Lithium, boron and chlorine as tracers for metasomatism in high-pressure metamorphic rocks: a case...

Miner Petrol (2009) 95:291–302DOI 10.1007/s00710-008-0032-3

ORIGINAL PAPER

Lithium, boron and chlorine as tracers for metasomatismin high-pressure metamorphic rocks: a case studyfrom Syros (Greece)

Horst R. Marschall · Rainer Altherr ·Katalin Gméling · Zsolt Kasztovszky

Received: 31 March 2008 / Accepted: 3 November 2008 / Published online: 20 November 2008© Springer-Verlag 2008

Abstract High-pressure metamorphic (HPM) rocks(derived from igneous protoliths) and their metaso-matised rinds from the island of Syros (Greece) wereanalysed for their B and Cl whole-rock abundancesand their H2O content by prompt-gamma neutron-activation analysis (PGNAA) and for their Li andBe whole-rock abundances by ICP-OES. In the HPMrocks, B/Be and Cl/Be ratios correlate with H2O con-tents and appear to be controlled by extraction of B andCl during dehydration and prograde metamorphism. Incontrast, samples of the metasomatised rinds show nosuch correlation. B/Be ratios in the rinds are solelygoverned by the presence or absence of tourmaline,and Cl/Be ratios vary significantly, possibly relatedto fluid inclusions. Li/Be ratios do not correlate withH2O contents in the HPM rocks, which may in partbe explained by a conservative behaviour of Li dur-ing dehydration. However, Li abundances exceed thevast majority of published values for Li abundances infresh, altered, or differentiated oceanic igneous rocksand presumably result from metasomatic enrichment ofLi. High Li concentrations and highly elevated Li/Be

Editorial handling: D. Harlov

H. R. Marschall (B)Department of Earth Sciences, Wills Memorial Building,Queen’s Road, University of Bristol, Bristol BS8 1RJ, UKe-mail: [email protected]

H. R. Marschall · R. AltherrMineralogisches Institut, Universität Heidelberg,Im Neuenheimer Feld 236, 69120 Heidelberg, Germany

K. Gméling · Z. KasztovszkyInstitute of Isotopes, Hungarian Academy of Sciences,P.O. Box 77, 1525 Budapest, Hungary

ratios in most metasomatised samples demonstrate anenrichment of Li in the Syros HP mélange during fluidinfiltration. This study suggests that B and Cl abun-dances of HPM meta-igneous rocks can be used to traceprograde dehydration, while Li concentrations seem tobe more sensitive for retrograde metasomatic processesin such lithologies.

Introduction

A number of studies have recently focused on themetamorphic, chemical and isotopic evolution of high-pressure metamorphic (HPM) rocks as geochemicalprobes into the subducting lithosphere (see Bebout2007, and references therein). Element budgets of HPMmetabasites are thought to represent the composition of(altered) oceanic crust modified by dehydration duringsubduction. Chemical signatures of metamorphic rockscan be related to specific pressure and temperature con-ditions, once the metamorphic and metasomatic historyhas been unravelled. Metamorphic geochemistry hasthe potential to disentangle the pathways and redistrib-ution mechanisms of material in subduction zones, andto put the interpretation of geochemical and isotopicsignatures of volcanic rocks on a much sounder basis.

It is, however, crucial for the correct interpreta-tion of HPM rocks to recognise processes that mayhave altered the geochemical signatures of the rocksat depth, such as cryptic or modal metasomatism. Thechemical and isotopic patterns of the rocks could other-wise be misinterpreted and lead to false conclusions onthe element flux during dehydration of the subductinglithosphere.

292 H.R. Marschall et al.

Lithium, B, and Cl have gained significant attentionamong geochemists during the last two decades, as theyare characterised by high mobility in hydrous fluids andstrong isotopic fractionation at low temperatures (e.g.Palmer and Swihart 2002; Tomascak 2004; Stewart andSpivack 2004). These trace elements, therefore, have ahigh potential as tracers of slab-mantle fluid transportand for recycled material in the Earth’s mantle (e.g.Ishikawa and Nakamura 1994; Straub and Layne 2003;Tomascak 2004). The behaviour of Li, B, and Cl dur-ing progressive high-pressure metamorphism has beeninvestigated using whole-rock abundances in HP ser-pentinites, metasedimentary, and meta-igneous rocks(e.g. Moran et al. 1992; Philippot et al. 1998; Beboutet al. 1999; Zack et al. 2003; Scambelluri et al. 2004;Bonifacie et al. 2008). These studies suggest that B andCl are generally extracted together with hydrous fluidsfrom the subducting lithosphere with increasing P andT. An important exception to this rule are tourmaline-bearing meta-sediments that retain B to high pressures(Nakano and Nakamura 2001, Marschall et al. 2008a, b).

