www.elsevier.com/locate/lithos

Lithos 75 (2004) 209–237

Feldspar from carbonate-rich silicate metasomatism in the shallow

oceanic mantle under Kerguelen Islands (South Indian Ocean)

G. Delpecha,b,*, M. Gregoirec, Suzanne Y. O’Reillya, J.Y. Cottinb,B. Moineb, G. Michonb, A. Giret

aGEMOC ARC National Key Centre, Earth and Planetary Sciences, Macquarie University, NSW 2109, Australiab ‘‘Equipe Transferts Lithospheriques’’-UMR 6524 ‘‘Magmas et Volcans’’, Universite Jean Monnet,

23, rue du Dr. P. Michelon, F-42023 Saint-Etienne Cedex, FrancecUMR 5562 ‘‘Dynamique Terrestre et Planetaire’’, Observatoire Midi-Pyrenees, 14, Avenue E. Belin, 31400, Toulouse, France

Received 10 February 2003; received in revised form 25 September 2003; accepted 18 December 2003

Available online 12 April 2004

Abstract

Some highly depleted harzburgitic mantle xenoliths from two different localities in the Kerguelen Islands display a

secondary mineral assemblage unusual for both oceanic and continental settings. Petrographic and chemical evidence indicate

reaction of primary orthopyroxene and spinel with an infiltrating melt to form Na–Cr-rich clinopyroxene + olivine +Cr-rich

spinelF apatiteF rutileF carbonate and a Si–Al–Na-rich phase, which is either a glass and/or a Na-rich plagioclase. Major-

element compositions of secondary minerals strongly indicate that their formation resulted from the infiltration of a sodium-rich

carbonate–silicate melt with a low water content through the host depleted harzburgite. The trace-element signatures of

metasomatic clinopyroxene and glass indicate that the metasomatic agent was enriched in Sr, P, Cl, Th, U, and LREE and low in

K, Rb and HFSE (Nb, Ta, Zr, Hf, Ti). The infiltration of this metasomatic agent led to heterogeneous crystallisation and wall-

rock reactions, which are recorded by the mode and the major-element variations of minerals. The trace-element composition of

clinopyroxene suggests that the carbonate to silicate melt proportion in the metasomatising agent was locally variable, resulting

in places in trace-element signatures typical of alkali silicate melt metasomatism. The migration of sodic, carbonate-rich silicate

melt is inferred to result in feldspar-bearing metasomatic assemblages formed by reaction of the melt with the host peridotite if

carbonate is not stable at shallow depth in the oceanic upper mantle. The metasomatic imprint was acquired shortly before

eruption and formed areas of felsic composition, heterogeneous at the mm scale. This study demonstrates the close relationship

between migrating carbonate and silicate melts in the oceanic mantle beneath Kerguelen Islands.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Carbonate-rich mantle metasomatism; Silicate mantle metasomatism; Mantle feldspar; Mantle xenoliths; Kerguelen lithosphere;

Lithospheric mantle processes

1. Introduction* Corresponding author. GEMOC ARC National Key Centre,

0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.lithos.2003.12.018

Department of Earth and Planetary Sciences, Macquarie University,

NSW 2109, Australia. Tel.: +1-61-2-9850-9676; fax: +1-61-2-9850-

8943.

E-mail address: gdelpech@els.mq.edu.au (G. Delpech).

URL: http://www.es.mq.edu.au/GEMOC.

Many geochemical mantle studies in the last decade

have focused on the major and trace-element compo-

sitions of minerals of metasomatic origin in order to

G. Delpech et al. / Lithos 75 (2004) 209–237210

assess the nature and origin of melts or fluids respon-

sible for modifying the composition of the lithospheric

mantle, in both oceanic and continental settings. The

possible metasomatising agents in mantle xenoliths

span a wide range of compositions ranging from

basaltic melts to carbonatitic melts (e.g. Wulff-Peder-

sen et al., 1996; O’Reilly and Griffin, 1988; Neumann

and Wulff-Pedersen, 1997; Hauri et al., 1993).

The oceanic upper mantle under the Kerguelen

Islands is dominated by depleted harzburgites that

experienced an extensive partial melting event and

underwent subsequent, widespread metasomatism by

highly alkaline, mafic silicate melts related to the

Kerguelen hotspot (Gregoire et al., 1997, 2000a;

Moine, 2000; Moine et al., 2001). Moine (2000)

showed that progressive reaction between carbonate-

bearing, highly alkaline silicate melts with and a host

peridotite may ultimately lead to small fractions of

evolved carbonate-rich melt acting as very mobile and

efficient metasomatising agents in the Kerguelen lith-

ospheric mantle. Moreover, Moine et al. (this issue)

documented another type of carbonate-rich metasoma-

tism in Kerguelen opx-free dunites. These rocks con-

tain interstitial patches of MgO-bearing calcite with

unusually high rare-earth element contents, which are

interpreted as quenched carbonate melts.

The characteristic features of carbonatitic metaso-

matism have been inferred from systematic studies of

mantle xenoliths from many worldwide occurrences

(Amundsen, 1987; Kogarko et al., 1995; Hauri et al.,

1993, Ionov et al., 1993, Rudnick et al., 1993;

Coltorti et al., 1999). Metasomatised lithospheric

mantle may contain carbonates, apatite, amphibole,

mica, secondary pyroxene and glass, resulting in

strong modifications of its pristine major and trace-

element composition. The ‘‘carbonatite imprint’’ is

commonly recorded by the bulk-rock composition

and in reacted minerals, such as clinopyroxene, and

is characterised by strong enrichments in LREE and

concomitant strong depletions in HFSE (Nb, Ta, Zr,

Hf, Ti). Experimental studies on the stability of

carbonate-rich melts in the mantle have shown that

a primary carbonatite melt must be dolomitic in

composition (Yaxley and Green, 1996; Dalton and

Presnall, 1998) and that at lower P and T, the

carbonate-rich melt becomes unstable in a lherzolitic

composition. It will then react with the host peridotite,

thus converting the original lherzolite or harzburgite

into an olivine–clinopyroxenite or a wehrlite and

liberating CO2. The percolating carbonate-rich mag-

ma will gradually change in composition with de-

creasing pressure, evolving from dolomitic (near its

source and down to f 70 km depth) towards more

calcitic compositions at shallow depth in the mantle

(f 35 km) (Lee and Wyllie, 2000).

We present in-situ major- and trace-element anal-

yses of two different secondary mineral assemblages,

which share some common geochemical features, and

aim to assess the nature of the metasomatic agent that

induced these strong geochemical modifications in the

depleted Kerguelen oceanic lithosphere.

2. Geological setting

The Kerguelen Islands belong to the northern part

of the Kerguelen Plateau, which is the second largest

oceanic plateau in the world after the Ontong Java

plateau (Coffin and Eldholm, 1993). The Kerguelen

plateau originally was situated near the South East

Indian Ridge (SEIR) when the emplacement of huge

volumes of flood basalts under the influence of the

Kerguelen plume started to build the Kerguelen Islands

some 45 Ma ago. The Kerguelen plateau then moved

away from the SEIR and the Broken Ridge Plateau and

26Ma after an early geodynamic setting similar to that

of the present Iceland hotspot it is now in an intraplate

setting within the Antarctic Plate. Magmatic activity

has been recorded for 45 Ma (Giret, 1993). The

magmatism was dominantly of tholeiitic-transitional

affinity in the early history of Kerguelen Islands and

became progressively alkaline to highly alkaline over

time (Gautier et al., 1990; Weis et al., 1993).

Ultramafic and mafic xenoliths are commonly

found in dykes, lava flows or breccia pipes related to

the young alkaline and highly alkaline volcanic activ-

ity (Gregoire et al., 1998). Mantle xenoliths of the

spinel harzburgite suite are protogranular Cr-diopside

harzburgites and poikilitic Mg-augite harzburgites.

Previous studies were mostly focused on xenoliths

from the SE peninsula (Gregoire et al., 1997, Fig. 1)

and recently on xenoliths containing volatile-rich-

bearing phases from the NW part of the Islands

(Moine, 2000; Moine et al., 2001). The clinopyrox-

ene-spinel-bearing harzburgites investigated here were

found at the Lac Michele locality (NW Kerguelen), in

Fig. 1. Simplified geological map of Kerguelen Islands modified after Gregoire et al. (1997). (1) Lac Michele locality; (2) Mont-Peeper locality.

G. Delpech et al. / Lithos 75 (2004) 209–237 211

a nepheline-hawaiitic dyke, and at the Mont-Peeper

locality (E Kerguelen), in a basanitic dyke.

Two of the samples from the Lac Michele (hereaf-

ter referred as to LM) suite (BY96-357 and 381) and

the sample GR97-225 from Mont-Peeper (MP) dis-

play the feldspar- and/or glass-bearing mineral assem-

blage described here.

3. Analytical methods

Major- and minor-element contents were deter-

mined with a CamecaR Camebax SX50 or CamecaRSX100 electron microprobe at the GEMOC ARC

National Key Centre in the department of Earth and

Planetary Sciences, Macquarie University. The operat-

ing conditions were: 15 kV accelerating voltage, a

sample current of 20nA, a beam diameter of f 1–2

Am for most primary and secondary minerals, and a

counting time of 20 s for each element. The beam was

defocused between 5 and 10 Am for feldspars and glass

to prevent alkali loss during analysis. Lower limit

detections (LLD) are 0.07 wt.% for SiO2, 0.03 wt.%

for Al2O3, 0.06wt.% forMgO, 0.08wt.% for FeO, 0.06

wt.% for TiO2, 0.09 wt.% for Cr2O3, 0.05 wt.% for

CaO, 0.04 wt.% for Na2O and K2O, 0.03 wt.% for

P2O5, 0.02 wt.% for Cl and 0.05 wt.% for F.

Trace elements in clinopyroxenes from BY96-357

and BY96-381 samples were analysed with an Agilent

7500 Series Quadrupole ICP-MS connected to a

Continuum Surelite I-20 Q-switched frequency qua-

drupled Nd:YAG laser that provides a beam with a

wavelength of 266 nm, operating at a frequency of 10

Hz and an energy of f 0.5 mJ for the work reported

G. Delpech et al. / Lithos 75 (2004) 209–237212

here. Trace elements in clinopyroxene and glass from

sample GR97-225 were analysed with the same ICP-

MS, connected to a Merchantek LUV-213 nm laser

ablation system, using a 5Hz frequency and an energy

of f 0.25 mJ. In both systems ablation is done in

pure He atmosphere and the analyte is carried to the

ICP torch by a mixture of He +Ar. The standard glass

NIST 610 was used as the external standard to

calibrate the instrument, with preferred values from

Norman et al. (1996), and Ca contents from EMP

were used as internal standard. Precision and accuracy

on trace elements in the minerals are given by

repeated analysis of international standard glass

BCR-2 during the different runs. Values are shown

in Table 2. Trace-element reductions were done with

the GLITTER software (Van Archterbergh et al.,

2001).

4. Petrographic observations

The two samples from the Lac Michele locality

(BY96-357 and BY96-381) are xenoliths about 10

cm across, and are mantle wall-rock harzburgites with

a dominant protogranular microstructure (Mercier and

Nicolas, 1975). Primary minerals include olivine,

orthopyroxene and spinel. However, the microstructure

may locally vary to poikiloblastic within one thin

section. Olivine in poikiloblastic areas consists of large

poikilitic olivine grains up to several cm and show un-

dulose extinction. Orthopyroxene in poikiloblastic

areas commonly has irregular and curvilinear grain

boundaries and may contain olivine inclusions. Dark

brown vermicular spinel is commonly associated with

orthopyroxene and sometimes olivine to form clusters.

