formation of podiform chromitite deposits-implications from pge

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Formation of podiform chromitite deposits: implications from PGE

abundances and Os isotopic compositions of chromites from the

Troodos complex, Cyprus

Anette Buchl a,b,*, Gerhard Brugmanna,c, Valentina G. Batanova a,d

a Max-Planck Institut fu r Chemie, Postfach 3060, 55020 Mainz, Germany b Institut fu r Mineralogie, Universita t Mainz, Becherweg 21, 55128 Mainz, Germany

c Institut fu r Mineralogie, Universita t Mu nster, Corrensstr. 24, 48149 Mu nster, Germanyd Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Kosygin Street 19, Moscow 117975, Russia

Abstract

Podiform chromitite deposits occur in the mantle sequences of many ophiolites that were formed in supra-subduction zone

settings (SSZ). We have measured PGE abundances and Os isotopic compositions of three major chromitite deposits

(Kannoures, Hadji Pavlou, Kokkinorostos) and associated mantle peridotites from the Troodos Ophiolite Complex in order to

investigate the petrogenesis of these rocks, and their genetic relationships and to examine the geochemical behaviour of the

PGE.

Spinels from the chromitite deposits have flat chondrite-normalized PGE patterns, but have distinct negative Pt anomalies.Thus, Pd, Os, Ru and Ir concentrations are very high compared to the Pt concentrations (Os: 13.7–104 ng/g, Ir 11.3–19.0 ng/g,

Ru 34.3–83.6 ng/g, Pt 0.41–9.07 ng/g, Pd 11.1–76.8 ng/g). With the exception of Pd, this appears to be a general feature of

chromitites from ophiolites worldwide. However, Pd concentrations determined in this study are high compared to other studies

where whole rock samples were analysed. There is no simple explanation for this difference because mass balance constraints

would not allow that this is solely due to Pd-depletion in the interstitial component. Rather, it implies that chromitites display

large variations of relative PGE abundances, even on a local scale.

Podiform chromitite deposits form as a result of the interaction of fluid-rich, percolating melts with surrounding mantle

peridotites. Osmium, Ir, Ru and Cr concentrations decrease systematically from harzburgite to dunite surrounding the deposits.

In addition, dunites and chromites have complementary PGE distribution patterns. Thus, the mantle peridotite is the source of

these metals in chromitites. This also indicates that these elements behave incompatibly and are mobilized during continuous

melt percolation. However, the low Pt concentrations in the chromitites suggest that Pt is not as effectively mobilized during

melt percolation. Uniformly high Pt concentrations in harzburgite and dunite (ca. 11 ppb) also imply that most Pt remains in themantle peridotite. This can be explained if residual Pt-rich phases, such as PtFe alloys, limit the mobility of Pt. PGE and Cr

become concentrated when chromite and sulfide liquids precipitate as a result of the mixing of percolating melts in magma

pools near the crust– mantle boundary.

The 187Os/ 188Os ratios of the chromite separates (0.1265–0.1301) are less variable than those of the associated peridotites

(0.1235– 0.1546). The average isotopic composition of the chromites (187Os/ 188Os: 0.1284F 0.0021) is superchondritic

compared with the carbonaceous chondrite value (187Os/ 188Os: 0.1260F 0.0013 after Geochim. Cosmochim. Acta 66 (2002)

0009-2541/$ - see front matter D 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.chemgeo.2004.04.013

* Corresponding author. Current address: Department Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol

BS8 1RJ, UK.

E-mail address: [email protected] (A. Buchl).

www.elsevier.com/locate/chemgeo

Chemical Geology 208 (2004) 217–232



329; Geochim. Cosmochim. Acta 66 (2002) 4187) and similar to the average value measured for podiform chromitites

worldwide (0.12809F 0.00085 after Geochim. Cosmochim. Acta 66 (2002) 329; Geochim. Cosmochim. Acta 66 (2002) 4187).

Radiogenic melts/fluids derived from the subducting slab trigger partial melting in the overlying mantle wedge and add

significant amounts of radiogenic Os to the peridotites. Mass balance calculations suggest that a melt/rock ratio of

approximately 17:1 (melt: 187Os/ 188Os: 0.163, Os: 0.01 ng/g, mantle peridotite: 187Os/ 188Os: 0.127, Os 4.2 ng/g) is necessary in

order to increase the Os isotopic composition of the chromitite deposits to its observed average value. This value implies a

surprisingly low average melt/rock ratio during the formation of chromitite deposits. The percolating melts likely have variable

isotopic composition and PGE concentration. However, in the chromitite pods the Os from many melts is pooled and

homogenized, which is the reason why the chromitite deposits show such a small variation in their Os isotopic composition. The

results of this study suggest that the187

Os/ 188

Os ratio of chromitites is not representative for the DMM, but only reflects an

upper limit.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Podiform chromitite deposits; Os isotopes; Platinum-group elements; Ophiolite; Troodos

1. Introduction

The formation of podiform chromitite deposits has

been discussed for many decades (e.g. Lago et al.,

1982; Paktunc, 1990; Prichard and Lord, 1990; McEl-

duff and Stumpfl, 1990; Zhou et al., 1994). A general

consensus has emerged that such deposits form in the

mantle section of ophiolites from SSZ environments

during melt/rock or melt/melt interaction (Zhou and

Robinson, 1997; Ballhaus, 1998; Zhou et al., 1994,

1996, 1998; Melcher et al., 1999). These processesmobilize Cr and platinum-group elements (PGEs), and

these elements are subsequently concentrated again

during pooling of the percolating melts and fluids

(e.g. Irvine, 1977; Matveev and Ballhaus, 2002).

