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Lithos 75 (2004) 239–252

Carbonatite melt in oceanic upper mantle beneath

the Kerguelen Archipelago

B.N. Moinea,b,*, M. Gregoirec, Suzanne Y. O’Reillyb, G. Delpecha,b, S.M.F. Sheppardd,J.P. Lorande, C. Renaca, A. Gireta, J.Y. Cottina,b

aDepartement de Geologie-Petrologie-Geochimie, Universite Jean Monnet, UMR–CNRS 6524,

23 rue du Dr. P Michelon, 42023 St. Etienne, FrancebDepartment of Earth and Planetary Sciences, GEMOC ARC National Key Centre, Macquarie University, NSW 2109, Australia

cLaboratoire Dynamique Terrestre et Planetaire, UMR–CNRS 5562, Observatoire Midi-Pyrenees, 31400 Toulouse, FrancedLaboratoire des Sciences de la Terre, UMR–CNRS 5570, Ecole Normale Superieure de Lyon, 69366 Lyon, France

eMuseum National d’Histoire Naturelle, FRE–CNRS 2456 Paris, France

Received 14 January 2003; received in revised form 13 August 2003; accepted 18 December 2003

Available online 2 April 2004

Abstract

Some mantle-derived Kerguelen harzburgite and dunite xenoliths have bulk-rock and mineral trace element compositions

that provide evidence of carbonatitic metasomatism similar to that described in some continental and other oceanic settings.

Rare xenoliths contain carbonates that are highly enriched in rare earth elements (REE), interpreted to be quenched, evolved

carbonatitic melts. One amphibole-bearing dunite mantle wall-rock containing carbonates in small interstitial pockets (100–500

Am across) has been studied in detail. Mg-bearing calcite (MgO: < 1.4 wt.%, XCa 0.96) with unusually high REE abundances

and strong light REE (LREE) enrichment occurs in the pockets and is sometimes associated with euhedral carbonates (dolomite

and Mg-free calcite), mafic silicate glass (low in alkalis) and with fine grains of spinel, sulfides and magnesio–wustite

concentrated near the boundaries of the carbonate pockets. The unusual metasomatic mineral assemblage, together with the

microstructural features and chemical composition of carbonates (with trace element contents similar to those of common

carbonatite magmas), suggests that the pockets of Mg-bearing calcite represent quenched carbonate melts rather than crystal

cumulates from carbonate-rich melts. The associated mafic silicate glass could represent the immiscible silicate fraction of an

evolved fluid produced by the dissolution–percolation of the original carbonate melt in the dunitic matrix and subsequent

unmixing as the xenoliths ascended to the surface. Clinopyroxene formed during the percolation event and is therefore inferred

to be in chemical equilibrium with the carbonate melt. This allowed calculation of clinopyroxene/carbonate melt partition

coefficients for a large set of trace elements at relatively low pressure (1 GPa). As a result, a significant pressure control on REE

partitioning between carbonate melt and silicate minerals was observed. This study provides further evidence for the occurrence

of carbonate melts and demonstrates that these melts can be preserved in hot oceanic uppermost mantle.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Mantle carbonate; Oceanic lithosphere; Carbonate melt partition coefficients; Mantle metasomatism; Kerguelen lithopsheric mantle

* Corresponding author. Departement de Geologie-Petrologie-Geochimie, Universite Jean Monnet, UMR–CNRS 6524, 23 rue du Dr. P

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

doi:10.1016/j.lithos.2003.12.019

Michelon, 42023 St. Etienne, France. Fax: +33-477-485-108.

E-mail address: bertrand.moine@univ-st-etienne.fr (B.N. Moine).

B.N. Moine et al. / Lithos 75 (2004) 239–252240

1. Introduction

We have studied the trace element distribution

in a mantle-derived dunite xenolith from the

Kerguelen Archipelago (south Indian Ocean). This

rock is a cpx-bearing spinel dunite containing

interstitial amphibole grains and carbonate pockets

and was carried to the surface by a 8–12-Ma-old

highly alkaline lava. This sample is clearly of

direct upper mantle origin and is not affected by

any crustal contamination. The carbonate and

silicate trace element compositions were deter-

mined in situ by laser ablation inductively coupled

plasma mass spectrometry. The in situ analyses

allow the study of mineralogical and chemical

microsystems in their spatial relationships as pre-

served in mantle rocks. These microsystems are

key indicators of the processes of reaction, perco-

lation and immiscibility, affecting the melts circu-

lating within the upper mantle. Silicate or/and

carbonatitic metasomatic melts have been clearly

identified in mantle xenoliths of continental and

oceanic origin by studies of microinclusions in

mantle minerals (e.g., Andersen et al., 1984;

Schiano et al., 1994; Frezzotti et al., 2002) and

more commonly by the studies of trace element

characteristics of mantle peridotites and their con-

stituent silicate mineral phases (e.g., Yaxley et al.,

1991; Dautria et al., 1992; Ionov et al., 1993;

Hauri et al., 1993). Some mantle carbonates in

peridotites have been interpreted as products of

fractional crystallization of silicate–carbonate melts

(Ionov, 1998; Lee et al., 2000). Evidence of

quenched carbonatite melts occurring in mantle

rocks is rare and occurrences include peridotite

mantle xenoliths from Spitsbergen (Amundsen,

1987), the Slave craton kimberlite (van Achterbergh

et al., 2002) and the Canary Islands (Kogarko et

al., 1995). The scarcity of carbonates in mantle

peridotite xenoliths has been attributed to the

decarbonation reaction occurring between metaso-

matic melt and mantle peridotite (Wyllie and

Huang, 1976; Green and Wallace, 1988; Dalton

and Wood, 1993) or to decompression en route

to the surface (Canil, 1990). Therefore, the preser-

vation of carbonates within mantle peridotites is

exceptional and requires specific physical–chemical

conditions.