Smaller spinel grains also occur along the rims of large

orthopyroxenes. Primary spinels in contact with feld-

spar and glass commonly show spongy or reacted rims,

which indicate disequilibrium between the primary

minerals and the secondary assemblage. These two

samples display a heterogeneously developed coarse

network of serpentine. There are no veins cross-cutting

these samples that could suggest infiltration by the host

basalt.

Sample GR97-225 from the Mont-Peeper locality is

a typical fresh protogranular harzburgite (f 10cm

wide) with olivine, orthopyroxene and spinel as pri-

mary minerals.

The secondary mineral assemblage in the three

investigated xenoliths consists of clinopyroxene, ol-

ivine, feldspar, glass, spinel (Fig. 2a–f), and in some

cases in LM samples, apatite and rutile. Carbonate has

been observed in LM samples only. Orthopyroxene is

never observed in the reaction zones. Clinopyroxene,

olivine, spinel and glass are found in the three

samples. Glass occurs in the MP sample and to a

minor extent in the LM samples, whereas feldspar is

restricted to the LM samples. In the LM samples, the

secondary minerals comprise two different types of

reaction zones.

Reaction zone A: The first type of reaction zone is

commonly observed around reacted primary spinel in

orthopyroxene-spinel clusters (Fig. 2a,b,d,e). The sec-

ondary mineral assemblage is composed of clinopyr-

oxene, olivine, spinel, feldspar and may contain glass,

apatite, carbonates and, in LM sample BY96-357,

small rutile needles. However, in LM sample BY96-

381, spongy clinopyroxene is concentrated around

reacting spinel, and is only rarely associated with minor

feldspar or other secondary minerals.

Reaction zone B: The second type of reaction zones

is disseminated through the peridotitic matrix and

concentrated around reacted primary orthopyroxene

grains (Fig. 2c). Clinopyroxene is the dominant phase

in these patches, associated with minor olivine, feld-

spar and secondary spinel. Small relics of primary

spinel grains may also occur. Apatite and rutile have

not been observed in reaction zone B.

In the MP harzburgite, the secondary assemblage

comprises clinopyroxene, olivine, spinel and glass

and is mainly developed around primary spinel

grains (similar to reaction zone A, Fig. 2f), but is

also developed around large orthopyroxene grains

(similar to reaction zone B). These reaction zones

may be interconnected along the orthopyroxene

boundaries by a network of small veins filled with

glass.

4.1. Clinopyroxene

Clinopyroxene in the three samples commonly

occurs in a spongy zone around reacted primary

spinels or primary orthopyroxene grains. In some

reaction zones, clinopyroxene occurs as small anhe-

dral grains within a feldspar (LM samples) or glass

matrix (MP sample). In the MP harzburgite, clino-

Fig. 2. Images in reflected light of secondary mineral assemblages in LM and MP harzburgites. (a) Reaction zone A in sample LM BY96-357.

Secondary clinopyroxene, olivine and spinel are embedded into a feldspar matrix. (b) Enlargement of (a) showing the contact between primary

spinel and the metasomatic assemblage fs + cpx + ol + sp. (c) Reaction zone B showing a primary reacted orthopyroxene being replaced by

cpx + ol + fs in LM sample BY96-357. Feldspar here is of minor occurrence interstitial to the spongy clinopyroxene. (d) Reaction zone A around

primary reacted spinel in LM sample BY96-357. Note the feldspar mantling reacted primary spinel and also interstitial to the spongy

clinopyroxene. (e) Reaction zone A in LM sample BY96-381 showing development of a spongy clinopyroxene around a primary reacted spinel

with minor feldspar. (f) Reaction zone A around reacted primary spinel in MP sample GR97-225 showing similar microstructural features to LM

sample BY96-357 but with secondary clinopyroxene, olivine and spinel in a glass matrix.

G. Delpech et al. / Lithos 75 (2004) 209–237 213

pyroxene more commonly displays sharp grain

boundaries and is subhedral to euhedral when it is

associated with glass. Small clinopyroxene needles

have recrystallised from the glass in some reaction

zones.

4.2. Feldspar

Feldspar is abundant in BY96-357 and much less

abundant in BY96-381. It occurs in the first type of

reaction zone as a matrix of anhedral grains in which

G. Delpech et al. / Lithos 75 (2004) 209–237214

the other secondary minerals are embedded (Fig.

2a,b). When associated with primary spinel, feldspar

commonly occurs between the spinel and the spongy

clinopyroxene growing around the primary spinel, as

well as in the interstices of spongy clinopyroxene

(Fig. 2d,e). In the second type of reaction zone,

feldspar associated with spongy clinopyroxene (with

little or no primary spinel involved) is only of minor

occurrence in the interstices of spongy clinopyroxene

(Fig. 2c). As shown in Fig. 2a,c,d,e, the modal

abundance of feldspar to clinopyroxene in the reac-

tion zone may vary greatly from one reaction zone to

another within the same thin section.

4.3. Glass

Glass is most abundant in the MP harzburgite and

its microstructural occurrence is analogous to that of

feldspar in the LM samples (see above and Fig. 2f)

and commonly mantles reacting primary spinel. Small

euhedral to subhedral clinoproxene grains and small

euhedral secondary spinels occur within in the brown-

ish glass. In LM and MP samples, glass also occurs

interstitially to the spongy clinopyroxene developed

around primary spinel and as small veins or films on

the rims of metasomatic patches, at the contact with

primary minerals. Thin veins or films of glass may

also cross-cut primary olivine or orthopyroxene (MP

sample), suggesting that the xenolith was brittle when

the inflitration of the metasomatic agent occurred.

4.4. Carbonates

Carbonates were found only in LM samples (Fig. 3).

Carbonate forms veins randomly distributed in the

samples (Fig. 3b) or are in contact with primary spinel,

spongy clinopyroxene, plagioclase and apatite in reac-

tion patches (Fig. 3a,c,d,e,f). Validation of the primary

nature of the carbonates is important as some of the

carbonates occur in serpentinised veins. Carbonates in

veins commonly display resorbed grain boundaries

when in contact with serpentine veins, indicating they

predated serpentinisation. Furthermore, carbonate

grains are never found in direct contact with minerals

formed in post-entrainment alteration. Finally, carbo-

nates are commonly associated with plagioclase (Fig.

3a,d,e), spongy clinopyroxene (Fig. 3f), apatite (Fig.

3a,b,c,f), secondary euhedral spinel (Fig. 3a–f) and

more rarely, primary spinel (Fig. 3f). The contact

between carbonate and feldspar grains (or with second-

ary euhedral spinels or spongy clinopyroxenes) is

commonly straight and sharp (Fig. 3a–d) and apatite

and carbonate grains are consistently closely related,

suggesting a contemporary magmatic origin of the

carbonate with the other secondary minerals.

4.5. Other minerals

Secondary olivine occurs as small anhedral grains

disseminated within the reaction zones and sometimes

located at the rims of the primary spinels (Fig. 2a–e).

Secondary spinel is generally euhedral and dispersed

within the feldspar matrix (LM samples) or in a glass

matrix (MP sample) (Fig. 2b,d,e,f). Spinel is also

developed at the boundary between clinopyroxene

and feldspar or clinopyroxene and glass. Rare spinel

inclusions occur in clinopyroxene.

Apatite occurs as euhedral needles f 1–2 Amwide

and are found only in the LM samples, and its occur-

rence in BY96-381 is rare. Apatite needles are dis-

persed either in the feldspar matrix, in glass or at grain

boundaries between clinopyroxene and feldspar or

glass. Bright yellow small rutile needles (size < 1 Am)

are also observed within the feldspar matrix in some

rare reaction zones and occur only in LM samples. X-

ray major elemental maps (using the electron micro-

probe) were obtained in different reaction zones in the

three samples and confirm that rutile is rare in the LM

samples and is restricted to someA-type reaction zones.

However, it has never been found in the MP sample.

Petrographic observations indicate that primary

spinel and orthopyroxene reacted to form the second-

ary assemblage according to the following reactions:

(i) Opx I + Sp I+(Cpx I ?) +melt!Cpx +Ol II + Sp

II +Glass + Fs +Ap +CarbFRu (LM samples);

(ii) Opx I + Sp I +Cpx I +melt!Cpx +Ol II + Sp

II +Glass (MP sample).

5. Major element compositions and variations

5.1. Primary minerals

Mineral compositions of the studied xenoliths are

given in Table 1. Primary olivine cores have mg#

Fig. 3. BSE images of carbonates in LM samples BY96-357 (a–e) and BY96-381 (f). (a) Calcite in vein closely associated with plagioclase,

secondary chromitic spinels and apatite. Note the sharp, straight boundaries between the feldspar and the carbonate (b) Calcite and apatite in a

vein cross-cutting a spongy clinopyroxene. (c) Calcite in contact with apatite in a Type A reaction zone subsequently weathered. (d) Small

calcite grain crystallising inside a feldspar. Note the sharp contacts, indicating a magmatic origin. (e) Remnant rounded calcite in contact with a

feldspar in a vein. (f) Calcite in contact with a spongy clinopyroxene and a primary spongy spinel in a reaction zone A. Carbonate contains

apatite + secondary spinel.

G. Delpech et al. / Lithos 75 (2004) 209–237 215

(90.9–91.9) and low CaO contents (0.01–0.11

wt.%) (Fig. 4). Orthopyroxene has mg# from 91.6

to 92.9 in LM xenoliths and 91.54 to 92.16 in the

MP xenolith. In LM sample BY96-357, cores and

rims of orthopyroxene in contact with reaction

zones show compositional variations. The rims

Table 1

Representative major element compositions of olivine in primary and secondary assemblages in LM and MP xenoliths