So far, no chromitite deposits have been observed

in abyssal peridotites. PGEs and chromium behave

compatibly during dry partial melting (e.g. Mitchell

and Keays, 1981; Dick and Bullen, 1984) and these

metals therefore show restricted mobility at mid-ocean

ridges. In contrast, mantle fluxing by hydrous fluids

and melts is a typical feature of supra-subduction zone(SSZ) environments. In this environment, mantle

peridotite can be melted to a higher degree than

beneath mid-ocean ridges, because the mantle wedge

is fluxed by fluids released from the subducting

oceanic lithosphere (e.g. Pearce et al., 1984; Roberts

and Neary, 1993). Thus, in order to understand the

formation of chromitite deposits, it is important to

know the nature of these fluids and the behaviour of

Cr and PGE during the interaction among fluids,

silicate melts and mantle peridotites.

Podiform chromitites are mainly composed of

spinel and olivine with occasional subordinate pyrox-

ene. Sulfide grains are common in most of the

chromitite deposits (e.g. McElduff and Stumpfl,

1990). Most podiform chromitites have Os, Ir and

Ru concentrations of between 0.1 and 0.01 times

chondritic (Page et al., 1982; Talkington and Watkin-

son, 1986; Leblanc, 1991) and lower chondrite-nor-

malized abundances of Pt and Pd. Very few podiform

chromitites are enriched in Pt and Pd relative to the

other PGEs. These exceptional cases include thechromitites of Greece (Konstantopoulou and Econo-

mou, 1991), the Zambales ophiolite in the Philippines

(Bacuta et al., 1990) and the Shetland ophiolite in

Scotland (Prichard et al., 1996). In these cases, the

high Pt and Pd values are believed to reflect a

contribution from magmatic sulfides (e.g. Prichard et

al., 1996).

Walker et al. (2002a,b) reviewed the Os isotopic

composition of chromitite deposits from ophiolites

worldwide and observed a well defined average187

Os/

188

Os value of 0.12809F

0.00085 (2r

). Theseauthors suggested that this value is representative of

the DMM and that the addition of 187Os from the

dehydrating oceanic crust has no significant effect on

the Os isotopic composition of the chromitites.

In this paper, we present PGE abundances, Os

isotopic compositions and Cr numbers of the chromi-

tite deposits and of surrounding mantle rocks from the

Troodos Ophiolite, Cyprus, in order to investigate the

influence of percolating fluids/melts on the behaviour

of PGEs and Cr in the Earth’s upper mantle. The study

A. Bu chl et al. / Chemical Geology 208 (2004) 217–232218



also re-evaluates the importance of the imprint of a

radiogenic signature from percolating melt on the

isotopic composition of mantle peridotites and chro-

mitites in ophiolites.

2. Samples and methods

2.1. Samples

The Troodos Ophiolite Complex represents ocean-

ic lithosphere, which formed in a supra-subduction

zone environment 90 Ma ago (e.g. Robinson and

Malpas, 1990; Mukasa and Ludden, 1987). The man-

tle sequence of the Troodos Ophiolite Complex can be

divided into two parts (Batanova and Sobolev, 2000).The eastern part (Unit 1 in Fig. 1) consists mainly of

spinel–lherzolite with subordinate cpx-bearing harz-

burgites and dunites. The western part (Unit 2 in Fig.

1) is composed of harzburgites and dunites and

contains three chromitite deposits: K annoures, Kok-

kinorotsos and Hadji Pavlou (Fig. 1). The chromitites

occur (e.g. at Kannoures) in dunite lenses in the

harzburgite and (e.g. at Kokkinorotsos) at the base

Fig. 1. Location of the chromitite deposits Hadji Pavlou, Kannoures, Kokkonoro tsos and associated mantle peridotites from the Troodos

Ophiolite Complex. Peridotite sample locations are shown with white squares circles (Buchl et al., 2002), chromitite samples by sample numbers.

A. Bu chl et al. / Chemical Geology 208 (2004) 217–232 219



of a dunite unit as shown in Fig. 1 (Greenbaum, 1977;

Prichard and Lord, 1990). Unit 2 has been overprinted

by melt percolation (Batanova and Sobolev, 2000;

Buchl et al., 2002). Major element compositions, Osisotopes and PGE abundances of spinel were deter-

mined in one chromitite sample from Kannoures, in

three samples from Kokkinorotsos and in two samples

from Hadji Pavlou (Fig. 1). In addition, one sample of

a dunite body from Unit 1 containing chromite

‘‘schlieren’’ was analysed (T14). Samples were

crushed, and fresh spinels were handpicked and

washed with deionised water in an ultrasonic bath

before analysis. The data of Buchl et al. (2002) and

Buchl et al. (2003, GCA in revision) complement the

present data set providing additional data for harzbur-

gites and dunites enclosing the chromitite deposits

(Fig. 1).

2.2. Analytical methods

Major and trace element concentrations were de-

termined on glass and powder pellets with a PhillipsPW 1404 X-ray fluorescence spectrometer at the

University of Mainz. Electron microprobe (Jeol JXA

8900 RL) analyses of spinel were made at the Uni-

versity of Mainz using the routine standard procedure.