The fate of Li during dehydration is more difficult toevaluate. Despite its high solubility in hydrous fluids, Lishows relatively high abundances in eclogites, HP meta-sediments and ultramafic rocks (Scambelluri et al. 2004,2006; Bebout 2007; Marschall et al. 2007b). This has inpart been attributed to the retention of Li in the rocksduring dehydration, due to the compatibility of Li in HPminerals (Woodland et al. 2002; Scambelluri et al. 2006;Marschall et al. 2007a). However, many HPM rocksshow Li concentrations that grossly exceed abundancesof their likely protoliths, and it has been argued that Limust have been added to the rocks during subductionor exhumation (Marschall et al. 2007b).

In order to investigate the behaviour of elementsduring progressive dehydration of meta-igneous rockswithin the subducting lithosphere, we determined Li,B, and Cl whole-rock abundances of HPM rocks fromtwo different groups from the island of Syros (Greece).The first group comprises HPM rocks that were formedby prograde metamorphism and dehydration and donot show signs of modal metasomatism in the form ofretrograde hydrous minerals. This group should pro-vide information on the behaviour of elements duringprogressive dehydration of meta-igneous rocks withinthe subducting lithosphere. Group 1 is further sub-divided on the basis of the respective protoliths andincludes eclogites, blueschists, metagabbros and meta-plagiogranites (Table 1). The second group includessamples that were significantly hydrated and recrys-tallised during exhumation (Marschall 2005; Marschallet al. 2006b; Miller et al. 2008). This group of rocksdemonstrates the effects of metasomatism of exhuming

HPM rocks on their Li, B, and Cl abundances.Group 2 is further subdivided into tourmaline-bearingand tourmaline-free samples.

In this study, we investigated the systematics of Li,B, and Cl in dehydrated HPM rocks and their metaso-matised equivalents. We evaluate the reliability of Li,B, and Cl as tracers for dehydration processes, and theway in which they are affected by cryptic and modalmetasomatism.

Geological background and investigated samples

The island of Syros displays a sequence of rocks be-longing to the lower unit of the Attic-Cycladic Crys-talline Complex (Dürr et al. 1978). The major partof the island is composed of interlayered schists andmarbles (e.g. Hecht 1984; Dixon and Ridley 1987;Seck et al. 1996). The most interesting formations areexposed in the northern part of Syros near Kámposand along the coastline around Hermoupolis and Kini(Fig. 1). These formations mainly preserve a blueschist-to eclogite-facies metamorphic overprint. The progradeP − T path is characterised by a high P/T ratio alonga geothermal gradient of ∼ 8 ◦C/km, typical for sub-duction zone metamorphism. Peak metamorphic con-ditions have been estimated at ∼ 470 − 520 ◦C and1.3 − 2.0 GPa by different authors for different rocktypes based on the stable mineral assemblages and ther-mobarometry (Dixon 1968; Ridley 1984; Okrusch andBröcker 1990; Trotet et al. 2001; Rosenbaum et al. 2002;Keiter et al. 2004). The exhumation path of the Syroshigh-pressure rocks is characterised by near-isothermaldecompression at ∼ 400 ◦C with variable rehydrationduring exhumation (Trotet et al. 2001; Marschall et al.2006b; Miller et al. 2008). However, HPM assemblagesand minerals have been preserved in many parts ofthe island.

The metabasites are interpreted as the HPM equiv-alents of different parts of ancient oceanic crust,based on major and trace element compositions, radio-genic isotope studies, and petrographic observations(e.g. Seck et al. 1996). The different rock types forma mélange with blocks of eclogite, omphacite-richfelsic rocks (meta-plagiogranites), serpentinite, andmetagabbro embedded in a matrix of chlorite schistand serpentinite (Dixon 1968; Hecht 1984; Dixon andRidley 1987; Okrusch and Bröcker 1990; Seck et al.1996; Bröcker and Enders 2001; Marschall et al. 2006b;Ague 2007). Contacts between the blocks and theirchemically and mineralogically contrasting matrix arecharacterised by reaction zones rich in OH-bearingminerals, which are referred to as “blackwalls”. The

Tracers for metasomatism in high-pressure metamorphic rocks 293

Tab

le1

H2O

conc

entr

atio

ns(w

t%)

and

Li,

Be,

B,a

ndC

labu

ndan

ces

(μg/

g)of

Syro

sw

hole

-roc

ksa

mpl

es

Gro

up1

H2O

Li

Be

BC

lB

/B

eL

i/B

eC

l/B

eG

roup

2H

2O

Li

Be

BC

lB

/B

eL

i/B

eC

l/B

e

Blu

esch

ists

Tou

rmal

ine-

free

blac

kwal

lsam

ples

SY3a

2.50

36.9

0.92

62.4

169

67.9

40.1

183

SY30

9A2.

3228

.00.

6711

.451

617

.041

.877

0SY

51.

5142

.60.

953.

1662

.63.

344

.865

.9SY

325

3.69

2.29

2.59

7.15

d.l.

2.8

0.9

<12

SY30

43.

177.

750.

4612

.021

726

.116

.947

1SY

328

2.85

65.3

0.88

7.58

107

8.6

74.2

122

SY30

62.

467.

140.

409.

9130

024

.817

.874

9SY

400A

2.34

69.2

3.30

n.a.

n.a.

21.0

SY34

21.

5027

.41.

004.

9262

.64.

