GEMOC ARC National Key Centre

Olivier Alard1, Laurie Reisberg2, William L. Griffin1,3, Jean-Pierre Lorand4, Suzane Y. O'Reilly1, Norman J. Pearson1 and Stuart Graham1

1 GEMOC Macquarie
2 CRPG, CNRS, France.
3 CSIRO Exploration and Mining
4 Laboratoire de Mineralogie, Museum National  d'Histoire Naturelle, France.

The Platinum Group elements (PGE), Au and Re form a coherent group of highly siderophile and variably chalcophile elements which behave very differently to lithophile elements during mantle processes. Therefore, these Highly Siderophile Elements (HSE) provide a different perspective on the formation and evolution of the lithospheric mantle. In addition, the Re-Os isotopic system may provide reliable melt depletion ages, which provide further understanding of the evolution of the lithosphere. The apparent reliability of the Re/Os isotopic system is based on the assumption that Os is strongly compatible, so that metasomatism due to percolation of "exotic" fluids through the mantle is unlikely to disturb or reset this isotopic system. This is in contrast to the lithophile isotopic systems such as Rb/Sr and Sm/Nd, where the parent and/or daughter elements may be strongly concentrated in metasomatic fluids that can seriously disturb the whole-rock isotopic systematics.

However, the distribution and behavior of the HSE in the mantle is poorly understood. Although base metal sulfides are thought to be the main host for these elements, little attention has been paid so far to sulfide abundance, dispersion, mineralogy and microstructural sites in mantle samples. Further understanding of the HSE behaviour in the mantle will depend on studies of the relationships between HSE distribution, Re/Os isotope systematics, and the abundance and composition of sulfide phases.

We have developed an in situ Laser Ablation ICP-MS procedure in order to further investigate distribution of HSE in mantle sulfides (Alard et al., 1999). The main results of this study are:
1: The extreme variability of the chondrite-normalised HSE patterns; Pd/Ir ranges from 0.002 up to 48.
2: The high solubility of PGE in base metal sulfides (BMS);  Os and Ir contents in mono-sulfide solid solution (MSS) can be as high as 1000 ppm. These data confirm the extreme compatibilitity of such elements in the BMS of mantle peridotites, and emphasise that the Os budget of mantle rocks is completely controlled by the sulfide phases.
3: The HSE patterns define two groups, which display differences in major element chemistry and microstructure. MSS enclosed in silicates show high Os and Ir contents but low to very low Pd/Ir, independent of metasomatic events undergone by the samples.  The chemistry, occurrence and HSE patterns of these enclosed sulfides  suggest that they are residual following partial melting. In contrast, interstitial sulfides, which typically are lower in S but higher in Cu (e.g. Cu-rich pentlandite) than the enclosed MSS, have low Os and Ir contents but high Pd/Ir.  These sulfides usually dominate in samples having very high S contents (330 to 600 ppm), commonly occur in reaction patches, and are interpreted as being metasomatically introduced.
4:  Both types of sulfide often occur in the same sample. Such samples have normal sulfur contents (e.g. 56<S< 153ppm), in contrast to the high values found in some samples where metasomatic sulfides are predominant.

We have determined the Os isotope compositions of peridotite xenoliths from the Massif Central and neighboring areas. S-poor samples (S<20ppm), where sulfide occurs only as silicate-enclosed MSS and which have whole rock Pd/Ir <chondrites, have unradiogenic Os isotope compositions (187Os/188Os = 0.110-0.125) consistent with ancient melt depletion and isolation from the convecting asthenospheric mantle for time periods similar to the age of the overlying crust.  These samples preserve such unradiogenic Os despite textural and mineralogical evidence of extensive melt/rock reactions, suggesting that the silicates have efficiently shielded the enclosed sulfide from reaction (Burton et al., 1999).  In contrast, highly S-enriched samples  have very radiogenic 187Os/188Os values up to 0.175. These samples are characterised by large enrichments of the incompatible elements without concomitant enrichment of the HFSE (Nb, Ta, Zr, Hf), suggesting metasomatism by a volatile rich or "carbonatitic" small volume melt (Dautria et al., 1992, Ionov et al., 1993). Among these S-rich samples, 187Os/188Os increases together with the Pd/Ir, Os/Ir and S contents, and these parameters are positively correlated with the La/Sm ratio. Such parallel variations of Os, Pd and S are consistent with the experimentally demonstrated affinity of these elements for S-bearing vapor phases at magmatic temperatures.

Samples with S contents between 20 to 150 ppm contain both interstitial and enclosed sulfide. They have 187Os/188Os between 0.125 and 0.140 indicative of mixing between depleted and enriched components as described above.  The interpretation of Re-Os model ages in such samples must be undertaken with great caution, and based firmly on understanding of sulfide distribution and chemistry.

Further work is in progress on the Os isotopic characteristics of sulfide, using the Nu-MC-ICPMS facility at GEMOC and alternate analytical procedures as developed by Shoenberg et al. (1999).

 These studies provide further evidence that metasomatic process(es) involving volatile-rich small melt fractions may significantly alter the relative abundance of the HSE and the Os isotopic composition of the lithospheric mantle, provided that sulfur is a mobile component.  The results also emphasize the necessity of an integrated, multi-component approach to understanding lithospheric evolution.
 

REFERENCES

Alard O., Griffin, W.L., Lorand J.P., Jackson S.E. and  O`Reilly S.Y., 1999, Highly siderophile elements in mantle sulfides: In-situ LAM-ICP-MS analysis. 9th V.M. Goldschmidt, August 22-27, 1999, Boston, U.S.
Burton, K.W., Schiano, P., Birck, J.L., Allègre C.J., Rehkämper M. and Halliday A., 1999, The distribution and behaviour of Re and Os amongst mantle minerals and the consequences of metasomatism and melting on mantle lithologies. 3Rd Orogenic lherzolites and mantle processes conference, Septemder 12-15, Pavia, italia, Ofioliti, 24:77.
Dautria, J. M., Dupuy, C., Takherist, D. and Dostal, J., 1992, Carbonate metasomatism in the lithospheric mantle: peridotitic xenoliths from a melilitic district of the Sahara Basin. Contrib. Min. Pet., 111:37-52.
Ionov, D.A., Dupuy, C., O'Reilly, S.Y., Kopylova, M.G. and Genschaft, Y.S., 1993, Carbonated peridotite xenoliths from Spitsbergen: implications for trace element signature of mantle carbonate metasomatism. Earth Planet. Sci. Lett., 119: 283-297;
Shoenberg R., Nägler T.F. and Kramers J.D., 1999, Precise Os isotope ratio and Re-Os isotope dilution measurements down to the picogram level using multicollector ICP-MS. In press.