Plots (b)–(f ) outline two major types of Spitsbergen clinopyroxenes basedon abundances and/or elemental ratios of REE, Sr, Ti and Pb.

relative to Type-2 clinopyroxene, and by high La/Ce types 1a (LREE depleted) and 1b (LREE enriched) ofKempton (1987).ratios (Fig. 6e). We emphasize that the terms Type-1

and Type-2 are used here simply to distinguish between The differences between the two types are furtherhighlighted on primitive mantle (PM)-normalized tracethe two most common trace element patterns in this

xenolith suite. They are not related to the classification element distribution diagrams (Fig. 7). Type-1 clino-pyroxenes have nearly flat HREE–MREE patterns (atof Frey & Green (1974), which would identify all those

samples as Type I (based on their high Mg#), or with >4 times the PM level) followed by moderate depletion

2232


Fig. 7. Primitive mantle-normalized (Hofmann, 1988) REE and multi-element abundance patterns of clinopyroxene: (a) and (b) Type-1; (c) and(d) Type 2; (e) and (f ) vein and other samples together with fields for Types 1 and 2. Cpx 4-90-9 (after Ionov, 1998) has higher HREE andLREE contents than any other Type-2 sample. Dashed lines are clinopyroxene compositions in residues of 5–25% of partial melting of primitivemantle, calculated using algorithm and partition coefficients from Takazawa et al. (2000). It should be noted that the residual clinopyroxenecompositions were calculated for amphibole-free rocks. Comparisons between these model compositions and clinopyroxenes from amphibole-bearing xenoliths should take into account REE partitioning from clinopyroxene into the metasomatic amphibole (Fig. 8c).

from Eu to Nd (Pr) and a steep La–Ce–(Pr) inflection pronounced negative anomalies of those elements (Fig.7d) and low Ti/Eu ratios. Both clinopyroxene types also(Fig. 7a). Type-2 clinopyroxenes have somewhat lower

HREE concentrations (>3 × PM) followed by a con- have a similar range of Pb concentrations, but high Ndand Ce in the Type-2 bring about conspicuous negativetinuous increase in normalized REE concentrations from

Ho to Ce (Fig. 7c). Both types of clinopyroxene have Pb anomalies (Fig. 7d). A Ce/Pb vs Ti/Eu diagramclearly distinguishes between the two types (Fig. 6f ).strong negative Nb anomalies and small to moderate

positive Sr anomalies (Fig. 7b and d). Type-2 clino- Clinopyroxenes 26a and 63-90-30 cannot be groupedwith either Type-1 or Type-2: 26a has very low abund-pyroxenes have similar or only slightly lower con-

centrations of Ti, Zr and Hf compared with Type-1 ances of all lithophile trace elements and a nearly flatREE pattern; 63-90-30 has a convex-upwards REEclinopyroxenes, but their high MREE levels result in

2233


Fig. 8. Primitive mantle-normalized (Hofmann, 1988) REE (a) and multi-element (b) abundance patterns of amphiboles. Three samplesrepresenting the range of compositional variations are presented in (b). It should be noted that the only Type-1 amphibole has a patterndistinct from those in the rest of the samples (Type-2 and vein). (c) Amphibole/clinopyroxene trace element ratios. (d) Primitive mantle-normalized element abundances determined in the cores of clinopyroxene and amphibole by LAM-ICP-MS, and core–rim profiles by ionprobe in sample 315-6. The lack of significant core–rim zoning and consistency of the LAM-ICP-MS and the ion probe data should benoted.

pattern (Fig. 7e and f ). Vein clinopyroxene 4-90-1 has Kempton et al., 1999; Tiepolo et al., 2001). Vein am-phibole has high Nb and Ta abundances, no negativean REE pattern resembling those of Type-2 clino-anomalies for Ti and Zr, and a negative Pb anomalypyroxenes but differs from them largely by its much(Fig. 8b). All the amphiboles are relatively low in Rbhigher HFSE (high field strength element) abundancesand Cs, with Rb/Ba and Rb/Sr ratios below those ofand lower HREE (Fig. 7d–f ).the primitive mantle.Only one Type-1 sample contains amphibole (315-6),

Ion probe analyses in profiles across mineral grainswhereas all Type-2 xenoliths are amphibole bearing.found slightly lower Ti and V in rims compared withHowever, it is the elemental abundances and ratioscores of clinopyroxene (but not in the single amphibolerather than presence of ‘hydrous’ phases that define andgrain analysed). Other elements show no significantcharacterize the two geochemical xenolith types. Thedifferences in abundances between the cores and theREE pattern of Type-1 amphibole 315-6 is clearly distinctrims (Table 5, Fig. 8d). The ion probe and LAM-ICP-from that of Type-2 amphiboles (Fig. 8a); it also hasMS data for the same samples agree well (Fig. 8d).unusually low abundances of K (Fig. 5d), Nb and Ta

(Fig. 8b). REE patterns in amphiboles are very similarto those of coexisting clinopyroxenes; these two mineralsalso have very similar abundances of Th, U, Zr and Hf

TRACE ELEMENT COMPOSITIONS(Figs 8d and 9b). However, the amphiboles have muchOF BULK ROCKShigher concentrations of the alkalis, Ba, Nb, Ta and Ti,

and moderately higher concentrations of Pb and Sr (Fig. Trace element abundances in whole-rock samples are8c), consistent with amph–cpx relationships observed in given in Table 6. Ionov et al. (1993b) and Ionov (1998)

found high incompatible trace element concentrations inother mantle rocks (e.g. Ionov & Hofmann, 1995;

2234


Tab

le5

:Io

nm

icro

prob

etr

ace

elem

ent

conc

entr

atio

nsin

ppm

for

min

eral

sin

thin

sect

ions

(cor

e–ri

mpa

irs)

Sam

ple

:31

5-6

63-9

0-18

39-8

6-2

4-36

-90∗

Gra

in:

Cp

x(r

im-t

o-c

ore

pro

file

)A

mp

hC

px

Cp

xC

px

(rim

-to

-co

rep

rofi

le)

Po

siti

on

:ri

min

t1in

t2co

reco

reav

.o

f5

rim

core

av.

of

2ri

mco

reav

.o

f2

rim

core

av.

of

2A

BC

Dav

.o

f4

K18

018

218

1n

.d.

n.d

.n

.d.

n.d

.

Sc

6566

6968

6466

4951

5066

6666

7076

73n

.d.

n.d

.n

.d.

n.d

.

Ti13

0513

4415

2815

0614

9814

3675

7675

1075

4317

2323

6120

4217

6022

3519

97n

.d.

n.d

.n

.d.

n.d

.

V21

021

523

122

724

922

643

042

242

627

027

627

327

129

028

1n

.d.

n.d

.n

.d.

n.d

.

Cr

3597

3800

4436

3940

3904

3935

7301

7314

7308

4909

4901

4905

4565

5141

4853

n.d

.n

.d.

n.d

.n

.d.

Rb

4·5

5·1

4·8

Sr

158

159

158

160

173

162

440

441

441

118

111

114

7271

7119

117

517

618

418

1

Y14

·614

·314

·714

·713

·814

·425

·125

·225

·115

·216

·315

·716

·417

·617

·014

·214

·714

·914

·614

·6

Zr

16·4

17·3

18·1

18·7

17·9

17·7

22·1

21·9

22·0

25·5

26·1

25·8

22·9

24·4

23·6

33·8

28·0

30·0

35·6

31·8

Nb

0·73

0·75

0·74

0·03

0·03

0·03

30·

040·

070·

054

Cs

0·18

0·19

0·19

n.d

.n

.d.

n.d

.n

.d.

Ba

1215

1312

1264

0·03

0·06

0·05

0·04

0·09

0·07

La14

·114

·213

·914

·514

·514

·221

·721

·121

·45·

14·

85·

02·

632·

682·

720

·619

·920

·922

·220

·9

Ce

9·8

9·5

9·9

10·1

10·0

9·86

14·8

15·3

15·0

7·2

6·6

6·9

4·0

4·1

4·0

31·7

30·4

32·3

34·5

32·2

Nd

3·3

3·2

3·2

3·1

3·1

3·20

5·1

5·2

5·16

3·6

3·2

3·41

3·2

3·6

3·36

6·75

6·21

6·32

7·22

6·62

Sm

1·22

1·24

0·98

1·26

1·26

1·19

1·64

1·77

1·71

1·27

1·40

1·34

1·18

1·25

1·21

1·43

1·42

1·42

1·53

1·45

Eu

0·48

0·47

0·46

0·52

0·53

0·49

0·52

0·55

0·54

0·53

0·56

0·54

0·58

0·59

0·66

0·62

0·62

Gd

1·83

1·85

2·52

1·98

2·00

2·04

2·90

2·83

2·86

2·06

2·31

2·19

2·17

2·26

2·21

2·31

2·23

2·31

2·40

2·31

Dy

2·41

2·44

2·59

2·52

2·32

2·46

4·22

4·43

4·32

2·74

3·04

2·89

2·87

3·13

3·00

2·37

2·54

2·74

2·72

2·59

Er

1·53

1·61

1·68

1·73

1·51

1·61

3·02

2·98

3·00

1·55

1·76

1·66

1·87

1·90

1·89

1·64

1·72

1·72

1·80

1·72

Yb

1·40

1·60

1·39

1·42

1·56

1·47

2·67

2·57

2·62

1·51

1·60

1·56

1·65

1·72

1·69

1·38

1·44

1·42

1·50

1·43

Si

was

use

das

inte

rnal

stan

dar

d.

n.d

.,n

ot

det

erm

ined

.∗A

nal

ysed

atM

on

tpel

lier.

2235


Tab

le6

:T

race

elem

ent

anal

yses

ofw

hole

rock

sby

solu

tion

ICP

-MS

(ppm

)

BIR

-1JP

-1Le

ach

ing

pro

du

cts

WR

per

ido

tite

sB

asal

ts†

R.V

.av

.o

f2

R.V

.av

.o

f2

25le

ach∗

25re

sid∗

25∗

26a

28b

311-

931

5-6

318

318-

131

8-1∗

39-8

6-1

39-8

6-2∗

4-25

-90

4-36

-90

63-9

0-18

63-9

0-30

H1-

25H

3-15

S35

a39

-86-

2

Sc

4448

·76

7·2

8·0

2·56

11·3

512

·60

9·4

12·3

12·6

13·9

8·8

16·0

14·6

13·2

n.d

.2·

110

·314

·313

·114

·615

·116

·114

·3

V31

336

5·2

27·6

18·1

n.d

.n

.d.

n.d

.32

5051

6334

6966

53n

.d.

4939

6349

n.d

.n

.d.

n.d

.n

.d.

Co

5156

·53

116

118

n.d

.n

.d.

n.d

.12

111

912

011

111

811

210

511

8n

.d.

117

118

110

121

n.d

.n

.d.

n.d

.n

.d.

