EVIDENCE FOR PT MOBILIZATION IN SERPENTINIZED PERIDOTITES FROM THE ST. ELENA OPHIOLITE IN COSTA RICA

We present in situ sulfide LA-ICPMS and bulk rock data on the platinum group element (PGE) and chalcophile element (namely Cu, Ag, Te, Au, Pb) systematics from partially serpentinized peridotites of the St. Elena ophiolite, Costa Rica. PGE are strong indicators of primary mantle processes, though their behavior during low temperature alteration processes such as serpentinization is not well understood. St. Elena sulfides are dominantly pentlandites which coexist with Fe-Ni alloys and native Cu. This indicates extremely low fO₂ and fS₂ conditions during serpentinization and low water-rock ratios. Depleted REE patterns suggest minimal refertilization, while very low U and Sr are also consistent with low water-rock ratios. Pb ranges from 0.01-31 ppm with the majority of sulfides <5 ppm, and an average concentration of 2.8 ppm (n=114). Assuming that this Pb concentration is representative of primary mantle sulfides, this suggests that Pb is not dominantly held in mantle sulfide. PGE-Re in sulfides are highly variable ranging from 1 – 100,000 times that of primitive mantle (PM). PM normalized PGE-Re patterns for these sulfides are dominated by strong Pt depletions relative to Pd and Re. Occasional Pt enrichments (~7% of the total sulfide population) correlate with Te and Au and weakly with Cu, consistent with hydrothermal fluid infiltration. Bulk rock PGE patterns are flat and concentrations are close to that of PM. The discrepancy in Pt between bulk rock and sulfide concentrations suggests that a significant fraction of Pt is decoupled from other PGE in sulfides. The presence of rare Cu-Pt-Te-Au alloys implies that Pt is primarily hosted in trace phases (“nugget effect”) resulting in the observed heterogeneity within single sulfides at the scale of a hand specimen. Excess Cu in these samples has been explained by hydrothermal fluid infiltration from proximal mafic lithologies (Schwarzenbach et al., 2014). Data suggests that hydrothermal fluids preferentially mobilized and locally redistributed Pt under relatively reduced conditions. These data point to short scale redistribution and fractionation of PGE during serpentinization.