www.elsevier.com/locate/chemgeo

Chemical Geology 208 (2004) 175–194

A multi-technique study of platinum group element systematic in

some Ligurian ophiolitic peridotites, Italy

Ambre Lugueta,*, Jean-Pierre Lorandb, Olivier Alardc,d, Jean-Yves Cottine

aDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015, USAbLaboratoire ‘‘Mineralogie, Meteorites, Manteau’’, FREE CNRS 2456, Museum National d’Histoire Naturelle,

61 rue Buffon, 75005 Paris, FrancecGEMOC ARC National Center, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia

dDepartment of Earth Sciences, The Open University (Walton Hall), Milton Keynes MK7 6AA, UKeDepartement ‘‘Geologie-Petrologie-Geochimie’’, Universite Jean Monnet, 23 rue du docteur Paul Michelon, 42023 Saint-Etienne, France

Abstract

Fe–Ni–Cu sulfide mineralogy has been investigated along with bulk-rock and in-situ PGE analyses by ICP MS and LA-ICP-

MS in eight lherzolites from the Internal (IL) and External Liguride (EL) ophiolites (Italy). The two EL lherzolites are fertile (2–

4% partial melting) and slightly serpentinized while the six IL cpx-poor lherzolites have experienced 5–10% of partial melting,

impregnation by instantaneous melt fractions [Geochim. Cosmochim. Acta 61 (1997) 4557] and have been highly serpentinized.

The EL lherzolites show broadly chondritic PGE relative abundances with a slight to pronounced enhancement of the light PGE

(Ru, Rh and Pd) relative to the heavy PGE (Os, Ir and Pt) (RuN/IrN = 1.13; RhN/IrN = 1.08–1.10; PdN/IrN = 1.24–1.62; N =CI-

chondrite normalized). Their magmatic sulfide modal abundances and S contents, similar to the orogenic peridotites values, are

consistent with their very low degree of partial melting. The occurrence of Cu–Rh–Pd-rich pentlandite, however, demonstrates

that, even for low degree of partial melting, a Cu–Ni-rich sulfide liquid can segregate, leaving the residual monosulfide solid

solution (Mss) (now transformed into Cu-poor pentlandite) depleted in Rh and Pd (RhN/IrN and PdN/IrN < 1).

The IL cpx-poor lherzolites display a broadly flat PGE patterns from Os to Pt with a slight enhancement of Ru and Rh (RuN/

IrN = 1.05–1.38; RhN/IrN = 1.01–1.31). PdN/IrN ratios range from chondritic to superchondritic (1.02–2.99) and cannot be

interpreted in terms of partial melting models. Rh–Pd–Cu–Ni-rich pentlandite grains are associated with large corroded cpx

crystals ascribed to exotic melt percolation by Rampone et al. [Geochim. Cosmochim. Acta 61 (1997) 4557]. It is concluded that

precipitation of Cu–Ni–Rh–Pd-rich sulfides has significantly enhanced the Pd concentrations as well as the magmatic sulfide

modal abundances. Such processes, previously documented in abyssal peridotites from slow-spreading mid-oceanic ridges,

characterize residues from low to moderate melting degrees (5–10%) of the oceanic mantle as a whole, either from mature ocean

or from short-lived oceanic basins.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Magmatic sulfides; Platinum-group elements; LA-ICP-MS; Liguride ophiolitic lherzolites

0009-2541/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.chemgeo.2004.04.011

* Corresponding author. Tel.: +1-202-478-8463; fax: +1-202-478-8821.

E-mail addresses: luguet@dtm.ciw.edu (A. Luguet), lorand@mnhn.fr (J.-P. Lorand), o.alard@open.ac.uk (O. Alard),

cottin@univ-st-etienne.fr (J.-Y. Cottin).

A. Luguet et al. / Chemical Geology 208 (2004) 175–194176

1. Introduction

Complementary to the widely used incompatible

trace element, platinum-group elements (PGE) sys-

tematics can provide insights into a great variety of

mantle processes, such as partial melting, melt perco-

lation and mantle metasomatism (e.g. Handler and

Bennett, 1999; Lorand et al., 1999, 2003; Buchl et al.,

2002; Lee, 2002 and references therein). Moreover, as

highly siderophile elements (HSE), they also have

major bearing on planetary differentiation processes

(e.g. core–mantle segregation) or the late-accretionary

history of the primitive Earth (Morgan, 1986; Pattou

et al., 1996; Morgan et al., 2001). Recent improve-

ments in the chemical separation procedures, coupled

with highly sensitive instruments for detection (e.g.

ICP-MS), allow platinum-group elements to be rou-

tinely analysed at typical mantle concentration levels

( < 10 ppb) with precision better than 7% (e.g. Gros et

al., 2002; Meisel et al., 2003).

Most recent studies focused on continental mantle

samples, which on average were less hydrothermally

altered during crustal emplacement than oceanic

peridotites. Only recently, high-quality PGE data

were published for abyssal peridotites (Rehkamper

et al., 1999; Brandon et al., 2000; Luguet et al.,

2003). The few high-quality PGE analyses of ophio-

litic peridotites addressed samples not representative

of the convecting asthenosphere (the MORB source)

(e.g. Buchl et al., 2002). Only Snow et al. (2000)

analysed Os isotopes and the six PGE ( +Au) in 15

Ligurian ophiolitic lherzolites resembling abyssal

peridotites. They inferred from the strong similarity

of PGE patterns characterized by enhancement of Ru

to Pd relative to Os and Ir that the depleted mantle

shows the influence of core-derived metal. However,

care must be exercised in interpreting bulk-rock PGE

analyses of oceanic mantle-derived peridotites for

two reasons. Firstly, these samples have a complex

petrogenetic history, which generally obscures their

relationships with the convecting asthenospheric

mantle. Petrogenetic models for these rocks now

recognize strong interactions between partial melting

residues with incompletely extracted melts or exotic

melt migration (Rampone et al., 1997; Seyler et al.,

2001). Secondly, unlike incompatible lithophile trace

elements that are hosted in major silicates, PGE

reside in minor or trace phases, mostly Cu–Fe–Ni

sulfides (base metal sulfides: BMS), that can be

mobile at subsolidus mantle temperatures. Using a

laser ablation (LA) sampling system coupled to an

inductively coupled plasma-mass spectrometer (ICP-

MS), Alard et al. (2000, 2002) determined PGE

contents of mantle BMS and were able to identify

a strong affinity of Os, Ir, Ru and Rh for refractory

Fe–Ni monosulfides while Pt, Pd and Au concen-

trate preferentially in Cu–Ni-rich sulfide melts.

Luguet et al. (2001, 2003) provided compelling

evidence that, during adiabatic partial melting be-

neath mid-oceanic ridges, such melts percolate the

residual lithosphere and precipitate as Cu–Ni-rich

sulfides, thus enhancing bulk-rock Rh, Pt and Pd

concentrations of abyssal peridotites.

The aim of the present paper is to reassess the

meaning of PGE systematics of ophiolitic lherzolites

and the connection between abyssal peridotite and

ophiolitic lherzolite. A multi-technique approach in-

cluding electron microprobe (EMP) analyses and

imaging, solution and laser ablation ICP-MS was

undertaken on eight lherzolites collected in the Lig-

urian ophiolites (Italy). These ophiolites are now

interpreted as an oceanic/continental lithospheric

package formed during the early rifting stages of the

Ligurian Tethys, in a tectonic setting analogous to

present-day passive margins or embryonic oceans

(e.g. the Red Sea) (Rampone et al., 1995; Rampone

and Piccardo, 2000).

2. Geologic setting

Ligurian ophiolites are located in Northern Apen-

nines (Italy, see Fig. 1 of Rampone and Piccardo,

2000) and are interpreted as remnants of the litho-

sphere that floored the Jurassic Liguro–Piemontese

ocean, a short-lived branch of the Tethys (Cottin,

1978). Ophiolite complexes crop out in the Internal

Ligurides (IL) and External Ligurides (EL) which

represent the oceanic formations and their periconti-

nental margins, respectively (Rampone and Piccardo,

2000 and references therein).