Rb

0·24

0·21

0·34

0·33

0·45

30·

223

0·78

70·

300·

180·

260·

420·

780·

070·

072

0·65

0·25

0·63

0·67

0·40

0·57

49·3

51·9

49·8

50·4

Sr

110

109·

70·

570·

648·

498·

0318

·26

1·25

11·0

35·4

29·8

51·1

44·2

45·0

26·1

13·2

67·8

57·9

18·6

7·7

1118

1174

1076

1060

Y16

·517

·35

0·10

0·10

7n

.d.

n.d

.n

.d.

0·06

1·62

1·65

2·43

1·26

2·51

2·54

2·30

2·11

1·91

1·58

2·47

1·74

27·2

27·5

26·7

25·5

Zr

14·5

15·9

05·

345·

490·

072

2·41

2·90

0·20

2·52

2·17

3·33

2·56

2·87

2·91

6·08

3·67

4·61

3·33

3·45

5·32

231

238

241

243

Nb

0·55

0·59

0·03

60·

040

0·00

40·

072

0·10

70·

084

0·03

62·

450·

131·

260·

961·

021·

620·

073

0·48

0·21

0·08

40·

111

77·1

77·8

80·1

79·6

Cs

0·00

50·

006

0·03

50·

040

n.d

.n

.d.

n.d

.0·

100·

017

0·02

60·

040

0·11

0·00

70·

006

10·2

00·

022

0·09

90·

067

0·16

0·13

1·05

1·15

0·91

0·93

Ba

6·4

6·8

9·8

9·9

1·32

1·05

3·89

0·83

11·0

14·9

22·7

31·7

13·6

13·4

11·1

6·0

16·7

20·9

4·4

3·0

930

988

962

960

La0·

580·

630·

030

0·02

80·

016

0·15

10·

207

0·04

20·

601·

061·

991·

311·

541·

521·

320·

362·

031·

300·

450·

155

71·0

74·8

67·6

67·1

Ce

1·85

1·95

0·05

40·

066

0·03

10·

284

0·41

10·

065

0·74

2·76

1·43

2·65

3·74

3·68

2·10

0·56

1·71

2·14

0·66

0·49

116

121

111

110

Pr

0·37

0·39

0·00

710·

0083

0·00

410·

058

0·07

50·

008

0·07

0·39

0·10

0·35

0·48

0·47

0·24

0·08

00·

170·

170·

080·

1011

·511

·911

·011

·0

Nd

2·35

2·47

0·03

30·

031

0·02

00·

320

0·40

30·

030

0·30

1·71

0·42

1·42

1·86

1·84

0·95

0·44

0·71

0·50

0·40

0·54

42·9

44·6

41·8

41·2

Sm

1·10

1·15

0·01

30·

009

0·00

660·

134

0·16

20·

006

0·12

0·36

0·15

0·27

0·35

0·33

0·23

0·17

0·19

0·13

0·16

0·22

8·28

8·62

8·12

8·13

Eu

0·52

0·53

0·00

340·

0025

0·00

240·

056

0·06

60·

002

0·04

80·

124

0·06

50·

095

0·12

20·

121

0·08

10·

074

0·07

10·

050

0·06

90·

081

2·81

2·91

2·74

2·74

Gd

1·97

1·90

0·01

30·

010

0·00

980·

217

0·23

70·

007

0·17

0·33

0·25

0·24

0·35

0·34

0·29

0·27

0·24

0·18

0·26

0·29

7·34

7·46

7·27

7·29

Tb

0·38

0·38

0·00

260·

0022

0·00

170·

043

0·04

70·

001

0·03

40·

049

0·05

00·

036

0·05

80·

058

0·05

20·

056

0·04

40·

034

0·05

20·

049

n.d

.n

.d.

n.d

.n

.d.

Dy

2·50

2·62

0·01

80·

014

0·01

240·

311

0·33

80·

008

0·24

0·27

0·35

0·20

0·38

0·37

0·34

0·39

0·29

0·24

0·36

0·29

5·36

5·43

5·17

4·96

Ho

0·57

0·61

0·00

430·

0035

0·00

280·

069

0·07

70·

002

0·05

50·

056

0·08

40·

042

0·08

60·

085

0·07

80·

098

0·06

50·

055

0·08

40·

060

0·95

0·97

0·93

0·88

Er

1·70

1·80

0·01

40·

013

0·00

850·

209

0·22

90·

008

0·17

0·16

0·26

0·12

0·26

0·25

0·23

0·29

0·20

0·17

0·26

0·17

2·23

2·22

2·12

2·10

Tm

n.d

.n

.d.

n.d

.n

.d.

0·00

140·

032

0·03

6n

.d.

n.d

.n

.d.

n.d

.n

.d.

n.d

.n

.d.

n.d

.0·

044

n.d

.n

.d.

n.d

.n

.d.

n.d

.n

.d.

n.d

.n

.d.

Yb

1·60

1·71

0·02

10·

020

0·00

900·

209

0·23

50·

019

0·18

0·16

0·27

0·12

0·26

0·25

0·24

0·29

0·20

0·17

0·26

0·15

1·69

1·71

1·53

1·43

Lu0·

250·

260·

0047

0·00

440·

0016

0·03

70·

039

0·00

40·

029

0·02

60·

042

0·02

00·

040

0·04

10·

038

0·04

90·

031

0·02

80·

042

0·02

20·

220·

230·

200·

19

Hf

0·56

0·59

0·12

0·11

30·

0017

0·08

40·

089

0·00

50·

077

0·04

80·

094

0·06

00·

088

0·08

80·

116

0·11

80·

110

0·08

00·

106

0·13

44·

875·

024·

975·

01

Ta0·

060·

040·

0036

0·00

400·

0002

0·00

300·

0046

0·00

20·

004

0·02

60·

005

0·02

40·

007

0·02

00·

089

0·00

50·

017

0·00

50·

004

0·00

33·

493·

683·

453·

39

Pb

3·0

2·26

0·09

00·

108

0·05

90·

082

0·28

20·

027

0·05

50·

110·

070·

130·

390·

580·

180·

207

0·12

0·08

0·06

0·04

5·6

6·2

5·6

5·5

Th

0·03

00·

033

0·01

20·

013

0·00

130·

0086

0·01

80·

005

0·09

70·

099

0·27

30·

127

0·08

10·

082

0·34

70·

044

0·46

0·19

60·

049

0·01

29·

5310

·06

8·64

8·39

U0·

010

0·01

30·

012

0·01

20·

0033

0·00

310·

009

0·01

20·

029

0·02

80·

095

0·04

50·

021

0·02

10·

106

0·01

60·

290·

153

0·01

60·

009

2·1

2·2

2·0

2·0

Nb

/Ta

29·1

23·9

23·0

35·3

10·1

93·0

25·9

53·2

128·

150

·018

·315

·427

·340

·718

·832

·322

·121

·123

·223

·5

Zr/

Hf

42·2

28·6

32·6

40·8

32·8

44·8

35·3

42·6

32·5

33·0

52·6

31·1

42·0

41·6

32·6

39·8

47·4

47·5

48·5

48·6

Th

/U0·

382·

772·

020·

483·

403·

542·

882·

853·

903·

863·

292·

711·

581·

283·

031·

394·

604·

564·

334·

22

Nb

/Th

3·6

8·5

6·1

15·3

0·4

24·7

0·5

9·9

11·9

12·5

4·7

1·7

1·0

1·1

1·7

9·1

8·1

7·7

9·3

9·5

Ba/

La83

·07·

018

·820

·118

·414

·111

·424

·38·

88·

88·

416

·78·

216

·09·

719

·313

·113

·214

·214

·3

Nb

/La

0·28

0·48

0·52

2·02

0·06

2·31

0·06

0·97

0·62

0·67

1·23

0·21

0·23

0·16

0·19

0·71

1·09

1·04

1·19

1·19

Sr/

Nd

424·

525

·145

·441

·836

·420

·771

·635

·923

·724

·527

·429

·796

·111

5·2

46·4

14·4

26·1

26·3

25·7

25·7

Zr P

M/(

Sm+

Eu

) PM

0·43

0·66

0·66

1·30

0·81

0·24

0·79

0·38

0·33

0·34

1·05

0·79

0·95

0·99

0·77

0·96

1·14

1·13

1·21

1·22

∗An

alys

edat

Mo

ntp

ellie

r.†A

nal

ysed

atH

ob

art.

Rec

om

men

ded

valu

es(R

.V.)

are

fro

mE

gg

ins

etal

.(19

97)f

or

BIR

-1an

dfr

om

Mak

ish

ima

&N

akam

ura

(199

7)fo

rJP

-1.C

on

tam

inat

ion

-co

rrec

ted

(Fig

.10)

abu

nd

ance

sar

e:0·

06p

pm

Uan

d28

pp

mS

rin

4-36

-90;

0·2

pp

mU

and

23p

pm

Sr

in4-

25-9

0;0·

05p

pm

Pb

in39

-86-

2;0·

08p

pm

Pb

in31

8-1.

2236


interstitial material within some Spitsbergen xenoliths, inparticular high Sr, Ba and U in the carbonate. Becausethis study is concerned with the composition of Spits-bergen peridotites before the late-stage formation of theinterstitial materials, it is clearly important to establishtheir relative role in the whole-rock budget. Table 6 andFig. 9a give results of a leaching experiment on sample25, similar to those reported by Ionov et al. (1993b).Xenolith 25 has abundant interstitial material and highermodal carbonate than most other samples studied in thiswork (Table 1) and represents a ‘worst case’ example oftheir combined effects on whole-rock compositions. Theleachate (in 10% HNO3) from the crushed rock has verylow abundances of REE, HFSE and Th but is stronglyenriched in Sr, Pb, U, Ba and Rb. It is obvious that thespikes for those elements in the whole-rock pattern aremainly related to the interstitial material (Fig. 9a). Onthe other hand, the residue after leaching has small butconspicuous positive anomalies for the same elementsindicating qualitative similarities between trace elementsignatures of the late-stage interstitial material and meta-somatic patterns in the rest of the rock, as discussed byIonov et al. (1993b).

We further address the role of interstitial ‘contaminants’by plotting abundances of incompatible trace elementsin the bulk rocks versus those in clinopyroxene andamphibole (Fig. 10). The majority of the data points plotalong well-defined trends consistent with enrichments inLREE, Sr, Pb, Th and U mainly hosted by clinopyroxeneand amphibole. Anomalously high Sr in two whole-rocksamples is clearly related to their high modal calcite(Table 1), as for sample 25 (Fig. 9a), also reflected inelevated CaO on the CaO–MgO plot (Fig. 3c). Overall,the cases of anomalous whole-rock enrichments as aresult of eruption-related metasomatism or post-eruptionalteration are rare and taken into account below. Whole-rock abundances calculated from mineral and modalcompositions commonly are close to those directly meas-ured by solution ICP-MS (Fig. 9c), except for elementsthat tend to reside in interstitial micro-phases (Eggins etal., 1998; Bedini & Bodinier, 1999; Garrido et al., 2000;Kalfoun et al., 2002).