2. Geological setting

The Kerguelen Archipelago is located in the

southern part of the Indian Ocean, on the huge

Kerguelen–Heard plateau (25� 106 km3). The Ker-

guelen Archipelago is dominated by flood basalts

(about 80% by volume) with minor plutonic rocks

(gabbro, syenite, alkali granite) which locally in-

trude the flood basalts and form volcano–plutonic

complexes (Giret et al., 1997). The magmatism has

evolved from tholeiitic-transitional to alkaline affin-

ities during the last 29 Myr (Gautier et al., 1990;

Yang et al., 1998; Nicolaysen et al., 2000). Dykes

related to the youngest alkaline volcanic activity of

the islands (Weis et al., 1993) outcrop in the

southeastern province of the Kerguelen Archipelago.

One basanite dyke from Phonolite Valley (Ronarc’h

Peninsula) contains few carbonated mantle xenoliths

(including the carbonate- and amphibole-bearing

dunite of this study, as well as amphibole- and

phlogopite-rich clinopyroxenites). The xenoliths dis-

play subrounded shapes and range in size from a

few centimetres to 15 cm. The xenolith suite from

Kerguelen has been studied in detail previously by

Gregoire et al. (1997, 1998, 2000), and Moine et al.

(2000, 2001) and can be divided into three main

groups: (i) clinopyroxene-bearingF amphibole-bear-

ingF phlogopite-bearing harzburgites and dunites

that represent mantle wall-rock; (ii) clinopyroxene-

rich lherzolites, websterites and metagabbros that

are Al-augite series or Type II xenoliths (Wilshire

and Shervais, 1975; Frey and Prinz, 1978) and

represent high-pressure crystallization products of

basaltic melts of tholeiitic-transitional affinity; and

(iii) ilmenite- and spinel-bearing clinopyroxenites

and metagabbros that correspond to deep magmatic

segregates of alkali basaltic melts. In fact, all

studied peridotites from Kerguelen display refractory

characteristics and are mainly harzburgites. At first,

the development of a harzburgitic mantle under the

north Kerguelen plateau was interpreted as the result

of a large partial melting event related to the

interaction of the South East Indian Ridge with

the Kerguelen plume and, therefore, to the build

up of Kerguelen–Gaussberg plateau 40 Myr ago

(Gregoire, 1994; Mattielli, 1996). However, modal,

major and trace element data reveal that harzbur-

gites and dunites cannot be interpreted solely as

Table 1

Average electron microprobe analysis of mineral phases from dunite MG140; n: number of analysis, (1 sigma expressed in terms of last

significant digits); ol: olivine; cpx: clinopyroxene; amp: amphibole; Mg–wus: magnesio–wustite; sp: spinel

MG91-140 ol MG91-140 cpx MG91-140 amp MG91-140 glass MG91-140 Mg–wus MG91-140 sp

n 10 6 4 5 3 4

SiO2 39.78 (25) 53.60 (77) 42.17 (36) 48.27 (94) SiO2 1.49 (49) 0.01 (1)

TiO2 – 0.45 (2) 2.65 (8) – TiO2 0.09 (7) 0.94 (4)

Al2O3 – 2.75 (2) 12.29 (2) – Al2O3 0.73 (58) 23.07 (18)

Cr2O3 – 0.75 (7) 1.69 (8) – Cr2O3 – 35.84 (65)

MgO 49.50 (49) 15.84 (28) 17.07 (27) 23.47 (99) MgO 13.37 (42) 12.89 (46)

FeO 10.50 (20) 2.81 (8) 4.94 (20) 13.48 (97) FeO 74.26 (19) 24.42 (83)

MnO 0.15 (3) 0.08 (2) 0.06 (2) 0.26 (7) NiO 0.28

NiO 0.31 (3) 0.05 0.08 (2) 0.52 (21) MnO 0.31 (6) 0.24 (3)

CaO 0.08 (3) 22.75 (12) 11.49 (4) 1.39 (14) CaO 0.60

Na2O – 1.15 (5) 3.44 (11) 0.09 (3) Sum 91.12 97.40

K2O – 0.01 0.83 (2) 0.05 (3) mg* 24.3 48.5

Sum 100.32 100.23 96.70 87.52 cr* 51

mg* 89.36 90.94 86.04 75.6

Fo% 89.3

XCa 0.48

XMg 0.47

XFe 0.05

*mg* =Mg/(Mg+Fe) and cr* =Cr/(Cr+Al).

B.N. Moine et al. / Lithos 75 (2004) 239–252 241

refractory mantle assemblages. Arguments body

developed by Gregoire et al. (1997) suggests that

Kerguelen harzburgites and dunites after partial

melting underwent moderate to large interaction

with alkaline basaltic melts and that melt–peridotite

reaction was similar to that proposed by Kelemen

(1990). The carbonate-bearing dunite xenolith

(MG91-140) that is the focus of this study is an

amphibole-, clinopyroxene- and spinel-bearing du-

nite, and thus belongs to Group 1, and so is

interpreted as mantle wall-rock.

Table 2

Electron microprobe analysis of three carbonate types occurring in dunite

MG91-140

Mgcc Mgcc Mgcc Mgcc Mgcc Mgcc Mgcc cc cc

MgO 1.52 1.01 1.04 1.22 0.95 1.22 1.18 – –

FeO 0.44 0.33 0.59 0.61 0.47 0.58 1.8 0.06 0.

MnO 0.53 0.37 0.53 0.36 – – – 0.02 0.

CaO 53.33 54.17 53.77 53.64 54.35 58.81 52.28 55.9 55.