For PGE analysis, 50 mg of handpicked and

washed spinel separates were completely dissolved

in a quartz vessel together with a mixed PGE/Re

isotope tracer (185 Re, 190 Os, 191 Ir, 101 Ru, 198 Pt,106Pd), conc. HCl and conc. HNO3 (2:3) for 16 h in

a high-pressure asher at 100 bar and 300 jC. The PGE

were separated from t he spinel matrix using the

procedure described by Brugmann et al. (1999). Os-

mium was separated from the sample solution by

Table 1

PGE abundances, Os isotopic compositions and spinel mineral data from handpicked spinels from the chromitite deposits from the Troodos

Ophiolite Complex

Sample mine T6 T4e 98-3 T10a 98-16 98-17 T11 T14

Hadji Pavlou Kokkinorotsos Kannoures U1 picked Primitive

mantlea

Whole rock 187Os/ 188Os 0.1301 0.1284 0.1270 0.1265 0.1265 0.1298 0.1294 0.1296

2 sigma 0.0002duplicate 0.1297 0.1289

Os (ng/g) 19.38 23.35 5.89 18.62 288.56 5.84 15.52 28.11 3.40

Os duplicate 13.99 12.01

Ir (ng/g) 11.49 18.99 11.41 24.31 3.20

Ir duplicate 17.92 11.11

Ru (ng/g) 34.87 83.62 34.30 19.26 5.00

Pt (ng/g) 0.41 9.07 1.41 6.04 7.10

Pt duplicate 0.03 0.08

Pd (ng/g) 11.13 28.61 76.80 48.03 3.90

Re (ng/g) 1.35 0.33 0.28

Ir (normalized) 0.03 0.04 0.02 0.05 0.007

Os (normalized) 0.04 0.04 0.02 0.06 0.007

Ru (normalized) 0.05 0.12 0.05 0.03 0.007

Pt (normalized) 0.00 0.01 0.00 0.01 0.007

Pd (normalized) 0.02 0.05 0.14 0.09 0.007

Re (normalized) 0.04 0.01 0.007

Spinel Cr# in spinel 0.69 0.64 0.76 0.75 0.75 0.76 0.75

Mg# in spinel 0.62 0.60 0.62 0.53 0.54 0.62 0.50

MgO 14.11 13.80 12.95 11.84 12.02 13.75 10.71

FeO 15.65 16.35 17.36 19.07 18.72 14.92 19.40

Cr 2O3 53.16 48.96 57.19 54.73 55.69 58.52 56.72

Al2O3 16.21 18.81 11.84 12.14 12.20 12.23 12.83

Abbreviations: U1= Unit 1; U2= Unit 2; Cr# = Cr/(Cr + Al); Mg#= Mg/(Mg+ Fe), hzb= harzburgite.

Spinel mineral data determined by electron microprobe, Os isotopes by N-TIMS and PGE abundances by isotope dilution with MC-ICPMS.a Data for the primitive mantle from McDonough and Sun (1995).




solvent extr action with bromine and purified by micro-

distillation (Birck et al., 1997). Afterwards, Ru, Pd,

Re, Ir and Pt were sequentially extracted from the

solution by using anion exchange columns, applying atechnique modified after Rehkamper and Halliday

(1997). Osmium isotope measurements were carried

out by N-TIMS with a Finnigan MAT 262 mass

spectrometer at the Max-Planck-Institut in Mainz.

Fourteen procedural blanks (determined by isotope

dilution) ranged from 67 to 558 fg for Os. The 2r

external precision (the reproducibility of the isotopic

ratios) of the 187Os/ 188Os was 0.3% based on repeated

measurements of a standard (n = 77) containing 35– 70

pg Os. Duplicate analyses of two samples were per-

formed, each starting with digestion of a separate

aliquant of sample powder. The results of these dupli-

cates agreed within 0.5% for 187Os/ 188Os and within

28% for Os concentrations. The Pt, Pd, Ir and Ru

concentrations were determined by isotope dilution

using the Micromass Isoprobe, a second-generation

multicollector ICPMS at the University of Munster.

The 2r external precision based on repeated measure-

ments of a standard solution were 0.36% for 19 8Pt / 19 4Pt, 0.24% for 10 6Pd / 10 8Pd, 0.16% for 101Ru/ 99Ru and 0.30% for 191 Ir/ 193 Ir. Procedural

blanks (spiked and determined by isotope dilution)

ranged for Ru (n = 5) from 0.12 to 0.84 ng, for Ir (n = 5)

from 0.009 to 0.034 ng, for Pt (n = 4) from 0.07 to 0.25

ng and for Pd (n = 4) from 0.26 to 0.79 ng. These blank

concentrations are at least 25 times lower for Os, Ru, Ir

and Pd and three times lower for Pt, than thoseobserved in the samples; thus blank corrections are

generally negligible. Repeated complete digestions of

a fresh aliquant of the UBN standard indicated a

reproducibility of 10% for the highly siderophile

elements (Brugmann et al., in preparation).

3. Results

The Cr numbers (Cr/(Cr + Al)) of the studied

spinels range from 0.64 to 0.76 (Table 1; Fig. 2)

and lie within the range previously measured in

chromitite deposits from ophiolites worldwide (e.g.

Zhou et al., 1996, 1998; Melcher et al., 1997). Fig. 2

shows that the Mg numbers (Mg/(Mg + Fe)) in spinel

vary significantly and are lower than those observed

in previous studies. The Mg# is dependent on the

relative proportions of spinel and olivine, because

Mg– Fe are exchanged between olivine and spinel.

A higher modal percentage of olivine leads to higher

Mg# in spinel. Hence, variation of the relative pro-

portions of olivine and chromite may explain the

variable Mg# in spinel.

Fig. 2. Cr number and Mg number in spinel of the chromitite deposits and the associated mantle peridotites. The Cr numbers of the chromites

are at the high end of the range displayed by harzburgites and dunites from the mantle sequence. Data for the mantle peridotites are from Buchl

et al. (2004, GCA in press).