The REE patterns of whole-rock xenoliths are similarFig. 9. Examples of trace element distribution between major mineralsto those of their clinopyroxenes (compare Figs 11a andand interstitial material of the whole-rock xenoliths. (a) Whole-rock,7a, and 11c and 7c). The Type-1 and Type-2 patterns residue after leaching and leachate (largely representing interstitial

can be easily identified from LREE–MREE relationships material) for carbonate-bearing sample 25. (b) Whole-rock, amphiboleand clinopyroxene compositions for sample 318. (c) Ratios of tracefor most of the xenoliths, including those from the earlierelement concentrations in three peridotites calculated from mineralwork (Ionov et al., 1993b). For some elements, in particularanalyses and modal compositions to the abundances obtained by whole-

HFSE, their whole-rock abundances relative to those of rock analyses. For most elements the ratios are within 1·0 ± 0·1,indicating that they largely reside in the major minerals. By contrast,adjacent REE are distinct from those for clinopyroxene,a large proportion of Cs, Rb, Nb, Ta and Pb in some samples maywhose composition is significantly affected by partitioningreside in the interstitial material.into coexisting amphibole (Fig. 8c) and/or orthopyroxene

(Rampone et al., 1991; Ionov et al., 1995; Eggins et al.,1998; Garrido et al., 2000; Tiepolo et al., 2001). Whole- no or only minor negative Ti–Zr anomalies (compare

Figs 7b and 11b). The low Ti/Zr values, common inrock Type-1 xenoliths, unlike their clinopyroxenes, have

2237


Fig. 10. Plots of Sr, La, Pb and Th concentrations in whole-rock xenoliths vs those in clinopyroxene and amphibole (minerals from the samesample are connected by dashed lines). Data points for most of the samples define trends (shown by arrows) consistent with element residencepredominantly in clinopyroxene and amphibole. Outliers to the right of those trends are samples with high proportions of a given element inthe interstitial material.

Type-2 clinopyroxenes (Fig. 7d) as a result of strong Ti interstitial material are discarded, Type-2 rocks showhigher Sr (as well as Pr, Nd, Sm and Eu) than Type-partitioning into coexisting amphibole, are not seen in

the bulk rocks. Similarly, whole-rock Type-2 xenoliths 1 rocks (Figs 12 and 13). The abundances of highlyincompatible elements (La, Ce, Pb, Th and Ba) overlapdo not have negative Nb anomalies, and some yield high

Nb/La and Nb/Th values. However, all Type-2 rocks in Types 1 and 2, but the two types are clearly dis-criminated on the La/Ce vs Nd/Yb plot; this dem-have strong negative Ta anomalies, and therefore, high

Nb/Ta ratios (Fig. 11d) consistent with data for am- onstrates the strong LREE fractionation in Type-1xenoliths. The two types are also distinguished on a plotphiboles 318 (Fig. 9b) and 318-1, and clinopyroxene

311-9 (Table 4). of La vs Th (or U) because of higher Th at similar Lain highly LREE-enriched Type-1 xenoliths (Fig. 13).The abundances of moderately incompatible V, Sc, Y

and HREE are lower than in fertile lherzolites worldwide The distribution of Nb is distinct from that of Ti andZr (Fig. 13). Ti and Zr are uniformly low in both rockand decrease with increasing MgO (Fig. 12). There are

no differences in HREE and Y levels at similar MgO (or types and do not seem to be affected by metasomatism.Nb abundances are usually much higher in Type-2Mg#) between amphibole-bearing and amphibole-free

peridotites, unlike for clinopyroxene (Fig. 6a). Similar to xenoliths, and show no correlation either with Zr orLa and Th, elements that have relative peridotite–meltTi, Zr abundances are much lower than in the primitive

mantle (Fig. 13) and do not seem to be related to MgO compatibility similar to that of Nb. The high Nb cannotbe attributed to the presence of amphibole in the Type-(Figs 3d and 12). Abundances of Sr, LREE, Nb, Th, U,

Ba and Rb vary widely and do not define coherent trends 2 xenoliths alone because amphibole-bearing xenolith315-6 is as low in Nb as other Type-1 rocks. Zr/Hf andwith MgO variations (Fig. 12). If samples with Sr-rich

2238


Fig. 11. Primitive mantle-normalized (Hofmann, 1988) REE and multi-element abundance patterns for whole-rock peridotites: (a) and (b) Type-1; (c) and (d) Type 2; (e) and (f ) other samples and fields for Types 1 and 2. Dashed lines in (e) are compositions of residues of 1–20% of partialmelting of primitive mantle (spinel lherzolite), calculated using algorithm and partition coefficients from Takazawa et al. (2000). Data for samplesSB-4, SB-5, 28 and 43-86 are from Ionov et al. (1993b) (leached whole-rock compositions are shown for the Type-1 rocks to minimize effects ofcarbonate and other late-stage interstitial components).

Nb/Ta range from subchondritic in some Type-1 rocks CHEMICAL RECORD OF DEPLETIONto superchondritic in the rest of the xenoliths and are

AND METASOMATISMparticularly high in Type-2 rocks (Fig. 13). Zr/Hf valuesEvidence for partial melting andare positively correlated with some incompatible ele-mineral–melt equilibria in chemicalments, e.g. Nd, but vary broadly at similar Zr. By contrast,compositionsNb/Ta increases with Nb abundances both in Type-1

and Type-2 rocks (Fig. 13). Ce/Pb is commonly below Abundances of major oxides and moderately in-the PM value in both Type-1 and Type-2 rocks, which compatible elements in whole rocks and minerals varyis also seen as negative Pb anomalies in Fig. 11d (when systematically with variations in MgO contents and Mg#

in most of the xenoliths (Figs 3, 4, 6a and 12). Suchinterstitial Pb contamination is taken into account).

2239


in several samples is due to late-stage precipitation ofinterstitial calcite. On Mg# vs Al2O3 and MgO vs FeOplots (Fig. 3a and b), the Spitsbergen peridotites fallwithin the compositional field of the Horoman peridotitesinferred to have been derived by polybaric melting at2·5–0·4 GPa. Takazawa et al. (2000) concluded from dataon Mg–Fe partitioning that the Horoman peridotiteswith olivine Mg# and whole-rock MgO similar to thosein the most depleted Spitsbergen xenoliths (26a and 318)have been produced by 19–25% of partial melting.Importantly, amphibole-bearing Spitsbergen peridotitesplot within the compositional fields defined by the ma-jority of the xenoliths and follow major oxide variationtrends related to partial melting, indicating that am-phibole precipitation was not accompanied by significantadditions of ‘basaltic’ components (FeO, CaO, Al2O3,TiO2). An apparent exception is sample 318 (with highestmodal amphibole, >4%), which has Al, Ca and Tiabundances too high relative to MgO to be an unalteredmelting residue; yet the very low FeO in sample 318(Fig. 3b) rules out amphibole formation from an evolved,Fe-rich basaltic melt.

We calculated trace element abundances in residuesafter incremental (1% steps) partial melting of primitivespinel lherzolite using an algorithm and partition co-efficients from Takazawa et al. (2000). Compositions ofclinopyroxene and whole-rock samples from this studyare compared with the calculation results in Figs 7e and11e. The whole-rock abundances of HREE and MREEand Zr (elements least affected by metasomatism) areconsistent with 3–11% of partial melting for Type-1xenoliths and 5–12% for Type-2 xenoliths. The very lowHREE values in harzburgite 26a cannot be reproducedby batch melting and require 20–25% of incrementalpartial melting (see also Fig. 12), consistent with its highwhole-rock MgO, low whole-rock Ca, Al and Ti, lowmodal clinopyroxene (2·9%), and low Al, Ti and Na in

Fig. 12. Trace element abundances (in ppm) vs MgO (in wt %) in the clinopyroxene (Fig. 4). Sample 318 has higher MgOwhole-rock peridotites: Χ, Type-1 amphibole-free; Β, other am-

and Mg# than 26a (Table 2). However, the degree ofphibole-free; Μ, Type-1 amphibole-bearing; Α, Type-2. +, com-positions of unmetasomatized fertile lherzolites from central Mongolia incremental melting estimated for amphibole-rich xeno-(D. A. Ionov, unpublished data, 2000). PM, primitive mantle lith 318 from HREE abundances (>12%) is much lower(Hofmann, 1988). Dashed lines are melt extraction trends at 1 GPa than for 26a, possibly as a result of REE enrichment bycalculated using algorithm and partition coefficients from Takazawa

metasomatism.et al. (2000).Many Spitsbergen xenoliths plot off calculated partial

melting trends on covariation diagrams of moderatelyincompatible elements vs MgO (Fig. 12). Similar featurestrends are commonly interpreted as indicating that thein other mantle peridotites were previously attributed toperidotites formed as residues from variable degrees ofmelting in the garnet stability field, variable amountspartial melting and melt extraction from a fertile lherzolite

source. This assertion is supported by abundant ex- of trapped melt, changes in melting regime, etc. (e.g.Takazawa et al., 2000). Other processes, such as solid–meltperimental and modelling results and data on natural

peridotites [e.g. a recent review by Takazawa et al. (2000)]. mixing (Elthon, 1992) and solid–melt reaction (Kelemenet al., 1992; Van der Wal & Bodinier, 1996) may haveMost of the Spitsbergen xenoliths plot close to cal-

culated evolution lines for residual spinel peridotite at accompanied or followed the partial melting and meltextraction. It is hardly possible, however, to better con-1–2·5 GPa (Niu, 1997) on major oxide covariation dia-

grams (e.g. MgO vs CaO; Fig. 3c). Relatively high CaO strain melting conditions for the Spitsbergen peridotites

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Fig. 13. Covariation plots for trace elements (in ppm) and their ratios in whole-rock peridotites. Symbols as in Fig. 12. Continuous grey linesshow trace element ratios in primitive mantle. Arrows show possible trajectories of chemical change during melting and metasomatism.

because of widespread overprinting of the initial melting wehrlite (Fig. 2c) may reflect different degrees of magma–host interaction in the cross-section of a magmatic chan-signatures by later metasomatism.