Na2O – 0.01 0.01 0.02 0.03 – 0.1 0.01 0.

CO2 44.19 44.1 44.05 44.15 44.21 44.40 44.65 44.01 43.

mg* 0.86 0.83 0.76 0.79 0.79 0.79 0.54 – –

XCa 0.956 0.97 0.966 0.961 0.97 0.962 0.945 0.999 0.

XMg 0.038 0.025 0.026 0.03 0.023 0.03 0.029 – –

XFe 0.006 0.005 0.008 0.009 0.006 0.008 0.025 0.001 0.

CO2 was calculated by stoichiometry. Mgcc: Mg-bearing calcite; cc: Mg-

3. Analytical techniques

Major-element compositions of the constituent

minerals of the xenolith were determined with the

CamecaR SX 100 microprobe at Clermont–Ferrand

(France), using a wavelength-dispersive spectrometric

(WDS) technique. The microprobe was used with 15

kV accelerating voltage, sample current of 20 nA, a

beam diameter of 2–3 Am, and natural and synthetic

minerals as standards. Matrix corrections were done

by PAP procedures. Counting times were 20–40 s and

MG91-140

cc cc cc dol dol dol dol dol

– – – 17.78 19.01 17.83 19.87 17.35

1 0.2 0.32 0.12 0.97 0.67 1.07 1.24 0.89

07 0.02 – – 0.06 0.14 0.05 0.03 0.15

81 55.72 55.7 55.83 34.07 32.91 33.97 33.68 0.06

03 0.03 0.01 0.04 0.08 0.06 0.06 0.07

99 44.03 43.97 44 47.05 47.2 47.02 47.41 47.86

– – – 0.97 0.98 0.97 0.97 0.97

999 0.997 0.996 0.998 0.572 0.549 0.57 0.523 0.576

– – – 0.415 0.442 0.416 0.461 0.413

001 0.003 0.004 0.002 0.013 0.009 0.014 0.016 0.012

free calcite; dol: dolomite; mg* =Mg/(Mg/Fe).

Table 4

In situ analyses of trace element contents (ppm) of each constituent

minerals of dunite MG91-140 obtained by laser ablation ICP–MS

MG91-140

Mgcc Mgcc cpx ol sp am

Mg 7080 7560

B.N. Moine et al. / Lithos 75 (2004) 239–252242

no values are reported below detection limits (0.01–

0.04 wt.%). Chemical compositions of the mineral

phases of dunite MG91-140 are given in Tables 1 and

2. X-ray phosphorus mapping was done with the same

CamecaR SX100 microprobe using a 15 kV acceler-

ating voltage, sample current of 200 nA and counting

Table 3

Whole-rock composition of carbonate-bearing spinel dunite MG91-

140 from Kerguelen Island

MG91-140 MG91-140

WR WR

wt.% ppm

ol 96.73 Sc 7.06

opx – V 31.9

cpx 0.30 Cr 5175

sp 1.70 Co 185

am 0.78 Ni 3230

Carb 0.49 Cu 7.75

Zn 75.1

Ga 1.58

SiO2 39.76 Cs 0.01

TiO2 0.03 Rb 0.25

Al2O3 0.55 Ba 2.39

FeO 10.30 Sr 19.48

MnO 0.14 Pb 0.29

MgO 46.61 Th 0.14

CaO 0.47 U 0.02

Na2O 0.17 Nb 0.16

K2O 0.01 Ta 0.01

P2O5 0.01 Ti 210

H2O+ 0.62 Zr 0.83

H2O� 0.11 Hf 0.01

CO2 0.44 La 1.87

Total 99.22 Ce 2.54

mg* 88.97 Pr 0.21

CaO/Al2O3 0.85 Nd 0.57

Sm 0.06

Eu 0.02

Gd 0.05

Tb 0.01

Dy 0.04

Ho 0.01

Er 0.03

Yb 0.04

Lu 0.01

Y 0.29

Major (wt.%) and trace (ppm) element contents were measured by

XRF and solution ICP–MS, respectively. Modal composition was

calculated using mass balance calculation based on the whole-rock

composition and the electron microprobe analyses of mineral phases

and refined using trace element contents of constituent mineral

phases; mg* =Mg/(Mg+Fe).

Al 74 33

Sc 11.3 11.6 230 220

Ti 5.06 1.51 3900 5.95 5.10 14,550

Rb 1.07 0.62 3.86 0.13 1.96 10.93

Sr 3500 3290 116 0.50 0.57 311

Y 36.1 34.9 7.9 0.12 0.03 11.16

Zr 1.00 0.49 48 0.10 0.66 58.9

Nb 0.012 0.014 0.65 0.03 0.49 14.62

Ba 138 143 12.3 0.30 0.76 264

La 413 392 7.41 0.03 0.76 22.10

Ce 467 439 15.94 0.03 0.22 40.9

Nd 94 86 6.55 0.14 18.55

Sm 8.89 8.57 1.26 3.14

Eu 2.67 2.41 0.36 1.00

Gd 5.28 5.20 1.1 2.20

Dy 4.66 4.41 1.2 0.04 2.40

Ho 1.16 1.22 0.3 0.70

Er 3.91 4.13 1.12 2.05

Yb 6.95 6.50 1.82 0.01 2.25

Lu 1.19 1.19 0.36 0.80

Hf 0.023 0.017 1.69

Ta 0.08 0.02 0.90

Pb 11.90 14.50 2.62 0.15 0.36 8.36

Th 1.78 2.02 3.92 0.02 5.95

U 0.76 0.44 1.4 0.01 0.02 2.11

Mgcc: Mg-bearing calcite; ol: olivine; cpx: clinopyroxene; amp:

amphibole; sp: spinel.

time of 300 ms per pixel with an image area of

512� 384 pixels.