Fig. 3. (A) PGE patterns of separated spinels from the chromitite deposits. The patterns are in the range of worldwide measured chromitite

deposits from ophiolites with Pt depleted relative to Os, Ir and Ru. However, Pd has an unusually high concentration compared to most other

studies. The data from Prichard and Lord (1990) and McElduff and Stumpfl (1990) are from whole rock chromitite samples. PGE concentrations

for the primitive mantle are from McDonough and Sun (1995). (B) PGE patterns of chromites from the chromitite deposits and surrounding

harzburgites and dunites (average concentrations revealed). Chromites and dunites show complementary PGE patterns, whereas the PGE

patterns of the harzburgites are mantle-like. Data for the harzburgites and dunites are from Buchl et al. (2004, GCA in press).




Osmium, Ir, Ru and Pd concentrations of chromites

from all three chromitite deposits and of the dunite

(T14) are very high compared with those of the

primitive mantle, but Pt concentrations are low (Table1; Fig. 3A). The Os concentrations vary from 5.8 to

288.5 ng/g, Ir from 11.2 to 24.3 ng/g, Ru from 19.2 to

83.2 ng/g, Pt from 0.4 to 9.0 ng/g, and Pd from 11.1 to

76.8 ng/g (Table 1). Such large variations in the PGE

concentrations are typical for chromitit e deposits (e.g.

Zhou et al., 1998; Melcher et al., 1999). High concen-

trations for Os, Ir and Ru, but low concentrations for Pt

and Pd have also been observed in whole rock samples

of chromitites from the Troodos Ophiolite by Prichard

and Lord (1990) and McElduff and Stumpfl (1990)

and from other ophiolite chromitite deposits (e.g.

Page and Talkington, 1984; Talkington and Watkin-

son, 1986; Leblanc, 1991; Crocket, 1981, 2002;

Agiorgitis and Wolf, 1978; Zhou et al., 1998).

The Pd concentrations in the chromites from this

study lie in the range of Os, Ir and Ru concen-

trations observed for deposits worldwide, but are

high if compared with results of the Troodos study

of Prichard and Lord (1990) and McElduff and

Stumpfl (1990).

The 187Os/ 188Os ratios of the chromites from three

chromitite deposits vary from 0.1265 to 0.1305, with

an average of 0.1284F 0.0021 2r (Fig. 4). This value

agrees well with the average of worldwide measured podiform chromitites (187Os/ 188Os: 0.12809F0.00085

2r, Walker et al., 2002a,b). It is somewhat higher than

estimates for the carbonaceous chondritic reservoir

(187Os/ 188Os: 0.127; Shirey and Walker, 1998;18 7Os / 18 8Os: 0.1260F 0.0013; Wa lk er e t a l. ,

2002a,b), and slightly lower than the value estimat-

ed for the primitive upper mantle (187Os/ 188Os:

0.1296F 0.0008 2r; Meisel et al., 2001), but

overlaps within uncertainty with the ratios pro-

posed for both of these reservoirs. The samples

show no correlation between major elements,

PGE concentrations and ratios and Os isotopic

compositions.

4. Discussion

In detail, it is still poorly understood how Cr and

PGE are mobilized and transported in the ophiolite

mantle and subsequently become concentrated in

Fig. 4. 187Os/ 188Os ratios and Os concentrations of the spinels from the chromitite deposits and associated mantle rocks. The chromites have a

much smaller range in their Os isotopic composition and higher Os concentrations compared with the peridotites. A mixing line between mantle

and melt suggest that a melt/ rock ratio of 17:1, on average, is necessary to explain the 187Os/ 188Os ratio of the chromitite deposits. Data for the

mantle peridotites are from Buchl et al. (2004, GCA in press).




chromitite deposits. This is also true for the role of

fluids during chromitite formation.

4.1. Origin of chromium in the chromitite deposits

In order to form chromitite deposits, large amounts

of Cr have to be mobilized in the Earth’s upper

mantle. The chromitite deposits of the Troodos

Ophiolite as well as most of the chromitite deposits

worldwide are enclosed in dunite envelops (Roberts,

1988, Prichard and Lord, 1993; Lago et al., 1982;

Melcher et al., 1999) and surrounded by moderately

depleted harzburgites (Pearce et al., 1984; Roberts,

1988; Nicolas, 1989; Leblanc and Nicolas, 1992).

Gradual lithological changes from dunites to harzbur-

gites to lherzolites have also been descri bed around

chromitite pods in a number of ophiolites (Zhou et al.,

1996). Such dunites formed by melt percolation and

not by partial melting, because the liquidus tempera-

ture of the dunite is not approached in the Earth’s

upper mantle during partial melting. During melt

percolation, dunites and harzburgites can form from

lherzolites by clinopyroxene dissolution and incon-

gruent melting of ortho pyroxene, and precipitation of

olivine from the melt (Kelemen et al., 1997; Suhr,

1999). Chromium behaves compatibly during igneous

fractionation processes as long as orthopyroxene

( K DCr

opx/liq = 4.6– 29 after Jones and Layne, 1997),

clinopyroxene ( K DCr

cpx/liq = 8.1– 36 after Jones and

Layne, 1997) or spinel ( K DCr

sp/liq = 77 after Ringwood,1970) are fractionating phases. However, Cr is incom-

patible in olivine ( K DCr

ol/liq = 0.58– 0.657 after Gaetani

and Grove, 1997). During partial melting in the pres-

ence of orthopyroxene, clinopyroxene and spinel Cr

will behave compatibly. Therefore, chromitite deposits

cannot form under these conditions. However, during

continuous melt percolation in the sub-arc mantle pure

Ol-residues form and Cr therefore could be mobilized

by the melts. In the mantle peridotites enclosing the

chromitite deposits of the Troodos Ophiolite, Cr con-

centration systematically decreases from harzburgites

(average: 2528 ppm) to dunites (average: 1758 ppm)

(Fig. 5). Thus, the dunites enclosing the chromitite

deposits and the dunite melt channels occurring

throughout the mantle section are the source for Cr in

the chromitite deposits.