927

.462

.6SY

401A

2.53

32.5

0.68

7.86

308

11.6

47.8

453

SY40

32.

9011

.51.

079.

1129

08.

510

.727

1SY

402A

0.97

71.0

2.04

12.9

48.3

6.3

34.8

23.7

Met

agab

bros

SY40

2B0.

6278

.01.

818.

2841

.44.

643

.122

.9SY

112

2.23

5.61

0.95

4.78

189

5.0

5.9

199

SY40

2C2.

3163

.31.

6113

.9d.

l.8.

739

.3<

19SY

344

1.87

18.8

0.54

5.05

56.1

9.4

34.7

104

SY40

49.

697.

500.

913.

75d.

l.4.

18.

2<

33SY

425

1.94

40.5

1.58

25.6

56.6

16.2

25.6

35.8

SY40

59.

2811

.30.

633.

8587

16.

118

.013

80SY

438

3.08

88.7

1.28

19.6

d.l.

15.3

69.3

<23

SY43

72.

7219

.13.

1815

.518

44.

96.

057

.9SY

443

2.83

16.9

0.61

4.88

130

8.0

27.7

212

Ecl

ogit

esT

ourm

alin

e-be

arin

gbl

ackw

alls

ampl

esSY

109

0.63

6.05

1.77

n.a.

n.a.

3.4

SY30

9B4.

5431

.70.

6539

6052

460

9048

.880

6SY

323a

0.78

45.8

2.08

4.77

59.9

2.3

22.0

28.8

SY40

0B2.

6476

.83.

1735

1d.

l.11

124

.2<

10SY

324

0.70

56.5

2.20

2.18

28.1

1.0

25.7

12.8

SY40

1B4.

6720

.20.

5115

300

366

3000

039

.671

8SY

411

0.50

72.3

2.88

3.80

34.8

1.3

25.1

12.1

SY41

23.

0760

.20.

7593

.455

.512

480

.374

.1M

eta-

plag

iogr

anit

esSY

420

4.14

36.2

0.68

8760

d.l.

1290

053

.2<

44SY

10.

929.

681.

7612

.053

.26.

85.

530

.2SY

81.

255.

583.

4311

.744

.63.

41.

613

.0T

ypic

alfr

esh

and

alte

red

MO

RB

(ave

rage

liter

atur

eva

lues

)SY

308

1.25

5.02

0.82

10.3

69.3

12.5

6.1

84.6

MO

RB

0.25

5.0

0.25

1.0

204

2080

SY41

50.

685.

623.

4610

.652

.03.

11.

615

.0A

OC

5.0

150.

550

500

100

3010

00SY

431

0.95

22.7

3.34

38.8

101

11.6

6.8

30.4

n.a.

nota

naly

sed,

d.l.

belo

wde

tect

ion

limit

,AO

Cal

tere

doc

eani

ccr

ust

a Sam

ples

SY3

and

SY32

3co

ntai

nac

cess

ory

Typ

e-I

tour

mal

ine.

Fre

shan

dal

tere

dM

OR

Bva

lues

are

from

Hof

man

n(1

988)

,M

cDon

ough

and

Sun

(199

5),

Jam

bon

etal

.(1

995)

,M

agen

heim

etal

.(19

95),

Lee

man

and

Siss

on(2

002)

,Rya

n(2

002)

and

Bon

ifac

ieet

al.(

2008

)


Fig. 1 Simplified geologicalmap of Syros after Hecht(1984), showing the studyarea, which encloses themafic-ultramafic mélangenear Kampos. Inset shows amap of Greece with thelocation of the island of Syros

KamposGrammata

Lia

Grizzas

Kini

Galissas

Finikas

Hermoupolis

Vari

N

5 km

Schists and marbles

Mélanges of metabasites± serpentinite; blueschist-to eclogite facies

Metabasites, pervasivegreenschist facies over-print

Vari unit (gneiss)Study area

Syros Crete

Athens

35˚N

40˚N

20˚E 25˚E

process of metasomatic blackwall formation on Syroshas been previously described by Dixon (1968) andother workers (Ridley 1984; Dixon and Ridley 1987;Okrusch and Bröcker 1990; Seck et al. 1996; Bröckerand Enders 2001; Marschall et al. 2006b; Miller et al.2008). Ague (2007) modelled the fluid flux through theSyros HP mélange and suggested that the schistosematrix and the margins of the blocks were subjected toa strong flux of hydrous fluids, which did not affect thecentral parts of the blocks (Fig. 2).

The block-and-matrix association in the mélangeallows for samples with variable degrees of metaso-matic overprint to be taken. Samples from the cores ofblocks show peak-metamorphic dominantly anhydrousassemblages of garnet + omphacite + quartz + rutile ±phengite with or without primary glaucophane and lesscommon epidote, lawsonite, chloritoid, or paragonite.

The rocks consist of porphyroblastic minerals oftendisplaying chemical growth zoning, typical for high-P/T rocks equilibrated during progressive subductionat low T. Thermobarometry on these rocks completedin previous studies is consistent with their equilibra-tion in the low-T eclogites field (Dixon 1968; Ridley1984; Okrusch and Bröcker 1990; Trotet et al. 2001;Rosenbaum et al. 2002; Keiter et al. 2004).