Bulk rock were analysed for major elements, Cr

and Ni by X-ray fluorescence spectrometry [XRF;

see O’Reilly and Griffin (1988) for methods] and for

trace elements by solution ICP–MS (Perkin Elmer

5100) at Macquarie University. The whole-rock

composition of the dunite MG91-140 is given in

Table 3.

Concentrations of 27 trace elements [rare earth

elements (REE), Ba, Rb, Nb, Ta, Pb, Sr, Zr, Hf, Y,

Sc, V, Co and Ni] in minerals were determined in

polished thick section (200 Am) using a Perkin

Elmer Elan 6000 ICP–MS coupled with a Contin-

uum Surelite I-20 Q-switched Nd:YAG (UV :266

nm) laser ablation system (LA–ICPMS) in the

GEMOC Key Centre Macquarie University. The

laser was operated with 1 mJ/pulse energy and 5

B.N. Moine et al. / Lithos 75 (2004) 239–252 243

Hz frequency with laser beam focused above sam-

ple surface. Crater size was 30–50 Am. Detection

limits range from 10 ppb for U and Th, to 2 ppm

for Ni. NIST 610 glass standard was used for

calibration of relative element sensitivities and each

analysis was normalized using CaO values for

carbonate and clinopyroxene and MgO values for

amphibole, olivine and spinel as internal standards

and determined by electron microprobe. A more

detailed description of laser operating conditions,

calibration values for the NIST 610 glass standard

and error analysis is given by Norman et al. (1998).

Signal intensity for indicative major and minor

elements was monitored during analysis to make

sure that the laser beam kept running within the

mineral grain selected and did not ablate inclusions.

For example, ablation of carbonates was aborted

when signal intensity for Mg, Al, Ti or Ni increased

well above background levels indicating the occur-

rence of spinel or sulfide microinclusions. In these

cases, carbonate analyses with indicative levels of

these elements were discarded. Trace element com-

positions of the mineral phases of dunite MG91-140

are given in Table 4.

4. Petrography

Modal composition of the carbonate-bearing spi-

nel dunite MG91-140 was calculated using mass

balance based on the whole-rock composition and

the electron microprobe analyses of mineral phases,

and refined using trace element contents of constit-

uent mineral phases. Carbonate content was also

determined manometrically after dissolution in phos-

phoric acid (at 25 and 50 jC). The sample is

dominated by olivine (96.7 wt.%) and contains a

low proportion of amphibole (0.8 wt.%), cpx (0.3

wt.%) and spinel (1.7 wt.%). Orthopyroxene is

absent. Carbonates represent 0.4 to 0.5 wt.%,

depending on the method of estimation.

The xenolith has an equigranular to tabular

coarse-grained microstructures in which the grain

size of olivine generally ranges from 1 to 2 mm.

Olivine grains commonly contain numerous CO2

fluid inclusions. Others constituent minerals (clino-

pyroxene, amphibole and spinel) vary from 0.05 to

0.2 mm across. Spinel occurs as small holly leaf

interstitial grains and as euhedral crystals included

in olivine. Interstitial spinel is sometimes associated

with amphibole that seems to corrode it. Amphibole

occurs as anhedral to euhedral grains, commonly

associated with spinel. Amphibole sometime reacted

with the infiltrated fluid to produce a secondary

silicate phase (Si-poor and Al-rich clinopyroxene)

and ilmenite (Fig. 1f). Interstitial, globular to

euhedral clinopyroxene (50 to 150 Am) occurs at

the grain boundaries of corroded olivine (Fig. 1g);

it is mostly concentrated in the vicinity of carbo-

nates pockets but only rarely in direct contact with

them.

Carbonates occur in interstitial pockets located

at triple junctions of olivine grains, in microveins

or as globular patches located at grain boundaries.

There are two types of carbonate-bearing pockets

and three different compositional types of carbo-

nates. The first pocket type and the most abundant

is characterized by relatively large (50 to 500 Am)

anhedral Mg-bearing calcite grains that have sharp

curvilinear boundaries with olivine and totally fill

the interstitial spaces (Fig. 1a and g). Globular or

euhedral clinopyroxene occurring close to these

pockets is not crosscut by carbonate microvein

(Fig. 1g). The second type of carbonate pocket is

less abundant; it is characterized by the presence of

reaction zones containing silicate glass (Fig. 1c,d,e)

which is associated with the other two carbonate

types recognised by back-scattered electron (BSE)

imaging. These pockets are characterized by the

presence of a silicate glass-bearing reaction zone

that occurs between Mg-bearing calcite and prima-

ry olivine or may completely fill the whole pocket

when Mg-bearing calcite is lacking. Dolomite

occurs as euhedral grains located at the margins

of the reaction zone and silicate glass (Fig. 1b,c,d)

and is associated with magnesio–wustite and sul-

fide grains. Mg-bearing calcite and dolomite are

always separated by silicate glass. Mg-free calcite

has only been observed as euhedral grains in the

pockets that are filled by silicate glass without Mg-

bearing calcite and that show strong reaction rims

against olivine (Fig. 1d), resulting in a very fine

grained ( < 1 Am) mixture of silicate and oxide

phases.