Olivine

Sample BY96-

357

BY96-

357

BY96-

357

BY96-

357

BY96-

381

BY96-

381

BY96-

381

BY96-

381

GR97-

225

GR97-

225

GR97-

225

GR97-

225

Assemblage I I II II I I II II I I II II

Analysis

Nber

36 10 10 17 18 74 7 11 123 32 4 27

core core core core core core core core core core core core

SiO2 41.58 41.31 41.32 40.88 40.67 41.70 39.88 40.81 41.32 40.92 40.13 40.62

TiO2 0.00 0.00 0.01 0.02 0.00 0.00 0.01 0.03 0.00 0.00 0.02 0.00

Al2O3 0.00 0.02 0.01 0.04 0.00 0.06 0.00 0.01 0.00 0.03 0.03 0.07

Cr2O3 0.00 0.00 0.10 0.35 0.01 0.00 0.15 0.46 0.04 0.06 0.32 0.11

NiO 0.37 0.36 0.27 0.27 0.33 0.42 0.33 0.42 0.27 0.40 0.40 0.25

MgO 49.62 50.62 50.16 50.81 50.28 50.82 50.13 50.46 50.09 51.20 51.20 51.57

FeO* 8.63 8.25 8.47 7.86 8.93 8.12 8.32 8.70 8.51 8.07 7.22 7.55

MnO 0.10 0.15 0.18 0.18 0.12 0.12 0.16 0.13 0.09 0.10 0.13 0.12

CaO 0.03 0.01 0.19 0.15 0.06 0.06 0.20 0.15 0.04 0.02 0.21 0.14

Na2O 0.01 0.00 0.04 0.01 0.00 0.03 0.00 0.01 0.00 0.02 0.02 0.02

K2O 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.02 0.00 0.01 0.03 0.00

P2O5 – – 0.11 – – – – – – 0.04 0.20 0.03

Total 100.34 100.71 100.85 100.57 100.39 101.31 99.16 101.19 100.36 100.93 101.93 100.49

mg# 91.11 91.63 91.34 92.02 90.94 91.78 91.49 91.18 91.21 91.88 92.67 92.41

Representative major element compositions of spinel in primary and secondary assemblages in LM

and MP xenoliths

Spinel

Sample BY96-

357

BY96-

357

BY96-

357

BY96-

381

BY96-

381

BY96-

381

GR97-

225

GR97-

225

GR97-

225

GR97-

225

Assemblage I I II I I II I I II II

Analysis

Nber

1 3 2 1 2 6 1 2 10 6

core rim core core rim core core core core core

SiO2 0.03 0.02 0.45 0.04 0.06 0.11 0.01 0.04 0.20 0.09

TiO2 0.04 0.03 1.87 0.09 0.15 0.08 0.05 0.02 0.21 0.20

Al2O3 31.09 22.23 12.97 29.28 29.50 18.07 30.87 20.34 20.24 21.76

Cr2O3 37.81 45.37 52.25 39.40 38.38 48.94 37.47 48.72 46.53 43.68

NiO 0.17 0.02 0.17 0.18 0.20 0.14 0.20 0.10 0.06 0.13

MgO 17.68 16.15 16.49 18.05 18.21 14.37 16.65 16.74 14.48 16.33

FeO* 12.56 14.81 16.04 12.87 12.74 16.82 14.56 13.99 17.48 16.48

MnO 0.17 0.17 0.17 0.26 0.13 0.28 0.19 0.16 0.23 0.18

CaO 0.00 0.01 0.17 0.01 0.08 0.15 0.00 0.00 0.05 0.17

ZnO 0.12 0.18 0.09 0.14 0.05 0.05 0.13 0.24 0.17 0.20

V2O5 0.10 0.11 0.27 0.13 0.09 0.14 0.13 0.11 0.29 0.32

Total 99.74 99.11 100.95 100.46 99.58 99.15 100.25 100.46 99.92 99.54

mg# 71.51 66.03 64.69 71.43 71.81 60.36 67.09 68.09 59.62 63.86

cr# 38.69 51.44 67.65 41.12 40.30 58.43 38.64 55.42 54.40 51.02

Representative major element compositions of orthopyroxene in primary and

secondary assemblages in LM and MP xenoliths

Orthopyroxene

Sample BY-96-

357

BY-96-

357

BY-96-

357

BY-96-

381

BY-96-

381

GR-97-

225

GR-97-

225

Assemblage I I I I I I I

Analysis

Nber

19 17 18 1 6 96 145

core core rim core core core core

SiO2 57.19 56.97 57.92 56.42 56.84 58.07 57.50

TiO2 0.00 0.00 0.03 0.04 0.04 0.01 0.01

Al2O3 1.86 1.87 1.00 2.13 2.08 1.42 1.78

Cr2O3 0.44 0.40 0.33 0.50 0.48 0.39 0.47

NiO 0.11 0.03 0.09 0.04 0.13 0.12 0.05

MgO 34.53 34.56 35.05 34.66 34.60 34.72 34.57

FeO* 5.22 5.50 4.96 5.41 5.30 5.51 5.70

MnO 0.12 0.12 0.13 0.15 0.05 0.12 0.22

CaO 0.57 0.52 0.53 0.67 0.75 0.38 0.35

Na2O 0.14 0.13 0.11 0.16 0.19 0.12 0.18

K2O 0.00 0.00 0.00 0.00 0.01 0.02 0.04

P2O5 0 0 0.04 – – – –Total 100.17 100.10 100.19 100.17 100.47 100.86 100.85

mg# 92.18 91.80 92.65 91.95 92.08 91.83 91.54

G.Delp

echet

al./Lith

os75(2004)209–237

216

Representative major element compositions of clinopyroxene in secondary assemblages in LM and MP xenoliths

Clinopyroxene

Sample BY96-

357

BY96-

357

BY96-

357

BY96-

357

BY96-

357

BY96-

357

BY96-

357

BY96-

357

BY96-

381

BY96-

381

BY96-

381

BY96-

381

BY96-

381

BY96-

381

BY96-

381

GR97-

225

GR97-

225

GR97-

225

GR97-

225

GR97-

225

GR97-

225

GR97-

225

GR97-

225

GR97-

225

Reaction

zone

A A A A A B B B A A A A B B B A A A A A A A B B

Assemblage II II II II II II II II II II II II II II II II II II II II II II II II

Analysis

Nber

25 30 55 15 19 34 8 10 89 94 49 50 71 62 63 15 107 102 152 10 12 80 85 86

in fs

matrix

in fs

matrix

spongy spongy spongy spongy spongy spongy spongy spongy spongy spongy spongy spongy spongy spongy spongy spongy spongy spongy spongy euhedral spongy spongy

core core core core rim core core rim core core core rim core core rim core core core core core rim in glass core core

SiO2 51.73 53.70 54.59 54.65 55.06 55.16 55.64 54.87 53.42 54.35 53.64 53.71 54.28 53.92 54.04 53.77 54.84 54.62 54.57 53.27 54.04 53.26 55.53 55.02

TiO2 0.08 0.08 0.02 0.03 0.06 0.00 0.00 0.02 0.08 0.04 0.15 0.10 0.09 0.05 0.07 0.02 0.27 0.15 0.11 0.09 0.11 0.14 0.07 0.05

Al2O3 5.44 2.76 1.92 5.08 2.52 1.67 2.60 1.35 5.36 2.95 3.68 2.73 3.41 2.91 2.77 2.58 3.30 1.89 1.52 2.89 1.81 3.19 3.28 2.21

Cr2O3 5.01 2.04 2.06 2.74 1.88 1.65 1.00 1.22 2.58 2.35 1.85 1.84 2.21 1.99 2.07 2.52 1.50 1.88 1.58 1.79 1.34 2.86 1.23 1.79

NiO 0.10 0.00 0.00 0.04 0.02 0.05 0.07 0.05 0.00 0.10 0.08 0.08 0.12 0.06 0.06 0.00 0.01 0.07 0.00 0.09 0.04 0.01 0.05 0.09

MgO 16.07 17.48 18.02 15.06 17.81 19.24 16.60 18.03 15.98 18.59 17.87 18.82 17.91 18.59 18.18 16.78 16.96 18.31 17.50 17.12 17.42 16.87 15.48 18.19

FeO* 2.17 2.28 2.57 2.49 2.39 2.52 2.29 2.32 2.58 2.96 3.02 3.05 2.79 2.84 2.91 2.45 2.51 2.45 2.39 2.19 2.11 2.31 2.26 2.66

MnO 0.00 0.05 0.06 0.06 0.08 0.08 0.05 0.05 0.00 0.00 0.07 0.11 0.03 0.05 0.08 0.01 0.11 0.02 0.02 0.05 0.07 0.06 0.06 0.08

CaO 18.57 20.99 19.34 17.74 19.36 19.24 20.93 21.99 17.51 18.11 18.68 18.70 18.59 18.67 19.53 19.87 19.96 19.34 21.18 20.86 22.41 20.87 19.87 19.73

Na2O 1.59 0.79 0.91 1.53 1.00 0.93 1.66 0.74 2.29 1.15 1.14 0.72 1.28 1.04 0.92 1.59 1.12 0.92 0.91 0.96 0.60 1.05 2.42 1.11

K2O 0.02 0.02 0.00 0.09 0.08 0.01 0.00 0.00 0.00 0.01 0.01 0.00 0.01 0.00 0.02 0.00 0.02 0.00 0.01 0.00 0.00 0.01 0.02 0.01

P2O5 0.00 0.06 – 0.03 0.18 0.04 0.09 0.09 – – 0.07 0.03 – 0.00 0.06 0.05 – – – 0.00 0.07 0.00 0.08 0.00

Total 100.77 100.23 99.50 99.56 100.48 100.59 100.95 100.75 99.80 100.59 100.30 99.88 100.71 100.12 100.70 99.63 100.61 99.65 99.80 99.33 100.08 100.63 100.35 100.95

mg# 92.96 93.20 92.60 91.53 93.00 93.15 92.82 93.27 91.70 91.80 91.35 91.67 91.96 92.10 91.77 92.44 92.34 93.03 92.88 93.30 93.63 92.87 92.43 92.42

CaTs 0.03 0.06 0.01 0.00 0.01 0.02 0.01 0.02 0.06 0.04 0.05 0.04 0.04 0.05 0.05 0.04 0.01 0.01 0.01 0.05 0.03 0.05 0.00 0.02

Al IV 0.04 0.07 0.02 0.00 0.01 0.02 0.01 0.03 0.08 0.05 0.07 0.06 0.05 0.06 0.06 0.05 0.03 0.02 0.02 0.06 0.04 0.08 0.01 0.03

(continued on next page)

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Table 1 (continued)

Representative major element compositions of felspar and apetite in secondary assemblages in LM and MP xenoliths

Feldspar Ap

Sample BY96-

357

BY96-357

BY96-

357

BY96-

357

BY96-

357

BY96-

357

BY96-

381

BY96-

381

BY96-

381

BY96-

381

BY96-

357

BY96-

357

Reaction

zone

A A A A B B A A A A A A

Assemblage II II II II II II II II II II II II

Analysis

Nber

28 26 38 22 24 32 3 6 12 14 4 2

core core core core core core core core core core

contact carb

SiO2 65.35 64.26 63.94 61.30 66.53 62.79 56.90 57.72 59.04 62.92 0.98 1.33

TiO2 0.00 0.06 0.13 0.01 0.05 0.05 0.16 0.13 0.19 0.27 0.01 0.01

Al2O3 20.72 20.73 22.21 22.70 20.93 22.67 24.56 25.38 25.51 21.96 0.23 0.15

Cr2O3 0.06 0.00 0.02 0.23 0.00 0.01 0.29 0.20 0.29 0.76 0.04 0.11

NiO 0.02 0.02 0.03 0.00 0.00 0.00 0.03 0.00 0.00 0.01 0.02 0.00

MgO 0.04 0.70 0.08 0.47 0.03 0.04 0.50 0.09 0.06 0.43 3.12 0.12

FeO* 0.31 0.29 0.35 0.29 0.24 0.32 0.29 0.29 0.29 0.43 0.41 0.38

MnO 0.00 0.04 0.03 0.01 0.01 0.00 0.04 0.00 0.01 0.04 0.02 0.05

CaO 2.05 2.56 3.64 4.87 2.17 4.20 8.16 7.70 6.98 3.58 44.79 51.09

Na2O 10.34 9.44 9.28 8.26 9.50 8.51 7.47 7.27 7.86 9.16 2.43 0.51

K2O 0.60 0.65 0.59 0.57 0.74 0.43 0.38 0.30 0.46 0.77 0.06 0.11

P2O5 0.01 0.11 0.08 0.06 0.09 0.09 – – – 0.17 44.32 40.43

Cl – – 0.04 – 0.00 0.00 0.03 0.01 0.01 0.02 0.00 4.11

F – – 0.06 – 0.00 0.01 0.03 0.03 0.02 0.02 0.02 0.42

BaO 0.05 0.10 – 0.17 – – – – – – –SrO 0.11 0.12 – 0.09 – – – – – – 0.25 0.46

Ce2O3 – – – – – – – – – – 0.92 0.14

Y2O3 – – – – – – – – – – 0.05 0.03

Total 99.65 99.06 100.48 99.03 100.29 99.12 98.84 99.11 100.71 100.52 96.40 97.81