In contrast, many samples from the schistose matrixdisplay lower-P assemblages dominated by hydrousminerals, such a chlorite, epidote, and glaucophaneintergrown with sphene (titanite) and albite. The rocksoften show large, homogenous blasts of chlorite, al-bite, and jadeite-rich omphacite intergrown in appar-ent petrographic equilibrium. Thermobarometry onthese assemblages revealed temperatures between 350and 450 ◦C, at pressures between 0.6 and 1.3 GPa


Tur

blackwall

eclogite/metagabbro

block

chlo

rite-ta

lc-se

rpentin

ite

mélan

ge mat

rix

block

modalmetasomatism

fluid fluxcrypticmetasomatism

Group-IIsamples

Group-Isamples

Fig. 2 Schematic drawing of the Syros block-matrix mélange inthe Kámbos area (Fig. 1) depicting the strong fluid flux in theschistose chlorite-talc-serpentine matrix and the hydrous reactionzones (“blackwalls”), which contains tourmaline (Tur) in places.The HPM blocks suffered little fluid flux without formation ofhydrous minerals, but certain elements may still be re-enriched(= cryptic metasomatism). (modified after Ague (2007))

(Trotet et al. 2001; Breeding et al. 2004; Marschall et al.2006b; Miller et al. 2008), i.e. significantly below the es-timated peak pressure conditions for Syros. All samplesinvestigated in this study are described in great detail inprevious publications (Marschall 2005, Marschall et al.2006a, b).

Partitioning and budget of Li, Be, and B in HPMrocks from Syros have been studied by Marschallet al. (2006a). Very high concentrations of lithium werefound in chlorite, glaucophane, and clinopyroxene andto a lesser extent in paragonite and phengite. Berylliumis mainly hosted by clinopyroxene, glaucophane, andwhite mica. In addition, Ca-amphibole, lawsonite, andalbite may also contain considerable amounts of Be.The only minerals (except for tourmaline) showing Bconcentrations in excess of 10 μg/g are phengite andparagonite. Chlorine partitioning in HPM rocks is lesswell established. Studies on altered oceanic basalts andon HP meta-gabbros have revealed high Cl concen-trations in amphibole (Vanko 1986; John and Schenk2003). Apatite is another likely mineral host for Cl(Brenan 1993), and chlorite is a major host of Cl inultramafic rocks (Scambelluri et al. 2004). However,electron-microprobe analyses of apatite, phengite, andglaucophane from Syros rocks did not reveal any de-tectable Cl abundances in these minerals (Marschall

2005). Philippot et al. (1998) have argued that mostof the Cl in HPM rocks will not be structurally boundin silicates or phosphates, but is hosted in saline fluidinclusions.

Tourmaline in the Syros rocks is related to twodifferent occurrences, which can be distinguished pet-rographically. Type I is found in HPM rocks as smallcrystals, typically as inclusions in garnet, glaucophaneor phengite (Marschall et al. 2008a). Type II appearsas aggregates of large grains, related to metasoma-tism during exhumation of the rocks (Marschall et al.2006b). Type II is restricted to the hydrous reactionzones formed at the contact between blocks and theultramafic matrix of the mélange. It formed after theonset of chloritisation and albitisation of the mélangeat retrograde P-T conditions (Marschall et al. 2006b).

Analytical methods

Whole-rock concentrations of Li in all samples weremeasured at the University of Bristol by ICP-OES(inductively-coupled plasma optical emission spec-trometry), using a Jobin Yvon Ultima 2 SequentialSpectrometer, operated by Chung Choi (University ofBristol). Lithium was analysed using the 670.784 nmline and calibrated with 9 international silicate ref-erence materials of basaltic to granitic compositionwith Li concentrations ranging from 4.8 to 93 μg/g(Marschall 2005). Beryllium was analysed by CavendishAnalytical Laboratory Ltd., Canada, by ICP-OES. Pre-cision on Li and Be measurements is ∼ 10 % rela-tive. Accuracy and reproducibility were controlled byanalysing international rock standards and repeats ofsamples and are estimated to ∼ 10 % relative for Li and∼ 20 % relative for Be. Detection limits for Li and Beare ∼ 0.1 μg/g and ∼ 0.05 μg/g, respectively. Details ofsample preparation and applied anlytical methods aregiven in Marschall (2005).