The convex-upward trace element patterns of whole- nel (Kelemen, 1990), whereas abundant amphibole andphlogopite may be due to significant water and alkalirock xenolith 63-90-30 (Fig. 11) and its clinopyroxene

(Fig. 7) cannot be produced by partial melting and contents of the magma and/or in situ crystallization oftrapped fluid. Estimates of the trace element compositionindicate equilibration with a melt that has lower Nb and

LREE, and higher HREE than the host basalt (based on of a basaltic liquid in equilibrium with clinopyroxene 4-90-1 show strong enrichments in highly incompatiblepublished cpx/meltD; Fig. 14). Several other features also

set that sample apart from the rest of the xenoliths: high elements, with Th, U, Nb and LREE several times higherthan in the basalts hosting the xenoliths (Fig. 14). Thewhole-rock Ca/Al, high Ti in the bulk rock (Fig. 3d) and

clinopyroxene (Fig. 4d), and high Cr# in clinopyroxene REE pattern of that hypothetical liquid is similar tocalculated liquids for clinopyroxene from several Type-relative to spinel (Fig. 4c). However, the high Mg# in

whole rock (0·909) and olivine (0·911) appear to rule out 2 xenoliths (e.g. 311-9, Fig. 14a), except for somewhathigher LREE and MREE and lower HREE for cpx 4-an origin from or equilibration with an evolved, Fe-rich

basaltic melt. 90-1. The minor differences between the patterns cal-culated for clinopyroxene from the vein and from theMineral compositions in composite xenolith 4-90-1 are

distinct from those in the other xenoliths, in particular Type-2 xenoliths could reflect local variations in meta-somatic melt compositions. Alternatively, they can below Mg# and high Ti (Figs 4–7). That sample appears

to be a fragment of a magmatic conduit system in the explained assuming that the vein clinopyroxene equi-librated with an initial LREE-enriched melt (flowingperidotite mantle. The modal gradient from dunite to

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Fig. 14. Primitive mantle-normalized (Hofmann, 1988) trace element abundance patterns of average basalts from Halvdan and Sverre (blackcrosses and continuous lines). Also shown are: (a) clinopyroxene from three xenoliths (open grey symbols and lines) with REE patterns thatindicate equilibration with melts; (a) and (b) hypothetical basaltic liquids in equilibrium with those clinopyroxenes (filled symbols and dashedlines) calculated using cpx–melt partition coefficients after Hart & Dunn (1993) and Vernieres et al. (1997).

in a conduit), whereas clinopyroxenes in the peridotite trace element zoning—even in large, porphyroblasticxenoliths record equilibration (either partial or complete) Type-1 clinopyroxenes (revealed by ion probe analyses,with a melt modified during percolation from a conduit Table 5)—indicates that the mineral grains were in-into the host peridotites. We conclude that a melt with ternally homogenized by diffusion during or after meta-REE abundances in equilibrium with vein cpx 4-90- somatism. The time required for homogenization is1 could be an appropriate metasomatic agent for the primarily dependent on diffusion coefficients and tem-Spitsbergen xenolith suite. perature as well as on the distance. We use here ex-

perimental data of Sneeringer et al. (1984) on Sr diffusionin natural diopside, which are consistent with Sr diffusion

Mineral zoning and the timing of coefficients obtained on clinopyroxene (and amphibole)metasomatism in other studies. Possible temperatures may range from

[1200°C for metasomatism by a silicate melt toStrong compositional gradients between the depletedΖ1100°C for metasomatism by a carbonate-rich meltcores of minerals and their rims (in direct contact withor fluid. The lower T values may be more appropriatean enriched melt) may develop during initial stages of

metasomatism of melting residues. The lack of significant in view of the experimentally determined amphibole

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stability field in pyrolite compositions (Ζ1050°C) (Niida incompatible trace elements cannot produce strongLREE fractionations and the diversity of enrichment& Green, 1999) if it is applicable to less fertile Spitsbergen

peridotites. The solution of Fick’s equation given by patterns found in the Spitsbergen xenoliths. We then usenumerical modelling to demonstrate that those featuresCrank (1975) for Sr homogenization (from 45 to 181

ppm, to 95% of the final equilibrium level) to occur in can be explained by chromatographic effects of meltpercolation in peridotite matrix. Two modelling tech-a grain with a radius of 1–2 mm yields about 106 years

for T in the range 1050–1100°C and [104 years at niques are employed: (1) one-dimensional (1-D) meltpercolation with fixed rock/melt ratios; (2) ‘plate’ models1200°C. These results further indicate that the meta-

somatism was not a very recent event related to formation of reactive porous flow (percolative fractional crys-tallization). The results are presented in order of in-of interstitial silicate glass and carbonate.creasing model complexity, beginning with the REEdistribution in spinel peridotites during melt percolationat constant rock/melt ratios. More complex models ex-

Trace element evidence for mechanisms plore the effects of trapped melt, variable rock/meltand sources of metasomatism and its ratios, distinct modal compositions of the peridotite, andinterpretation the role of amphibole in element fractionation. TheseNearly all the xenoliths studied in this work show evidence models are first run for REE to constrain their abundancesfor modal and/or cryptic metasomatism, which must have in the initial metasomatic melt and outline the mostbeen widespread in the source regions of the xenoliths. In likely physical parameters of melt percolation. We thenthe discussion below we will focus on the nature of the introduce a broader range of elements (HFSE, Th, Umetasomatism, leaving aside the few samples that do not and Pb) assuming that their ratios to adjacent REE inbear well-defined enrichment signatures (26a, 63-90-30). the initial melt are close to those in the primitive mantle.

Geochemical evidence outlines two major types of Finally, alternative melt compositions (with anomaliesthe metasomatized rocks. One of them (Type-1) shows on PM-normalized diagrams for certain elements) aresmaller and highly variable degrees of metasomatism, considered to provide the best fit between the modelsand only one xenolith from that group contains am- and the Spitsbergen xenoliths. For simplicity, a single setphibole (with an unusually low K concentration). In of mineral–melt partition coefficients ( mineral/meltDElement) iscontrast, all Type-2 xenoliths contain amphibole (some used (Table 7), except for Nb and Ta in amphibole.also have apatite) and show strong enrichments in someincompatible elements, with very similar enrichment pat-terns (Figs 6–8 and 11). Although trace element patterns

The origin of strong LREE enrichment andand covariation plots (e.g. Fig. 13) clearly distinguishfractionation in the Spitsbergen xenolithsbetween the two groups, one cannot rule out that inter-The strongly fractionated LREE patterns of several Type-mediate compositions also exist in the mantle beneath1 xenoliths with extremely high La/Ce, La/Pr and La/Spitsbergen, but are not represented in this sample series.Nd ratios provide key evidence for their mode of origin.Because both groups have similar equilibration tem-These extreme fractionations between individual REEperatures, they appear to come from the same depthdo not appear to have analogues among any type ofrange in the uppermost mantle, but we have no in-terrestrial magma. Therefore, such REE patterns information on specific spatial relationships between the

two rock types, e.g. with relation to the sources of xenoliths cannot be modelled by simple mixing betweenLREE-depleted residues of partial melting and a mag-metasomatic media.

This study has to address two principal questions: (1) matic liquid. Figure 15 demonstrates that mixtures of amodel LREE-depleted peridotite with either host basaltwhether the two rock types were formed by two different

mechanisms of metasomatism or, alternatively, reflect or a hypothetical highly LREE-enriched liquid (in equi-librium with vein 4-90-1, Fig. 14) cannot reproducetwo distinct stages of the same mechanism (event); (2)

whether distinct compositions (and sources) of meta- high La/Ce or Sr/Nd ratios in typical Type-1 rocks.Furthermore, the ratios of highly incompatible elementssomatic media were involved in the formation of the two

rock types. The second question can be reworded as how in the mixing models are nearly uniform for moderatelyto highly enriched compositions (resulting in subparallelto distinguish between metasomatic features produced

by fractionation processes versus those that should be LREE–MREE patterns) because they are dominated bythose in the liquid. By contrast, the LREE patterns andattributed to distinct metasomatic melt–fluid com-

positions. La/Ce, La/Nd and other ratios vary broadly amongindividual Type-1 xenoliths and seem to require a specificThese questions are addressed below using theoretical

modelling of metasomatism. We first establish that simple liquid composition for each sample in the mixing models.Finally, Navon & Stolper (1987) have demonstrated thatmixing of depleted peridotites with melts enriched in

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Table 7: Mineral–melt partition coefficients and initial peridotite and melt compositions (ppm) used in the

modelling

Partition coefficients (Dmineral/melt) Peridotite matrix Percolating melt

cpx opx olivine spinel amph fertile refractory vein model ‘best fit’

Th 0·012 0·00003 1E − 07 1E − 08 0·0145 0·00038 0·00003 47·9 47·9

U 0·0103 0·00004 1E − 07 1E − 08 0·0125 0·00016 0·00001 11·8 11·8

Nb 0·0077 0·0005 0·0002 0·001 0·20 0·01 0·0007 299 299

Ta 0·02 0·001 0·0004 0·002 0·20 0·001 0·00007 17·2 5·7

La 0·054 0·0002 0·0001 0·0004 0·086 0·033 0·003 234 234

Ce 0·086 0·0004 0·0002 0·0005 0·138 0·160 0·013 391 391

Pb 0·072 0·0012 0·0002 0·0007 0·18 0·024 0·002 31·5 15·7

Pr 0·139 0·0006 0·0003 0·00065 0·222 0·045 0·0053 36·5 36·5

Sr 0·128 0·0003 0·0001 0·0001 0·32 4·22 0·57 2500 4853

Nd 0·187 0·001 0·0004 0·00058 0·299 0·34 0·054 118 118

Zr 0·13 0·005 0·004 0·005 0·156 3·36 0·672 784 280

Hf 0·2 0·01 0·006 0·01 0·24 0·105 0·025 21·6 4·6

Sm 0·291 0·003 0·00044 0·00048 0·466 0·16 0·044 16·6 16·6

Eu 0·35 0·004 0·00056 0·00045 0·56 0·067 0·023 4·2 4·2

Ti 0·33 0·07 0·007 0·15 1·32 520 195 27300 19500

Gd 0·4 0·0128 0·00076 0·00042 0·64 0·26 0·107 10·0 10·0

Tb 0·429 0·0186 0·00104 0·00041 0·686 0·051 0·025 1·21 1·21

Dy 0·442 0·0261 0·0014 0·0004 0·707 0·36 0·192 6·36 6·36

Ho 0·439 0·0356 0·00184 0·00041 0·702 0·088 0·047 1·09 1·09

Er 0·436 0·0474 0·00236 0·00042 0·698 0·267 0·144 2·45 2·45

Tm 0·433 0·0617 0·00296 0·00045 0·693 0·042 0·023 0·36 0·36

Yb 0·43 0·0787 0·00364 0·00048 0·688 0·28 0·158 2·37 2·37

Lu 0·427 0·0986 0·0044 0·00053 0·683 0·043 0·024 0·35 0·35

References: D cpx/melt values for REE, Nb, Pb and Sr are from Hart & Dunn (1993); D cpx/melt values for Zr, Hf and Ti are fromJohnson (1998); D cpx/melt values for Th and U are from Hauri et al. (1994); D cpx/melt values for Pr, Eu, Gd, Tb, Ho and Tm wereobtained by interpolation of data for adjacent REE; D cpx/melt for Ta is from Lundstrom et al. (1998), consistent with averageDNb/DTa ratios from Forsythe et al. (1994) and Blundy et al. (1998). Dopx/melt, Dol/melt and D spl/melt values are based on theD cpx/melt combined with D cpx/opx, D cpx/ol and D cpx/spl values from peridotite xenolith studies after Bedini et al. (1997) and Verniereset al. (1997). Some of the original Dmineral/melt values were rounded up or slightly modified to provide a consistent and smoothset of peridotite–melt partition coefficients. D amph/melt values were obtained from D cpx/melt based on mean amph/cpx ratios inSpitsbergen xenoliths for different groups of elements: 1·2 for Th, U, Zr and Hf; 1·6 for REE; 2·5 for Pb and Sr; 4 for Ti; 10for Ta; 26 for Nb.

plots of LREE vs MREE for peridotite suites with LREE course of an enrichment event from a single initial LREE-enriched liquid.enrichment patterns similar to those in this work do not

form linear arrays expected in the case of simple mixing. The REE patterns of Type-1 xenoliths are very similarto those produced by modelling chromatographic effectsSimilarly, it is unlikely that an exotic metasomatic fluid

with extreme LREE fractionations and very high LREE/ during melt percolation (Navon & Stolper, 1987; Bodinieret al., 1990; Vasseur et al., 1991; Takazawa et al., 1992) andHREE (implied in mixing models) could form in equi-

librium with any common mantle rock type. It follows appear to show evidence for interaction with percolatingLREE-rich melts (Fig. 16). The ‘chromatographic’ mod-that possible origins for the trace element patterns of

Type-1 xenoliths may be related to enrichment processes els imply that the behaviour of elements during meltpercolation in residual peridotites is controlled by min-rather than to unusual metasomatic source compositions.