The mantle sequence of IL ophiolitic complex

mostly consists of spinel–plagioclase peridotites and

numerous intrusive gabbros. The peridotite–gabbro

basement is uncomfortably overlain by late Jurassic

MORB-type basalts, which are not linked to the

Fig. 1. Microphotographs of magmatic and hydrothermal sulfides in Liguride lherzolites (plane polarized reflected light). A: Large sulfide grain

displaying lobate grain boundaries and partly enclosed in a large orthopyroxene. B: Copper-rich magmatic sulfide located at grain junctions

between orthopyroxene (Opx), olivine (Ol), clinopyroxene (Cpx) and plagioclase (Pl). C: Magmatic sulfide grain displaying a serpentinization-

derived paragenesis consisting of pentlandite (Pn), mackinawite (Mw) and magnetite (Mt). Note that this sulfide grain is adjacent to a small

spinel grain. D: Magmatic sulfide grain displaying a serpentinization-derived paragenesis made up of Pn, Mt and native copper (Cu). E:

Network of ‘‘pseudo-inclusions’’ trapped inside cleavage planes at the periphery of a cpx. F: Large hydrothermal sulfides made up of pyrrhotite

(Po), chalcopyrite (Cpy) and accessory pentlandite.

A. Luguet et al. / Chemical Geology 208 (2004) 175–194 177

peridotites by a genetic melt–residue relationship

(Rampone et al., 1996, 1998). The peridotites are

clinopyroxene-poor spinel lherzolites (1.7–2.5 wt.%

Al2O3, Rampone et al., 1995, 1996, 1997) which, in

terms of major and trace element chemistry, strong-

ly resemble slow-spreading abyssal peridotites

(Rampone and Piccardo, 2000). On the other hand,

these lherzolites bear evidences of impregnation by

instantaneous melt fraction very depleted in incom-

patible trace elements; this impregnation occurred at

low temperature, when the IL peridotites were in a

lithospheric thermal regime (Rampone et al., 1997).

The evidence consists of plagioclase blebs/veins

confined along grain boundaries not related to

spinel; orthopyroxene + plagioclase replacing intersti-

tial clinopyroxene; and rim composition of such

A. Luguet et al. / Chemical Geo178

reacted clinopyroxene showing Al-depletion (3.9–

5.1 wt.% Al2O3), Ti-enrichment (0.3–0.5 wt.%

TiO2), overall REE enrichment and a slight Eu

negative anomaly.

EL ophiolites are distinguished from IL ophiolites

by (i) a mantle basement of lherzolites containing no or

very little gabbros, (ii) being stratigraphically covered

by MORB-type volcanics and marine sedimentary

rocks (Abbate et al., 1994), and (iii) a spatial associ-

ation with Hercynian continental crust (Marroni et al.,

1998). The lherzolites are spinel–plagioclase lherzo-

lites (2.9 <Al2O3 < 4 wt.%). Their modal, bulk-rock

major and trace element compositions are less depleted

than the IL cpx-poor lherzolites and bear strong

similarities to some circum-Tethys orogenic lherzolites

(e.g. Lanzo, Ronda, Beni Bousera; Bodinier et al.,

1991). The EL lherzolites are interpreted as Protero-

zoic subcontinental lithospheric mantle tectonically

emplaced in the continental margin of the Ligurian

oceanic basin by passive extension of the lithosphere

(Beccaluva et al., 1984; Rampone et al., 1995; Ram-

pone and Piccardo, 2000).

Table 1

Major element contents of the Liguride peridotites analysed for PGE and

Location External Ligurides Internal Ligurides

Monte Maggiorasca Val Gravelia–Monte Bo

Sample LI Xa LI 129a LI 48a LI 52a

wt.%

SiO2 44.98 45.66 46.17 45.38

TiO2 0.17 0.12 0.06 0.06

Al2O3 3.87 3.24 2.11 1.95

FeOtot 7.77 7.68 8.36 7.95

MnO 0.13 0.12 0.13 0.13

MgO 37.79 39.15 41.63 42.17

CaO 3.68 2.95 0.80 1.18

Na2O 1.48 0.64 0.69 0.42

K2O 0.03 0.02 0.02 0.01

P2O5 0.02 0.02 0.01 0.01

LOI 3.36 2.83 12.9 13.26

CaO/Al2O3 0.95 0.91 0.38 0.60

Mg# 90.6 91.0 90.8 91.3

ppm

Cr 2526 2487 2387 2962

Ni 1669 1784 2035 2021

LI referenced as AL in Cottin (1978).a X-ray fluorescence (Open University, O. Alard analysts).b wet chemical analyses (Cottin, 1978); tr: traces; – : not analysed.

2.1. Main petrographic features of the analysed

sample

Our samples were collected from both the IL and

the EL units (Cottin, 1978; Table 1). LI X and LI 129

are two weakly serpentinized ( < 30%; estimated on

loss on ignition values; Table 1) spinel–plagioclase

lherzolites collected at Monte Maggiorasca in the

External Ligurides. These rocks display typical

coarse-granular textures characterized by a network

of centimetric rounded orthopyroxene and clinopyr-

oxene crystals immersed in an olivine and plagioclase

matrix that may contain relict Cr-spinel. Cr-spinel

may also be associated with the pyroxenes in clusters

typical of protogranular textures. The bulk-rock major

element analyses, clinopyroxene (Cpx) core compo-

sitions (4.6–7.0 wt.% Al2O3, 0.6–0.9 wt.% Na2O,

0.8–1 wt.% TiO2) and olivine Fo content (89.9) are

symptomatic of low-degree ( < 5%) melting residues,

as are incompatible trace element data for the Cpx,

determined by in-situ LA-ICP-MS analyses (LaN/

YbN = 0.33; YbN = 5.92, N: primitive mantle normal-

logy 208 (2004) 175–194

Au

cco Bracco–Monte di San Agata

LI 27-2b LI 27-5b LI 87b LI 138a

45.23 44.77 45.48 44.95

tr tr tr 0.07

2.16 2.15 2.15 2.73

8.11 8.43 8.56 8.43

0.13 0.13 0.11 0.15

41.83 40.72 39.99 40.78

1.85 2.09 1.11 1.38

tr tr tr 1.12

– – – 0.03

– – – 0.02

12.65 12 12 12.44

0.85 0.97 0.51 0.50

91.1 90.5 90.2 90.5

– – – 2633

– – – 2000

A. Luguet et al. / Chemical Geology 208 (2004) 175–194 179

ized after McDonough and Sun, 1995; Luguet and

Alard, unpublished data).

All the other samples come from the Internal

Ligurides (Bracco–Val Gravelia unit). They are high-

ly serpentinized (80–100%, estimated on loss on

ignition values; Table 1) cpx-poor lherzolites with

coarse-granular to porphyroclastic textures. Olivine

and orthopyroxene (Opx) are pervasively altered.

Cr-spinel may sometimes be replaced by ferrit–chro-

mite, especially in sample LI 138 that also displays

incipient alteration of clinopyroxene. Where unal-

tered, clinopyroxene may occur as clusters of several

undeformed crystals remote from the orthopyroxene.

Interstitial Cpx with convex-inward grain boundaries

inside vein forming plagioclase/orthopyroxene aggre-

gates have also been observed. Some clinopyroxene

rim compositions (3.2–4.9 wt.% Al2O3, 0.3–0.4

wt.%TiO2) agree well with reacted clinopyroxene

rim compositions that Rampone et al. (1997) ascribed

to exotic melt percolation.

3. Analytical methods

Sulfide petrography was investigated from pol-

ished thin sections using reflected light microscopy

at magnifications up to 1500. Sulfide modal abundan-

ces were determined from digitized plane-polarized

reflected light microphotographs. Up to three polished

thin sections per sample were used. Fe–Ni–Cu sul-

fides were analysed using Cameca SX 50 electron

microprobes (GEMOC, Macquarie University, Syd-

ney, Australia and Camparis, Paris VI University,

Paris, France) operating in WDS mode. Cu, Co and

Ni concentrations were determined with pure metal

standards, S and Fe with a natural pyrite and O with

iron oxides. The operating conditions for the GEMOC

electron microprobe were as follows: 15 nA sample

current, 20 kV accelerating voltage, 10 s counting

times (10 s/peak, 10 s/background) for Ni, S, O and

60 s counting times (60 s/peak, 60 s/background) for

Fe, Cu and Co. For the Camparis electron microprobe,

these conditions were as follows: 15 nA sample

current, 15 kVaccelerating voltage, 6 s counting times

(6 s/peak, 6 s/background).

Bulk-rock S concentrations were determined by

iodometry of the SO2 produced by combustion at 950

jC of 500 mg powder aliquots. They reflect S con-

tents both bound as sulfides (hydrothermal and mag-

matic) and as sulfates. Three replicate analyses of the

international standard sample SY2 (CANMET) yield

119F 5 ppm (1r), in good agreement with the rec-

ommended value (0.011%) (Lorand, 1990). The de-

tection limit is c 5 ppm.