4.2. Origin of PGEs in the chromitite deposits

With the exception of Pd, the PGE patterns of the

spinels determined in this study are similar to those of

whole rock chromitites observed in other studies of

Fig. 5. Cr and Os concentrations from peridotites associated with the chromitite deposits. Cr and Os both behave incompatibly during the melt

percolation process. Data are from Buchl et al. (2004, GCA in press).




the Troodos Ophiolite (McElduff and Stumpfl, 1990;

Prichard and Lord, 1990) (Fig. 3A). They also are

within the range of PGE patterns o bserved worldwide

in chromitite deposits (Fig. 3A) (e.g. Zhou et al.,1998; Melcher et al., 1999). The depletion of Pt

relative to Ir, Os and Ru is also a characteristic feature

of chromitites from ophiolite complexes. This sug-

gests that podiform chromitite deposits concentrate

the PGE by similar processes and under comparable

thermodynamic conditions. Variable relative abundan-

ces of Os, Ir and Ru may reflect the presence and

heterogeneous distribution of Os–Ir alloys and sulfide

phases. For example, the presence of laurite inclusions

could provide an explanation for the high Ru concen-

tration in sample 10a (Fig. 3A). This also provides an

explanation for the non-chondritic element ratios of

the samples (Table 1).

Our spinel samples have higher Pd concentrations

(0.02–0.15 times C-1) than those observed in previ-

ous studies of the Troodos Ophiolite Complex (0.0015

and 0.01 times C-1; McElduff and Stumpfl, 1990;

Prichard and Lord, 1990) (Fig. 3A). All previous

studies analyzed whole rock powder, whereas in this

study, handpicked and purified spinel samples were

analyzed. The enrichment in Os, Ir and Ru relative to

Pt and Pd observed in previous studies has been

explained by the early removal of Os, Ir and Ru withchromite from the melt, whereas the more incompat-

ible Pt and Pd remain in the silicate melt. Alterna-

tively, it has been suggested that the low Pd and Pt

contents require that the source material lost Pd and Pt

prior to the formation of the chromi te deposi ts

(Melcher et al., 1999; Crocket, 1981; Barnes and

Naldrett, 1985; Edwards, 1990; Keays, 1995; Zhou

et al., 1998). Our study shows that in fact the

enrichment of Pd is similar to that of Os, Ir and Ru

and it is not necessary to invoke such processes in

order to explain the PGE pattern. However, it isdifficult to explain the different Pd abundances in

mineral separates and whole rock samples. It appears

to be obvious that the matrix between the spinel grains

makes the difference. If this is the case, then the

matrix should be depleted in Pd relative to Ir, Os and

Ru. The matrix between the fresh spinel grains is

strongly altered, mainly serpentinised. Thus, it is

likely that the most mobile PGE, namely Pd, has been

lost during the alteration process. Alternatively, the Pd

depletion is a primary feature as suggested by previ-

ous studies (Melcher et al., 1999; Crocket, 1981;

Barnes and Naldrett, 1985; Edwards, 1990; Keays,

1995; Zhou et al., 1998). However, mass balance

constraints indicate that Pd depletion in the matrixcannot be the only reason for the difference. The

measured whole rock Pd/Pt ratios of Troodos chromi-

tites are about 10 times lower than those of the

chromite separates. This implies that the great major-

ity of the Pd, and presumably the other PGE, was

hosted by the matrix before alteration. As the modal

chromite content of these rocks is about 85%, this

means that the matrix must have been highly enriched

in PGE relative to the chromite. For example, if the

whole rock chondrite-normalized Pd content before

alteration was 0.1, and after alteration was 0.01, and

we assume that all of the remaining Pd is in the

chromite, then the matrix originally had a chondrite-

normalized Pd content of f 0.6 compared to a value

of 0.0118 in the chromite. If this original Pd content is

representative of the chondrite-normalized contents of

Os, Ir, and Ru, then the chromitite whole rock con-

centrations of these elements should be about 10 times

higher than those of t he chromite separates. Many

whole rock samples in Fig. 3a have such high Ir, Os

and Ru concentrations. The concentrat ion range of

whole rocks from the Troodos complex (Prichard and

Lord, 1990; McElduff and Stumpfl, 1990), however,overlaps with that of our chromite separates. There-

fore, Pd depletion in the matrix is not the complete

explanation for the difference between the whole rock

and chromite separate results.

Prichard and Lord (1990) suggested that the high

PGE concentrations in the chromitite deposits are the

result of enhanced partial melting of the peridotites.

However, during partial melting, Os, Ru, and Ir

behave compatibly even at the high degrees of partial

melting necessary to form komatiitic and picritic melts

(>30%; Morgan, 1986; Brugmann et al., 1987, 2000;Lorand et al., 1999). Enhanced partial melting, there-

fore, cannot be responsible for the high PGE concen-

tration of the chromitite deposits.

In the mantle peridotites from Troodos, Ir, Os, and

Ru concentrations systematically decrease from the

harzburgites to the dunites as observed by Buchl et al.

(2002) and shown for Os in Fig. 5. In fact, chromites

and dunites have complementary PGE patterns (Fig.

3b). This pattern also supports our suggestion that Pd

is mobilized in the mantle peridotites as well as Os, Ir




and Ru. Buchl et al. (2002) suggested that these

elements tend to behave incompatibly during melt

percol ation bec ause first sulfide, and eventuall y

pyroxenes and chromite are dissolved by the perco-lating melt. This is supported by the observation that

the dunites (no sulfides visible) have a lower sulfide

content than the harzburgit es ( < 0.001% after Bata-

nova and Sobolev, 2001). Prichard and Lord (1990)

also noted the virtual absence of visible sulfides in the

Troodos ultramafic rocks.