Ab 79.60 74.61 68.74 60.31 76.52 64.71 46.65 47.61 51.39 67.80 – –An 15.78 20.26 26.93 35.57 17.48 31.99 51.00 50.41 45.62 26.52 – –Or 4.62 5.13 4.33 4.12 5.99 3.31 2.35 1.98 2.99 5.68 – –

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Representative major element compositions of glass in secondary assemblages in LM and MP xenoliths

Glass

Sample BY96-

381

BY-96-

357

BY-96-

357

BY-96-

357

GR97-

225

GR97-

225

GR97-

225

GR97-

225

GR97-

225

GR97-

225

Reaction

zone

A A A A A A A A A A

Assemblage II II II II II II II II II II

Analysis

Nber

31 29 20 43 38 16 10 161 25 21

SiO2 62.37 65.65 63.99 62.82 59.11 62.73 61.22 66.52 68.71 71.93

TiO2 0.35 0.04 0.01 0.12 0.40 0.36 0.29 0.07 0.23 0.23

Al2O3 19.79 16.23 20.28 21.49 19.48 20.48 21.29 19.42 19.66 18.12

Cr2O3 0.04 0.21 0.29 0.19 0.81 0.38 0.02 0.14 0.27 0.22

NiO 0.00 0.00 0.00 0.00 – 0.00 0.02 0.06 –MgO 1.76 3.36 1.36 1.28 4.42 2.00 2.28 2.62 1.83 1.21

FeO* 0.37 0.71 0.40 0.63 3.31 1.78 2.34 2.39 1.61 1.29

MnO 0.02 0.00 0.00 0.00 0.10 0.02 0.03 0.02 0.08 0.06

CaO 3.12 2.66 2.45 3.90 5.41 3.31 3.99 3.38 2.63 1.88

Na2O 9.05 9.30 9.36 9.08 7.20 9.06 7.65 5.85 5.07 4.83

K2O 1.54 1.26 0.75 0.41 0.52 0.60 0.59 0.37 0.62 0.65

P2O5 0.94 0.14 0.13 0.28 – 0.32 0.62 – – –Cl – 0.00 0.16 0.02 – – 0.15 – – –F – 0.00 0.23 0.00 – – 0.01 – – –BaO 0.00 – – – – 0.15 – – – –SrO 0.00 – – – – 0.08 – – – –Total 99.35 99.55 99.40 100.22 100.75 101.26 100.51 100.85 100.71 100.42

Normative compositions of glasses from LM an MP xenoliths

Qz – 0.5 2 – – – 6 20 29 36

Pl 84 77 88 94 83 91 80 67 57 51

Or 9 8 4.5 2.5 3 4 4 2 4 4

Ne 1.5 – – – – – – – – –

Corundum – – 1 – – – 3 3 6 6

Di – 10 – 0.3 5 0.1 – – – –

Hy – 4 3 0.8 5 0.6 6 6.5 4.5 3

Ol 3 – – 1.5 2 3 – – – –

Na2SiO3 – 0.5 – – – – – – – –

Ru 0.2 – – – – – 0.3 0.1 0.2 0.2

Ilm – – – – – – 0.1 – – –

Ap 2 0.3 0.3 0.7 – 0.7 1.5 – – –

Perovskite 0.2 – – – – – – – – –

Chr 0.1 0.4 0.5 0.4 1.5 0.7 – 0.3 0.5 0.4

Ky – 0.1 – 0.3 1 1 – – – –

Total 100.1 100.3 99.4 100.2 101.2 100.7 99.6 100.2 100.4 100.3

Assemblage I refers to the primary mineralogy and II to the secondary mineralogy. FeO*: all as FeO, mg# = (MgO/MgO+FeOt), cr# = (Cr2O3/Cr2O3 +Al2O3), mg# and cr# in moles.

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219

Fig. 4. CaO versus mg# (Mg/(Mg+ FeOt)) contents in primary

olivine cores, and secondary olivines in reaction zones in LM and

MP harzburgites. Circle: sample LM BY96-357; triangle: sample

LM BY96-381; diamond: sample MP GR97-225.

Fig. 5. cr# (Cr/(Cr +Al)) versus mg# (Mg/(Mg+ FeOt)) in primary

spinel core, rims and secondary spinels in reaction zones in LM and

MP harzburgites. Symbols as in Fig. 2.

G. Delpech et al. / Lithos 75 (2004) 209–237220

commonly have lower Al2O3 but higher CaO than

the cores. However, in LM sample BY96-381,

orthopyroxene rims show a slight increase in CaO

and a slight increase in Al2O3. Primary spinel cores

have mg# between 66.82 and 73.18 and Cr# rang-

ing from 37.73 to 50.84. Spongy rims of the

primary spinels extend to slightly lower mg#

(65.2–71.3) and higher Cr# (40.3–55.42) (Fig. 5),

and may be higher in TiO2.

5.2. Secondary minerals

Secondary olivine displays greater compositional

variations in LM xenoliths (mg# ranging from 91.2 to

92) than in the MP xenolith (mg# from 92.2 to 92.7).

In LM xenoliths, secondary olivine does not have

systematically higher mg# than primary olivine,

whereas in the MP harzburgite secondary olivine

always has a higher mg# (Fig. 4). CaO contents are

generally higher than in primary olivine ranging from

0.09 to 0.22 wt.%. Cr2O3 content is also higher than in

primary olivine (0.05–0.61 wt.%) and up to 0.98

wt.% in olivine occurring near reacting primary spinel

(GR97-225).

Secondary spinel displays lower mg# and higher

cr# (Fig. 5) than primary spinel cores and rims, and

have also higher Ca and Ti contents.

Clinopyroxene in the reaction zones is a diopside

with high mg# (91.4–94.6) in LM xenoliths and

slightly lower mg# in MP xenolith (92.4–93.4) (Fig.

6). It has low Ti (0–0.27 wt.%) and high but variable Cr

contents (1.17–5.00 wt.%). Na2O and Al2O3 are var-

iable, ranging from low concentrations (0.6 and 0.84

wt.%, respectively), to higher concentrations (3 and

5.44 wt.%, respectively). Clinopyroxene displays com-

positional variations according to its position in the

metasomatic zones (especially in LM sample BY96-

357. Clinopyroxene in reaction zone A (i.e. associated

to reacted primary spinel, Fig. 2d,e,f) displays higher

Al, Cr, Na contents and slightly lower Si, Mg contents

than clinopyroxene in reaction zone B (i.e. associated

with reacting orthopyroxene only, Fig. 2c). This is

consistent with the release of Al and Cr by the reacting

primary Cr-spinel in the proximity of reaction zones A.

Microstructural evidence indicates that clinopyroxene

crystallised before plagioclase in the reaction zones,

thus hosting the highAl and Cr contents available in the

vicinity of the reacting Cr-spinel I. In reaction zones A

where feldspar and/or glass is more abundant (e.g. Fig.

2a,b), the small anhedral clinopyroxenes have the

highest Al (up to 5.44 wt.%), Cr (up to 5.01 wt.%),

Na (up to 2.50 wt.%) contents and the lowest Si, Mg

and Ca (down to 17.11 wt.%) contents. Their compo-

sition may be heterogeneous on the scale of a single

patch (especially Na2O and Al2O3). In reaction zones B

(e.g. Fig. 2c), clinopyroxenes have commonly lower

Al, Cr, and Na contents, together with higher Si, Mg,

Ca contents. Rims of these clinopyroxenes usually

contain lower Al2O3, Cr2O3, Na2O and higher CaO,

Fig. 6. Major elements versus mg# (Mg/(Mg+ FeOt)) in clinopyroxenes from LM and MP harzburgites. Medium grey field: clinopyroxene

compositions in depleted protogranular harzburgites, the least metasomatised xenolith suite in Kerguelen (Gregoire et al., 2000a). Light grey field:

clinopyroxene compositions associated with highly alkaline mafic silicate metasomatism in poikilitic harzburgites (Gregoire et al., 2000a). Dark

grey field: clinopyroxenes compositions in other depleted harzburgites fromLacMichele locality (similar to clinopyroxenes from the protogranular

suite). Square: clinopyroxene compositions associated to carbonatitic metasomatism in opx-free dunites (Moine, 2000). Note the difference

between LM and MP clinopyroxenes and clinopyroxenes from the poikilitic and carbonated suite in the Cr2O3 versus Al2O3 diagram and also

Cr2O3 versus Na2O.

G. Delpech et al. / Lithos 75 (2004) 209–237 221

Fig. 7. Feldspar compositions in the An–Ab–Or diagram. Dark

grey: field for feldspar compositions in silicate and carbonate zones

associated to immiscibility between a calcio-carbonatitic melt and a

silicate melt (Chalot-Prat and Arnold, 1999). Light grey: field for

feldspar compositions from different localities world-wide (see text

for references). Square: feldspar compositions from Ti-rich highly

alkaline metasomatism in Kerguelen (Gregoire et al., 2000b). Circle:

feldspars from LM sample BY96-357; triangle, feldspars from LM

sample BY96-381.

Fig. 8. Na2O+K2O versus SiO2 for glasses in LM and MP harzburgite

diamond, MP sample GR97-225. Light grey field, melt inclusions hosted in

1994). Medium and dark grey fields, glasses in trachytic and rhyolitic lav

G. Delpech et al. / Lithos 75 (2004) 209–237222

MgO contents than the cores. FeO usually decreases

towards the rim.

These compositions are significantly different

from those of ‘‘primary’’ clinopyroxenes in other

LM harzburgites, from Cr-diopside in protogranular

harzburgites and Mg-augite in poikilitic harzburgites

from other Kerguelen Islands localities (Gregoire et

al., 2000a; Fig. 6). The secondary clinopyroxene

described here is, however, systematically higher in

Cr2O3 than the Mg-augite in the poikilitic harzbur-

gite suite.

Feldspar from LM samples is compositionally

variable but is always a plagioclase within the range

of labradorite to oligoclase (An16–51Ab47–80Or2–6)

(Fig. 7). It displays variation in composition from

An12–51Ab47–68Or2–6 in BY96-381 to An16–36Ab60–

80Or3–6 in BY96-357. Feldspar in contact with car-

bonates is always an oligoclase within the range

An17–19Ab75–77Or5–6. Compositional heterogeneity

is common between different feldspar grains on a

small scale inside both types of reaction zones.

Feldspar has previously been found in harzburgites

from Kerguelen (Gregoire et al., 2000b) but in con-

s. Circle, LM sample BY96-357; triangle, LM sample BY96-381;

olivine and orthopyroxene in Kerguelen harzburgites (Schiano et al.,

as from Rallier du Baty Peninsula.

G. Delpech et al. / Lithos 75 (2004) 209–237 223

trast to that in the LM xenoliths, it shows a continuum

in composition from plagioclase to K-feldspar. Such

high-Na plagioclases are not common and differ

markedly from those from localities worldwide in-

cluding Hamar Daban (Ionov et al., 1995), Zabargad

Fig. 9. Major elements versus SiO2 in glasses from LM and MP harzburgit

(1994). Grey circle: sample LM BY96-357; grey triangle: sample LM BY9

225. Each different symbol represents glass in a different reaction zone. The

al. (1994) have TiO2 up to 1.7 wt.%.