Whole-rock B, Cl, and H2O concentrations weredetermined using prompt gamma neutron activationanalysis (PGNAA) at the facility installed at theBudapest 10 MW research reactor, equipped with acold neutron source (20 K). Thermal equivalent neu-tron flux at the target position during the measurementswas ∼ 5 · 107 cm−2s−1 (increased to 1 · 108 cm−2s−1 afterupgrade in 2007). The beam area was set to 4 cm2, andexposure time was 1 to 4 h for most of the samples.Energy spectra ranging from 30 keV to 11 MeV weremeasured using a high-purity germanium semiconduc-tor (HPGe) bismuth germanate (BGO) scintillatordetector system in Compton-suppressed mode. Thedata acquisition was performed by a Canberra S100


multichannel analyser. Gamma spectra were evaluatedusing the Hypermet PC program (Révay et al. 2001).Analytical details of the Budapest PGNAA facility aregiven in Révay et al. (2004), Szakmány and Kasztovszky(2004), and Molnár (2004), and references therein. Ac-curacy of B and Cl analyses by PGNAA has beenchecked by measurements of geological reference ma-terials performed at the Budapest Research Reactor(BRR) and is ∼ 10 % relative (Gméling et al. 2005).Precision for B analyses is better than 1.5 % relativefor concentrations > 5 μg/g and better than 1.7 % rel-ative in the range of low concentrations (1.9 − 5 μg/g).For Cl, precision is < 5 % relative at concentrations> 100 μg/g, and < 20 % relative at concentrations< 100 μg/g. For H2O, precision is < 2.5 % relativeat concentrations > 0.5 wt%. The calculated detectionlimits for B, Cl, and H2O are 0.3 μg/g, 30 μg/g and0.02 wt%, respectively, for the standard setup. Furtheranalytical details on PGNAA of high-pressure meta-morphic rocks at BRR are given in Marschall et al.(2005).

Results

Group 1: high-pressure metamorphic rocks

H2O contents of the blueschists and metagabbros arerelatively high (1.5 − 3.2 wt%; Table 1), reflecting thehigh modal abundance of hydrous minerals, such asglaucophane and phengite. The meta-plagiogranitesand eclogites, which are dominated by anhydrousminerals, such as clinopyroxene, garnet, and quartz,show low H2O concentrations (0.68 − 1.25 wt% and0.50 − 0.78 wt%, respectively; Table 1).

Since it is hardly possible to give an independentestimation on the enrichment of elements in the sam-ples at the seafloor and the subsequent depletion dur-ing subduction-zone dehydration, a documentation ofthe net effect of alteration and subduction-zone dehy-dration seems to be sensible. In addition, subductionof oceanic lithosphere consisting of fresh MORB andrestitic mantle will not affect the composition of themantle, as those two components segregated from theupper mantle in the first place by melting at MOR.In order to estimate the impact of subduction on thetrace-element chemistry of the mantle, it is thereforeimportant to focus on rocks that deviate from normalMORB. Hence, element abundances in the investigatedsamples are compared here with typical concentrationsin (differentiated) MORB.

The analysed meta-igneous rocks display Li con-centrations between 5.0 and 88.7 μg/g, independent

from rock type and H2O content (Table 1). Whole-rock abundances of Li in most samples are significantlyhigher than abundances of Li in fresh oceanic igneousrocks (∼ 3 − 30 μg/g; Ryan and Langmuir 1987; Niuand Batiza 1997; Bach et al. 2001). MORB-normalisedelement abundance patterns of most Group-1 samplesdisplay positive Li anomalies ((Li/Yb)N > 2; Marschall2005), pointing to an enrichment of Li by inter-action with fluids, rather than by magmatic differ-entiation. An exception are the meta-plagiogranites,showing low Li concentrations of ∼ 5 − 10 μg/g (onesample with 22 μg/g Li), and negative Li anomaliesin MORB-normalised element abundance patterns((Li/Yb)N < 0.6; Marschall 2005), suggesting selectivepost-magmatic removal of Li.

In order to decrease the influence of magmatic dif-ferentiation of the precursor rocks on the geochemicalsignal of Li, B, and Cl, different authors have dis-cussed fluid-mobile vs fluid-immobile element ratios,using Be as a tracer with a relatively low mobilityin hydrous fluids compared to B, Cl and Li (Ryan2002). Element ratios of incompatible elements areless affected by magmatic differentiation than are ab-solute abundances. Fluid-immobile elements, such asBe, are employed to estimate pre-metasomatic abun-dances of fluid-mobile elements (cf. Bebout 2007).Beryllium concentrations in the Syros blueschists andmetagabbros are between 0.40 and 1.58 μg/g (Table 1),i.e. the same range as Be in fresh and altered MORB(∼ 0.1 − 2.5 μg/g; Ryan and Langmuir 1988; Niu andBatiza 1997; Regelous et al. 1999; Danyushevsky et al.2000) and hence, were probably not influenced bymetamorphism or dehydration. Concentrations of Bein eclogites (∼ 1.8 − 2.9 μg/g) and meta-plagiogranites(∼ 0.8 − 3.5 μg/g; Table 1) are higher than in fresh andaltered MORB, and are closer to Be concentrationsin OIB (∼ 0.5 − 10 μg/g Ryan 2002). Li/Be ratios ofthe investigated HPM rocks scatter between 1.6 and 69(Table 1) and show no correlation to H2O concentra-tions (Fig. 3a).