The temperature of equilibration of MG91-140

dunite can be estimated only using the olivine-spinel

Fig. 1. Back-scattered electron imaging of carbonate-bearing dunite MG91-140 and X-ray mapping of phosphorus. (a) Anhedral Mg-bearing

calcites that have sharp curvilinear boundaries with olivine and totally fill the interstitial spaces; (b) pockets of carbonate consisting of Mg-

bearing calcite and small fraction of glass at olivine boundaries; (c) pocket of Mg-bearing calcite with silicate glass and rim reaction zone that

consists of very fine grain of silicate phases, magnesio–wustite, euhedral crystal of dolomite and Ni-sulfide; (d) enlarged view of reaction rim;

(e) silicate glass-rich pocket showing development of the wide reaction rim and crystallization of euhedral crystal of Mg-free calcite in the centre

part; (f) interstial grains of clinopyroxene and reacted amphibole in the olivine matrix; (g) Mg-bearing calcite pocket and microvein associated

with interstitial and globular clinopyroxene; (h) X-ray mapping of phosphorus in the pocket c described above; note that the highest content

corresponds to the Mg-bearing calcite area.

B.N. Moine et al. / Lithos 75 (2004) 239–252244

B.N. Moine et al. / Lithos 75 (2004) 239–252 245

geothermometer (Fabries, 1979; Ballhaus et al.,

1991). Results give temperatures ranging between

900–950 jC and pressure conditions from 0.6 to

1.5 GPa (O’Neill, 1981; Gasparik, 1984), lying in

the stability field of spinel peridotite. When the

calculated T values are referred to the empirical,

xenolith-derived geotherm for the Kerguelen mantle

(Moine, 2000), a pressure range of 0.9–1.1 GPa is

obtained.

5. Whole-rock composition

The dunite has low CaO (0.47 wt.%), Na2O (0.17

wt.%), Al2O3 (0.55 wt.%) and TiO2 (0.03 wt.%)

Fig. 2. Rare earth (a) and trace element (b) patterns of whole-rock,

clinopyroxene and Mg-bearing calcite (trace element contents

normalised to primitive mantle; McDonough and Sun, 1995).

contents similar to those of other dunite xenoliths

from Kerguelen (Gregoire et al., 1997, 2000) and

indicating a refractory character compared with the

estimated composition of the primitive mantle (CaO:

3.23–3.60 wt.%; Na2O: 0.33–0.61 wt.%; Al2O3: 4–

4.46 wt.%; TiO2: 0.18–0.22 wt.%, Jagoutz et al.,

1979; McDonough and Sun, 1995). However, its mg

[mg = 100(Mg/[Mg + Fe])] is 89, and thus at the low

end of the range for primitive mantle (89.9) and

Kerguelen mantle harzburgites (89–92; Gregoire et

al., 1997, 2000). The low P2O5 (0.01 wt.%) content is

consistent with the absence of apatite, an important

consideration for the residence sites of rare earth

elements (REE), especially in the carbonate pockets.

Whole-rock REE contents normalized to primitive

mantle values show a U-shaped pattern characterized

by a large relative enrichment in light REE (LREE;

[La/Sm]N: 19) and a slightly heavy REE (HREE)

enrichment over heaviest middle REE (MREE; [Dy/

Yb]N: 0.67; Fig. 2a). The trace element pattern shows

relative enrichment in U (0.02 ppm), Th (0.14 ppm)

and (to a lesser extent) for Ti (210 ppm), and a relative

depletion in Nb (0.16 ppm), Ta (0.01 ppm), Zr (0.83

ppm) and Hf (0.01 ppm; Fig. 2b).

6. Mineral and glass compositions

6.1. Major elements

Olivine has mg close to 89.4, a common value

for mantle dunites and similar to those of other

anhydrous dunites from Kerguelen (Gregoire et al.,

1997). The CaO content of olivine is low (0.08

wt.%) but slightly increases in the vicinity of

carbonates and glass pockets (V 0.14 wt.%; Fig.

2). Clinopyroxene (En47 Wo48 Fs5) displays homo-

geneous compositions with mg ranging from 90 to

91.4. It is low in TiO2 (0.45 wt.%) and relatively

high in CaO (22.8 wt.%), Cr2O3 (0.75 wt.%) and

Na2O (1.2 wt.%). Amphibole is a Cr2O3- and Na2O-

rich titanian-pargasite.

Mg-bearing calcite (XCa: 0.945–0.97) shows low

Na2O ( = 0.1 wt.%) and MnO ( = 0.53 wt.%) contents;

the average mg is 0.80 (range 0.76–0.86) if the lowest

value (0.54) is disregarded. Dolomite (XCa: 0.52–

0.58) displays FeO content ranging from 0.67 to

1.24 wt.% and very low Na2O ( = 0.08 wt.%) and

B.N. Moine et al. / Lithos 75 (2004) 239–252246

MnO ( = 0.14 wt.%) contents. Mg-free calcite (XCa:

0.99) contains Na2O at trace levels (0.01–0.04 wt.%)

and a relatively low FeO content (0.06–0.32 wt.%).

The mafic silicate glass displays high MgO

(23.5F 1 wt.%), FeO (13.5F 1 wt.%) and SiO2

(48.3F 1 wt.%) contents, relatively high NiO

(0.5F 0.2 wt.%) contents and low CaO (1.4 wt.%)

and alkali contents (Na2O: 0.09 wt.%; K2O: 0.05

wt.%). It contains no detectable Al2O3, TiO2, Cr2O3

and has a similar composition to those described for a

dunite–wehrlite sample from the Canary Islands

(Frezzotti et al., 2002) and in a lherzolite sample from

Spitsbergen (Amundsen, 1987; Ionov et al., 1993,

1996). The mafic silicate glass also contains high

amounts (z 10 wt.%) of volatiles (H2O and/or

CO32�).

The X-ray phosphorus mapping of carbonate pock-

ets (Fig. 1h) shows that phosphorus is contained in the

Mg-bearing calcite (at a few ppm level) and not in

silicate glass or in microapatite grains.