(Bonatti et al., 1986; Piccardo et al., 1988), Northern

Apennines (Beccaluva et al., 1984; Rampone et al.,

1995), Yitong (Xu et al., 1996) and the Veneto

Volcanic Province (Bonadiman et al., 2001) (Fig. 5).

However, their compositions resemble closely some

es. Black filled circles: melt and glass inclusions from Schiano et al.

6-381. All the other symbols represent glass from MP sample GR97-

arrow in (f) indicates that melt and glass inclusions from Schiano et

Table 2

Major element composition of carbonates in LM harzburgites

Sample BY96-

357

BY96-

357

BY96-

357

BY96-

357

BY96-

381

BY96-

381

BY96-

381

Point# 5 6 30 14 48 54 59

Type Cc Cc Cc Cc Cc Cc Cc

SiO2 0.02 0.05 0.03 0.65 0.08 0.03 0.01

TiO2 0.00 0.00 0.00 0.00 0.00 0.00 0.01

Al2O3 0.01 0.00 0.01 0.22 0.00 0.01 0.00

Cr2O3 0.01 0.02 0.00 0.01 0.21 0.12 0.05

NiO 0.00 0.04 0.01 0.04 0.00 0.00 0.00

MgO 0.00 0.00 0.00 0.00 0.00 0.00 0.00

FeO* 0.18 0.20 0.25 0.31 0.15 0.11 0.18

MnO 0.07 0.04 0.15 0.08 0.02 0.01 0.02

CaO 56.04 53.59 52.57 55.45 54.67 54.49 52.40

Na2O 0.00 0.00 0.03 0.09 0.01 0.01 0.00

K2O 0.01 0.01 0.03 0.03 0.01 0.00 0.00

P2O5 0.06 0.01 0.00 0.04 0.01 0.02 0.00

F 0.02 0.01 0.01 0.06 0.00 0.02 0.00

Cl 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Total 56.42 53.98 53.10 56.97 55.17 54.83 52.68

X Ca 0.997 0.996 0.995 0.994 0.997 0.998 0.997

X Fe 0.003 0.004 0.005 0.006 0.003 0.002 0.003

X Ca=(Ca/(Ca + Fe +Mg)); X Fe=(Fe/(Ca + Fe +Mg)).

G. Delpech et al. / Lithos 75 (2004) 209–237224

plagioclase in harzburgite from Romania (Chalot-Prat

and Arnold, 1999) associated with calcio-carbonatitic

metasomatism.

Glasses in LM and MP samples have trachytic

to rhyolitic compositions (Figs. 8 and 9a–f). They

are silicic (59.11–71.93 wt.%), aluminous (16.23–

21.93 wt.%) and sodium-rich (4.80–9.36 wt.%) but

have low TiO2 (0–0.5 wt.%) and K2O (0.37–1.54

wt.%). Glass compositions mainly lie within the

range from 0.4 to 0.7 K2O wt.% except for two

examples from the LM locality with K2O>1 wt.%.

The glass is mostly plagioclase and quartz norma-

tive with one being nepheline-normative (Table 1).

Glass in LM xenoliths generally has higher alkali

contents (Na2O +K2O) and lower Ti and Fe con-

tents than MP glasses with restricted silica contents

of f 62–65 wt.% (Fig. 9a–f). In the LM samples,

the few glass areas analysed are always closely

associated with feldspar and have major element

compositions very similar to the average plagioclase

major element composition. The glass in the MP

sample shows a compositional range within the

same sample, with silica from f 59 to 72 wt.%.

Glass analysed in different melt pockets has con-

trasting major-element compositions. Glass compo-

sitions within a single melt pocket may be

heterogeneous or homogeneous (Fig. 9) and there

is also no apparent spatial correlation of glass

major-element composition within the melt pockets.

Indeed, the most extreme glass compositions

(f SiO2 59 wt.% and SiO2 70–71 wt.%) are found

adjacent to reacting primary spinel.

Carbonates analysed in the two LM samples are

calcite (Table 2), with minor FeO and MnO con-

tents and no detectable MgO. Their composition is

very homogeneous and compositionally similar in

both veins and reaction zones A. Calcite from LM

sample BY96-357 has slightly higher FeO contents

(0.18–0.35 wt.%) than in LM sample BY96-381

(0.11–0.18 wt.%). Calcite from LM sample BY96-

357 also has MnO contents ranging from 0.15 to

0.28 wt.%, whereas calcite from LM sample

BY96-381 has no detectable MnO. Calcite from

LM sample BY96-381 in contact with primary

reacting spinel displays Cr2O3 up to 0.21 wt.%

(Fig. 3f).

Apatite may be Cl-rich (up to 4.87 wt.%) and

has high Ce2O3 (up to 0.92 wt.%). Apatite ana-

lysed in LM samples resembles hydroxychlorapatite

compositions, which may have significant CO2

substitution (O’Reilly, 1987; O’Reilly and Griffin,

2000).

6. Temperatures estimates

Equilibrium temperatures for the primary minerals

were calculated using the Ca-in-opx thermometer

from Brey and Kohler (1990) and the opx-sp ther-

mometer of Sachtleben and Seck (1981). T (Ca-in-

opx) yielded 840–895 jC for GR97-225 and BY96-

357, respectively and a slightly higher temperature

for BY96-381 (1020 jC). The opx-sp method

yielded a temperature range of 830 to 960 jC. Usingthe empirical geotherm for Kerguelen of Moine

(2000), the inferred equilibrium pressures range

from f 0.6 to f 0.7 GPa using the Ca-in-opx

temperature estimates for GR97-225 and BY96-357

and f 1.2 GPa for BY96-381. Equilibrium pres-

sures of f 0.6 to 1 Gpa were obtained for the three

harzburgites using the opx-sp temperatures estimates.

A high pressure limit for the Lac Michele xenoliths

is given by the upper stability limit of An-plagio-

G. Delpech et al. / Lithos 75 (2004) 209–237 225

clase in a peridotite and this occurs around 0.8–0.9

GPa at 800 jC and 1.1–1.2 GPa at 1200 jC in the

CMAS system (Green and Hibberson, 1970; Jaques

and Green, 1980). However, plagioclase with higher

sodium content can be stable in a lherzolite to higher

pressures (ca. f 1.8 GPa at 1170 jC; Windom and

Unger, 1988).

7. Trace-element compositions

Trace elements have been analysed in clinopyr-

oxene of all the samples and in the glass of the MP

sample. The clinopyroxenes analysed in LM and MP

harzburgites are spongy clinopyroxenes developed

around primary orthopyroxene and spinel grains.

Due to the fine grain-size of the secondary minerals

Table 3

Trace-element compositions of clinopyroxene and glass in reaction zones

Sample BY96-

357

BY96-

357

BY96-

357

BY96-

381

BY96-

381

BY9

381

Reaction zone B A A A A A

Mineral cpx-1 cpx-2 cpx-3 cpx-1 cpx-2 cpx-3

Rb 1.23 0.33 1.46 2.83 3.00 2.1

Ba 120 108 111 84 147 22

Th 1.44 1.19 3.88 0.73 1.21 0.6

U 0.83 0.14 1.58 0.25 0.24 0.1

Nb 1.74 0.08 1.64 2.20 5.73 2.0

Ta 0.17 0.04 0.16 0.18 0.34 0.2

La 12.17 15.68 49.12 22.27 24.04 26.5

Ce 32.51 38.15 99.29 59.53 60.42 72.3

Pb 1.51 1.93 nd nd nd nd

Pr nd 4.99 nd nd nd nd

Sr 442 268 520 491 349 327

Nd 19.72 17.69 38.22 57.83 52.06 58.9

Zr 17 1.08 12 311 266 290

Hf 0.76 < dl 0.41 3.24 3.66 3.7

Sm 3.27 2.31 6.04 13.99 12.17 14.1

Eu 1.35 0.94 1.61 4.22 3.31 3.9

Ti 400 89 420 881 1510 802

Gd 2.80 1.62 3.82 13.35 10.98 12.6

Tb nd 0.30 nd nd nd nd

Dy 1.58 1.95 2.68 8.43 8.46 6.9

Y 10.20 8.38 13.94 28.79 27.92 27.9

Ho 0.60 0.25 0.63 1.21 1.15 1.2

Er 0.79 0.70 1.16 2.54 2.10 2.4

Tm nd 0.11 nd nd nd nd

Yb 1.08 0.65 1.14 1.42 1.12 1.6

Lu 0.29 0.15 0.21 0.20 0.23 0.1

La/Yb 11.27 24.12 43.09 15.68 21.46 16.5

< dl: value below detection limit; nd: not determined.

and the occurrence of small accessory minerals such

as apatiteF rutile, the accuracy of trace-element

compositions for clinopyroxene and glass was estab-

lished during ablation runs by monitoring several

elemental isotopes (i.e. Al, P, La, Th, U, Nb, Ti).

Data are shown in Table 3 and CI-chondrite-normal-

ised REE patterns and primitive mantle-normalised

plots in Fig. 10a,b (McDonough and Sun, 1995). In

Fig. 10a, clinopyroxene displays different types of

REE patterns. LM sample BY96-357 and MP sample

GR97-225 shows LREE enrichment from Gd to La

with an almost flat slope from Gd to Lu, whereas

LM sample BY96-381 has an upward convex REE

pattern. Fig. 10b shows that the spongy clinopyrox-

ene is highly enriched in the most strongly incom-

patible trace elements such as Th, U, and LREE

(La = 12.17–24.55 ppm) but displays significantly

6- GR97-

225

GR97-

225

GR97-

225

GR97-

225

BCR-

2G Std

1 S.D.

A A A A

cpx-1 cpx-2 glass-1 glass-2 Average n= 9

3 < dl < dl 2.25 2.88 49.21 1.53

2 6 169 214 677 11

9 8.40 7.05 7.73 12.63 6.13 0.15

4 1.12 1.07 1.34 2.52 1.86 0.06

9 0.23 0.08 1.57 1.40 13.47 0.33

5 < dl < dl 0.11 < dl 0.80 0.06

1 24.35 24.55 39.03 44.67 24.62 0.52

4 48.58 46.23 80.45 74.50 51.32 1.03

2.49 2.68 6.46 7.14 10.87 0.39

4.77 4.54 8.68 7.49 6.86 0.13

523 448 1092 1061 344 4

6 13.78 14.44 27.64 20.05 29.49 0.63

14 11 12 11.02 192 7

9 0.38 0.25 0.32 0.25 5.00 0.33

8 1.59 1.76 3.24 2.49 6.55 0.22

7 0.45 0.68 1.17 0.71 1.95 0.05

356 359 1464 1446 14266 415

4 1.00 1.30 2.01 1.45 6.49 0.36

0.18 0.24 0.30 0.15 0.92 0.03

4 1.45 1.39 1.82 1.12 6.39 0.41

9 9.55 9.49 11.18 8.03 34.20 1.29

9 0.39 0.32 0.36 0.28 1.27 0.06

4 0.62 0.99 0.92 0.80 3.60 0.16

0.17 0.13 0.15 0.09 0.48 0.03

0 1.31 1.07 1.18 1.04 3.40 0.20

9 0.20 0.18 0.17 0.15 0.49 0.03

7 18.59 22.94 33.08 42.95 – –

Fig. 10. (a) REE patterns for LM and MP spongy clinopyroxenes in reaction zones (normalising values from McDonough and Sun, 1995). (b)

Trace-element patterns of the LM clinopyroxenes compared to other clinopyroxenes from Kerguelen harzburgites and dunites. Light grey field,

field for clinopyroxenes (Mg-augite) from poikilitic harzburgites referring to metasomatism by volatile-rich highly alkaline silicate melt

(Gregoire et al., 2000a). Medium grey field, field for clinopyroxenes in Kerguelen opx-free dunites metasomatised by carbonate-rich melts

(Moine, 2000). Dark grey field, field for clinopyroxenes from the MP harzburgite (see Fig. 11). Dotted line, field for clinopyroxenes in others

depleted harzburgites from LM locality (G. Delpech, unpublished data). Note the general poor enrichments in trace elements in these

clinopyroxenes compared to those of this study.