The details of the analytical procedure used for the

bulk-rock PGE and Au analyses can be found in Gros

et al. (2002). Briefly, bulk-rock concentrations of Os,

Ir, Ru, Rh, Pt, Pd and Au were determined from 10 to

15 g powder aliquots, the PGE and Au were separated

from these aliquots by NiS fire assay preconcentration

followed by Te coprecipitation. The Te precipitate was

collected on a cellulose filter, which was then dis-

solved in a concentrated HNO3–HCl solution. This

solution was diluted and analysed by ICP-MS oper-

ated with external calibration standards, using the VG

353 plasmaQuad turbo 2+ instrument, jointly owned

by the university of Montpellier II and the MNHN.

Procedural blank data are reported Table 4.

In-situ PGE analyses were performed using the

GEMOC LA-ICP-MS (Alard et al., 2000). The six

PGE, Au, Se, Te, As and Bi were determined using a

custom-built laser ablation system (designed by S.E.

Jackson) linked to a HP 4500 (RF power 1350 W).

The laser is a Continuum Surelite I-20 Q-switched

quadrupled frequency Nd:YAG laser delivering a 266

nm UV beam (see Norman et al., 1996). Sulfur was

determined by electron microprobe (Cameca SX 50,

GEMOC, Macquarie University) and used as internal

standard. Ablation was done in a pure He atmosphere

(0.85 l/min). More details on the analytical conditions

were reported by Alard et al. (2000), Lorand and

Alard (2001) and Luguet et al. (2001).

4. Results

4.1. Sulfide petrography

All the Ligurian peridotites studied here contain

trace amounts of magmatic sulfides (Table 2). These

latter are easily identified by their shapes indicating

crystallization from a former immiscible sulfide melt

(oval to rounded habit, larger grains with convex-

inward grain boundaries; Fig. 1) and the predomi-

nance of pentlandite, a typical subsolidus decomposi-

tion product of high-temperature magmatic sulfides

Table 2

Modal abundances and petrography of magmatic sulfide assemblages

Sample N Magmatic sulfide modal

abundances (wt.%)

Pn Hz Tr Mw Cp Bo Cu Vall Mt

LI X 3 0.045F 0.019 + + (+) (+) (+) (+) + + +

LI 129 1 0.056 + + (+) (+) (+) (+) (+) +

LI 48 1 0.040 + + + (+)

LI 52 2 0.006F 0.006 + (+) (+) +

LI 27-2 1 0.020 + + (+) (+) +

LI 27-5 1 0.027 + + (+) + (+) (+)

LI 87 2 0.065F 0.03 + + + (+) (+) +

LI 138 3 0.05F 0.0046 + + (+) (+) +

N: number of polished thin sections taken into account for image analysis; ++: abundant, +: widespread but small grain size, (+): scarce and of

very small size (10 Am across). Pn: pentlandite, Hz: haezlewoodite, Tr: troilite, Mw: mackinawite, Cp: chalcopyrite, Bo: bornite, Cu: native

copper, Vall: valleriite, Mt: magnetite.

A. Luguet et al. / Chemical Geology 208 (2004) 175–194180

(e.g. Lorand, 1989; Luguet et al., 2003). Magmatic

sulfides modal abundances range between 0.006 and

0.065 wt.%, within the ranges of orogenic lherzolites

(Lorand, 1989) and abyssal peridotites (Luguet et al.,

2003). Standard deviations may be high (e.g. LI X; LI

52), reflecting both large variations of sulfide grain

size (10–1000 Am), their heterogeneous distribution

at the thin section scale and, in the case of the highly

serpentinized samples (e.g., LI 52), the uncertainty

regarding the origin of some sulfide grains. Table 2

indicates no correlation between modal sulfide con-

tents and fertility index. However, rather important

differences of sulfide texture, sulfide mineralogy and

sulfide–silicate textural relationships are to be noted

between EL and IL lherzolites.

The two EL lherzolites (LI X and LI 129) are

sulfide-rich (0.045–0.056 wt.%). Magmatic sulfides

are coarse-grained intergranular sulfides, 100� 100

Am on average. The largest grains are partly engulfed

in large orthopyroxene porphyroclasts (Fig. 1A), or

surrounded by olivine neoblasts, developing dihedral

angles >90. The smaller sulfide grains are dissemi-

nated at spinel–olivine and spinel–plagioclase con-

tacts, and less commonly, in the vicinity of matrix

clinopyroxene (Fig. 1B). Primary sulfide inclusions in

silicates are very scarce and observed only in the cpx.

The few inclusions isolated from serpentine display a

pentlandite + chalcopyrite + pyrrhotite assemblage,

pentlandite being always predominant. Interstitial sul-

fide grains in contact with serpentine are always

surrounded by magnetite, and composed of an iron-

rich pentlandite (Fe/Ni = 1.38–2.15, Table 3), coex-

isting with troilite, mackinawite, native copper and/or

vallerite (Fig. 1C,D). This metal-rich assemblage is

characteristic of the strongly reducing conditions that

are generated by incipient serpentinization of olivine

at low temperature ( < 300 jC; Lorand, 1985; Abra-jano and Pasteris, 1989). LI X shows a greater amount

of Cu-bearing opaque minerals (i.e. chalcopyrite,

bornite, native copper and valleriite) than LI 129.

Chalcopyrite/bornite grains are trapped in plagioclase

or as very small inclusions in cpx and, thus, were

protected from redox reactions. Chalcopyrite/bornite

exsolutions have been observed in cleavage plane of

some coarse intergranular pentlandite grains. Native

copper commonly occurs as veinlets infiltrated in

silicate grain boundaries from coarser sulfide grains.

Valleriite replaces both chalcopyrite and native cop-

per. All these textures testify to a highly mobile, Cu-

rich phase in this sample.

The IL cpx-poor lherzolites are on average poorer

in magmatic sulfides (0.006–0.065 wt.%). Magmatic

sulfides consist of a Co-poor pentlandite (Co = 2.4–

2.6 wt.%; Fe/Ni = 1–1.2, Table 3), often replaced by

heazlewoodite and surrounded by serpentinization-

derived magnetite. Chalcopyrite is small-sized while

bornite and valleriite are scarce (Table 2). Despite

their strong degree of serpentinization, the IL cpx-

poor lherzolites do not contain native metals, suggest-

ing less strongly reducing serpentinization conditions

than for EL lherzolites. Magmatic sulfides are gener-

ally attached to vermicular spinel and to the large

orthopyroxene porphyroclasts, and are sometimes

located at plagioclase/orthopyroxene contacts. In LI

Table 3

Representative electron microprobe analyses of magmatic sulfides and native copper

Sample LI X LI X LI X LI 129 LI 138 LI 129 LI X

** * ** ** * ** *

Mineral Pn Pn Pn Pn Pn Tr Cu

Assemblage Pn + Tr +Mt Pn +Cu+Mt Pn +Tr +Mt Pn +Mw+Mt Pn +Mt Pn +Tr +Mt Pn +Cu+Mt

Fe (wt.%) 37.0 39.8 42.0 44.1 33.1 62.5 2.1

Ni 28.4 25.1 23.1 21.5 31.1 0.1 0.9

Co 0.6 0.6 0.3 0.3 2.5 0.0 0.2

Cu 0.1 0.3 0.9 0.7 0.0 0.4 97.1

S 32.8 33.9 33.1 33.3 33.5 36.1 0.0

O 0.5 n.a. 0.5 0.4 n.a. 1.0 n.a.

Total 99.4 99.7 99.9 100.3 100.2 100.1 100.3

Fe (at.%) 29.5 32.2 33.4 35.3 26.8 48.3 2.4

Ni 21.4 19.3 17.4 16.4 24.0 0.1 0.9

Co 0.5 0.5 0.2 0.3 1.9 0.0 0.2

Cu 0.1 0.2 0.6 0.5 0.0 0.5 96.4

S 45.5 47.8 46.0 46.4 47.3 48.6 0.1

O 1.5 n.a. 1.3 1.1 n.a. 2.8 n.a.

Fe/Ni 1.38 1.67 1.92 2.15 1.12

*: Camparis (Univ. Paris VI, France), **: GEMOC (Macquarie Univ., Sydney, Australia). Abbreviations of mineral names as in Table 2. n.a.:

not analysed.