The key idea is that some metasomatic mechanisms may eral–melt partition coefficients. Elements with higherrock/meltD values, such as HREE, are selectively removedgenerate a range of fractionated REE patterns in the

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patterns ‘sample’ different zones in percolation systems(Fig. 16c). This hypothesis will be tested below usingnumerical modelling.

MECHANISMS AND SOURCES OFMETASOMATISM IN THE CONTEXTOF NUMERICAL MODELLINGOne-dimensional melt percolation modelsand the origin of Type-1 and Type-2 REEpatternsWere Type-1 and Type-2 metasomatic patterns formedin the same event and by the same process? To addressthat question we performed numerical simulation of REEand Sr distribution during 1-D porous melt flow inperidotite (Vasseur et al., 1991) suited to the xenolithseries in this work. The principles of the modelling areillustrated in Fig. 16a and b; modelling parameters andresults are presented in Fig. 17 (‘1-D.p’ models). Twoinitial LREE-depleted peridotite compositions rep-resentative of the modal variation range in the Spits-bergen xenoliths were used (Table 7), one ‘fertile’ (11%cpx) and one ‘refractory’ (6% cpx), based on estimatesof element abundances in partial melting residues (Fig.11e). The presence of amphibole in the rocks does notsignificantly affect comparisons with modelling results for

Fig. 15. Primitive mantle-normalized (Hofmann, 1988) REE abund- REE and can be simply considered equivalent to higherance patterns of mixing products (grey lines) of a model LREE-depleted modal clinopyroxene because REE patterns in amphiboleperidotite with: (a) average host basalt (melt proportion in wt %); (b)

are nearly parallel to those in clinopyroxene (Figs 8c andcalculated melt in equilibrium with vein cpx 4-90-1 (see Fig. 14).Comparison with representative whole-rock xenoliths (continuous black d and 9b). For percolating melt compositions we usedlines and symbols) shows that peridotite–melt mixing models cannot that of average host basalt and a hypothetical melt inreproduce either Type-1 or Type-2 patterns.

equilibrium with vein clinopyroxene 4-90-1 (Fig. 14,Table 7). These two compositions have smooth, LREE-enriched patterns and cover a range of PM-normalized

from the percolating melt by interaction with minerals abundances (from 3–5 for Lu to 100–500 for La). Sr wasin the host peridotite so that the melt at the percolation included in some of these models because its residencefront becomes progressively enriched in the most in- and behaviour in peridotites is similar to that of thecompatible elements relative to the less incompatible LREE and because nearly all xenoliths have positive Srelements. As a consequence, an advancing metasomatic anomalies.percolation front first produces enrichments in the most The modelling using the host basalt composition (notincompatible elements followed by enrichments in the shown) failed to yield high LREE–MREE abundances,less incompatible elements, thus creating transient meta- like those in Type-2 and many Type-1 samples. Forsomatic zoning with different enrichment patterns as a example, a melt in equilibrium with typical Type-2function of the distance from the magma source (Fig. sample 311-9 (probably located close to the melt source)16b). must have LREE–MREE abundances much higher than

At an advanced stage of metasomatism, xenolith com- those in the host basalt (Fig. 14). This modelling outcomepositions approach chemical equilibrium with the mi- is not surprising because the ‘constant porosity’ modelgrating liquid (e.g. Navon & Stolper, 1987; Bodinier et we used does not involve fractional crystallization of theal., 1990; Vasseur et al., 1991), with REE patterns similar percolating melt and therefore cannot yield an increaseto those of Type-2 xenoliths. It may be possible therefore in its element abundances. The element fractionation into explain the whole range of REE patterns in the Type- such models is produced because the abundances of more1, Type-2 and vein samples by a single mechanism of compatible elements in the melt near the percolation

front decrease (because of their partitioning to the matrix)metasomatism assuming that xenoliths with different REE

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Fig. 16. A scheme and REE pattern diagrams illustrating the principles of one-dimensional (1-D) porous flow modelling. (a) Melt percolatesaway from its source (e.g. a vein) in the intergranular space of host peridotite, which is assumed to have constant porosity and modal composition.Melt and peridotite equilibrate by diffusion; no minerals precipitate from the melt. The model calculates average compositions of clinopyroxene,bulk matrix (peridotite without percolating melt) and percolating melt at a given time after the start of percolation in a given number of ‘cells’(equal distance intervals) within a percolation ‘column’ of a given total length (Z ). (b) Host peridotite near the melt source (cell 1) soon reachestrace element equilibrium with the initial melt, whereas far from the source (cell y) both percolating melt and host peridotite are selectivelyenriched in highly incompatible elements relative to elements with lower compatibility as a result of ‘chromatographic’ effects of mineral–meltinteraction (see text). (c) Comparison of REE patterns in Type-1, Type-2 and vein clinopyroxene in Spitsbergen xenoliths with results of 1-Dnumerical simulation of porous melt flow (using a model LREE-enriched melt composition) in an LREE-depleted lherzolite (13·5% cpx) forselected cells (numbered) after Bedini et al. (1997). (d) A schematic illustration of the principles of the reactive porous flow model used in thiswork (Vernieres et al., 1997). Unlike in 1-D modelling, the matrix porosity decreases as pyroxenes precipitate as a result of melt–matrix reactions,the total distance and duration are not defined, and the cell size decreases from the melt source to the end of the ‘column’.

faster than those of less compatible elements. It follows 17a) approach equilibrium with the initial melt for allREE and may correspond to vein mineral associationsthat the concentrations of least compatible elements (e.g.

La) in the initial melt should be sufficient to yield, (as in xenolith 4-90-1) rather than to Type-2 peridotites.The models with the refractory matrix also yield Type-1by mineral–melt equilibria, the strongest metasomatic

enrichments in those elements far from the melt and -2 REE patterns, but the highest LREE abundancesthey produce are only half of those with the fertile matrixsource.

The models using the ‘vein melt’ composition (with because of lower modal cpx in the refractory peridotite(Fig. 17b).higher LREE and MREE) yield good results for the

fertile initial matrix, producing (at different distances This fairly simple method of modelling melt porousflow does not address some aspects of that process, butfrom the melt source) both REE pattern shapes and

concentrations typical of Type-1 and Type-2 rocks (Fig. nevertheless clearly demonstrates several fundamentalconsequences of melt percolation in peridotites. First, a17a). It should be noted that mineral compositions for

cells next to the melt source (e.g. cells 1 and 2 in Fig. single initial liquid fractionates to produce a range of

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Fig. 17. Comparison of REE + Sr primitive mantle-normalized (Sun & McDonough, 1989) patterns in whole-rock xenoliths with results of 1-D numerical simulation of porous flow of an initial melt (similar to that in equilibrium with vein 4-90-1, as in Fig. 14) in a ‘fertile’ matrix (left:11% cpx, 24% opx, 63% ol, 2% spl) and a ‘refractory’ matrix (right: 6% cpx, 22% opx, 70% ol, 2% spl). REE and Sr abundances in the matrixwere calculated for appropriate residues after incremental partial melting (Fig. 11e). The mineral–melt partition coefficients are given in Table7 and largely follow those from Vernieres et al. (1997). Diffusivity in minerals is 10−14 cm2/s; diffusivity in melt is 10−8 cm2/s. Average radii ofminerals are 0·05 cm for cpx and 0·2 cm for opx and olivine. Other model parameters are shown on each graph: �, matrix porosity (meltfraction); Z, total percolation distance in cm; t, time after the start of percolation (in years); V, melt percolation velocity (in cm/year). Calculationsin each model are performed for 20 distance increments (cells) within a given total distance, ranging from 5 cm (1-D.p7 and 12) to 500 cm (1-D.p10 and 15); some cells are numbered on the plots. Dashed grey lines show compositions of cells 1–10 (closest to melt source); continuousgrey lines are compositions of cells 11–20 (far from melt source). (See Fig. 16 for explanations.)

element patterns, thus eliminating the need for multiple Melt percolation over short distances, e.g. from a veininto host peridotite, is likely to involve partial meltmelt–fluid compositions in simple mixing models (Bod-

inier et al., 1990). Second, highly fractionated element crystallization and decreasing effective porosity alonga temperature gradient. These effects cannot be fullypatterns are derived from ‘conventional’ initial melt com-

positions and do not imply the existence of exotic mantle reproduced in the 1-D porous flow model we used.However, an important step to more realistic modellingsources capable of generating a highly fractionated liquid

by equilibrium melting. Finally, strong element frac- can be made assuming that percolation stops at a certaintime and the fractionated melt present in the matrix istionation can be produced over relatively short distances,

i.e. the same order of magnitude as metasomatic aureoles ‘trapped’, to be incorporated into the peridotite afterrecrystallization. Figure 18 presents modelling resultsaround magmatic veins in peridotite massifs (Bodinier et

al., 1990; Navon et al., 1996; Zanetti et al., 1996), contrary analogous to those given in Fig. 17 except that the‘trapped melt’ was added to whole-rock compositionsto the view that long percolation distances are required

to produce significant ‘chromatographic’ effects (Nielson (‘1-D.tm’ models). Because the trapped melt has muchhigher abundances of LREE and MREE than the matrix& Wilshire, 1993). Modelling percolation over distances

from 1 to 100 m (Fig. 17a) has yielded similar results (because of low cpx/meltD), these models may producemarked enrichments of these elements in the rocksassuming that the distance to porosity ratio (Z/�) remains

the same. For instance, strong fractionation within 1 m compared with 1-D.p models, in particular for cpx-poor peridotites. The trapped melt effects are roughlyis possible at >0·01% porosity, a low but physically

realistic value (McKenzie, 1989). proportional to matrix porosity and become significant