6.2. Trace elements

Clinopyroxene shows a U-shaped trace element

pattern (Fig. 2) characterized by slight enrichment in

LREE ([La/Sm]N: 3.6; [Sm/Yb]N:0.8) and very high

relative enrichment in U (1.4 ppm), Th (3.92 ppm)

and a slight enrichment in Pb (2.62 ppm), Hf (1.69

ppm) and Ti (3900 ppm) . The pattern also shows

relative depletion in Nb (0.65 ppm), Ta (0.08 ppm)

and Zr (48 ppm).

Trace-element contents of Mg-bearing calcites are

unusual for mantle carbonates with very high REE

(SREE: f 900 ppm), average of 3400 ppm Sr, of 140

ppm Ba, of 1.9 ppm Th, of 0.6 ppm U and of 13.2

ppm Pb. The U shaped trace element pattern of the

Mg-bearing calcite (Fig. 2) therefore shows very high

enrichment in LREE ([La/Sm]N: 28) and large deple-

tion in HFSE (Nb:0.01 ppm, Zr: 0.75 ppm, Hf: 0.02

ppm) and Ti (1.5–5 ppm).

7. Discussion

7.1. Origin of Kerguelen carbonate melts

Experimental studies strongly suggest that carbo-

natites can be mantle derived (e.g., Green and

Wallace, 1988; Dalton and Presnall, 1998a). It is

therefore likely that mantle-derived carbonate-rich

melts can become trapped in mantle xenoliths.

Based on the experimental work of Dalton and

Wood (1993) and Dalton and Presnall (1998a,b),

primary carbonate melts should be dolomitic, but

the majority of carbonate inclusions observed in

mantle peridotites or megacrysts are calcic (Lee et

al., 2000). This is interpreted as the result of

reaction between dolomitic melt and the peridotitic

mineral assemblage, especially with orthopyroxene

and clinopyroxene (Wyllie and Huang, 1976; Dal-

ton and Wood, 1993; Kogarko et al., 1995), accord-

ing to the following reactions:

CaMgðCO3Þ2Melt

þ 2Mg2Si2O6

orthopyroxene

¼ CaMgSi2O6

clinopyroxene

þ 2Mg2SiO4 þ 2CO2

forsterite

and

3CaMgðCO3Þ2Melt

þ CaMgSi2O6

clinopyroxene

¼ 4CaCO3þ2Mg2SiO4

Melt

þ 2CO2

forsterite

These reactions lead to the production of forster-

ite, calcic carbonate melt and CO2 in equilibrium

with a wehrlitic assemblage. Moreover, Dalton and

Wood (1993) have shown that at decreasing pressure,

carbonate-rich melt in equilibrium with wehrlitic

assemblages shifts towards more a Ca-rich composi-

tion, with a maximum calculated XCa (Ca/[Ca +Mg+

Fe]) near 0.96. The Kerguelen dunite MG91-140 has

a high modal content (96.7 wt.%) of magnesian olivine

with a relatively high mg (89.4) and the wehrlitic

mineral assemblage Ol +Cpx + Sp ( +Amp) with no

orthopyroxene. Its carbonates have XCa ranging from

0.945 to 0.97. Moreover, one of the features of this

sample is the occurrence of a clinopyroxene high in

CaO (22.8 wt.%), Na2O (1.2 wt.%), low in Al2O3 (2.75

wt.%) and characterized by a high mg (90.9) and by

high LREE enrichment. O’Reilly et al. (1991), Yaxley

et al. (1991, 1998) and Rudnick et al. (1993) proposed

B.N. Moine et al. / Lithos 75 (2004) 239–252 247

‘‘carbonatite mantle metasomatism’’ to explain the

same features in similar suites of xenoliths, dominantly

clinopyroxene-bearing dunites from southeastern Aus-

tralia and Tanzania, respectively. The review of Ionov

and Harmer (2002) and Lee et al. (2000) about trace

element compositions of carbonates occurring in man-

tle peridotite xenoliths highlights the very low REE

contents and moderate LREE enrichment of the mantle

carbonates, except for one sample from Mongolia.

These chemical features, together with the textural

characteristics of the mantle carbonates, support previ-

ous interpretations (e.g., Lee et al., 2000) that these

carbonates are crystal cumulates deposited from car-

bonate-rich, silica-bearing melts. By contrast, the Mg-

bearing calcites from the studied Kerguelen dunite

show very high REE contents. Their chemical and

textural features, in addition to the presence of the

coexisting mafic silicate, support an origin as a

quenched carbonate melt within the Kerguelen oceanic

mantle lithosphere.

Carbonate-bearing xenoliths and xenoliths dis-

playing evidence of carbonate metasomatism are

rare in the Kerguelen lithospheric mantle samples.

Most peridotite mantle xenoliths from Kerguelen

display evidence of metasomatism by highly alka-

line mafic silicate melts (Gregoire et al., 1997,

2000; Moine et al., 2000, 2001). The phlogopite-

and amphibole-bearing dunites from Kerguelen are

characterized by clinopyroxene showing progressive

LREE, Th and U enrichments and progressive

depletion in HFSE correlated with a decrease in

the modal proportion of clinopyroxene (Gregoire et

al., 2000; Moine, 2000). The less LREE-enriched

and HFSE-depleted clinopyroxene have a highly

alkaline mafic silicate melt signature while the most

LREE-enriched and HFSE-depleted clinopyroxenes

show a carbonate melt signature. Process of reactive

melt transport is considered to take place progres-

sively during the percolation of carbonate-bearing,

highly alkaline mafic silicate melts within the re-

fractory Kerguelen mantle (Gregoire et al., 1997).