A. Luguet et al. / Chemical Geology 208 (2004) 175–194 181

52, the sulfide-poorest IL lherzolite, they are dissem-

inated in the olivine matrix. Large (up to 500 Amacross) magmatic sulfide grains tend to concentrate in

the vicinity of the largest intergranular clinopyroxene

crystals (often recrystallized into polycristalline aggre-

gates along with euhedral to subhedral spinels). The

clinopyroxene may be penetrated by minute negative

inclusions arrays connected to each other by sulfide

veinlets. Polyedral sulfide ‘‘pseudo-inclusions’’ are

trapped inside cleavage planes at the periphery of

the cpx (Fig. 1E). These smallest sulfide droplets

inside late-magmatic cpx escaped reduction from

serpentinization fluids and thus preserved a Cu–Ni-

rich (i.e. pentlandite + chalcopyrite + bornite) assem-

blage; other, cracked inclusions show incipient re-

placement of primary magmatic Cu sulfides by

digenite/chalcocite.

The IL cpx-poor lherzolites also contain hydrother-

mal sulfides recognizable by their Fe-rich assemblage

(essentially pyrrhotite) and their preferential location

in serpentine matrix, silicate cracks and hydrothermal

veins crosscutting the samples. They are euhedral

Co–Ni-rich pentlanditeF chalcopyrite, 10–20 Amacross, scattered within the serpentine matrix or inside

fracture planes of silicates, or even inside Cr-spinel

when this mineral is replaced by ferrit–chromite. In

addition, two samples (LI 138; LI 87) are crosscut by

hydrothermal veins sealed by large (up to 2 mm

across) Ni-free polycristalline pyrrhotite grains (Fig.

1F) sometimes associated with chalcopyrite and gen-

erally pentlandite-free. Where associated with hydro-

thermal pyrrhotite, pentlandite occurs as highly

corroded residual grains inside the pyrrhotite, suggest-

ing that it reacted with S-rich hydrothermal fluids to

produce the pyrrhotite.

4.2. Bulk-rock analyses

Total sulfur concentrations range between 195 and

1844 ppm. In fact, only the two EL lherzolites show a

good agreement between measured S concentrations

and S concentrations recomputed from magmatic

sulfide modal abundances, provided the large error

on some modal analysis is taken into account (e.g. LI

X; Table 2). The two S data for these lherzolites also

agree very well with those measured in orogenic

lherzolites (Lorand, 1991; Hartmann and Wedepohl,

1993). The IL cpx-poor lherzolites have much higher

S contents than expected from the modal abundances

of magmatic sulfides. Indeed, the S-richest samples LI

A. Luguet et al. / Chemical Geology 208 (2004) 175–194182

138 and LI 87 (1363–1844 ppm S), apart from

magmatic sulfides which account for 167–200 ppm

S (Table 4), contain numerous coarse-grained hydro-

thermal sulfides but are devoid of sulfates. The S

budget of these rocks is thus mainly accounted for by

hydrothermal sulfides. Petrographic data for this sam-

ple indicate that Fe–Cu–S-rich hydrothermal fluids

reacted with magmatic pentlandite, precipitating pyr-

rhotite and, in a lesser extent, chalcopyrite.

Duplicate analyses of PGE are available for three

IL samples (LI 48, LI 87 and LI 138). For LI 48 and

LI 138, all the PGE data reproduce within 10–20%

except Pt (25%) (Fig. 3). For LI 87, reproducibility of

PGE concentrations are lower (30–40% for Ir and Ru,

45–50% for Pt and Pd) while the PGE ratios are much

more reproducible (10–20%). One may thus infer that

all PGE were at one time concentrated in a single

mineral carrier which is heterogeneously distributed at

the hand-sample scale. Gold data reproduce poorly,

varying by a factor 4 within a single sample (e.g. LI

138), supporting the idea that Au is hosted in hetero-

geneously distributed discrete phases different from

the PGE carriers.

Platinum-group elements concentrations, even gold

and palladium, the most sensitive to secondary remo-

bilization by hydrothermal fluids, show no relation-

ship with measured S concentrations. The two EL

lherzolites have absolute concentrations and CI-nor-

malized PGE patterns similar to the orogenic lherzo-

Table 4

Bulk-rock PGE, Au and S concentrations

Sample Os

(ppb)

Ir

(ppb)

Ru

(ppb)

Rh

(ppb)

Procedural

blank (n= 4)

0.007F 0.003 0.040F 0.010 0.136F 0.013 0.005F 0.00

LI X 3.31 3.07 5.4 1.03

LI 129 3.86 3.69 6.5 1.22

LI 48 3.41 3.04 6.9 1.21

* 3.60 3.49 5.7 0.95

LI 52 4.24 4.00 5.7 1.23

LI 27-2 2.97 3.08 5.8 0.96

LI 27-5 2.52 2.57 5.5 0.80

LI 87 3.85 3.26 5.8 1.12

* 2.63 2.54 4.4 0.82

LI 138 2.81 2.95 5.3 1.04

* 3.26 3.16 5.6 1.02

*: Replicate analysis from a different powder aliquot.a Calculated from modal abundances of magmatic sulfides (Table 2),

lites analysed so far (Pattou et al., 1996; Schmidt et

al., 2000; Lorand et al., 1998; Pearson and Woodland,

2000). They display slightly enhanced light PGE

abundances (Ru, Rh, Pd) relative to heavy PGE (Os,

Ir and Pt) when compared to CI-chondrites (Fig. 2).

Ru/Ir, Rh/Ir and Pd/Ir ratios are higher by 13%, 9–

10% and 24–62% compared to CI-chondrites whereas

OsN/IrN and PtN/IrN are broadly chondritic (0.98–

1.01; 1.02–1.06, respectively; N: CI-chondrites nor-

malized). However, sample LI X differs from LI 129

by a coupled Pd and Au enrichment relative to the

other PGE.

The IL cpx-poor lherzolites display CI-normalized

patterns with broadly flat Os to Pt segments as

observed for the EL lherzolites (Fig. 2). Os and Rh

positively correlate with Ir (r = 0.96 for both ele-

ments). The mean OsN/IrN ratio (0.97F 0.06) is

chondritic while the mean RhN/IrN ratio is slightly

suprachondritic (1.06F 0.04). Slight correlations be-

tween Pt and Ir and Ru vs. Ir (r = 0.73 and r = 0.78,

respectively) are also noted; the regression lines

define a chondritic PtN/IrN ratio (0.97F 0.09) but a

suprachondritic RuN/IrN ratio (1.17F 0.15). The

improvements made by Gros et al. (2002) to the

NiS fire assay and Te coprecipitation method have

been shown to considerably decrease interferences on

Ru. Besides, the good agreement between Ru/Ir ratios

obtained by ID-ICP-MS and NiS fire assay and Te

coprecipitation for a Pyrenean harzburgite (1.05–1.07

Pt

(ppb)

Pd

(ppb)

Au

(ppb)

Smeasured

(ppm)

Scalculated(ppm)a

2 0.102F 0.034 0.086F 0.020 0.57F 0.11

7.1 6.03 1.9 256 150F 63

8.2 5.55 1 1.95 186

6.4 3.90 0.4 821 134

5.4 4.33 1.2

8.1 4.97 0.8 1163 20F 20

7.0 4.42 1.5 800 70

5.2 4.02 0.9 716 90

7.3 11.82 2.4 1363 220F 100

4.8 8.40 1.9

8.0 6.08 < 0.3 1844 167F 16

6.3 5.81 1.0

assuming 34 wt.% S in 100 wt.% sulfides.

Fig. 2. CI-chondrites normalized PGE patterns in Liguride

lherzolites. Normalizing values after Anders and Grevesse (1989).

Dashed-line patterns denote duplicate analysis (D).

A. Luguet et al. / Chemical Geology 208 (2004) 175–194 183

against 1.09; Ambre Luguet, unpublished data) seem

to rule out any analytical bias for Ru. Concentrations

of Au and Pd are more variable, as are the Pd/Ir and

Au/Ir ratios. LI 87 shows a strong enrichment in both

Pd and Au relative to the IPGE (PdN/IrN = 2.8F 0.1;

AuN/IrN = 2.66, Fig. 3). LI 138 is moderately enriched

in Pd (PdN/IrN = 1.6F 0.1) but does not show any Au

enrichment. It is worth noting that both samples are

rich in magmatic sulfides. In contrast, the other IL

cpx-poor lherzolites, poorer in magmatic sulfides, are

depleted in Pd relative to these previous two IL

samples and show Au concentrations varying irre-

spectively with Pd. However, none has subchondritric

PdN/IrN (1.02–1.28). No correlation between Pd/Ir

and fertility index is observed.

4.3. In-situ LA-ICP-MS analyses of sulfides

4.3.1. Magmatic sulfides

Sulfide grains large enough to be analysed by LA-

ICP-MS are the pure pentlandite grains and complex

intergrowths between pentlandite, troilite, native cop-

per, valleriite and magnetite, mostly observed in the

EL lherzolites. The contribution of native copper or

Cu sulfides could be monitored by the Cu/Ni ratio of

the analyses, while the magnetite contribution in the

Cu-poor grains can be detected from low S contents in

the analyses.