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Fig. 18. Same as Fig. 17 except that modelling results were recalculated to obtain bulk compositions of the host matrix together with ‘trapped’percolating melt (‘1-D.tm’ models). Data for Type-2 Spitsbergen xenolith 43-86 are from Ionov et al. (1993b).

at �[0·1%. 1-D porous flow models with trapped melt through precipitation of clinopyroxene and ortho-pyroxene as a result of olivine–melt reaction. Comparedcontents between 0·1 and <1% provide the best fit with

REE data for the xenoliths in this work (Fig. 18). They with 11% cpx and 24% opx in the initial fertile matrix, thereaction products contain 11·3–12% cpx and 24·4–25·4%can explain both strong LREE fractionation in Type-1

sample 315-6 (model 1-D.tm10) and a continuous in- opx, implying only very moderate changes in bulk-rockmajor oxide contents (e.g. increasing CaO by Ζ0·2%crease in MREE and LREE abundances (model 1-

D.tm15) in highly metasomatized Type-2 sample 43-86 and Al2O3 by Ζ0·12%).The plate models produce both LREE enrichments(containing amphibole and apatite; Ionov et al., 1993b)

in a single event. and strong LREE fractionation far from the melt sourceas a result of ‘percolative fractional crystallization’ of theinitial melt (Godard et al., 1995; Bedini et al., 1997), evenfor liquids with relatively moderate LREE abundances

Reactive porous melt flow (plate models) like the host Spitsbergen basalts. However, as in theVernieres et al. (1997) developed a method of numerical 1-D modelling, combined LREE–MREE enrichmentssimulation of porous melt flow, which they called a ‘plate typical for Type-2 rocks are not likely to be generatedmodel’, to account for variable matrix porosity, modal with the host basalt as the initial melt. By contrast,compositions and mineral precipitation or dissolution the modelling using the hypothetical vein melt yieldsthrough matrix–melt reactions. The method assumes compositions that match very well those of both Type-1instantaneous melt equilibration as it moves from one and Type-2 rocks (Fig. 19). We conclude that the initialmatrix cell to another and therefore, by comparison metasomatic melt was probably strongly enriched inwith the 1-D models, disregards melt velocity, time and LREE and MREE, with REE abundances similar todistance. A simplified version of that method is used here those in our ‘vein model’. That inference only refers to(Fig. 16d) as an alternative to the 1-D ‘trapped melt’ the REE abundances and does not imply that the majormodels. The plate modelling presented in Fig. 19 for the and trace element composition of the metasomaticfertile and refractory matrices implies a decrease in medium was identical to that of the parental melt of the

vein 4-90-1.porosity from 1% near the melt source to near zero

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Fig. 19. Results of numerical simulation of reactive porous flow (plate model of Vernieres et al., 1997) in comparison with REE + Sr primitivemantle-normalized (Sun & McDonough, 1989) patterns in whole-rock xenoliths. Calculations are performed for 100 increments (cells), with cellsize decreasing from>10% of the column length in cell 1 to >0·008% in cell 100. Initial matrix porosity of 1% decreases to near-zero as meltis consumed in the sliding reactions ranging from {1 melt + 0·18 olivine = 0·91 melt + 0·162 opx + 0·108 cpx} at the base of the column(cell 1) to {1 melt+ 0·52 olivine= 0·74 melt+ 0·468 opx+ 0·312 cpx} at the top of the column (cell 100). The changing mineral proportionsin these reactions were selected to provide for an evenly distributed decrease in porosity along the column. Initial melt and matrix compositionsand mineral–melt partition coefficients are same as in Figs 17 and 18. For simplicity, results for cells 1–4 are grouped together as ‘near meltsource’ field, results for cells 15–80 are grouped as ‘far from source’ field.

The cell size in the plate model we used decreases kilometres (‘sampled’ by two volcanoes) and may haveultimately produced metasomatism in a large portion ofaway from the melt source and therefore the number

of cells with strong fractionation near the end of the the lithospheric mantle. The metasomatic medium wasstrongly LREE enriched (with higher LREE and MREEpercolation column is not representative of their fairly

small volume. To illustrate relative proportions of cor- abundances than in the host basalts).Various 1-D and plate percolation models for REEresponding rock types in the column, we calculated

average compositions of clinopyroxene in 10% intervals and Sr yield minor positive Sr anomalies far from themelt source but cannot reproduce high Sr spikesof the total percolation column for model PM-50 (Fig.

19). The results (Fig. 20) show that some 40% of the common in Spitsbergen xenoliths (Figs 17 and 18).One possible explanation is that the cpx/meltD for Sr iscolumn near the melt source (compositions 1–4) has a

Type-2 REE pattern, whereas the far half of the column close to that of Ce (rather than Pr in our modelling)as suggested by some experimental studies (Skulski et(compositions 6–10) has a Type-1 pattern. This is roughly

consistent with the proportions of the two rock types in al., 1994; Foley et al., 1996). That inference implies amore incompatible behaviour of Sr during meta-our xenolith suite.

Overall, either 1-D porous flow models with trapped somatism than follows from compilations based ontrace element systematics in oceanic basalts (Hofmann,melt or plate models (percolative fractional crystallization)

can satisfactorily match the principal REE pattern types 1988; Sun & McDonough, 1989) and appears to besupported by data on some cryptically metasomatizedand enrichment levels in the Spitsbergen xenoliths. We

conclude that REE patterns in the majority of the xeno- mantle peridotites (e.g. Ionov et al., 1994; Vannucci etal., 1998; Lenoir et al., 2000). Plotting Sr between Laliths can be explained by a single event and the same

process: reactive porous flow. That process probably and Ce on PM-normalized diagrams eliminates the Sranomaly in many Type-1 samples. Regardless ofoperated beneath an area on a scale of at least several

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Fig. 20. (a) Results of numerical simulation of reactive porous flow (same as in Fig. 19) followed by calculation of average clinopyroxenecompositions for 10 successive equal column volumes each making up 10% of the total. Volume 1 is closest to the melt source and approachesequilibration with the melt, volume 10 is farthest from the melt source. (b) Comparison of modelling results from (a) with fields of REE primitivemantle-normalized (Sun & McDonough, 1989) patterns in Type-1 and Type-2 clinopyroxene (see Fig. 7). LREE-rich cpx 4-90-9 is shownseparately from other Type-2 samples. Model compositions 1–4 are similar to those of Type-2 cpx; compositions 5–10 match those of Type-1cpx. It should be noted that REE abundances in some Type-2 clinopyroxenes may be lower than in the model results because of (1) preferentialREE partitioning into coexisting amphibole and (2) the use of fertile initial matrix in this model whereas many Type-2 xenoliths are morerefractory.

whether that observation is relevant, consistently high Zr, Hf, Th and U) than clinopyroxene (Fig. 8c). ToSr/Nd values in strongly metasomatized Type-2 xeno- explore the role of various factors in the behaviour ofliths (Fig. 13) appear to require a superchondritic Sr/ these elements in percolation metasomatism, we ranNd in the initial metasomatic liquid because Type-2 several 1-D porous flow models with and without am-rocks approach equilibrium with that liquid. Lower phibole. Element abundances in the fertile and refractorycpx/meltDSr values than those used in our modelling would peridotite matrix were derived from those of adjacentyield even higher Sr/Nd in the metasomatic liquid. REE to yield smooth depleted patterns on PM-nor-Alternatively, cpx/meltDSr values could have been higher malized diagrams. Similarly, the composition of the per-during the formation of Type-2 rocks than of Type-1 colating liquid was fixed to yield smooth enrichmentrocks (for example, as a result of changes in melt patterns on PM-normalized diagrams, with Th/U, Nb/composition). However, variations in mineral/meltD values, Ta and Zr/Hf values equal to those in the primitivee.g. those related to evolving melt compositions or mantle (Sun & McDonough, 1989) and no anomaliestemperature changes, were not considered in our relative to adjacent REE. The matrix and melt com-modelling, to keep the models as simple as possible. positions and partition coefficients are listed in Table 7.

The models without amphibole based on mineral–meltequilibria alone (no trapped melt) yield only moderateenrichments in Th, U and Nb (a1 and b1; Fig. 21), whichEffects of amphibole on REE–HFSEcannot match the high contents of these elements inrelationshipsstrongly metasomatized xenoliths. The mismatch is evenThe example of Sr highlights multiple problems involvedhigher than for LREE (discussed in previous section)in modelling the behaviour of elements during meta-because of very low cpx/meltD values for Th, U and Nb.somatism in comparison with REE of similar com-This problem is partially resolved if the interstitial meltpatibility. Whereas REE patterns in initial melts areis incorporated into the matrix, providing a good fit withexpected to be smooth, other elements may define an-the xenolith data for Th and U at matrix porosityomalies relative to adjacent REE. Furthermore, partition0·1–0·5% (a2 and b2; Fig. 21). Nevertheless, the ‘trappedcoefficients of some elements are less precisely constrainedmelt’ models cannot produce negative HFSE anomaliesor may vary with melt composition or other parametersbecause, first, the initial liquid does not have them and,in a different manner than those for REE (Lundstrom etsecond, the behaviour of elements during percolation isal., 1998). Metasomatic minerals may be strongly en-largely controlled by their cpx/meltD values, whichriched in certain elements and therefore control theirconsistently decrease from HREE to Th and are not verybudget even when present only in small quantities (Kal-different from those of adjacent REE (Table 7).foun et al., 2002). In particular, amphibole, a common

The only obvious way to fractionate HFSE from ele-metasomatic mineral in Spitsbergen xenoliths, has muchhigher abundances of Nb, Ta, Ti, Pb and Sr (but not ments with similar cpx/meltD values by porous melt flow

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Fig. 21. Results of 1-D numerical simulation of reactive porous flow in comparison with primitive mantle-normalized (Sun & McDonough,1989) trace element patterns (REE, Sr, HFSE, Th, U, Pb) in whole-rock xenoliths. Models 2 and 4 include trapped melt. Peridotite matrix inmodels 3 and 4 contains amphibole: 3% amph, 8% cpx in ‘fertile’ and 3% amph, 3% cpx in ‘refractory’. Modelling parameters: porosity, 0·5%;distance, 50 m; time, 5000 years; melt velocity, 1 cm/year. (See Table 7 for partition coefficients.) Symbols are the same as in Figs 17 and 18.It should be noted that mineral compositions for cells next to the melt source (commonly cells 1 and 2) approach equilibrium with the initialmelt for all REE and may correspond to vein mineral associations (as in xenolith 4-90-1) rather than to Type-2 peridotites.