Concentrations of B display a large variation inblueschists and metagabbros (∼ 3−26 μg/g B; Table 1),and are significantly higher than in fresh MORB(< 2.3 μg/g B for MORB with > 7 % MgO; Ryanand Langmuir 1993; Chaussidon and Jambon 1994;Perfit et al. 1999; Kamenetsky et al. 2000). Eclogites(∼ 4 − 6 wt% MgO; Marschall 2005) show low B abun-dances of ∼ 2 − 5 μg/g B (Table 1). Boron concen-trations in meta-plagiogranites (∼ 0.3 − 1.6 wt% MgO;Marschall 2005) are higher (∼ 10 − 12 μg/g B, oneoutlier; Table 1). However, the protoliths of themeta-plagiogranites are highly differentiated magmaticrocks, which were probably enriched in incompatible


(c) Cl/Be

(a) Li/Be

(b) B/Be

(d) Li/Be

1

10

100

0 1 2 3 4

B/B

e

MORB

Glaucophane schists

Eclogites andmeta-plagiogranites

Metagabbros

Li/B

e

1

10

100

0 2 4 6 8 10 12 14

Srp + Tlc

0.1

1

10

100

1000

10,000

100,000

0 2 4 6 8 10 12 14

B/B

e

ChlSrp + Tlc

Phe

Gln

Omp

Tur

tourmalineformation

chloriteformation

Tur-bearingblackwall rocks

Tur-freeblackwall rocks

(e) B/Be

(f) Cl/Be

H2O (wt %)

Cl/B

e

1

10

100

1000

0 1 2 3 4

MORB

H2O (wt %)

Cl/B

e

1

10

100

1000

0 2 4 6 8 10 12 14

Srp + Tlc

Li/B

e MORB

1

10

100

0 1 2 3 4

Fig. 3 a–c Li/Be, B/Be and Cl/Be ratios, respectively, vs H2Ocontent of HPM rocks, which lack retrograde hydrous minerals.Grey fields displays data of fresh N to E-MORB from literature(Michael and Cornell 1998; Magenheim et al. 1995; Jambon et al.1995; Perfit et al. 1999; Danyushevsky et al. 2000; Kamenetskyet al. 2000; le Roux et al. 2006). d–f Li/Be, B/Be and Cl/Beratios, respectively, vs H2O content of blackwall rocks, whichwere metasomatised during exhumation. For comparison, thenon-metasomatic samples enclosed in the loops in a, b and c areshown by small circles and the dark grey fields in d, e, and f,

respectively. e Compositional variation of rock-forming minerals(Tur, Omp, Gln, Phe, Chl) from HPM rocks are displayed forcomparison (data from Marschall et al. 2006a). Boxes labelled“Srp-Tlc” represent whole-rock analyses of serpentinite (±talc-bearing). Tourmaline-bearing samples (squares) show a strongincrease in B/Be ratios along a trend marked by the arrow.Tourmaline-free blackwall samples (diamonds) show constantB/Be ratios, but a strong increase in H2O contents, marked bythe horizontal arrow

trace elements during magmatic differentiation, and Bconcentrations of ∼ 10 − 12 μg/g are typical for oceanfloor magmatic rocks with MgO contents below 2 wt%(Ryan and Langmuir 1993; Perfit et al. 1999).

B/Be ratios of all meta-igneous samples are < 30with one tourmaline-bearing blueschist (SY3) being the

only exception (Table 1). B/Be ratios decrease fromblueschists and metagabbros, which show the largestvariation (∼ 3 − 25), to meta-plagiogranites (∼ 3 − 13)to eclogites (< 2.5). This decrease is correlated with adecrease in H2O content of the different rocks from∼ 3.2 wt% to ∼ 0.5 wt% (Fig. 3b).


Chlorine concentrations are high in the blueschistsand metagabbros (56 − 300 μg/g Cl; Table 1), whichis higher than in fresh, undifferentiated MORB(∼ 5 − 100 μg/g Cl; Michael and Cornell 1998) andmore typical for enriched MORB (∼ 50 − 500 μg/g Cl;Michael and Cornell 1998) and altered oceanic crust(∼ 20 − 2700 μg/g; Magenheim et al. 1995). The eclog-ites and meta-plagiogranites have low Cl abundances(< 70 μg/g Cl, one exception; Table 1). Cl/Be ratiosshow a strong positive correlation with H2O contents(Fig. 3c), and are particularly low in the most dehy-drated HPM rocks, i.e. Cl/Be < 30 in almost all eclog-ites and meta-plagiogranites (Table 1).

Group 2: metasomatised blackwall rocks

Lithium abundances of most tourmaline (Tur)-bearingand Tur-free Group-2 samples are high (20 − 80 μg/g;Table 1), but do not show systematic variation withH2O content. Be abundances show a large scatter(0.5 − 3.3 μg/g; Table 1) in both Tur-bearing and Tur-free rocks. Li/Be ratios are high in most blackwallrocks (6 − 80 in most samples; Table 1), without a cor-relation to H2O contents (Fig. 3d).

