Geochemistry of sulfides in Hawaiian garnet pyroxenite xenoliths: Implications for highly siderophile elements in the oceanic mantle

Two dominant petrographic types of sulfides occur in Hawaiian garnet pyroxenite xenoliths — Type I occurs as globular, poikilitic inclusions only in clinopyroxenes, and Type II occurs interstitially between silicate phases. Chemically, the two types are essentially identical and their compositions are consistent with Fe–Ni monosulfide solid solution (Fe = 55–57%, Ni = 3–10%, Cu = 0.5–2.0%, S = 35–37% and Zn = 0.01–0.5%). Both types of pyroxenitic sulfides have a factor of 10 to 1000 lower PGE contents than sulfides found in peridotites elsewhere (e.g. Os ∼ 1 to < 0.01 ppm), show fractionated PGE patterns (Pd(n)/Ir(n) ∼ 1–35; n = chondrite-normalized) and very high Re(n)/Os(n) ratios (∼ 10–400). The Ni and the total PGE (Os + Ir + Pt + Pd) contents of the sulfides correlate positively with the Mg# of clinopyroxene in the host rock, which points to a genetic link between the sulfides and silicates and a high-temperature mantle origin. The Hawaiian sulfides formed from an immiscible sulfide melt that separated from a silicate melt, which formed the host garnet pyroxenite. Experimental constraints place this sulfide–silicate melt immiscibility at 1530 ± 100 °C and 3.1 ± 0.6 GPa, i.e. near the base or slightly below the Pacific lithosphere. Melt/mineral partition calculations suggest that the parental silicate melts (prior to immiscibility) were similar to the Honolulu Volcanics (HV) alkali lavas that host the xenoliths, and that limited (0.1–0.3%) sulfide fractionation from a parental HV-type melt can account for the observed HSE variability in the sulfides. The relatively low Pt/Re ratios (< 10) suggest that, upon “aging”, such sulfides cannot generate the coupled 186Os–187Os isotope enrichments observed in Hawaiian lavas and some komatiites. Therefore, recycling of mantle sulfides of pyroxenitic parentage is unlikely to explain the enriched Pt–Re–Os isotope systematics of plume-derived lavas.