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(assuming smooth distribution patterns in both initial Nb/La ratios (i.e. positive Nb–Ta anomalies) close to thematrix and melt) is by means of a mineral with higher source, as in Type-2 rocks. Models with higher modalDHFSE values than for clinopyroxene. Recent experimental amphibole (5%) and higher amph/meltDNb,Ta values (0·5)work (Tiepolo et al., 2000b) has yielded generally higher yield similar results (not shown). We emphasize that theamph/meltD values for Nb and Ta (0·1–1·6) than for La and negative Nb–Ta anomalies far from the melt sourceTh and also confirmed that DNb,Ta and DTi are much are a signature of the fractionated melt produced byhigher for amphibole than for clinopyroxene, as indicated percolation through the amphibole-bearing matrix closerearlier by natural amph–cpx pairs in mantle rocks (Ionov to the source and are not related to the presence or& Hofmann, 1995). We derived amph/meltD values (Table absence of amphibole in that zone. Therefore, both7) from those for clinopyroxene based on mean amph/ amphibole-bearing and amphibole-free peridotites farcpx ratios in Spitsbergen xenoliths for different groups from the source should have low Nb/Th and Nb/La, asof elements: 1·2 for Th, U, Zr and Hf, 1·6 for REE, 2·5 in the Type-1 xenoliths.for Pb and Sr, and 4 for Ti. amph/meltD values for Nb and The modelling of Nb–Ta behaviour during porousTa were both fixed at 0·2. These values are 10–26 times melt flow hinges critically on assumptions about thehigher than for clinopyroxene, yet are conservative in

partitioning of these elements (in comparison with LREE,view of the generally very high amph/cpx ratios (Fig.Th and U) between mantle minerals and metasomatic8c). The bulk matrix/meltD values are somewhat higher formedia. Experimental work has shown that DHFSE valuesTa than for Nb, both in amphibole-bearing and inmay be strongly affected by major element compositionamphibole-free peridotites, because of higher cpx/meltDof minerals and the silicate melt (Lundstrom et al., 1998;values for Ta than for Nb.Tiepolo et al., 2000b), proportion of carbonate componentWe prefer to derive amph/meltD values from cpx/meltD valuesin the melt (Blundy & Dalton, 2000) or a transition fromand the amph/cpx ratios in the xenoliths (rather than usea silicate melt to a supercritical fluid. Combined effectsexperimental amph/meltD data), so as to have an internallyof these factors are hard to assess but there is little doubtconsistent set of partition coefficients for these two min-that amph/meltD values for Nb and Ta are significantlyerals that host most of the REE and the HFSE in thehigher than cpx/meltD. Notably, xenolith studies consistentlyperidotites. This work aims to constrain general featuresreport very high amph/cpx ratios for Nb and Ta (in-of trace element behaviour during melt percolation (baseddicating high amph/meltDNb,Ta), whereas experimental workon a generalized set of mineral/meltD values) rather than

to evaluate amph/meltD values from specific experimental yields moderate DNb,Ta values, at least for Mg-rich am-studies. Experimental amph/meltD values vary broadly as a phiboles typical for mantle peridotites (Green, 1995).function of SiO2 in the melt and Mg# and TiO2 in Ionov & Hofmann (1995) suggested that precipitationamphibole (Tiepolo et al., 2000a, 2000b). Few data are of Nb,Ta-rich vein amphibole (and phlogopite) fromavailable for amphiboles with high Mg# and, in par- migrating melts may produce negative Nb–Ta anomaliesticular, low TiO2 as in the Spitsbergen xenoliths. Never- in residual liquids whereas Tiepolo et al. (2000b) arguedtheless, the amph/meltD values we used are within the ranges that experimentally determined amph/meltDNb values for Mg-reported by Tiepolo et al. (2001) for amphiboles with rich systems are not high enough relative to amph/meltDLaMg# [0·74: 0·2 vs 0·06–0·23 for Nb and Ta (DNb/Ta to effectively fractionate these elements by peridotite–melt0·7–1·2); 0·16 vs 0·15–0·51 for Zr; 0·24 vs 0·24–0·95 for equilibria. Our modelling shows that melt percolation inHf. The higher DHFSE values of Tiepolo et al. (2001) amphibole-bearing peridotites can fractionate Nb fromare for Ti-rich (>6% TiO2) amphiboles, whereas the La and Th even at small differences in bulk matrix–meltamphiboles in the Spitsbergen peridotite xenoliths have partition coefficients (in the same manner as La–Ce–Pr<1·5% TiO2. are fractionated, Figs 17–19). This happens becauseThe 1-D models were rerun with 3% amphibole and

chromatographic effects enhance the effects of differences8% clinopyroxene in the fertile matrix and 3% amphibolein matrix/meltD values and thus produce a wide range ofand 3% clinopyroxene in the refractory matrix. TheNb/(La, Th) values at different distances from the meltmodels with amphibole (a3, a4, b3 and b4; Fig. 21) showsource (a3–b4; Fig. 21). Similarly, Bedini et al. (1997)little difference for LREE, Th and U but indicate ashowed that porous melt flow may create negative Nb–Tadramatic change in the behaviour of Nb and Ta. Becauseanomalies far from the melt source if accessory mineralsDNb,Ta values for amphibole are higher than for clino-with high DNb,Ta values (e.g. rutile) are present, even inpyroxene, the bulk matrix–melt partition coefficientsvery small amounts, in the percolation column. Wefor Nb and Ta increase and ‘slow down’ the Nb–Taconclude that melt percolation through amphibole-bear-enrichment front in the percolating melt relative to othering rocks will inevitably decrease the abundances of Nbhighly incompatible elements. This produces negativerelative to those of Th and La and thus is likely toNb anomalies far from the melt source, similar to those

in Type-1 xenoliths, and moderate to high Nb/Th and produce negative Nb anomalies in the melt.

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strongly metasomatized Type-2 rocks without significantTrace element composition of the initialincreases in Zr abundance. The situation is different formetasomatic meltthe Nb–Ta pair because Nb has negative anomalies inThe modelling with amphibole yields relatively highType-1 xenoliths but positive anomalies in some Type-values for all HFSE and Pb in peridotites close to the2 rocks. Moreover, Nb abundances and Nb/Ta valuesmelt source (cells 1–5 in Fig. 21). By contrast, most Type-are positively correlated (Fig. 13), consistent with both2 xenoliths have negative anomalies for Ta, Zr, Hf, Tihigh Nb and very high Nb/Ta in the melt.and Pb, and only few samples have moderately high Nb/

The assumption of very high Sr and Sr/Nd in theTh ratios. We have failed to reproduce the negativeinitial melt does not provide a completely satisfactoryanomalies for these elements in Type-2 xenoliths withmatch of modelling results with the xenolith data. Themodels that assume a fixed initial melt composition andmodels yield positive Sr anomalies close to the meltvary other parameters within reasonable limits, evensource (Fig. 22) and therefore provide a better matchthough these models yield a good match for other ele-with Type-2 samples than models in Fig. 21. However,ments. If the initial melt has no anomalies for giventhey typically yield higher Sr abundances and Sr/Ndelements, the only way to produce the negative anomaliesvalues than in Type-2 xenoliths, indicating that the Sr/close to melt source in the percolation models is toNd in the initial melt was overestimated. On the otherassume low matrix/meltD values for those elements (relativehand, even those high Sr/Nd values in the initial meltto adjacent REE). As discussed above, that is not likelyfail to result in marked positive anomalies far from thefor HFSE in amphibole-bearing rocks and, furthermore,melt source to match the very high Sr/Nd in most Type-

low matrix/meltDHFSE values may result in positive HFSE 1 rocks. In general, positive anomalies for a highlyanomalies far from the melt source, which are not ob- incompatible element both near and far from the meltserved in the Type-1 rocks. More complex models, in- source do not appear to be a likely feature of thevolving intermittent precipitation of various accessory metasomatism by melt percolation.minerals (Bedini et al., 1997), variable partition co- Overall, the theoretical modelling strongly suggestsefficients, transition from a melt to fluid within a per- that both Type-1 and Type-2 trace element patterns incolation column (Ionov & Hofmann, 1995), etc. are the Spitsbergen xenoliths may have been produced bybeyond the scope of this paper. the same metasomatic mechanism and possibly in the

However, 1-D percolation models using a liquid with same event. The Type-1 REE patterns may have beennegative Ta, Zr, Hf, Ti and Pb anomalies, a positive Sr derived by small-scale reactive porous flow metasomatismanomaly and high Zr/Hf and Nb/Ta (‘best-fit’ in Table at the percolation front, whereas large-scale rock–melt7) have reproduced both Type-1 and Type-2 patterns interaction close to the melt source, probably approachingfor nearly all elements (Fig. 22). This leads us to conclude equilibration with the percolating melt, produced thethat the simplest way to explain the negative anomalies Type-2 patterns. Evidence from other elements is quant-in Type-2 rocks is for the initial melt to be markedly itatively consistent with such a model, assuming am-depleted (relative to adjacent REE) in Ta, Zr, Hf, Ti phibole precipitation and an initial melt depleted in Ta,and Pb and to assume that they approached equilibrium Zr, Hf, Ti and Pb and enriched in Sr (relative to adjacentwith that liquid (e.g. 311-9 in Fig. 14b). In other words, REE). It is likely that crystallization of amphibole inthe distribution of Ti, Zr, Hf, Ta and Pb in Type-1 and wall-rock peridotite (Type-2) exhausted water from theType-2 xenoliths can be explained if one assumes that percolating liquid so that at larger distances from thethese elements are not significantly affected by meta- melt source the water-poor liquid or fluid producedsomatism (because of their low abundances in the per- largely ‘anhydrous’ Type-1 metasomatic assemblages.colating liquid). In such a case, increasing REE contents The melt/rock ratio may have continuously decreasedimposed by an REE-rich liquid would result in negative as well.anomalies of Ti, Zr, Hf, Ta and Pb (whose contents donot increase), typical of Type-2 rocks, and even deeperanomalies in clinopyroxene (as a result of partial re-

The nature of the metasomatic media anddistribution to amphibole).their fingerprints in the metasomatic rocksThe assumption of high Zr/Hf in the initial melt can

also explain the positive Nd–Zr/Hf correlation and the The nature of the percolating melts can be constrainedlack of Zr–Zr/Hf correlation in Fig. 13. Zr/Hf must be from the mineralogical and chemical compositions of thesubchondritic in partial melting residues (as in some xenoliths. It is unlikely that the melt or fluid responsibleType-1 rocks) because of lower cpx/meltD values for Zr than for the formation of Type-1 rocks was rich in alkalis andfor Hf (Lundstrom et al., 1998; Takazawa et al., 2000; water because ‘hydrous’ phases are normally not presentKalfoun et al., 2002) and is increasingly buffered by the and the only amphibole from a Type-1 sample is very

low in K. An Fe–Ti-rich silicate melt is not a likely optionmetasomatic liquid with superchondritic Zr/Hf in more