Boron abundances of Tur-bearing and Tur-free sam-ples are strongly contrasting, as expected (Table 1).Tur-free rocks have relatively low B contents of∼ 4 − 15 μg/g independent from H2O contents. Tur-bearing samples show a strong enrichment in B. Modalabundances of tourmaline range from ∼ 0.3 wt% inretrogressed eclogite SY412 to ∼ 30 wt% in phengite-tourmaline schist SY420 and ∼ 50 wt% in sampleSY401B (a tourmaline-rich layer within a glauco-phane schist). B/Be ratios of Tur-bearing and Tur-freesamples define two discrete trends with increasingH2O contents as displayed in Fig. 3e. Tur-bearingsamples show a strong increase in B/Be ratios withincreasing H2O content up to values on the orderof 104. Even relatively low amounts of tourmaline of1 wt% already increase the B/Be ratios of the wholerock analysis to > 100. In contrast, Tur-free samplesare enriched in H2O to various degrees (∼ 0.6−9.7wt%), but do not show any preferential enrichmentin B. All samples show B/Be ratios < 10, which iseven lower than the average of the blueschists ofgroup 1.

Chlorine abundances in the Tur-bearing and Tur-free blackwall rocks scatter widely from below thedetection limit (30 μg/g) to 871 μg/g, without any cor-relation to the H2O content (Table 1). Cl/Be ratiosrange from < 10 to 1380 and are not related to the H2Ocontent (Fig. 3f).

Discussion

The correlation of B/Be and Cl/Be ratios with H2Ocontents of Group-1 rocks (Fig. 3b, c) is probably pro-duced by the loss of B and Cl with hydrous fluids duringprogressive dehydration. However, it has to be consid-ered that the suite of Group-1 rocks does not representdifferent degrees of dehydration of a uniform pre-subduction lithology, but that each of the investigatedsamples probably has had a different B, Cl, and H2Ocontent before the onset of subduction-zone metamor-phism. Hence, higher B/Be and Cl/Be ratios of rockswith higher H2O contents may be explained by more in-tense alteration of the protoliths at the seafloor. In anycase, H2O will be lost during prograde metamorphismwhen hydrous minerals become unstable. Abundancesof elements with low rock/fluid partition coefficientswill than decrease and preserve the positive correlationdisplayed in Fig. 3b and c. In contrast, elements thatshow a less fluid-mobile behaviour will be retained andthe respective plot will show an array of data points tolow H2O contents at constant element abundances, asis the case for Li in Fig. 3a. Hence, the absence of low-H2O rocks with high B/Be and Cl/Be ratios supportthe hypothesis that B and Cl are quantitatively releasedfrom meta-igneous rocks by the time they reach theanhydrous eclogite stage.

The results from Syros are consistent with abun-dances of < 20 μg/g B and B/Be ratios of < 30 inmafic HPM rocks determined in previous studies(Moran et al. 1992; Bebout et al. 1993, 1999). Onlyone tourmaline-bearing sample (SY3) shows a higherB content and a higher B/Be ratio, demonstrating theimpact of accessory tourmaline (Tur B/Be ≈ 106) on Bretention.

Philippot et al. (1998) estimated from the abun-dance and salinity of fluid inclusions in eclogites thata minimum of 100 − 200 μg/g Cl is transported intothe deep mantle with subducting oceanic crust. Thelower abundances of 28 − 69 μg/g Cl determined bydirect measurement in the majority of dehydrated rocksin this study suggest that significantly less Cl may berecycled into the mantle.

The wide spread of Li/Be ratios over the wholerange of H2O contents in Group-I rocks could, in part,be explained by a conservative behaviour of Li duringdehydration, due to its compatibility in most HP min-erals (Woodland et al. 2002; Scambelluri et al. 2006;Marschall et al. 2006a, 2007a). However, Li abundancesin excess of 35 μg/g in meta-igneous HPM rocks arehigher than the vast majority of published values for Liabundances in fresh (3 − 8 μg/g Li; e.g. Niu and Batiza1997), altered (1 − 35 μg/g Li; e.g. Chan et al. 2002), or


differentiated (10−30 μg/g Li; e.g. Ryan and Langmuir1987) oceanic crust and presumably result from addi-tion of Li after the onset of subduction, most probablyduring eclogitisation or exhumation (Marschall et al.2007b).

Group-2 samples, representing metamorphic rocksthat were strongly metasomatised by hydrous fluidsduring exhumation can be used to monitor the effects ofretrograde metasomatism during exhumation on traceelement abundances. Li concentrations of > 50 μg/g inmany blackwall samples are much higher than in freshand altered oceanic crust, demonstrating an enrichmentin Li after the onset of subduction. Similarly high Liabundances and Li/Be ratios in eclogites of Group Isuggest metasomatic enrichment in these rocks. Meta-somatism within the HPM rocks was much less intensethan in the blackwalls and did not lead to a significantformation of hydrous minerals (modal metasomatism).However, the high Li abundances in eclogite demon-strate that cryptic metasomatism was sufficient to en-rich the blocks in this trace element (Fig. 2). Hence, Liabundances in eclogites apparently are highly sensitiveto metasomatic processes.