G. Delpech et al. / Lithos 75 (2004) 209–237226

G. Delpech et al. / Lithos 75 (2004) 209–237 227

lower enrichments in HREE (Yb = 0.15–0.29 ppm).

It also has pronounced negative anomalies in HFSE

elements (Nb, Ta, Zr, Hf, Ti) and high Sr contents

(268–523 ppm), although Sr may be depleted rela-

tive to Ce and Nd in some clinopyroxenes, most

probably due to the presence of plagioclase in the

metasomatic assemblage. It is characterised by im-

portant differences among trace elements, especially

high La/Yb (11.27–24.12) and low HFSE/REE ra-

tios (i.e. Zr/Sm; Nb/La; Ti/Eu). Clinopyroxene in

samples BY96-381 and GR97-225 are quite homo-

geneous in trace-element compositions, in contrast to

the heterogeneity in that of BY96-357 (Fig. 10a,b

and Table 3). In the latter, the lowest trace-element

Fig. 11. Trace-element compositions of glasses from the MP harzburg

McDonough and Sun, 1995). The bulk-rock trace-element pattern of the h

contents occur in spongy clinopyroxene in a B-type

reaction zone (cpx + fsF sp IIF ol II), whereas the

most enriched are in spongy clinopyroxenes in

reaction rims of primary spinels in an A-type reac-

tion zone. The most enriched clinopyroxenes are

found in sample BY96-381 where feldspar is rare.

These clinopyroxenes have greater MREE, HREE

and HFSE enrichment but similar LREE contents

and significantly lower Th and U contents than those

of the feldspar-rich and glass-rich sample.

Compositions of ‘‘primary’’ clinopyroxene in oth-

er depleted harzburgites from Lac Michele are shown

in Fig. 10b for comparison. They only contain small

abundances of trace elements and do not show

ite and their coexisting clinopyroxenes (normalising values after

ost basanite is shown for comparison.

G. Delpech et al. / Lithos 75 (2004) 209–237228

enrichment in the most incompatible elements.

Trace-element compositions of clinopyroxenes from

LM and MP xenoliths are comparable to those of

Mg-augite in poikilitic harzburgites (related to highly

alkaline silicate metasomatism; Gregoire et al.,

2000a; Fig. 10b). However, they have much higher

Th, U, Sr contents and very pronounced HFSE

depletions (except sample BY96-381 for Nb, Ta,

Zr, Hf). They also display higher LREE contents

and may have slightly lower HREE contents. On the

other hand, LM and MP clinopyroxenes resemble

those associated with carbonatitic metasomatism

recorded in Kerguelen mantle xenoliths (Moine,

2000) (Fig. 10b), and those from worldwide locali-

ties (e.g. Coltorti et al., 1999).

The glass in GR97-225 has a concave upward REE

pattern with marked LREE enrichment from Gd,

similar to that of clinopyroxene (Fig. 11a). It is

enriched in the strongly incompatible trace elements

and especially in LREE Th, U and Sr (1061–1092

ppm), whereas HFSE (Nb, Ta, Zr, Hf, Ti) and HREE

show lower abundances (Table 2, Fig. 11a,b). The

glass contains small amounts of Ba (169–214 ppm)

and Rb (2.6 ppm). It has similar Th, U and La

contents to the host lava but significantly lower

contents for most of the MREE and HREE. Moreover,

the host basanite does not show any HFSE negative

anomalies. The trace-element pattern of the glass is

similar to that of the clinopyroxenes, with a slight

enrichment for the most incompatible elements (Th,

U, LREE), comparable HREE contents and higher La/

Yb ratios (33.08–42.15).

8. Discussion

8.1. Feldspar and glass formation

8.1.1. Feldspar in LM samples

Feldspar may form in the shallower lithospheric

mantle through sub-solidus re-equilibration of spinel

peridotites in the plagioclase stability field (Jaques

and Green, 1980) and this reaction may be assisted by

the infiltration of a metasomatic melt. Such feldspars

in mantle xenoliths have been reported from several

locations worldwide (Dautria et al., 1992; Bonadiman

et al., 2001; Ionov et al., 1995, 1999; Xu et al., 1996;

Gregoire et al., 2000b; Chalot-Prat and Arnold, 1999)

and commonly show a continuum in composition

from plagioclase to alkali feldspar (Fig. 7). The

restricted compositional range of Na-rich plagioclase

described here is not so common but has been

reported from silicate melt zones in harzburgites

metasomatised by calcio-carbonatitic melts (Chalot-

Prat and Arnold, 1999). Meen (1987) also experimen-

tally showed that reaction between carbonate-bearing

alkali melts with low water content and peridotite

minerals can produce sanidine and feldspathoıd at 1.4

GPa, if the peridotite–H2O–CO2 solidus is crossed

and carbonate is no longer a stable phase. The micro-

structural position of the plagioclase in the reaction

zones (mantling reacted spinel or as a matrix for other

metasomatic minerals) strongly suggests that the feld-

spar is a product of the disequilibrium reaction be-

tween the host peridotite and a metasomatic agent.

8.2. Glass in LM-MP samples

The glass in the three harzburgites have silicic

(59.11–71.93 wt.%), aluminous (16.23–21.93 wt.%)

and sodic (4.80–9.36 wt.%) compositions (Figs. 8 and

9). The occurrence of glass in mantle xenoliths is well

documented worldwide and has been attributed to

many processes. Experimental studies on low degree

melting of peridotite have shown that silicic, alumi-

nous and alkaline (especially felsic sodic) melts are in

equilibrium with a lherzolitic or harzburgitic mineral-

ogy at 1–3 GPa and between 850 and 1350 jC(Draper and Green, 1997, 1999). Although some

glasses from LM and MP xenoliths have compositions

resembling those of experimentally determined melts

in equilibrium with peridotite, the extensive reaction

relationships with the host peridotite minerals indicate

this process is not likely for the generation of the LM-

MP glasses. Decompression melting during rapid

ascent to the surface of pre-existing alkali-bearing

metasomatic silicates can produce glass (e.g. Yaxley

et al., 1991; Yaxley and Kamenetsky, 1999; Ionov et

al., 1993; Chazot et al., 1996; Kogarko et al., 1995;

Dautria et al., 1992; Hauri et al., 1993; Laurora et al.,

2001). However, incongruent melting of former am-

phibole and phlogopite is highly unlikely because

these minerals (nor any remnants thereof) are not

observed. Dissolution of orthopyroxene in Si-under-

saturated alkaline melts has been studied experimen-

tally (Shaw, 1999) and may produce Si-rich alkaline

G. Delpech et al. / Lithos 75 (2004) 209–237 229

glasses, together with secondary olivine and clinopyr-

oxene. Infiltration by the Si-undersaturated host lava

is documented in some other mantle xenoliths from

Kerguelen Islands and results in clinopyroxene over-

growths on reacting primary orthopyroxene. The

composition of these clinopyroxenes is markedly

different from those reported here, with significantly

higher Ti and Fe contents (1–2 and 3–4.5 wt.%,

respectively), lower Na contents, and lower mg# of

f 88–90. The K2O content of the MP host lava (2.28

wt.%) is close to that of Shaw’s experiments and if the

glass originated from the host basanite infiltration, a

K-bearing glass would result. In contrast, the glass has

very low K and very high Na2O/K2O compared with

the Na2O/K2O of the host lava. In addition, the trace-

element contents of the glass and the host lava are

very distinct in terms of HFSE/LREE and HFSE/

MREE ratios. However, several models show that

the generation of highly silicic glasses in mantle

xenoliths (e.g. Zinngrebe and Foley, 1995; Wulff-

Pedersen et al., 1996, 1999; Neumann and Wulff-

Pedersen, 1997) can derive from progressive reaction

between infiltrating basaltic or alkaline melts and the

host peridotite (AFC-type processes). In these models,

dissolution of orthopyroxene by the Si-undersaturated

infiltrating basaltic or alkaline melt and reaction with

the host peridotite is an important process in generat-

ing the highly silicic glasses. Such a model involving

infiltration of a melt and reaction with the host

peridotite could explain the generation of the highly

silicic glasses in the MP harzburgites because the MP

orthopyroxene (and clinopyroxene) are incongruently

melting. However, both the high Na2O/K2O ratios and

trace-element abundances of the glass (especially the

negative HFSE anomalies in the glass) and the MP

host magma are too different to be consistent with this

mechanism if involving an infiltrated basaltic or

alkaline melt (Neumann and Wulff-Pedersen, 1997;

Wulff-Pedersen et al., 1999; Zinngrebe and Foley,

1995). If such a process had occurred, the trace-

element composition of the infiltrating melt must have

had low abundances in HFSE elements and must have

been enriched in the remaining trace elements. Zinn-

grebe and Foley (1995) also pointed out that such

interaction processes could lead to modification of the

major-element composition of the secondary minerals

(olivine, clinopyroxene) similar to that commonly

observed during carbonatitic metasomatism. The

microstructural and major-element variations de-

scribed in this study are similar to those described

by Coltorti et al. (1999) for the generation of Si–Al–

Na-rich glasses in relation with carbonatitic metaso-

matism. Microstructural features and compositional

variations in the glass in LM-MP harzburgites support

an origin due to the reaction of primary orthopyrox-

ene, spinel and clinopyroxene with a metasomatic

melt, as proposed for the origin of the feldspar in

LM xenoliths. Reaction of the primary minerals may

be due to infiltration of a Si-undersaturated melt with

low HFSE contents and high contents of other trace

elements (especially LREE, Th, U, Sr), or a carbon-

ate-rich melt.

The rare and relatively homogeneous glass in LM

xenoliths occurring within the felspar matrix has

significantly higher Na2O wt.% (FK2O) and lower

FeOt, TiO2 than most of the glasses from MP xenolith

(Fig. 9) as described above. The simplest explanation

is that they represent frozen residual melt trapped in

the interstices after extensive crystallisation of feld-

spar, clinopyroxene, olivine, Cr-rich spinels and apa-

tite from the infiltrating melt. The lower TiO2 and FeO

contents of these glasses may reflect the crystallisation

of oxide minerals (rutile, chromite) from the reacting

melt.

8.3. Nature of the metasomatic agent

The refractory compositions of the primary miner-

als (high mg# in ol–opx–sp) indicate that LM and

MP xenoliths have undergone a previous high degree

partial melting, similar to that described by Gregoire

et al. (1997, 2000a). The nature of the secondary

assemblage and its major-element and trace-element

compositions records subsequent reactions between

the primary minerals and a metasomatic melt.