The PGE pattern of the spinels show a distinct

depletion in Pt compared with the other PGEs and this

is a typical feature of chromitit es from ophiolite

complexes worldwide (Fig. 3A). Interestingly, the

associated dunites and harzburgites have very high

Pt concentrations varying from 9.11 to 14.95 ng/g. In

addition, Pt concentrations in harzburgites (11.8 ng/g)

and dunites (10.7 ng/ g) are similar on average (Buchl

et al., 2002; Fig. 3B). This suggests that Pt was not

mobilized with the other PGEs during melt migration

through mantle peridotites. The discrete behaviour of

Pt relative to t he other PGEs has been described by

other authors. Handler and Bennett (1999) suggested,

based on separated spinel and silicates of Australian

peridotite xenoliths, that Pt and Pd occur mostly as

discrete Pt- and Pd-rich PGE phases that are the cause

for the poor reproducibility of Pt and Pd whole rock analyses and the lack of correlation between these

elements and Ir in bulk rock analyses. Laser-ablation

analyses of mantle rocks and abyssal peridotites from

the Mid-Atlantic and South West Indian ridges

showed that the whole rock budget of Os, Ir, Ru,

Rh and Pd is balanced by the concentrations measured

in low-temperature sulfide assemblages (Alard et al.,

2000; Luguet et al., 2001). In contrast, Pt shows a

deficit if compared to the measured whole rock

concentrations. These authors also observed Pt con-

centration peaks along laser-ablation profiles in Cu-rich pentlandite. Alard et al. (2000) suggested that Pt

occurs as disseminated Pt-rich micronuggets, too

small in size to be properly analysed, and that these

may be relatively low-temperature exsolution prod-

ucts. However, Luguet et al. (2001) concluded that

they could also represent primary igneous minerals.

Lorand and Alard (2001) suggested, based on laser-

ablation analyses of Massif Central xenoliths, that if

Pt-rich discrete microphases really exist they probably

exsolved during subsolidus decomposition of mantle

sulfide. Pendlandite may theoretically accommodate

Ru, Rh, and Pd in its octahedral sites while rejecting

Pt (cf. Mackovicky et al., 1986; Czamanske et al.,

1992; Ballhaus and Ryan, 1995; Ballhaus and Syl-vester, 2000). Thus, Pt alloys could be stable in the

presence of pendlandite ( <600 jC; f S2 < 10À 7 atm;

Vaughan and Craig, 1978). Its f S2 dependency could

result in a complex partitioning behaviour of Pt

bet wee n Mss, Cu-sul fid es, and perhaps a small

amount of Pt alloys. At 900 jC, the Cu– Ni-rich

sulfide liquid, the high-temperature precursor phase

of coarse-grained pentlandite and Cu-sulfides, can

dissolve up to 15% Pt (Mackovicky et al., 1986),

and there is no large differ ence of Dmss/liq for Pt and

Pd (Li et al., 1996). The Eggler and Lorand (1993)

sulfide barometer calibrated for the P – T conditions of

the lithospheric mantle (950 jC, 1.2 GPa) supports the

occurrence of Pt as sulfide rather than as alloys.

However, the dunite PGE pattern of this study (Fig.

3B) indicates that Pt has been retained in the absence

of sulfide. In detail, however, the formation and

solubility of discrete PGE minerals as a function of

pressure, temperature, f S2 and f O2 is not well known.

The present study shows that the mobilization of Pt

has been inhibited during the melt percolation process

in the mantle peridotites of the Troodos complex. This

suggests that Pt-rich phases are also present in theupper mantle.

We suggest that the PGEs (except Pt) and Cr are

mobilized in the mantle by percolating melts. The

incompatible behavior of Ir, Os, Ru and Pd during

melt percolation could be due to the presence of

fluids in the supra-subduction zone environment.

Johan and Le Bel (1978) and Johan (1986) also

suggested that chromitite deposits form during the

interaction of magma with reducing fluids. Matveev

and Ballhaus (2002) suggested that basaltic melts

parental to podiform chromitite deposits need to bewater saturated in order to produce podiform chromi-

tite deposits. Their experiments indicate that in con-

jugate basalt – water systems (at 1150– 1200 jC and

0.5 GPa hydrostatic pressure) silicates, oxides, and

metallic phases crystallize together and may be frac-

tionated from each other by purely physical process-

es. For example, the authors observed that dispersed

PGE nuggets are mechanically concentrated along

with chromite in the exsolving fluid phase. However,

this model does not explain the Pt-depletion observed




in podiform chromitites. In addition, Pd is not known

to form discrete mineral phases in such deposits.

Thus, the similar enrichment of Pd, Ir, Os and Ru

consistently observed in our study does not support an accidental concentration of different PGE phases.

It rather suggests the contemporaneous concentration

of all PGE and that the PGE distribution in chromites

reflects that of the percolating melts. The only known

mantle phase which has high and similar partition

coefficients for all PGE is a sulfide liquid. We

therefore propose that sulfide liquids play a major

role as collectors of PGE during the formation of

chromitite deposits.

Chromitite deposits form in the mantle if chromite

becomes the major liquidus phase of migrating melts.

Irvine (1977) proposed that the chromium solubility in

silicate melts depends on the silica activity. This can

be effectively changed by assimilating silica during

contamination with continental crust, which is, how-

ever, not a feasible process in the ophiolite mantle.

Alternatively, silica activity can be changed during

magma mixing. This process has been claimed to

produce chromite layers in mafic– ultramafic intru-

sions (Irvine, 1977). Similarly, experiments made by

Ballhaus (1998) showed that chromitite deposits can

form during mingling and mixing of silicate magmas.