Boron is not enriched in Tur-free blackwall rocks,despite the fact that Tur-free and Tur-bearing black-walls occur in close vicinity and display very similarmodal compositions – apart from the tourmaline. Theywere formed during the same metasomatic event andhydrating fluids were probably B-rich in both cases.Low B/Be ratios and low B concentrations in Tur-free blackwall rocks are consistent with very low par-tition coefficients of B between silicates and hydrousfluids (∼ 0.0001 − 0.02; Brenan et al. 1998; Marschallet al. 2007a). Formation of tourmaline (∼ 3 wt% B)by reaction of B-bearing fluids with blackwall silicates(chlorite,glaucophane; Marschall et al. 2006b) led toan effective retention of B in the rocks and stronglyincreased their B/Be ratios. This demonstrates thatB concentrations and B/Be ratios are not elevated inmetasomatised meta-igneous HPM rocks unless newtourmaline is formed. Therefore, the B abundance ofmeta-igneous rocks cannot be used as an indicator formetasomatic processes in Tur-free rocks. It has beendemonstrated experimentally that B abundances influids of ∼ 150 μg/g to less than 620 μg/g are sufficientto form Tur from amphibole or chlorite-bearing as-semblages (Weisbrod et al. 1986; Morgan and London1989). These experiments demonstrate that the criticalfactor for the formation of metasomatic Tur is thepH of the fluid. While Tur readily crystallises fromacidic fluids, it does not form from neutral or basicfluids (Morgan and London 1989). Thus, the occurrenceof tourmaline (and the fixation of B) in the strongly

metasomatised blackwalls may depend on local changesin fluid pH caused by hydration-dehydration reactionsand the formation and breakdown of pyrite.

Cl/Be ratios are highly elevated in some but stronglyreduced in many other Group-II rocks. This apparentlyrandom behaviour of Cl could not be pinned down tothe appearance of any specific mineral (e.g. apatite,amphibole) in this study. Hence, it may be related to theabundance and salinity of retrograde fluid inclusions.The bulk method of PGNAA on whole-rock powdersemployed in this study does not distinguish elementspresent in liquid solutions from those bound in crystalstructures. Fluid inclusion studies have shown that earlyHP fluids in Syros HPM rocks have very low salini-ties, while late-stage fluids trapped after hydration ofthe silicates are close to halite saturation (Barr 1990).Similar to B, it can be concluded that Cl whole-rockabundances are not a reliable indicator for retrogradefluid influx in meta-igneous HPM rocks.

Conclusions

Group-1 samples contribute information on the impactof dehydration of (altered) igneous oceanic crust onwhole-rock abundances of different trace elements. Indetail, the abundances of Li and Li/Be ratios do notcorrelate with H2O contents, which is attributed to a re-enrichment of Li in the HPM rocks without a significantformation of hydrous minerals. In contrast, B and Clconcentrations and B/Be and Cl/Be ratios are posi-tively correlated to the H2O contents of the HPM rocks,suggesting a significant loss of B and Cl with hydrousfluids during subduction and progressive dehydration.This overall behaviour of the three elements are verysimilar to their observed behavior in ultramafic rocksduring HP metamorphism by Scambelluri et al. (2004).These authors found that B and Cl are progressivelydepleted during prograde dehydration of serpentinites,while they argue that high Li abundances require anincome of Li into the rocks from external sources.Eclogite Cl abundances of < 70 μg/g suggest that abun-dances of 100 − 200 μg/g Cl estimated for eclogites by(Philippot et al. 1998) from the salinity and abundanceof fluid inclusions may overestimate the subductionflux of Cl into the mantle by a factor of 2 to 4.

Group-2 samples provide information on the effectsof metasomatism of HPM rocks during exhumation.Lithium shows very high abundances in many samples,suggesting a strong enrichment during metasomatism.The enrichment of B is connected to the occurrenceof tourmaline, which in turn is probably controlled bythe pH of the fluid rather than its B concentration.


Chlorine abundances in the metasomatised samples arenot correlated to H2O contents, which may be relatedto variable abundance and salinities of fluid inclusionsin the different samples. These results demonstrate thatthe Li abundance in meta-igneous rocks can be usedas a tool for tracing metasomatic enrichment processes,while B in such rocks is enriched only in the case oftourmaline formation.

This study demonstrates that B and Cl abundancescan be used to trace the progressive dehydration ofmeta-igneous HPM rocks, while Li concentrations aremore sensitive for metasomatic enrichment processes.

Acknowledgements This paper benefited from discussions withStefan Prowatke, Jens Paquin, and Thomas Zack. Chung Choiis thanked for performing ICP-OES analyses. Helpful and con-structive comments by Marco Scambelluri and an anonymousreviewer helped to clarify the presentation of this study. Financialsupport by the Deutsche Forschungsgemeinschaft (DFG, grantsKA 1023/8-1 and AL 166/15-3) and by a European Union Marie-Curie Fellowship awarded to HRM (ID 025844: “Isotopes insubduction zones – the metamorphic perspective”) is greatlyacknowledged. PGNAA and ICP-OES work was financed by theEuropean Community Access to Research Infrastructure frame-work, Contract HPRI-1999-CT-00099, awarded to G.L. Molnár,and Contract HPRI-1999-CT-00008, awarded to B.J. Wood.

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