In all samples pentlandite concentrates Os, Ir and

Ru (IPGE) in roughly chondritic proportions, at con-

centration levels between 600 and 5000� primitive

mantle (PM) (Fig. 4). Osmium and ruthenium posi-

tively correlate with Ir (r = 0.85 and 0.98 over 33

analyses). Although some in-situ analyses display

subchondritic Os/Ir and Ru/Ir ratios, the average Os/

Ir and Ru/Ir ratios of pentlandite are similar within

error with the bulk-rock Os/Ir and Ru/Ir ratios. Con-

versely, the native copper-rich grains are IPGE-de-

pleted; their bulk-rock normalized PGE pattern

display a positive segment from Os (20 x PM) to Ir

(20–200� PM) and Ru (100–200� PM). Magnetite,

abundant in some LI 138 grains free of native copper,

also has a diluting effect on Os and Ir concentrations

(down to 200� PM) but concentrates Ru, which

generates positive anomalies in the patterns of mag-

netite–pentlandite intergrowths (dashed-line pattern

in Fig. 4C).

Rhodium, platinum and palladium (the PPGE)

concentrations in pentlandite span a much larger

range compared to the IPGE, from two orders of

magnitude for Rh (400–8000� PM) to three orders

of magnitude for Pt and Pd (4–3000 and 10–

10,000� PM, respectively). Only two grains of the

EL lherzolites of the 33 pentlandite analysed display

nearly flat normalized patterns with slight positive

Pd anomalies reproducing the shapes of the CI-

normalized bulk-rock patterns. All the other pent-

landite patterns display more or less pronounced

Fig. 3. Variations of Os, Ru, Rh, Pt, Pd and Au vs. Ir. Solid square: External Liguride lherzolites, open circle: Internal Liguride cpx-poor

lherzolites. Average values, instead of duplicates, have been reported for samples LI 138, LI 87 and LI 48. Dashed line delineates the

compositional range for chondrites (carbonaceous, ordinary and enstatite; Wasson and Kallemeyn, 1988; Jochum, 1996; McDonald et al., 2001;

Horan et al., 2003). Plain line corresponds to the CI-chondrites ratio after Anders and Grevesse (1989).

A. Luguet et al. / Chemical Geology 208 (2004) 175–194184

negative Pt anomalies (Fig. 4). Taken as a whole, Pd

and Pt in pentlandite from IL cpx-poor lherzolites

positively correlate; Pd/Pt ratios are quite constant

(5F 3) apart from two analyses (the richest in Cu)

that display higher Pd/Pt ratios. Pentlandite in EL

lherzolites shows much more variable Pd/Pt ratios

(1–500). In the Pt-richest pentlandites of EL lher-

zolites, laser ablation profiles revealed during abla-

tion of Cu-rich area, Pt concentration peaks while

the other PGE are more regularly distributed

throughout the grain. According to Alard et al.

(2000) and Luguet et al. (2001), such profiles

indicate the occurrence of Pt as microphases while

the other PGE are likely sited in the pentlandite

Fig. 4. Primitive mantle-normalized PGE abundances in magmatic (A, B, C) and hydrothermal sulfides (D) obtained by LA-ICP-MS.

Normalization values after McDonough and Sun (1995). Solid circle: Type I pentlandite, open square: Type II pentlandite, triangle: chalcopyrite.

Arrows indicate value below detection limits. C: Grey symbol stand for LI 52 magmatic pentlandites and the dashed line pattern for the LI 138

magnetite–pentlandite intergrowths.

A. Luguet et al. / Chemical Geology 208 (2004) 175–194 185

lattice. Several native copper-rich analyses yield Pt

contents below detection limits ( < 40 ppb).

The PM-normalized Rh/Ir and Pd/Ir ratios allow

two different pentlandite patterns to be distinguished,

in close agreement with the patterns previously de-

fined in magmatic sulfides from abyssal peridotites.

One (Type I in Luguet et al., 2001) shows RhPM/

IrPM < 1 and PdPM/IrPM ratios < 1 and the other (Type II

in Luguet et al., 2001) RhPM/IrPM and PdPM/IrPMz 1,

respectively. Type I pentlandites are systematically

depleted in Cu relative to Ni (Cu/Ni < 0.06) whereas

Type II pentlandites show highly variable Cu/Ni

(0.001–0.3), LI 138 Type II pentlandites being gener-

ally Cu-poor (Cu/Ni < 0.007). The ‘‘purest’’ native

copper grain analysed has high Rh and Pd contents

sometimes coupled with high Ir content. Drilling

through this mineral likely explains the lack of any

correlation between Pd/Ir and Cu/Ni (nor between Rh/

Ir and Cu/Ni). Statistically, Type II pentlandite patterns

are more common in LI X than in LI 129 and in LI 138

than in LI 52, respectively. Moreover, the PM-normal-

ized Pd concentrations are a factor of 10 lower in LI

129 Type I and Type II pentlandites (30–300 and

1000–4000) compared to the same types in LI X and

LI 138 (>300 and up to 10,000, respectively). These

intrinsic variations reflect quite well the higher Pd

concentrations in these latter two samples.

Gold contents of pentlandite are generally very low

( < 0.1 ppm), often below detection limits. They corre-

late neither with Cu (or the chalcophile elements Se, Te,

As) nor with any other precious metals including Ag.

Native copper shows gold contents close to detection

limits and the highest Ag contents (Table 5). In the EL

Lherzolites, Ag positively correlates with Cu (r = 0.87

for LI 129 and r = 0.64 for LI X).

Apart from the Cu-rich grains, the Se concentra-

tions (110F 10 ppm) and the S/Se ratios (c 3000) are

typical of mantle sulfides (Guo et al., 1999; Lorand

and Alard, 2001) (Fig. 5). Arsenic contents range

between 0.3 and 1 ppm. LI 138 magmatic sulfides

Table 5

Representative in-situ analyses of sulfides

Point Assemblage S

(%)

Cu

(%)

Ni

(%)

Os

(ppm)

Ir

(ppm)

Ru

(ppm)

Rh

(ppm)

Pt

(ppm)

Pd

(ppm)

Au

(ppm)

Ag

(ppm)

As

(ppm)

Se

(ppm)

Te

(ppm)

Bi

(ppm)

Magmatic sulfides

External Ligurides

LI 129

DLI129-9 Pn 32.7 0.62 13.46 7.1 8.10 11.5 0.93 13.91 1.27 < 0.04 1.66 0.62 126.3 22.66 1.27

1.0 0.03 0.41 1.4 0.60 0.63 0.05 0.06 0.50 0.01 0.08 8.6 0.83 0.04

HLI129-3 Cp + Pn + 31.7 2.21 17.58 < 0.05 < 0.04 0.51 0.68 < 0.08 9.27 < 0.04 8.18 0.86 61.4 1.77 0.06

Mw 1.0 0.12 0.54 0.09 0.06 0.04 0.01 0.15 5.3 0.18 0.01

ILI129-2A Pn+Mt 32.0 1.18 17.42 6.2 4.8 8.33 0.65 < 0.05 24.65 < 0.04 5.74 < 0.25 104.5 2.19 0.09

1.0 0.07 0.54 1.3 0.4 0.54 0.04 0.02 0.01 8.2 0.12 0.01

ILI129-15 Pn +Mw 27.2 1.79 14.5 6.0 5.6 6.66 1.73 31.1 11.3 1.59 10.8 < 0.6 96.5 93.2 2.09

2.2 0.22 2.0 1.2 1.0 1.16 0.32 5.6 1.6 0.27 1.2 16.9 10.7 0.18

JLI129-10 Pn +Mt + 30.9 0.52 17.8 4.10 3.18 4.76 0.31 0.41 < 0.12 0.47 2.69 0.33 113.5 1.31 0.09

Cu 2.5 0.07 2.5 0.82 0.62 0.88 0.07 0.16 0.11 0.34 0.16 20.1 0.37 0.03

LI X

ELIXA-13 Pn +Mt 32.0 2.35 16.4 9.1 8.6 17.8 2.00 0.22 5.01 0.24 6.44 < 0.3 157.1 4.73 1.26

2.4 0.25 1.9 1.4 1.3 2.5 0.31 0.14 0.65 0.07 0.64 22.7 0.65 0.12

FLIXA-10 Pn +Mt 28.1 0.35 17.3 2.10 3.21 3.57 1.11 0.18 15.0 < 0.1 1.28 < 0.5 89.2 4.68 0.51

