Variations of olivine Fo–Ni contents and highly chalcophile element abundances in arc ultramafic cumulates, southern Alaska

Recent drilling in the Duke Island Complex shows that some olivine clinopyroxenites from Hall Cove contain low-S but high Pt + Pd. In contrast, dunites from the Duke Island Complex and the Yellow Hill intrusion on nearby Annette Island are low in Pt + Pd as well as S. New data from this study show that the low-S ultramafic rocks from Duke and Annette Islands are extreme cumulates of olivine and/or clinopyroxene and all contain trace to minor amounts of cumulus sulfide. As a result, the ratios of the least-mobile, highly chalcophile elements in these rocks mainly reflect those of original sulfide liquids which in turn reflect those of their parental magmas. Our data also show that all dunite samples from Annette Island contain Ni-depleted, primitive olivine with Fo contents > 90 mol% and Ni contents < 1200 ppm, indicating a primitive but significantly Ni-depleted parental magma for these rocks. Whole-rock compositions indicate that the parental magma was not depleted in platinum-group elements (PGE). The mantle-normalized PGE patterns of these samples are remarkably similar to dunites from some Ural-type ultramafic intrusions in Ural, Russia. They are all characterized by mantle-like Ru/Ir ratio plus significant Pt enrichment relative to other PGEs. Given their common occurrences in the world, we propose a Ni-depleted, Pt-enriched mantle component as the source for the parental magma of this type of dunite. A comparison of olivine and whole-rock compositions between different types of rocks from the Duke Island Complex reveals that significant Ir–Ru depletions relative to Pt–Pd in olivine clinopyroxenites from Hall Cove are most likely due to Ir-alloy and laurite segregation from their parental magma before it reached sulfide saturation.