The ultimate stage of these processes is the forma-

tion of a highly evolved carbonate-rich melt still

containing a silicate melt component. The overall

geochemical signatures of this latter largely depend

on the composition of the silicate component in the

original carbonate-rich melt and on the amount of

chemical components added through reaction with

the silicate minerals of host peridotite during per-

colation. These silicate melts, preserved as glass

inclusions in olivine grains and interstitial pockets,

have been previously described by Schiano et al.

(1994) and more recently by Delpech et al. (2004),

and are trachytic, trachy–andesitic or dacitic in

composition.

8. Mafic silicate melt coexisting with carbonates

A mafic silicate melt similar to that described in

the present study has been observed in dunite and

pyroxenite xenoliths from the Canary Islands by

Frezzotti et al. (2002). These authors argued that

these volatile-rich mafic silicate melts originated by

immiscibility and/or decarbonation processes from a

primary carbonate melt (dolomitic) stable in equi-

librium with lherzolites at near-solidus conditions

(1.5–2 GPa and 1150 jC). Indeed, the composition

obtained by mixing 85 wt.% of Mg-bearing calcite

and 15 wt.% of mafic silicate melt is in relative

accordance with experimental products (Dalton and

Wood, 1993), which also display low Al2O3 and

alkali contents but possess higher SiO2 content.

However, such carbonate melts, formed at pressure

ranging between 1.5–2 GPa, are required to perco-

late 15 km of the highly reactive, refractory Ker-

guelen mantle before reaching the level of sampling

without significant modification of their primary

chemistry. This seems highly unrealistic. Moreover,

the mafic silicate melts with low aluminium and

alkali contents described by Frezzotti et al. (2002)

and in this study are strictly different from those

described in glass inclusions and pockets (Schiano

et al., 1994; Delpech et al., 2004). On this ground,

we believe that alternative interpretations could

better explain the observed silicate–carbonate melt

association.

In lherzolite xenoliths from Spitsbergen, Amund-

sen (1987) and Ionov et al. (1993, 1996) described

similar silicate glasses associated with quenched car-

bonate melt. Ionov et al. (1993) interpreted these

glasses as the product of reaction between melted

carbonates and the peridotite constituent minerals

during ascent of the host lava. Ionov et al. (1996)

postulated a process of olivine resorption into a

preexisting mantle carbonate melt during the heating

Table 5

Lattice strain parameters for cpx–M2 site from fits of partitioning

data for REE3 + to Brice’s (1975) equation: D3þi ¼ D3þ

0 exp�4pE3þ

M2NA r3þ

0ðri�r3þ

0Þ2þ1

3ðri�r3þ

0Þ3½ �

RT

� �(fitted values from Blundy and

Dalton, 2000)

Valence3 + DC15a K2b MG91-140

T , P 1648 jK, 3 GPa 1373 jK, 2 GPa 1223 jK, 1 GPa

EM2 (GPa)c 280 283 281.5

r0 (A) 0.985 (0.971) 1.0182 (1.009) 1.011

D0 1.08 0.44 0.3

a Data from Blundy and Dalton (2000).b Data from Klemme et al. (1995).c Young’s moduli forM2 site calculated fromWood and Blundy’s

(1997) equation: E3þM2 ¼ 318:6þ 6:9P ðGPaÞ � 0:036T ðjKÞ.

B.N. Moine et al. / Lithos 75 (2004) 239–252248

and decompression processes related to the ascent. A

similar interpretation could explain the high MgO and

low Al2O3, Cr2O3, TiO2 and alkali contents of the

mafic silicate glasses in the Kerguelen xenoliths, as

the carbonate-rich melt percolated from a deeper level

below the site of sampling. Experimental results

shows that fast infiltration of carbonate melt in dunite

is controlled by dissolution of olivine at grain edges

(Hammouda and Laporte, 2000). The dissolved ele-

ments are then transported by diffusion into the

carbonate melt reservoir and are concentrated in some

carbonate pockets. During the percolation, the car-

bonate melt interacts with amphibole and crystallizes

clinopyroxene. Then, the small fraction of silicate

melt contained in some pockets could become immis-

cible during ascent as Lee and Wyllie (1998) have

shown that Ca-bearing carbonate–silicate melts are

miscible at pressure z 1 GPa. Thus, we propose that

the mafic silicate glass coexisting with carbonate in

Fig. 3. Plot of REE partition coefficient (Di) between cpx and

carbonate melt (Mg-bearing calcite) versus VIII-fold coordinated

ionic radius (ri; Shannon, 1976). Present work data are compared

with experimental results at 2 GPa (Klemme et al., 1995) and 3 GPa

(Blundy and Dalton, 2000).

some pockets in the dunite MG91-140 originated by

immiscibility with subsequent quenching during as-

cent and extrusion of the host lava. The exsolved

mafic silicate melt was volatile-rich and contained a

large amount of dissolved CO32 � that resulted in

crystallization of dolomite, magnesio–wustite in close

association with quenched silicate glass. This assem-

blage documents the dolomitic nature of the miscible

carbonate fraction in the mafic silicate melt in the

vicinity of olivine and its consistency with low P

experimental data ( < 1 GPa and 1100 jC) obtained by

Byrnes and Wyllie (1981).

8.1. Partition coefficients for clinopyroxene and

carbonate melt at 1 GPa and their implications

The chemical features of the clinopyroxene in the

carbonate-bearing dunite, i.e., high mg (90.9), high

Na2O (1.2 wt.%) and low Al2O3 (2.75 wt.%), as well

as those of the quenched carbonate melt (XCa: 0.945–

0.97) are similar to those of experimental products

(Dalton and Wood, 1993; Klemme et al., 1995). The

clinopyroxene, always interstitial and in close associ-

ation with Mg-bearing calcite, seems to be stable in

the metasomatic mineralogical assemblage, in contrast

to amphibole that shows reaction textures. Clinopyr-

oxene show U-shaped REE pattern that is uncommon

in Kerguelen mantle rocks (Gregoire et al., 1997,

2000; Moine et al., 2000) and similar to the REE

pattern of Mg-bearing calcite. These observations

suggest that clinopyroxene formed as product of the

percolation process operated by the carbonate melt

and is therefore in chemical equilibrium with the Mg-

bearing calcite.