Naldrett and Duke (1980) and Naldrett et al. (1990)suggested that this process may also trigger the

formation of an immiscible sulfide liquid that collects

the PGE. Thus, we propose that that during the

pooling and homogenization of percolating melts near

the mantle– crust boundary chromite and sulfides

coprecipitate from the hybrid magmas. These phases

may be further concentrated in a fluid phase, as

proposed by Matveev and Ballhaus (2002), eventually

forming a podiform chromitite deposit. Variable

amounts of droplets of immiscible sulfide liquids will

be scavenged by chromite during its crystallizationand this explains the PGE pattern and variable metal

abundances observed in our spinels. Post-magmatic

recrystallization of chromite would cause desulfuriza-

tion due to the transfer of Fe2 + from the sulfides into

vacancies of the chromite (Naldrett et al., 1989). This

would favour the formation of metal alloys as inclu-

sions in chromite and explains the occurrence of

discrete mineral phases of Ir, Os, and Ru often

observed in podiform chromitites. In contrast, sulfides

in the groundmass between chromite grains suffer a

strong alteration caused by late stage fluids whereby

sulfides and Pd are mobilized again.

5. Implications of the Os isotope systematics in

chromitite deposits

Chromites from ophiolites worldwide have supra-

chondritic Os isotopic compositions (average 187Os/ 188Os: 0.12809F 0.00085 (2r) after Walker et al.,

2002a,b) if compared with the carbonaceous chondrite

ratio of 0.126. Only the Jormua Ophiolite has sub-

chondritic 187Os/ 188Os ratios probably due to the

involvement of subcontinent al lithospheric mantle

(SCLM) (Tsuru et al., 2000). Walker et al. (2002a,b)

argued that the Os isotopic composition of chromitites

represents an integrated value of the depleted mantle.

As described above, chromitites may form during the

pooling and mixing of percolating melt. These melts

most proba bly have radiogenic Os isotope composi-

tions (e.g. Woodland et al., 2002; Borg et al., 2000).

Likewise the boninites (187Os/ 188Os: 0.163) and py-

roxenite veins (cpx-veins: 0.129 – 0.130, opx-veins:

0.166–0.184) (Buchl et al., 2003, GCA in revision)

from the Troodos Ophiolite have a radiogenic initial

Os isotopic composition. Even though most of their

chromitite samples are from SSZ environments, Walk-er et al. (2002a,b) believe that radiogenic melts or

fluids derived from subducting slabs cannot cause a

significant bias towards higher 187Os/ 188Os.

We suggest that the influence of radiogenic melts

from the subducting slab on the isotopic composition

of the mantle wedge cannot be ignored. Osmium can

be mobilized during the dehydration of the oceanic

crust, and the influence of slab derived 187Os on the

Os isotopic composition of mantle peridotites has

been demonstrated by several studies (e.g. Brandon

et al., 1996, 1999; Parkinson et al., 1998; McInnes et al., 1999; Borg et al., 2000). For example, Brandon et

al. (1996) explain a 16% increase of 187Os in some

peridotite xenoliths from areas lying above recent

subduction zones by adding slab-derived Os. Mass

balance calculations which include the whole mantle

wedge (Walker et al., 2002a,b) tend to overestimate

the amount of melts/fluids necessary to overprint the

Os isotopic composition of the mantle peridotites.

Melting in the mantle wedge is caused by lowering

the melting temperature of the peridotites due to the




invasion of melt/fluids. Therefore, partial melting

occurs only in those parts of the mantle that are

invaded by melt/fluids. In this case, the influence of

the radiogenic melts from the subducting slab on the peridotites is not negligible. The proportion of melt

necessary to elevate the Os isotopic composition of

the mantle wedge peridotites can be calculated from

the following mixing equation:

amix ¼ ðaendmember1 Â C endmember1 Â ð1 À f Þ

þ aendmember2 Â C endmember2 Â f Þ

=ðC endmember1 Â ð1 À f Þ þ C endmember2 Â f Þ

The variables are defined as follows: amix repre-

sents the Os isotopic ratio of the mixture of melt and

peridotite, here the average 187Os/ 188Os ratio of the

chromitite deposits from the Troodos Ophiolite:

0.1284; aendmember1 is the Os isotopic ratio of the

melt, here the initial (90 Ma) of boninites from the

Troodos Ophiolite (187Os/ 188Os: 0.163). Comparison

of the trace element composition of the cpx from a

websterite vein from the mantle section enclosing the

chromitite deposits with that of cpx phenocrysts from

the upper pillow lava boninites from the Troodos

Ophiolite shows a close match (Buchl et al., 2002).This observation allows us to suggest that a melt with

a composition similar to that of the boninites has

percolated through this part of the mantle section;

aendmember2 is the Os isotopic ratio of the peridotite,

here the spinel– lherzolite of the Troodos Ophiolite

(most primitive mantle rock in the mantle section)

(187Os/ 188Os: 0.127); C endmember1 is the concentration

of Os in the melt, here the boninites from the Troodos

complex with 0.01 ng/g Os; C endmember2 is the con-

centration of Os in the peridotite, here the spinel –

lherzolite from the Troodos Ophiolite with 4.2 ng/gOs; f : is the mass fraction of endmember 2.

The melt/rock ratio necessary to elevate the Os

isotopic composition of the mantle wedge peridotites

(spinel– lherzolite from the Troodos Ophiolite with187Os/ 188Os of 0.127) to the average value of the

chromitite deposits of 0.1284 is 17:1, on average. Our

calculations imply that the chromitites form during

percolation processes with a melt/rock ratio of < 1:1 –

41:1 (Fig. 4). These values were obtained using the

simple two-component mixing equation given above;

more realistic melt percolation models will in fact

yield slightly lower melt/rock ratios. Kelemen et al.

