Platinum Group Element Abundances in the Hawaiian Mantle: Constraints from in-Situ Sulfide and Bulk Rock Analyses of Garnet Pyroxenite Xenoliths from Oahu

Here we report the first in-situ sulfide and bulk rock analyses of PGE in garnet pyroxenite xenoliths from Oahu, Hawaii. The sulfides occur as globular inclusions (10-90 microns) mostly within clinopyroxene and as interstitial phases (100 - 500 microns) in the xenoliths. They were analyzed for major elements by electron microprobe. PGE analyses were carried out in situ by LA-ICPMS, and bulk rock analyses were performed by the carius tube digestion procedure and analyzed by solution ICP-MS. The sulfides are monosulfide solid solutions (MSS, Fe = 54%, Ni = 7%, Cu = 1.3%, S = 37%, Zn = 0.18%) plotting in the 1000 – 1100°C MSS field in the Fe-Ni-Cu-S system and two-pyroxene geothermometry of these xenoliths suggests similar final mantle equilibration temperatures around 1100 - 1250°C. The PGE contents of the sulfides are highly variable (Os 0.009-1.42, Pt 0.006-0.669, Ir 0.003-1.43, Pd 0.026-1.044 and Ru 0.054-2.994 ppm) and are lower by a factor of 10 to 100 than the typical peridotitic sulfides. A positive correlation of (Pd/Ir)_N vs. (Re/Os)_N and Ir vs. Os indicates the PGE pattern in the sulfides to be a primary feature. The (Pd/Ir)_N ratio of the bulk rock has a strong positive and negative correlation with the Al₂O₃ and MgO content of the clinopyroxene respectively.

Mass balance calculations between the sulfides and bulk rock PGE contents indicate that sulfides are the major repository of PGEs in these pyroxenites. From petrography and geochemical data we interpret these sulfides as immiscible liquids, separated from a more voluminous silicate melt that crystallized the garnet and clinopyroxene. Calculated PGE abundances for the parental melt are very similar to that of erupted picritic and post erosional alkalic Hawaiian lavas, suggesting a parental melt broadly similar to Hawaiian lavas.