2.1 0.04 2.1 0.37 0.52 0.57 0.18 0.12 1.8 0.15 13.7 0.64 0.06

GLIXA-7 Cu+ Pn 29.5 8.07 19.7 0.30 0.90 0.83 4.58 < 0.11 1.51 < 0.1 7.83 < 0.3 87.2 9.0 0.20

2.3 0.92 2.5 0.08 0.18 0.20 0.74 0.25 0.82 13.8 1.1 0.04

Internal Ligurides

LI 52

DLI52-B Pn 28.7 0.10 24.6 7.2 7.7 21.5 2.09 < 0.2 1.58 0.09 0.18 0.88 178.3 32.0 0.15

1.4 0.09 2.7 1.4 1.0 2.6 0.31 0.26 0.06 0.05 0.22 9.9 2.2 0.05

LI 138

FLI138A Pn+Mt 27.0 0.03 22.1 4.61 4.89 8.6 2.23 4.34 20.9 < 0.1 0.83 2.63 30.0 5.45 0.20

1.4 0.01 2.7 0.84 0.72 1.2 0.35 0.77 2.0 0.11 0.34 5.1 0.65 0.05

ILI138C Pn +Mt 40.8 0.26 36.0 3.30 3.12 7.46 0.81 0.60 0.86 < 0.04 2.11 2.00 39.6 6.47 0.24

2.3 0.01 1.3 0.30 0.21 0.55 0.08 0.10 0.11 0.15 0.23 3.7 0.47 0.03

JLI138C Pn +Mt 27.4 0.03 25.4 14.1 13.20 23.0 6.59 10.84 33.5 < 0.04 0.95 3.13 23.9 5.20 0.27

1.6 0.01 1.0 1.2 0.77 1.4 0.42 0.63 2.4 0.08 0.14 2.5 0.32 0.03

Hydrothermal sulfides

ELI138B Po 33.4 < 0.30 0.72 < 0.02 < 0.04 < 0.1 < 0.03 < 0.1 < 0.07 < 0.04 0.9 < 0.2 12.2 < 0.2 0.03

1.9 0.01 0.02 0.05 1.6 0.01

FLI138B Po 33.4 0.30 1.27 < 0.02 < 0.02 0.20 0.03 0.37 0.29 0.09 3.25 0.12 9.09 0.26 0.31

1.9 0.01 0.04 0.04 0.01 0.05 0.03 0.01 0.13 0.03 0.73 0.05 0.02

GLI138B Po 33.4 0.30 0.75 < 0.02 < 0.02 0.06 0.02 < 0.04 0.36 0.05 2.95 < 0.04 6.42 0.24 0.36

1.9 0.01 0.03 0.03 0.01 0.04 0.01 0.13 0.71 0.06 0.03

Pn: pentlandite, Po: Pyrrhotite, Mw: mackinawite, Mt: magnetite, Cp: chalcopyrite, Cu: native copper.

F 1r in italic.

A. Luguet et al. / Chemical Geology 208 (2004) 175–194186

show higher As contents (0.26–3.1 ppm) balanced by

much lower Se contents (27.4 ppm on average). These

features and their S/Se ratios>10,000 are likely hy-

drothermal fingerprints, as suggested by the analyses

of hydrothermal sulfides (see below and Fig. 5).

Tellurium concentrations of pentlandites are generally

low in IL cpx-poor lherzolites ( < 6.5 ppm, except

DAL52B :32 ppm Te) and more variable in EL

lherzolites (0.7–93.2 ppm). Highest Te concentrations

in EL lherzolites correspond to pentlandite with the

highest Pt contents. In fact, the ablation profiles

through such pentlandite reveals the existence of Te

concentration peaks stacked with Pt and Bi concen-

tration peaks. In LI 129, two analyses of a single

pentlandite grain gave highly variable Pt and Te

contents (0.3–10.7 and 5.6–50.5 ppm, respectively).

Table 6

Mass balance calculation of the contribution of magmatic sulfides to

the bulk-rock PGE budget for LI 129 (concentration in ppm)

Os Ir Ru Rh Pt Pd Au

Calculated

sulfidesa6.84 6.54 11.53 2.16 14.54 9.84 1.77

Measured 5.98 6.16 8.12 1.68b 3.85 7.83b 0.32

Sulfides (2.5) (3.4) (3.31) (1.4) (6.1) (7.0) (0.54)

– – – 3.07c – 12.56c

(1.1) (5.6)

Although arbitrary, this latter procedure emphasizes the large degree

of uncertainty resulting from the impossibility of determining

accurately the respective proportions of the two types of pentlandite

patterns in Ligurides peridotites. Propagated errors (one standard

deviation) are given between brackets.a Assuming 0.056 wt.% magmatic sulfides in the rock.b Calculated by averaging all the pentlandite analyses.c Calculated assuming 50% Type I pentlandite + 50% Type II

pentlandite.

Fig. 5. Variation of S as a function of Se in magmatic and

hydrothermal sulfides of Liguride lherzolites. Open square: External

Ligurides magmatic sulfides, open circle: LI 52 magmatic sulfides,

open triangle: magmatic sulfides of LI 138 lherzolite, solid triangle:

hydrothermal sulfides of LI 138 lherzolite.

A. Luguet et al. / Chemical Geology 208 (2004) 175–194 187

Taken as a whole, these observations suggest that Te

is bonded to Pt as unevenly distributed discrete Pt–Te

microphases. The Te-rich grains generally occur in

assemblages containing a Cu-rich phase (sulfides or

native copper); these grains are also enriched in Bi.

4.3.2. Hydrothermal sulfides

The three analyses performed on the coarse hydro-

thermal sulfide grains associated to a vein in sample LI

138 indicate very low PGE and Au contents (2–

40� PM for the IPGE, Rh and Pt; 20–90� PM for

Pd and 100–200� PM for Au) compared to magmatic

sulfides. These patterns are different from the mag-

matic ones but also from each other. The three hydro-

thermal sulfides show Os and Ir abundances lower

than the other PGE and Au. FLI138B pattern displays

a positive slope from Ru to Au with a slight negative

Rh anomaly while GLI138B displays a general posi-

tive slope from Ru to Pd but showing a strong negative

Pt anomaly. Hydrothermal sulfides also have higher

than chondritic S/Se ratios (7.1–11.1�103), due to the

sharp decrease of Se contents ( < 12 ppm). Those

sulfides are also characterized by low As, Te and Bi

contents ( < 0.4 ppm).

5. Discussion

5.1. PGE and Au carriers in liguride ophiolitic

lherzolites

LA-ICP-MS analyses show that pentlandite strong-

ly concentrates Os, Ir, Ru, Rh and Pd while rejecting

Au and to a lesser extent Pt, in good agreement with

the experimental data of Makovicky et al. (1986).

These results conform with previous in-situ LA-ICP-

MS analyses of pentlandite in mantle rocks, whether

orogenic lherzolites (Alard et al., 2000), basalt-borne

xenoliths (Lorand and Alard, 2001) and abyssal

peridotites (Luguet et al., 2001). Native copper may

also be a significant host for Pd and Rh while

rejecting the other PGE as well as Au.

Mass balance calculation of the contribution of

magmatic sulfides to the bulk-rock PGE budget has

been done for samples LI 129 and LI X, the least

serpentinized samples for which the amount of mag-

matic sulfides can be determined with the best level of

confidence from modal estimates and bulk-rock S

contents. The calculated sulfide, able to reproduce

the bulk-rock PGE concentrations assuming all the

PGE residing in pentlandite, agrees well with in-situ

analyses of pentlandite for Os and Ir (Table 6). The Ru

content of the calculated sulfide is lower by 20%,

although both the calculated and the measured Ru

concentrations overlap at the 1r level. This deficit

A. Luguet et al. / Chemical Geology 208 (2004) 175–194188

may be explained by the crystal-chemical compatibil-

ity of Ru in secondary magnetite (Capobianco et al.,

1994) which is abundant in LI X and LI 129, exsolved

Ru-rich phases being highly unlikely in base-metal-

rich lherzolites (cf. Brenan and Andrews, 2001). For

Rh and Pd, the interpretations are complicated by the

highly variable Rh and Pd contents of the two types of

pentlandite (defined above). Type II pentlandites,

which are Rh- and Pd-rich, are required to balance

the measured bulk-rock Rh and Pd contents. Native

copper which concentrates Pd and Rh also likely

contributes to the bulk-rock Rh and Pd budget,

although its exact contribution cannot be quantified

because of the lack of accurate modal abundance data

for this phase (and for troilite and valleriite too). On

the other hand, neither pentlandite nor native copper

can balance the bulk-rock Pt-budget. The same is also

true for Au.