(1995, 1997) suggested an average melt/rock ratio for

dunite conduit formation of 8:1 to 20:1. Our calculat-ed mean value lies within this range. Kelemen et al.

(1995, 1997) also determined that the integrated melt/

rock ratio for both chromitites and surrounding dun-

ites in the Oman Ophiolite must have been >300. This

is based on the fact that the solubility of Cr-spinel is

low in silicate melts and chromitites must have

scavenged Cr from 300 to 400 times their mass of

liquid (Leblanc and Ceuleneer, 1992). While the exact

value of this estimate depends on the solubility of Cr-

spinel and thus perhaps on the fluid content of the

melt phase, the main point is unlikely to change. That

is, it is easy to attain very high melt/rock ratios in the

mantle wedge environments where most chromitites

form. The melt/rock ratio required to significantly

alter the Os isotopic ratio of the peridotite will of

course depend on the Os concentrations and isotopic

compositions chosen for the endmembers. Thus, as-

suming lower Os concentration ( < 0.01 ng/g) or/and a

less radiogenic Os isotopic composition for the melt

component (187Os/ 188Os < 0.163) would result in a

significantly higher melt/rock ratio (e.g. 180 for a

melt Os concentration of 0.001 ng/g). Nevertheless,

given the very extensive degree of melt percolation proposed on the basis of Cr solubility, the melt/rock

ratios required to significantly modify the 187Os/ 188Os

ratios seem quite plausible.

In summary, we propose that melts passing through

harzburgite and dunite conduits mobilize Os when all

mantle sulfides are dissolved. During the formation of

the chromitite deposits melts with different Os signa-

tures are pooled, mixed and homogenized on a large

scale. This is the reason for the relatively homoge-

neous Os isotopic composition in the chromitite

deposits. The Os isotopic composition of the chromi-tite deposits is buffered by the Os component of the

peridotites. However, in detail, the 187Os/ 188Os ratios

of the spinels in the chromitite deposits show some

variation (Fig. 4), depending on the relative amount of

Os of melts and peridotites and their Os isotopic

compositions. The range in 187Os/ 188Os ratios is

mainly a function of the initial compositions of melts

and peridotites and the melt/rock ratio. Therefore, we

conclude that podiform chromitites do not represent

the Os isotopic composition of the upper convecting




mantle, but that they define an upper limit. This

implies that prior to melt percolation, the mantle

source of the ophiolites had a significantly lower 187

Os/ 188

Os than the PUM.

6. Conclusions

We present a data set of PGE abundances, Os

isotopic compositions, and Cr numbers of chromites

from chromitite deposits and associated mantle peri-

dotites from the Troodos Ophiolite Complex.

This study shows that the surrounding mantle

peridotites are the sources of the PGE and Cr in the

chromitite deposits. This is because Cr and PGE—

with the exception of Pt—are mobilized in peridotites

during the interaction with percolating, probably flu-

id-rich, melts. At higher stratigraphic levels, the

metals are precipitated from the melt and become

concentrated when chromite and sulfide liquids pre-

cipitate as a result of magma mixing in magma pods

now represented by the chromitite deposits. Thus, the

PGE distribution observed in a chromitite reflects that

of the average percolating melt.

With the exception of Pd, the PGE patterns of

spinels separated from chromitites of the Troodos

complex are similar to those of whole rock analysesof podiform chromitite deposits worldwide. Typical

features are the high concentrations of Ir, Os, and Ru

and negative Pt anomalies. The stability of Pt-rich

phases in the ophiolite mantle inhibits Pt mobilization

during melt percolation and eventually causes the

negative Pt anomalies in the chromitite deposits.

However, Pd concentrations in separated spinel are

higher than those of whole rock chromitites. Pd-

depletion in the matrix cannot entirely explain this

difference and this implies significant variations of

PGE abundances in chromites even on a local scale.The 187Os/ 188Os ratios of the spinel from podiform

chromitite deposits from the Troodos Ophiolite Com-

plex range from 0.1265 to 0.1305. The average value

is similar to that of chromitite deposits worldwide

(Walker et al., 2002a,b). The variation of the187Os/ 188Os ratio of the chromitite deposits reflects a

mixture of Os derived from the mantle peridotites

with Os from the subducting slab. The latter compo-

nent is transported along with percolating melts and

fluids and probably has a radiogenic composition

typical of many arc basalts. Model calculations imply

that a melt/rock ratios of < 1:1– 41:1 are necessary in

order to increase the 187Os/ 188Os of the spinel–lher-

zolites from the Troodos Ophiolite to the observedvalues in the chromitites. These values are similar to

or lower than independent estimates of melt/rock

ratios in ophiolites. Thus, podiform chromitites do

not represent the Os isotopic composition of the upper

convecting mantle. However, they do define the upper

limit, and this implies that the ophiolite mantle has a

significantly lower 187Os/ 188Os than PUM and has

therefore suffered a long-term depletion of Re.

Acknowledgements

We thank Klaus Mezger (University of Munster)

for providing access to the Isoprobe. This study also

benefited from many discussions with Chris Ballhaus

and Alexander Sobolev. G. Brugmann is most grateful

to Costas Xenophontos for introducing him into the

geology of the Troodos Ophiolite. We would also like

to thank the editor Laurie Reisberg and the reviewers

Jim Crocket, Monica Handler, and an anonymous

reviewer for their critical comments which signifi-

cantly helped to improve the quality of the manu-

script. This work has been supported by the

Graduierten Kolleg ‘‘Stoffbestand und Entwicklung

von Mantel und Kruste’’ at the Johannes Gutenberg

University of Mainz. [RR]

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