GEMOC ARC National Key Centre

Alard O.1, Pearson N.J.1, Griffin W.L.1, Lorand, J.P.3, Jackson, S.E. 1 and O'Reilly S.Y1.
1. GEMOC, School of Earth and Planetary Sciences, Macquarie University, 2109 NSW
2. Laboratoire de Mineralogie, Museum National d'Histoire Naturelle, Paris, France

The geochemistry of the siderophile and chalcophile elements potentially carries a large amount of information on processes in mantle-derived rocks.  We have determined the HSE (PGE + Au) contents of single mantle sulfide grains using a custom-built Nd:YAG 266nm UV laser-ablation microprobe linked to a Perkin Elmer ELAN6000 or HP4500s ICPMS. Ablation was done in a pure He atmosphere (0.85 l/min). Laser operating conditions of 4 Hz frequency and a beam energy of ~0.5 mJ produced 40-60 µm diameter pits.  A combination of NIST 610 and a synthetic Ni-sulfide doped with PGE was used for trace element calibration.  Raw data were processed on-line using the GLITTER software package and included major element argide corrections on trace elements (e.g. CuAr on Rh).

A technique has been developed for the in-situ measurement of Os isotopes in sulfides using a Merchantek LUV266 laser microprobe and a Nu Plasma multi-collector ICP-MS. Repeated LAM analysis of a synthetic NiS bead with 200ppm Os demonstrates the precision expected for peridotite sulfides as a function of signal intensity, laser spot size and analysis duration.  Precision equivalent to N-TIMS is achieved using an all-Faraday collector array for Os > 200ppm and a Faraday-IC (ion counter) setup for Os > 50ppm.

The HSE in sulfides mantle peridotites patterns define two groups that are related to differences in major element chemistry and microstructure:
(i) MSS enclosed in silicates show high Os and Ir contents but low to very low Pd/Ir.  The chemistry, occurrence and HSE patterns of these enclosed sulfides suggest that they are residual following partial melting. This type of sulfides has unradiogenic Os isotope compositions consistent with ancient melt depletion and isolation from the convecting asthenospheric mantle for time periods similar to the age of the overlying crust.
(ii) interstitial sulfides, which typically are lower in S but higher in Cu (e.g. Cu-rich pentlandite) than the enclosed MSS, have low Os-Ir contents but high Pd/Ir. Their 187Os/188Os display radiogenic values up to 0.175 and have extremely variable 187Re/188Os. These sulfides are interpreted as being metasomatically introduced.
Both types of sulfide often occur in the same sample and the  mixture of the two types has major implications for interpretation of whole-rock PGE and Re-Os data. The major advantage of the LAM techniques lies in providing in-situ analyses with the spatial and microstructural context required for meaningful interpretation.