Os-isotope variability within sulfides from podiform chromitites

In situ laser ablation MC-ICPMS was used to measure the Os-isotope compositions of single sulfide grains (≥ 5 μm in diameter) included in unaltered primary chromite from the podiform chromitite of Caridad (eastern Cuba). The analyzed phases include PGE-sulfides (laurite and PGE-rich monosulfide solid solution) and base-metal sulfides (BM-sulfides; millerite, heazlewoodite and pentlandite). The results show that PGE-sulfides and the BM-sulfides have distinctly different Os-isotope compositions. Significant variations in Os-isotope composition are found among sulfide grains forming parts of single composite aggregates smaller than 50 μm in diameter. A similar degree of heterogeneity was observed in some chemically and isotopically zoned sulfide grains. At such small scales, the formation of sulfides with variable Os-isotope signatures requires a heterogeneous genetic process in which individual pulses of isotopically and chemically different melts coexisted in space and/or time. This could be achieved if the podiform chromitites formed as a result of mantle-melt reactions and the subsequent melt mixing/mingling of several batches of melt.

► PGM and BM-sulfides in podiform chromitites may have different Os-isotopic compositions.
► Sulfides with distinct Os-isotope composition may occur in composite aggregates.
► Sub-grain Os isotopic heterogeneity implies different batches of melts.
► The origin of the sub-grain Os isotopic heterogeneity is due to melt mixing/mingling.