Previous studies have recognised widespread meta-

somatism by volatile-rich, highly alkaline mafic sili-

cate melts and more rarely by volatile-poor, Ti-rich

highly alkaline mafic silicate melts in the Kerguelen

oceanic mantle (Gregoire et al., 2000a,b; Moine et al.,

2001). Metasomatism is evidenced by strong local

enrichments in the most incompatible trace-element

associated with crystallisation of Mg-augiteF phlo-

phlogopiteFTi-pargasite (Gregoire et al., 2000a;

Moine et al., 2001) or, more rarely, by the presence

of veins composed of plagioclase +K-feldspar +Ol

G. Delpech et al. / Lithos 75 (2004) 209–237230

II + Sp II + Ti-oxides (Rutile–Ilmenite–Cr–Armalco-

lite) (Gregoire et al., 2000b). However, the metaso-

matic mineral assemblage and the clinopyroxene

major and trace-element compositions in LM and

MP xenoliths from this study differ significantly from

those previously described (Figs. 6, 7 and 10b),

especially in terms of HFSE negative anomalies,

low HFSE/LREE ratios and high Th, U, Sr contents.

Microstructural evidence suggests that the infiltra-

tion of a metasomatic melt, with a silica activity too

low to be in equilibrium with primary orthopyroxene,

spinel and clinopyroxene of the lithospheric mantle,

triggered reaction of these minerals. The major-ele-

ment compositions recorded in the secondary assemb-

lages do not show the typical Fe–Ti enrichment that

usually accompanies infiltration and reaction of a

basaltic melt. However, the metasomatic assemblages

bear major-element and trace-element compositions

that are compatible with a carbonate-rich silicate melt

rather than a carbonatitic melt sensu stricto. The

formation of secondary olivine with higher mg# and

Ca content, Cr-rich spinel, and Na–Cr clinopyroxenes

in the LM-MP harzburgites are consistent with meta-

somatism by such melts (Yaxley et al. 1991, 1998;

Hauri et al., 1993; Ionov et al., 1996, Rudnick et al.,

1993; Dautria et al., 1992; Kogarko et al., 1995). The

occurrence of primary carbonates in the LM samples

further support the hypothesis that the metasomatic

agent was carbonate-bearing. Increasing carbonatitic

metasomatism should increase the jadeite component

of the clinopyroxene, but should not drastically affect

the Al content of the clinopyroxene, as shown by

Yaxley et al. (1991, 1998). However, high Na–Cr–Al

clinopyroxenes associated with carbonatitic metaso-

matism have also been described in peridotite from

Spitsbergen (Ionov et al., 1993, 1996). In the samples

studied here, not all clinopyroxenes display a clear

increase in jadeite component and most of those

clinopyroxenes are Na–Cr–Al-rich. This is especially

true for some of the spongy clinopyroxene crystallis-

ing near reacting spinel I (Type A reaction zones), and

for the small clinopyroxenes crystallising in the feld-

spar/glass matrix around reacting spinel I (Type A

reaction zones). On the other hand, the spongy Na–Cr

clinopyroxenes in reaction zones B have a higher

jadeite component and display in most of the cases

lower Al content, with a clear decrease of Al towards

the rims, and increase in Ca, Mg. These clinopyrox-

enes more likely reflect interaction with a carbonate-

rich agent. Some of the high Na–Al–Cr spongy

clinopyroxenes described in reaction zones A likely

reflect a combination of two processes, namely the

signature of the percolating melt and the overprint

from the local melt/rock interaction processes occur-

ring around the reacting primary spinel (higher Al, Cr

contents); therefore, their composition cannot be used

to directly infer the nature of the percolating agent.

However, the melt infiltrating the xenoliths cannot

be a carbonatitic melt sensu stricto. If we compare the

modal abundance of feldspar in LM sample BY96-

357 (f 1.8 vol.%) with that of clinopyroxene (f 3

vol.%), it seems unrealistic that such amount of

feldspar can be produced only by reaction of the

mantle wall-rock with a carbonatitic melt sensu

stricto.

Finally, the major-element composition of the glass

also indicates an origin of the metasomatic assem-

blage by interaction of a carbonate-rich silicate melt

and the host peridotite. Coltorti et al. (2000), reviewed

glass compositions from anhydrous mantle xenoliths,

and showed that glasses related to carbonatitic meta-

somatism has distinctive signatures from those related

to Na–K alkali silicate melt metasomatism. In partic-

ular, they have usually high CaO contents, high Na2O/

K2O ratios and tend to have lower SiO2 and

TiO2 +K2O contents (Fig. 12a,b). According to the

discrimination diagram of Coltorti et al. (1999), the

LM and MP glasses have a clear carbonatitic signature

(Fig. 12a). Their high Na2O/K2O ratios are also a

characteristic feature of carbonatitic metasomatism.

However, Fig. 12b shows that a majority of them

have lower CaO and higher SiO2 and lie in the alkali

silicate field, suggesting the presence of some silicate

component as well. Some of the glass compositions in

the LM and MP xenoliths (and particularly the Si–Al

rich glasses from the MP sample) closely resemble

those of glass associated with carbonatitic metasoma-

tism in oceanic mantle from Gran Comore (Coltorti et

al., 1999), Samoa (Hauri et al., 1993) and Fernando de

Noronha, Brazil (Kogarko et al., 2001). Schiano et al.

(1994) described migrating carbonate-rich melts in the

Kerguelen oceanic mantle on the basis of cogenetic

relationships between a silicate melt, a carbonate-rich

melt and CO2 fluid inclusions. Silicate melt from

Schiano et al.’s (1994) inclusions have homogeneous

compositions close to some of the low Si-glasses from

Fig. 12. Comparisons of LM and MP glasses with metasomatic

agents I (carbonatitic melt), II (Na-silicate melt), and III (K-silicate

melt) deduced from world-wide glasses (Coltorti et al., 2000). (a

(TiO2 +K2O) versus (CaO+Na2O) diagram for LM and MP

glasses; (b) CaO versus SiO2 diagram for LM and MP glasses

Light grey field, compositions of glasses related to Na–K silicate

metasomatic agent II and III; medium grey field, compositions o

glasses related to metasomatic agent I, i.e. carbonatitic metasoma

tism. Symbols as in Fig. 2.

G. Delpech et al. / Lithos 75 (2004) 209–237 231

)

.

f

-

LM and MP harzburgites but generally lower Na and

higher Ti (FCa) contents (Fig. 9). Our study con-

firms the close relationship between carbonate-rich

and silicate melt in the Kerguelen oceanic mantle

described by Schiano et al. (1994).

Therefore, on the basis of major element compo-

sition of minerals, there is evidence for introduction of

a carbonate-rich melt with a silicate component into

the depleted harzburgites. This melt must also be Na-

rich (and very low in K) as evidenced by the abun-

dance of Na-rich plagioclase and Na-rich glass. The

occurrence of feldspar in the LM harzburgites, togeth-

er with the absence of hydrous minerals (amphibole,

phlogopite) in the secondary assemblage, further indi-

cates that the metasomatic agent had a low water

content, as suggested in several studies on metasoma-

tism involving feldspar formation (Ionov et al., 1995,

1999; Chalot-Prat and Arnold, 1999).

The spongy clinopyroxenes and glass have large

negative anomalies in HFSE (Nb, Ta, Zr, Hf, Ti),

strong enrichments in LREE ([La/Yb] = 11.27–

33.08), Th, U and Sr contents (Fig. 10b). Experimental

studies have shown that compositional variations in

clinopyroxene, especially variation in its AlIV content

and CaTs component, may produce large variations in

mineral–melt partition coefficients for REE and HFSE

(e.g. Hill et al., 2000; Lundstrom et al., 1998; Wood

and Blundy, 2001). Considering the relatively low

AlIV and CaTs component in the clinopyroxenes from

this study (Table 1), it is unlikely that the high trace-

element contents in clinopyroxene are due to variation

in mineral/melt partition coefficients caused by com-

positional effects in the clinopyroxene. A careful

consideration of crystal-chemical effects on the

trace-element compositions would be to take into

account the melt composition/structure, which also

influences the mineral/melt partitioning coefficients

(Vannucci et al., 1998). However, this discussion is

beyond the scope of the present study and is not

developed here. From the trace-element compositions

of clinopyroxenes, it can be inferred that the melt that

infiltrated the LM and MP harzburgites had high

LREE, Sr, Th, U contents and low Rb, Ba and HFSE

contents. The low HFSE contents of the clinopyrox-

enes in the LM locality may have been enhanced by

the precipitation of smalll amounts of rutile. Accessory

phases, such as rutile, will preferentially deplete the

HFSE from the melt (especially for Nb and Ta),

whereas much of Zr and Hf may also reside in pyrox-

enes (Kalfoun et al., 2002). Thus, the concentration of

HFSE in the spongy clinopyroxene may not directly

reflect the HFSE signature of the original percolating

melt. However, the metasomatic agent most probably

had low original contents in HFSE despite the pres-

ence of rare rutiles in LM samples. This is supported

by the bulk-rock trace-element contents of the most

metasomatised sample LM BY96-357, which display

a trace-element pattern similar to that of the clinopyr-

oxene (i.e. negative anomalies in HFSE and strong Th,

U, LREE and Sr enrichments).

Fig. 13. (a) (Ti/Eu) versus (La/Yb)N in clinopyroxenes from LM and

MP harzburgites. Empty triangle, LM sample BY96-381. Empty

circle, LM sample BY96-357. Empty diamond, MP sample GR97-

225. Empty square, clinopyroxenes from other depleted harzbur-

gites from Lac Michele locality. Filled squares, clinopyroxene

associated with highly alkaline mafic silicate metasomatism in

poikilitic harzburgites (Gregoire et al., 2000a). Filled circles,

clinopyroxenes in depleted protogranular harzburgites, the least

metasomatized xenolith suite in Kerguelen (Gregoire et al., 2000a).

Note the high (La/Yb)N and low (Ti/Eu) in LM and MP

clinopyroxenes compared with clinopyroxenes from other LM

samples or from the poikilitic and protogranular suite. b) (Nd/Hf )Nversus (Gd/Ti)N in clinopyroxenes from LM and MP harzburgites.

Dark grey field, field for protogranular hazburgites in the Kerguelen

mantle xenoliths suite. Light grey field, field for poikilitic

harzburgites, metasomatised by highly alkaline mafic silicate melt.

Increase in (Nd/Hf )N and (Gd/Ti)N shows the effect of carbonatitic

metasomatism on the trace-element compositions of the clinopyr-

oxenes. Symbols for LM and MP clinoproxenes same as in (a).

Trace-element ratios are normalized to primitive mantle values

(McDonough and Sun, 1995).