Fig. 4. Comparison of trace element patterns between whole rocks

of MG91-140 and SB-4 (Ionov et al., 1996, 2002).

B.N. Moine et al. / Lithos 75 (2004) 239–252 249

This allows us to calculate partition coefficients for

a pressure (0.9–1.1 GPa) that is not yet experimentally

investigated. We plotted REE partition coefficients (D)

versus REE ionic radius (Onuma diagram) and com-

pared the resulting ‘‘parabola’’ (Fig. 3) with those

obtained using the experimental data from Blundy

andDalton (2000) andKlemme et al. (1995) at different

P and T conditions. It is interesting to note that the

Young’s modulus (E3 +) is similar for our data and 2

GPa experiments but not for 3 GPa experiments. The

data for 3 GPa and higher T result in a lower radius r0,

that in turn means crystal–chemical effects linked

either to an increasing flexibility of the M2 site of

clinopyroxene or/and in a lower solubility of REE in

carbonate melt. However, a better set of data for

heaviest rare earth at high pressure would allow to

confirm this observation. The fitting of our data to the

Blundy and Wood’s (1994) model results in signifi-

cantly lower D03 + (0.3; Table 5) with respect to the 2

and 3 GPa partitioning data; this decrease of D03 +

values could be tentatively ascribed to pressure effects

in agreement with previous modelling (Wood and

Blundy, 1997). All REE show very high incompatibil-

ity at 1 GPa. At higher pressures, the heavy REE

become more compatible and the LREE/HREE frac-

tionation is enhanced.

8.2. Mechanism of mantle metasomatism

The dunite sample MG91-140 has whole-rock

trace element patterns (Fig. 4) similar to those for

lherzolite xenoliths from Spitsbergen (Ionov et al.,

1996, 2002) that are interpreted as products of chro-

matographic evolution during percolation of a silicate

melt. In the Spitsbergen lherzolites, clinopyroxene

trace element contents control the REE and Sr con-

tents of the whole rock. The whole rock shows high

[La/Ce]N and [Sr/Nd]N ratios which cannot be

explained by clinopyroxene equilibrated with any

known type of terrestrial magma and are clearly not

in equilibrium with the silicate–carbonate melt pres-

ent in those samples. High [La/Ce]N, [La/Nd]N and

[Sr/Nd]N ratios in LREE-enriched metasomatic peri-

dotites and in constituent clinopyroxene are consistent

with chromatographic fractionation during porous

melt flow. However, REE and Sr patterns for

MG91-140 dunite can be modelled by simple mixing

between a previously LREE-depleted mantle rock and

the quenched, evolved carbonate melt now repre-

sented by the Mg-bearing calcite. A mixing model

between very LREE depleted peridotite with a highly

LREE and Sr-enriched liquid can reproduce the ob-

served high [La/Ce]N or [Sr/Nd]N ratio in such sam-

ple. This is confirmed by comparison of the measured

and calculated whole-rock composition with or with-

out Mg-bearing calcite (Fig. 5). Moreover, in this

case, clinopyroxenes equilibrated with a LREE- and

Sr-enriched carbonate melt do not display high [La/

Ce]N, [La/Nd]N and [Sr/Nd]N ratio (Fig. 2).

9. Conclusions

(1) This work documents the occurrence of carbonate

melt preserved in a shallow oceanic lithospheric

mantle dunites from Kerguelen as shown by

interstitial patches of Mg-bearing carbonate with

unusually high REE contents.

(2) The carbonate-rich melt has undergone reaction

with wall-rock peridotite silicates as it percolated

through the shallow mantle. The original melt

underwent significant fractionation as well as

addition of a silicate component from this reaction.

(3) The evolved carbonate-rich melt underwent im

miscible separation probably during ascent. This

resulted in the formation of melt pockets of

quenched carbonate melt (Mg-bearing calcite

patches) and the mafic silicate patches that are

associated with dolomite and Mg-free calcite.

Fig. 5. Comparison of trace element patterns of measured (cross and

dash line) and calculated (. and plain line) whole rock of MG91-

140 without (a) and with (b) Mg-bearing calcite.

B.N. Moine et al. / Lithos 75 (2004) 239–252250

(4) The occurrence of secondary clinopyroxene

inferred to be in equilibrium with the carbonate-

bearing melt allows the calculation of D for cpx/

melt at 1 GPa (the inferred depth of origin of the

dunite). Using existing experimental work, we

have demonstrated that cpx/melt D is pressure

dependent.

Acknowledgements

This work has been made possible by the generous

assistance and technical expertise of N.J. Pearson, A.

Sharma and C. Lawson (GEMOC Geochemical

Analysis Unit) and M. Veschambre (Clermont–

Ferrand). This work was supported by the French

UMR–CNRS 6524, Australian Research Council

Large and Small Grants (S.Y. O’Reilly) and Macquarie

University Research funding. We thank for their

support the French Polar Research and Technology

Institute (IFRTP, Brest, France), and also Hilary

Downes, an anonymous reviewer and Riccardo

Vannucci for their comments that improved this work.

This is publication number 329 in the ARC National

Key Centre for Geochemical Evolution and Metal-

logeny of Continents www.es.mq.edu.au/GEMOC/).

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