Recent mass balance calculations performed on

mantle rocks containing abundant pentlandite reached

the same conclusion about Pt (Alard et al., 2000;

Luguet et al., 2001). Both authors reported that laser

ablation profiles through pentlandite revealed Pt con-

centration peaks corresponding to discrete Pt-rich

refractory microphases. Similar Pt peaks associated

with Te and sometimes Bi concentration peaks have

also been observed in some Pt-rich pentlandites (LI

129-15) of the EL lherzolites. This provides evidence

that this missing platinum is in fact Pt–Te inclusions

containing subordinate amounts of Bi (10 < Te/

Bi < 50). From the size of the laser beam ( < 70 Am)

and the final concentrations of Te and Pt in the LA-

ICP-MS analyses, it is expected that the size of these

inclusions is about or less than 1 Am which explains

the difficulty of their identification by SEM techni-

ques. Pt–TeFBi compounds such as moncheite are

common Pt-rich minerals in serpentinized mantle

rocks whether ophiolites (e.g. Ohnenstetter, 1992)

orogenic lherzolites (Jedwab, 1992) and oceanic ul-

tramafic cumulates (Prichard et al., 1996). In all cases,

Pt–TeFBi compounds were interpreted as exsolved

during hydrothermal alteration of primary base metal

sulfides. However, the fact that Pt–Te inclusions are

suspected only in complex pentlandite–native copper

intergrowths from the EL lherzolites suggests that

their formation is closely related to the crystallization

history of pentlandite–chalcopyrite-rich assemblages.

Such assemblages belong to the S-poor part of the

Cu–Fe–Ni–S system. Their crystallization history at

magmatic temperatures (from a sulfide melt) involves

the separation of monosulfide solid solution and a

Cu–(Ni)-rich sulfide melt. On cooling below ca.

860–900 jC, the latter precipitates various propor-

tions of an intermediate solid solution (now referred

as isocubanite) and a high-temperature heazlewoodite

phase (Ni3F� S2) (Craig and Kullerud, 1969; Ball-

haus et al., 2001). Chalcopyrite, crystallizing from the

isocubanite below 557 jC, is commonly reported as

Pt-poor ( < 1 ppm; Oberthur et al., 1997; Ballhaus and

Sylvester, 2000; Luguet et al., 2001) whereas isocu-

banite may dissolve up to 4.9 ppm Pt (Lorand and

Alard, 2001). It is well known that Pt–Pd–Au–Cu–

Te and Bi concentrate in Cu-rich derivative sulfide

liquids residual after Mss crystallization (Fleet et al.,

1993; Theriault and Barnes, 1998; Barnes et al., 2001

and reference therein). Theoretical upper stability

limit of Pt–Te compounds is >875 jC (Makovicky,

2002 and references therein) so that this mineral could

have crystallized directly from the Cu-rich derivative

sulfide melt. Soft ligands (large ion and low valence)

like Te are thought to stabilize Pt atomic clusters in

natural sulfide melts (e.g. Tredoux et al., 1995), thus

enhancing the nucleation of Pt–Te micro-precipitates

at magmatic temperature. However, any attempt to

recompute the Pt budget (with an acceptable level of

confidence) is prevented by the lack of precise infor-

mations on the chemical composition and modal

abundances of these Pt–Te–Bi microphases. On the

other hand, the positive correlation between Pd and Pt

in LA-ICP-MS analyses of pentlandite from IL cpx-

poor lherzolites demonstrates that Pt is partly residing

in pentlandite.

The purest native copper analysed (HLI129-3;

GLIXA-7) displays very low gold contents. More

generally, the poor correlation between Au and Cu

excludes Au–Cu alloys (auricupride), as reported in

Corsican ophiolitic peridotites (e.g. Ohnenstetter,

1992). The most likely explanation of the absence

of gold in magmatic sulfides highlighted by our mass

balance calculation (Table 6) is the occurrence of

native gold, not associated with base metal sulfides.

The detection of these gold particles by SEM still

remains unsuccessful. Isocubanite contains 8.7 ppm

Au (Lorand and Alard, 2001; Theriault and Barnes,

1998); however, the few chalcopyrite analysed by

accelerator mass spectrometry (AMS) or LA-ICP-

A. Luguet et al. / Chemical Geology 208 (2004) 175–194 189

MS were unsuccessful in detecting a significant

amount of gold (Wilson et al., 1995; Luguet et al.,

2001). Thus gold was likely exsolved during subso-

lidus decomposition of high-temperature Cu sulfides

into low-temperature chalcopyrite. The positive Ag

vs. Cu correlation points to a location of silver into

native copper which is likely inherited from the

chalcopyrite precursor as Ag commonly occurs as a

lattice substitution for Cu in chalcopyrite (Harris et al.,

1984).

5.2. Petrogenetic models

Snow et al. (2000) PGE data of Internal and

External Liguride lherzolites show an enriched seg-

Fig. 6. Comparison of CI-normalized platinum-group element ratios in

determined by Snow et al. (2000) (solid symbols). Square: Mean compos

the External Liguride samples. CI-normalization values after Palme an

compositional range of chondrites without distinction between carbona

compositional ranges of the different chondrites group were distinguished

LL: ordinary chondrites, EH, EL: enstatite chondrites. Compositional rang

Jochum (1996) and Horan et al. (2003). PGE in the other classes of cho

(2001) and Horan et al. (2003).

ment from Ru to Pd, characterized by superchondritic

Ru/Ir, Rh/Ir, Pt/Ir and Pd/Ir ratios, but low Os concen-

trations with subchondritic Os/Ir ratios, with respect to

chondritic meteorites (Fig. 6). These authors, in the

continuity of previous results obtained on abyssal

peridotites (Snow and Schmidt, 1998), proposed that

these selective enrichments were due to outer core

material addition. However, according to Meisel et al.

(2003) data, Snow et al. (2000) PGE data may be

problematic. In fact, PGE analyses of reference mate-

rial UB-N of Snow and Schmidt (1998) are systemat-

ically lower than Meisel et al. (2003) ones except for Pt

which are higher and more variable (deficit of c 50%

for Os, Ir, Ru and 20–25% for Rh and Pd). This makes

the hypothetical metallic core contribution in mantle

Liguride peridotites between this study (open symbols) and those

ition of the Internal Liguride samples, circle: mean composition of

d Beer (1993). Gray field in Os to Pt columns delineates the

ceous, ordinary and enstatite chondrites. In the Pd column, the

using double-sided arrows. CI: CI-carbonaceous chondrites, H, L,

e for CI after Wasson and Kallemeyn (1988), Anders and Grevesse,

ndrites are from Wasson and Kallemeyn (1988), McDonald et al.

Fig. 7. Models of PtN/IrN and PdN/IrN in the Liguride lherzolites

based on mass balance equations for fractional melting of the sulfide

phase, assuming all the PGE reside in the base metal sulfides and

using sulfide liquid–silicate liquid Nernst partition coefficients (D)

(e.g. Lorand et al., 1993, 1999; Handler and Bennett, 1999). N: CI-

chondrites normalized Anders and Grevesse (1989). Symbols as in

Fig. 4. When available, duplicate values have been plotted instead

of average values. Two mantle sources were chosen for modeling

Pd/Ir, one CI-chondritic (A) and the other similar to the Enstatite

chondrites (B) (Horan et al., 2003). A constant sulfide melt– silicate

melt partition coefficient was selected for Ir (26,000) from the Fleet

et al. (1999) database. For PtN/IrN ratio models, plain, dashed and

dotted model lines were built with DPt = 30000, 10000 and 7000,

respectively. For the Pd/Ir ratio models, dashed and dotted model

lines were built with DPd = 10000 and 5000, respectively. Partial

melting degree ( F) of Liguride lherzolites was estimated from bulk-

rock major element composition (Handler and Bennett, 1999). For

further explanations, see Luguet et al. (2003).

A. Luguet et al. / Chemical Geology 208 (2004) 175–194190

protoliths of Ligurides lherzolites highly doubtful as

this Snow et al. (2000) interpretation was based on

erroneous bulk-rock PGE data.

No significant difference is observed between our

bulk-rock PGE concentrations of External and Internal

Liguride lherzolites. The data presented here as well

as all the previous PGE analyses of oceanic peridotites

(using ID-ICP-MS or fire-assay ICP-MS procedures)

yields CI-normalized PGE/Ir ratios (except for Ru) in

the uncertainty range of chondritic meteorites (Fig. 6).

Our two EL lherzolites fit well the Morgan et al.