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Fig. 22. One-dimensional percolation models for amphibole-bearing peridotite matrix with and without trapped melt for the ‘best-fit’ initialmelt composition (with anomalies). Other modelling parameters are the same as in Fig. 21.

either because rocks with the highest LREE enrichments proposed as likely metasomatic agents for peridotite xeno-liths worldwide (Green & Wallace, 1988; Yaxley et al.,and high modal amphibole do not show concomitant

enrichment in Fe and Ti (Fig. 3b and d). In particular, 1991; Dautria et al., 1992; Hauri et al., 1993; Rudnick etal., 1993) including those from Spitsbergen (Ionov et al.,Type-2 rocks contain a high incompatible element con-

tribution from the metasomatic media and must have 1993b). Those inferences were based on comparisons oftrace element patterns in metasomatized xenoliths withbeen formed by deep interaction of peridotitic residues

with melts. The lack of Fe–Ti enrichments in these rocks those of carbonatites exposed in the crust (Nelson et al.,1988; Woolley & Kempe, 1989), experimental evidenceis a strong argument for carbonate-rich metasomatic

media because primary carbonatites are characterized on element partitioning between immiscible carbonateand silicate liquids (Hamilton et al., 1989; Veksler et al.,by elevated Mg# values in the range 0·8–0·9 (Dalton &

Wood, 1993; Sweeney, 1994). Carbonate-rich melts are 1998) and compositional variations in mantle-derivedvolcanic rocks (Dupuy et al., 1992). Some of these con-believed to be very efficient metasomatizing agents (Green

& Wallace, 1988) because of a combination of high straints are poorly defined (Laurora et al., 2001), e.g. thecompositions of the crustal carbonatites may differ frommobility (Hunter & McKenzie, 1989) and high in-

compatible element abundances imparted by low degrees those of primary carbonate liquids generated by mantlemelting (Eggler, 1989; Ionov et al., 1993b; Ionov &of partial melting during their generation. For a silicate–

carbonate liquid, a consequence of its reaction with host Harmer, 2002), and liquid immiscibility may not be amajor factor in the carbonatite genesis (Lee & Wyllie,peridotite would be continuous exhaustion of its silicate

component and increasing proportion of the carbonate 1998). Direct evidence for trace element fingerprints ofmantle-derived carbonate-rich melts was recently pro-component in its composition (Mattielli et al., 1999),

because carbonates are not stable in the shallow spinel vided by experimental results of Blundy & Dalton (2000)and Adam & Green (2001). Their work demonstratedperidotite mantle at moderate to high temperatures (Dal-

ton & Wood, 1993). that cpx–melt partition coefficients for HREE, Ti, Zrand Rb are higher by factors of 5–200 for carbonateCarbonate-rich melts, which may be strongly enriched

in LREE and Sr and depleted in HFSE, were earlier melts than for silicate melts, whereas partition coefficients

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for LREE, Sr and Nb show less fractionation. Those shown that marked trace element variations in the Type-1 peridotites could have been accompanied by only minorworkers inferred that low degrees of partial melting ofmineralogical and major element effects (precipitation ofcarbonated mantle lherzolite should produce carbonate-<1% clinopyroxene). It follows, for example, that therich melts with elevated incompatible element con-absence of characteristic major element (high Ca/Al) orcentrations, steep REE patterns and marked depletionstrace element (low Ti/Eu or Zr/Sm) signatures inferredof Ti and Zr (but not necessarily Nb), consistent withfor high degrees of ‘carbonatite’ metasomatism in rockssignatures inferred for the initial metasomatic liquid infar from the melt source (like Spitsbergen Type-1 xeno-this study.liths) does not rule out a carbonate-rich composition ofWe would like to emphasize that the reactive porousthe initial metasomatic medium.flow is an essential mechanism to produce strong LREE

LREE enrichments and negative HFSE anomalies infractionation (high La/Ce, La/Nd) and highly variablemantle rocks can possibly be implanted by subduction-enrichment patterns in metasomatic peridotites (re-related silicate melts or water-rich fluids (Zanetti et al.,gardless of whether the metasomatic melt was carbonatite1999). These melts or fluids, in particular, are believedor silicate). As discussed above, no known type of mantle-to be strongly depleted in Nb and Ta (high Th/Nb andderived melt, including carbonatites, is likely to produceLa/Nb ratios) and enriched in Pb, Sr, Ba and alkalisREE + Sr patterns similar to those in Spitsbergen(Nakamura et al., 1985; Hofmann, 1988; Woodhead,Type-1 xenoliths, either by simple mixing or equilibrium1989). However, negative Pb anomalies (high Ce/Pb)mineral–melt partitioning, without the chromatographicand high Nb abundances in Type-2 xenoliths (Figs 11effects of melt percolation (see also Ionov et al., 2002).and 13) appear to argue against such an alternative.Conversely, porous melt flow can fractionate silicatePeridotites enriched by subduction-related fluids shouldbasaltic liquids to produce extremely high La/Ce andprobably also show stronger enrichments in Rb and CsLa/Nd ratios (Fig. 19). Therefore, it is more correct to(Churikova et al., 2001) than most of the Spitsbergenattribute the U-shaped or ‘spoon-shaped’ REE patternsxenoliths (note fairly low Rb and Cs contents in thein mantle rocks [fig. 5g and h of Downes (2001)] toamphiboles, Tables 4 and 5, Fig. 8b and d). Notably, a‘percolation’ metasomatism rather than ‘carbonatite’combination of depletion in Pb and enrichment in Nbmetasomatism even if carbonate-rich melts are consideredis an unlikely feature to be found in mantle rocks meta-the most likely candidates for the metasomatic media.somatized by subduction-related fluids.Similarly, negative HFSE anomalies (e.g. low Nb/Th

and Nb/La in Type-1 Spitsbergen xenoliths) can berelated to chromatographic effects of porous melt flow(as a result of precipitation of amphibole near the meltsource) and not be direct signatures of the initial melt

SUMMARY(compare high Nb/Th in Type-2 xenoliths, Figs 11 and13). These examples further demonstrate that frac- (1) The protolith for the lithospheric mantle beneathtionation processes may be as important as melt com- Spitsbergen was created by moderate to high degrees ofpositions in determining trace element signatures of the partial melting and melt extraction from a fertile source.metasomatized mantle (Laurora et al., 2001). The melting residues were then metasomatized with

In general, the relative effects of the source composition melts or fluids that intruded the upper levels of theand fractionation mechanisms in the metasomatism may lithospheric mantle via a network of veins and high-change with distance from the melt source in a generalized porosity zones in the wall-rock peridotite.‘metasomatic column’: (1) peridotites hosting the feeder (2) The initial melt was probably a carbonate-richchannel may undergo strong transformation (e.g. dis- silicate liquid, strongly enriched in highly incompatiblesolution of orthopyroxene and formation of wehrlites as trace elements, with negative anomalies of Zr, Hf, Ta,a result of reaction with carbonatites); (2) rocks in the Ti and Pb and a small positive anomaly of Sr relative tovicinity of the feeder will approach trace element equi- adjacent REE, and with high Nb/Ta and Zr/Hf ratios.librium with the melt or fluid and therefore acquire An involvement of subduction-related melt source is notstrong chemical fingerprints of the metasomatic agent likely.(Type-2 Spitsbergen xenoliths); (3) trace element patterns (3) The reactive porous flow in the vicinity of conduitsfarther in the metasomatic ‘column’ will be increasingly that involved crystallization of amphibole created meta-controlled by metasomatic fractionation mechanisms somatic patterns approaching equilibrium with the initial(Type-1 Spitsbergen xenoliths). Overall, different sig- melt. Melt composition evolved continuously as its frac-natures of the metasomatic medium in a percolation tion decreased as a result of percolative fractional crys-column may be spatially detached, depending on relative tallization. At larger distances from the local melt sources

the evolved liquid or fluid produced largely ‘anhydrous’matrix–melt element partition coefficients. We have

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mantle: a case study from the East African Rift. Earth and Planetaryassemblages with highly fractionated LREE. The meta-Science Letters 153, 67–83.somatic zoning in wall-rocks was ‘frozen’ when local

Bernstein, S., Kelemen, P. B. & Brooks, C. K. (1998). Depleted spinelsources of percolating melt were exhausted.harzburgite xenoliths in Tertiary dykes from East Greenland: restites(4) Very high and variable La/Ce, La/Nd and possiblyfrom high degree melting. Earth and Planetary Science Letters 154,

Sr/Nd ratios in LREE-enriched metasomatic peridotites 219–233.worldwide are consistent with ‘chromatographic’ frac- Blundy, J. D. & Dalton, J. A. (2000). Experimental comparison of tracetionation during reactive porous melt flow rather than element partitioning between clinopyroxene and melt in carbonate

and silicate systems, and implications for mantle metasomatism.simple mixing with an enriched melt (either silicate orContributions to Mineralogy and Petrology 139, 356–371.carbonatite). Negative Nb–Ta anomalies far from the

Blundy, J. D., Robinson, J. A. C. & Wood, B. J. (1998). Heavy REEmelt source can be produced by melt percolation throughare compatible in clinopyroxene on the spinel lherzolite solidus.amphibole-bearing peridotites.Earth and Planetary Science Letters 160, 493–504.(5) Chemical and mineralogical fingerprints of a meta-

Blythe, A. E. & Kleinspehn, K. L. (1998). Tectonically versus climaticallysomatic medium are strongest near its source (e.g. a vein), driven Cenozoic exhumation of the Eurasian plate margin, Svalbard:whereas trace element patterns farther in the metasomatic fission track analysis. Tectonics 17, 621–639.‘column’ are increasingly controlled by metasomatic frac- Bodinier, J. L., Vasseur, G., Vernieres, J., Dupuy, C. & Fabries, J.

(1990). Mechanisms of mantle metasomatism: geochemical evidencetionation mechanisms.from the Lherz orogenic peridotite. Journal of Petrology 31, 597–628.

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ACKNOWLEDGEMENTS H. V. (eds) Secondary Ion Mass Spectrometry, SIMS IX. Chichester: JohnD.A.I. thanks E. Takazawa for access to ICP-MS and Wiley, pp. 413–416 and 927–930.

Bottazzi, P., Tiepolo, M., Vannucci, R., Zanetti, A., Brumm, R., Foley,XRF facilities and assistance with analytical work andS. F. & Oberti, R. (1999). Distinct site preferences for heavymodelling, M. Norman for generously providing ICP-and light LREE in amphibole and the prediction of Amph/LDREE.MS analyses of basalts, Yu. Genshaft and M. KopylovaContributions to Mineralogy and Petrology 137, 36–45.for contributing samples, N. Pearson, A. Sharma, G.

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lherzolites II. New thermobarometers, and practical assessment ofKempton are highly appreciated. This research wasexisting thermobarometers. Journal of Petrology 31, 1353–1378.supported by funding from the Australian Research

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