G. Delpech et al. / Lithos 75 (2004) 209–237232

The trace-element characteristics of the clinopyr-

oxenes are consistent with features of carbonatitic

metasomatism ((Figs. 10b, 11b and 13a); Rudnick et

al., 1993; Coltorti et al., 1999, 2000; Klemme et al.,

1995). They also closely resemble clinopyroxenes

related to metasomatism by carbonate-rich melt found

in Kerguelen mantle xenoliths (Gregoire et al., 2000a;

Moine, 2000, Mattielli et al., 1999) (Fig. 10b), but

have higher Ba contents. However, the trace-element

compositions of the glass, if compared with most

glass related to carbonatitic metasomatism worldwide,

do not display such high Nb contents, and display

slightly lower LREE, Ba contents. The REE patterns

of clinopyroxene from LM sample BY96-381 are

similar to that in equilibrium with a silicate melt,

indicating that the metasomatic agent had greater

MREEFHREE contents, and was possibly less de-

pleted in HFSE elements. This implies that the meta-

somatic agent may have been locally heterogeneous in

both the trace-element content and the relative pro-

portions of silicate and carbonate components. A plot

of (Gd/Ti)N versus (Nd/Hf)N (Fig. 13b) shows little

fractionation of these ratios in protogranular and

poikilitic harzburgites, which were metasomatised

by basaltic to highly alkaline melts at high melt/rock

ratios. However, clinopyroxenes from LM sample

BY96-357 and MP sample GR97-225 show a clear

increase of (Nd/Hf)N with increasing (Gd/Ti)N, clearly

suggesting metasomatism by a carbonate-rich melt. In

contrast, clinopyroxene from sample BY96-381

shows a strong increase in (Gd/Ti)N, but not in (Nd/

Hf)N, thus probably reflecting larger amounts of the

inferred silicate component.

The trace-element characteristics typical of carbo-

natitic metasomatism may be produced by other

metasomatic processes, which do not necessarily

imply the presence of a carbonate-rich melt. These

include reactive porous flow (e.g. Bedini et al., 1997;

Rivalenti et al., 2000) and metasomatism by silicate

melts (Zanetti et al., 1999). However, the samples of

this study show strong microstructural, mineralogical

and geochemical evidence for reaction of a sodic

carbonate-rich silicate fluid with the depleted harz-

burgites. A distinctive feature of this sodic carbon-

ate-rich silicate metasomatism in the Kerguelen

oceanic mantle is the relatively high Sr (200–500

ppm), Th (up to 13.05 ppm), and U (up to 1.12 ppm)

abundance in clinopyroxenes relative to clinopyrox-

G. Delpech et al. / Lithos 75 (2004) 209–237 233

enes related to other carbonatitic metasomatism

worldwide (see review by Coltorti et al., 1999 and

references therein).

8.4. Carbonate-rich silicate melt/wall-rock peridotite

reactions

8.4.1. LM samples

The relative abundance of metasomatic patches

and minerals indicates that harzburgite LM BY96-

381 reacted to a lesser extent with the metasomatic

agent than harzburgite LM BY96-357. The progres-

sive reaction of the metasomatic agent with wall-

rock peridotite is recorded by the compositional

variations of feldspar (Fig. 7). This latter is Na-rich

(oligoclase) in the highly reacted LM sample BY96-

357 which contains abundant apatite, and Ca-rich

(labradorite) in the less reacted LM sample BY96-

381, which contains only minor Ab-rich plagioclase

and accessory apatite and has clinopyroxene with

trace-element signatures reminiscent of larger

amounts of a silicate component in the infiltrating

melt. The differences in metasomatic mineral modes

and their composition likely reflect locally heteroge-

neous melt/rock reactions at small melt/rock ratio

with the infiltrating melt and variable relative pro-

portions of the carbonate and silicate components in

this latter.

The reaction-crystallisation process observed in

LM harzburgites describes the reaction between

primary peridotitic minerals assemblage (opx, sp,

cpx) and the carbonate component of the infiltrating

melt. A carbonate-rich melt at low pressure is not

stable in a peridotitic assemblage and reacts with the

wall-rock mantle, especially if containing orthopyr-

oxene. Examples of these reactions (Yaxley et al.,

1991; Dalton and Wood, 1993) in the samples

studied may be:

ð1Þ CaMgðCO3Þdolomite

þ 4MgSiO3

enstatite

¼ 2Mg2SiO4

olivine

þ CaMgSi2O6

diopside

þ 2CO2

ð2Þ 9MgSiO3

enstatite

þMgðCr;AlÞ2O4

spinel

þ Na2CO3

Na� carb

¼ 5Mg2SiO4

olivine

þ 2NaðCr;AlÞSi2O6

jadeite� ureiite s:s:þ CO2

ð3Þ 3CaMgðCO3Þ2dolomite

þ CaMgSi2O6

diopside

¼ 2Mg2SiO4

olivine

þ 4CaCO3

calcite

þ 2CO2

The reaction-crystallisation process in the LM

harzburgites ultimately leads to crystallisation of a

silicate assemblage (ol, cpx, fs), carbonates and ac-

cessory minerals, including spinel II, apatite and

rutile. The dissolution of orthopyroxene, spinel and

clinopyroxene through the reaction releases Si, Al, Cr,

Mg, which will be added to the reacting carbonate–

silicate melt. The residual melt, depending on local

crystallisation of secondary olivine and clinopyroxene

in the pockets, may become Si-, Al- and Na-rich and

may crystallise various amount of plagioclase on

cooling. A remaining carbonate component of this

melt may be preserved if not in contact with ortho-

pyroxene, thus ultimately forming crystalline cumu-

lates of nearly pure calcite. Parts of this residual

calcitic melt may also migrate away from the silicate

portion to form veins. The occurrence of nearly pure

calcite as crystalline precipitates is in accordance with

experimental studies (Dalton and Wood, 1993; Lee

and Wyllie, 2000) at low pressure (f 1–1.5 GPa) in

the lithospheric mantle.

8.4.2. MP sample

In the MP harzburgite, the negative correlation of

major elements with silica content is contrary to

fractional crystallisation trends during melt/rock reac-

tion processes. For instance, the Na content of the

glasses drops by almost a factor of two between the

two extreme glass compositions. As no Na-bearing

mineral crystallised (clinopyroxene does not incorpo-

rate enough Na), Na2O should increase with SiO2 if

fractional crystallisation was the dominant process.

Yet there is only local evidence in some pockets that

fractional crystallisation played a role during the melt/

rock reaction (Fig. 6). Decreasing CaO, FeO and MgO

with increasing SiO2 is consistent with the formation

of secondary clinopyroxene and olivine from the

reacting melt. The negative correlation of Na2O with

increasing SiO2 is not clear. The large variations of

major elements with silica content in Fig. 9 probably

result from a heterogeneous reaction-recrystallisation

process of the percolating melt reacting with the host

G. Delpech et al. / Lithos 75 (2004) 209–237234

peridotite. A reaction involving Opx I + Sp I+(Cpx

I) + low SiO2 melt!Ol II + Cpx II + higher SiO2–

Al2O3 melt (e.g. Zinngrebe and Foley, 1995) can

explain the preferential reaction of orthopyroxene

and spinel, and can account for the silica oversatura-

tion and the high Al contents of the glasses. This

reaction is enhanced by a carbonate component,

which is also highly reactive with orthopyroxene

and spinel. The absence of carbonate in the MP

harzburgite, in contrast to some other locations world-

wide (e.g. Amundsen, 1987; Kogarko et al., 1995;

Ionov et al., 1993, 1996) may simply indicate that the

carbonate fraction reacted completely with the prima-

ry harzburgitic assemblage to produce the secondary

olivine, clinopyroxene and spinel, or that the remain-

ing carbonate fraction and/or CO2 percolated away. In

any case, this metasomatic event must have happened

shortly before the eruption in order to preserve both

the disequilibrium features between primary minerals

and the metasomatic agent and the fresh glass patches

(by rapid quenching of the glass during transport to

the surface).

In summary, we suggest that small volumes of

sodic carbonate-rich silicate melt, percolating through

the oceanic mantle, reach the LM and MP depleted

harzburgites and trigger incongruent melting of pri-

mary orthopyroxene, spinel and clinopyroxene. Vary-

ing degrees of melt/rock reactions associated with

different carbonate/silicate melt ratio in the migrating

melts, locally re-fertilised depleted harzburgites into a

more wehrlitic assemblage (ol–cpx–sp) and also

produced abundant interstitial Na-rich plagioclase

and/or Si–Al–Na-rich glass. Our findings illustrate

the model of Bailey (1987) who argues that deep-

seated, volatile-bearing mafic melt migrating upward

along an oceanic geotherm will percolate and react

with the surroundings mantle wall-rock, and may

precipitate sodic plagioclase when reaching the ‘‘fel-

sic stability zone’’ at shallow depth in the upper

mantle. However, from this study, the origin and

original composition of the metasomatising fluid

cannot be specified, as it is inferred to have undergone

a series of melt/rock interactions in the lithospheric

mantle, which would have modified its original com-

position. This study demonstrates the close relation-

ship between carbonate-rich and silicate melts in the

oceanic mantle beneath the Kerguelen Islands, first

recognised by Schiano et al. (1994).

9. Conclusions

(1) This work demonstrates that the major- and trace-

element variations in some mantle–derived harz-

burgite xenoliths from Lac Michele and Mont-

Peeper, Kerguelen Islands, result from interaction

between a percolating sodic carbonate-rich silicate

melt and the depleted lithospheric mantle wall-

rock. The inferred percolating melt was enriched

in P, LREE, Th, U, Sr and Na and low in K, Rb,

Ba, and HFSE.

(2) The interstitial metasomatic phases in the silicate

assemblage resulting from this interaction between

the melt and the mantle wall-rock comprise a Na-

rich plagioclase and/or Si–Al–Na-rich glass. This

suggests that during carbonate-rich silicate meta-

somatism with low water content in the shallow

part of the oceanic mantle, Na-rich plagioclase is

stable in the metasomatic silicate assemblage.

These felsic metasomatic patches (on a mm to

cm scale) have highly enriched trace-elements

contents.

(3) This metasomatic event must be recent and close

to the time of eruption in order to preserve the

disequilibrium microstructural and chemical fea-

tures and the glass patches in LM and MP

harzburgites. Metasomatic agents are probably

related to the magmatism responsible for the small

volumes of youngest mafic alkaline melts on

Kerguelen. This metasomatic agent may be related

to the magmatism responsible for the small

volumes of youngest mafic alkaline melts on

Kerguelen.

Acknowledgements

The authors would like to thank Tom Bradley for

the polished thick sections, Ashwini Sharma, Suzy

Elhlou and Carol Lawson for assistance with the

analytical work and William Griffin and Norman

Pearson for ever-ready advice, assistance and dis-

cussion during the analytical work and the preparation

of this manuscript. The manuscript has benefited from

constructive reviews by T. Andersen, G. Rivalenti,

and an anonymous reviewer. Comments by the editor

R. Vannucci have also been much appreciated. We

thank the French Polar and Technology Institute

G. Delpech et al. / Lithos 75 (2004) 209–237 235

(IFRTP, Brest, France) for their assistance during

fieldwork. This work is part of a ‘‘cotutelle’’ project

between GEMOC, Macquarie University, Sydney and

Equipe ‘‘ Transferts Lithospheriques ’’, UMR 6524

‘‘Magmas et Volcans’’, Universite Jean Monnet,

Saint-Etienne, France. G.D. thanks the support of

the Region Rhone-Alpes with the program ‘‘EURO-

DOC’’. Support for this project was provided from an

ARC Discovery Grant (S.Y.O’R and others), the

GEMOC ARC National Key Centre, an ARC IREX

Grant (S.Y.O’R and W.L. Griffin), a Macquarie

University Postgraduate Research Grant (G.D), and

a Macquarie University International Postgraduate

Scholarship (G.D). This is publication number 331 in

the GEMOC ARC National Key Centre www.es.mq.

edu.au/GEMOC/).

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