(2001) ‘‘high-Pd’’ primitive mantle reservoir estimate

both for the bulk-rock PGE and S concentrations. This

agreement supports theoretical models describing the

behaviour of PGE during mantle melting processes

(cf. Lorand et al., 1999; Handler and Bennett, 1999;

Rehkamper et al., 1999; Luguet et al., 2003). For low

values of F such as those recorded by LI X and LI 129

(1–4%), and assuming similarly high sulfide melt/

silicate melt partition coefficients (Fleet et al., 1999),

bulk-rock Pd, Pt and Ir should not fractionate from

each other (Fig. 7). However, in-situ analyses point

out a Rh and PPGE fractionation at the hand-sample

scale, while LI X differs from LI 129 by higher bulk-

rock Pd and Au contents, coupled with a greater

amount of Pd-rich pentlandite and Cu-rich opaque

minerals. Phase diagrams in the Cu–Fe–Ni–S system

(cf. Craig and Kullerud, 1969) and observations in

basalt-borne xenoliths (cf. Lorand and Conquere,

1983) indicate that mantle sulfide starts melting well

before anhydrous silicates of a four-phase lherzolite.

The first sulfide liquid below ca. 1000 jC is a Cu–Ni-

rich sulfide liquid (cf. Ballhaus et al., 2001 and

reference therein) that strongly concentrates Pt, Pd,

Au and the most incompatible chalcophile elements

such as Te–Sb–Bi (Theriault and Barnes, 1998; Alard

et al., 2000; Lorand and Alard, 2001), leaving mono-

sulfide solid solution (Mss), the most refractory sul-

fide phase in the Cu–Fe–Ni–S system, depleted in

these elements (Li et al., 1996; Ballhaus et al., 2001).

In line with the Luguet et al. (2001) interpretation of

the two pentlandite patterns of abyssal peridotites, the

Pd-depleted Type I pattern corresponding to Cu-poor

pentlandite grains is likely a subsolidus decomposi-

tion product of the refractory Mss. Their occurrence in

the vicinity of or inside corrosion gulfs in opx are

common features of residual sulfides from low to

moderate degrees of melting (Lorand, 1988, 1989;

A. Luguet et al. / Chemical Geology 208 (2004) 175–194 191

Luguet et al., 2001). By contrast, the Type II Pd-

enriched pattern mostly identified in pentlandite with

variable Cu contents and exsolved Pt–Te–Bi discrete

phases are undoubtedly entrapped Cu–Ni-rich partial

melt. Unfortunately, the extensive reduction of mag-

matic sulfide assemblages by retrogressive serpentini-

zation makes the subsolidus history of this partial melt

impossible to describe in detail. The higher bulk-rock

Pd and Au contents of LI X is evidence of a greater

contribution of the entrapped Cu–Ni-rich sulfide melt.

As LI X is more fertile than LI 129, one may speculate

that this liquid, which is characterized by better

wetting capacity than Fe sulfides (Ballhaus et al.,

2001) is rapidly exhausted from melting residues for

F values < 5%.

Melting model curves of Fig. 7 dictate that the

IL cpx-poor lherzolites which have experienced a

higher degree of partial melting (6–11% as estimat-

ed from bulk-rock Al2O3 contents) during the

opening of the Liguro–Piemontese oceanic basin

should display either similar or slightly lower bulk-

rock PtN/IrN and PdN/IrN ratios compared to the EL

lherzolites. Likewise, as S behaves incompatibly

during partial melting, the S contents should de-

crease sharply if these rocks had seen only a partial

melting process (cf. Lorand, 1991). Despite the high

variability of PtN/IrN in individual samples obvious-

ly resulting from the strong affinity of Pt for

discrete microphases, bulk-rock PtN/IrN roughly fol-

lows the model melting curves. For PdN/IrN, both

LI 52 and LI 48 fit the model curves rather well

while others such as LI 138, LI 87 have much too

high PdN/IrN (Fig. 7). Since the bulk-rock S con-

centrations have been significantly enhanced by

ocean floor hydrothermal activity (see Section

4.2), only magmatic sulfide modal abundances

may provide reliable informations on the behaviour

of S during the adiabatic upwelling of IL cpx-poor

lherzolites. The S concentrations thus recalculated

(20–350 ppm) are too dispersed over the restricted

range of degree of partial melting of the IL cpx-

poor lherzolites, to be interpreted in terms of partial

melting models, if uncertainties in the S contents of

the asthenospheric mantle and of the partial melts

are taken into account. It must be recalled at this

stage of the discussion that, in addition to pentlan-

dites adjacent to the orthopyroxene, the most S-rich

samples (LI 48, LI 87, LI 138) display a class of

large Cu-rich sulfides intergrown with corroded

intergranular cpx. Pd-enriched, Type II pentlandite

has been identified in that latter sulfide population.

All these features support the idea of a metasomatic

sulfide enrichment. Similar arguments were used to

support a model of continuous refertilisation of

abyssal peridotites from the Kane Fracture Zone

(mark area, Mid-Atlantic Ridge) by sulfides

exsolved from incompletely extracted partial melts

that precipitated clinopyroxene + spinel interstitial

aggregates (Luguet et al., 2003; see also Rehkamper

et al., 1999). As discussed above, Cu–Ni-rich

sulfide melts with the highest Pd/Ir are expected

to segregate at the very beginning of the partial

melting process, when residual Mss surviving in the

solid residue buffers the IPGE and Rh contents.

One may infer that IL cpx-poor lherzolites were

infiltrated by low-degree ( < 5%) partial melts as

concluded by Rampone et al. (1997) from incom-

patible trace element characteristics of reacted cli-

nopyroxene rims. The latter authors postulated

instantaneous melt fractions infiltrated from deeper

mantle levels trapped in a lithospheric thermal

regime. Sulfide liquid immiscibility may have been

triggered by (1) the incompatible behaviour of S in

the trapped melt pockets crystallizing silicates, (2)

the temperature decrease at the asthenosphere–lith-

osphere boundary, or both.

6. Conclusion

The External Liguride lherzolites have all character-

istics of the ‘‘high-Pd’’ mantle reservoir defined in the

subcontinental lithospheric mantle by orogenic lherzo-

lites: (1) 200–250 ppm S and (2) a slight enrichment in

the light PGE, especially Pd relative to Os and Ir and

compared to CI-chondritic materials (PdN/IrN = 1.5–

1.9). Petrographic data, coupled with bulk-rock and

LA-ICP-MS analyses of PGE, indicate that a low-

melting Cu–Ni-rich sulfide melt concentrating the

most incompatible PGE (Pt, Pd, Au) and the highly

incompatible chalcophile elements (Se, Te, Bi) can

segregate during degrees of melting as low as 2–4%,

This is attested by local concentrations of interstitial

Ni–Cu-rich sulfides. Retrogressive serpentinization

effects were too strong to allow a detailed discussion

of the primary mineralogy of these minerals.

A. Luguet et al. / Chemical Geology 208 (2004) 175–194192

The Internal Liguride cpx-poor lherzolites show

indisputable similarities with abyssal peridotites from

slow-spreading ridges. Hydrothermal sulfides account

for most of the bulk-rock S budget. The bulk-rock

PGE budget and the distribution of magmatic sulfides

bear strong evidence for a contribution from melt

infiltration from deeper zones of the mantle and

trapped when the peridotites were in a lithospheric

regime. This contribution is observed as high Pd/Ir

and S contents unrelated to fertility index, and, in thin

section, as Pd-enriched Cu–Ni-rich interstitial sul-

fides preferentially associated with large corroded,

interstitial Cpx. Our data suggest that these processes

previously documented in abyssal peridotites from

slow-spreading mid-oceanic ridges characterize resi-

dues from low to moderate melting degrees (5–10%)

of the oceanic mantle as a whole, whether from

mature ocean or from short-lived oceanic basins.

Hence, care must be exercised in using Pd for spec-

ulating on the PGE systematics of the asthenospheric

mantle.

Acknowledgements

AL thanks S.Y. O’Reilly and the staff of the

GEMOC for their help in this project and their

availability during her stay at the Macquarie Univer-

sity. Financial support was provided by a CNRS-INSU

grant (Interieur de la Terre) to JPL (98-IT-13, 01-IT-

08), the Australian Research Council, GEMOC and a

Macquarie Post-Graduate Research Fund for sulfide

laser ablation developement to OA. Michel Gros is

also gratefully acknowledged for his help during PGE

analyses. Finally, the authors thank Harry Becker and

Thomas Meisel for their thorough reviews, Maryse

Ohnenstetter for editorial comments and Mary F.

Horan for the final check of the English. This is

publication no. 340 of the GEMOC Arc National KEY

Center (www.es.mq.edu.au/GEMOC/). [RR]

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