Insights into The PGE and Re-Os Systematics of the Lithospheric Mantle from in-situ Analysis of Sulfide Phases.

The highly siderophile element (Platinum Group elements (PGE)+ Au+Re) form a coherent group which behave very differently to lithophile elements and thus 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. However, the distribution and behaviour of the HSE in the mantle is poorly understood. Although base metal sulfides are thought to be the main host for HSE, little attention has been paid to sulfide abundance, distribution, mineralogy and microstructural sites in mantle. Further understanding of HSE behaviour in the mantle will depend on the studies of the relationships between HSE distribution, ReOs isotope systematic, and sulfide petrographic characteristics. In-situ techniques as LA-ICPMS or LA-MC-ICPMS allow to investigate the distribution of HSE and variability of Os isotopes in mantle sulfides. The main results of these studies are: 1, the extreme variability of the chondrite-normalised HSE patterns; 2, Base metal sulfides control the PGE budget of mantle rocks; 3, The HSE patterns define two sulfide groups, which reflect also differences in major element chemistry and microstructural position. Silicate-enclosed MSS show arch-shaped primitive mantle normalised HSE patterns with low PdIr, suggesting that they are residual after melting. Unradiogenic Os isotope compositions, agree indeed with ancient melt depletion and long-term isolation from the convecting mantle. In contrast, interstitial sulfides, typically S-poor and Cu-rich have lower Os-Ir contents but high PdIr. Petrographic and geochemical features indicate that they have been metasomatically introduced. Their ¹⁸⁷Os/¹⁸⁸Os display usually radiogenic values and show variable ¹⁸⁷Re/¹⁸⁸Os. Both types of sulfide often occur in the same sample; such samples have normal sulfur contents (e.g. S< 200ppm). The in-situ analysis of PGE and Os isotopes provides crucial information on the behaviour and distribution of the HSE and on the evolution of the lithosphere. Although the in-situ Re/Os method is subject to larger errors than TIMS analysis, its spatial resolution preserves petrographic information allowing the data to be interpreted with less ambiguity. Since many of the mantle samples studied contain the two types of sulfide we therefore suggest that many of the whole-rock analyses reflect mixing between several sulfide generations. It is thus hazardous to draw conclusions on Earth formation and differentiation processes based upon whole rock PGE abundances or Os isotopes without at least basic information on sulfides. These data suggest that sulfide could become a key phase in unravelling the formation and evolution of the mantle lithosphere.