Origins of Os-isotope and platinum-group element compositions of metasomatized peridotite and cumulate pyroxenite xenoliths from Kharchinsky Volcano, Kamchatka

Abstract

Platinum-group element (PGE) + Re abundances and Os isotopic compositions were determined for metasomatized peridotite and cumulate pyroxenite xenoliths from Kharchinsky Volcano, Kamchatka. Two peridotites have moderately high PGE concentrations with 2.9–3.3 ppb Os and smoothly variable PGE patterns similar to abyssal peridotites and encompassing primitive and depleted mantle compositions. One of these has unradiogenic Os (187Os/188Os = 0.1148) at the lower limit of compositions observed in abyssal peridotite which results in a time of Re depletion (TRD) model age of ∼1.8 Ga. These contrasts Kharchinsky peridotites that are Pt-Ir enriched with sharply fractionated PGE patterns with Pt/Pd = 132–168 and Ir/Ru ∼ 4.0. This pattern of enrichment in the heavier and more highly siderophile Pt and Ir over the lighter and more siderophile-chalcophile Ru and Pd (Fleet and Stone, 1991; Fleet et al., 1991) is consistent with sulfur depletion under oxidizing metasomatic conditions that leave a residue with PGE host minerals that are dominantly alloys. A single Ru-enriched peridotite with low PGE abundances is interpreted to reflect advanced stages of paragenesis driven by metasomatism and resulting in the loss of all PGE and Re relative to Ru. All Kharchinsky peridotites included in this study have relatively unradiogenic Os (187Os/188Os = 0.1148–0.1314) but some are coupled to PGE and lithophile element characteristics that require oxidizing metasomatic conditions (Ce/Ce*≪1.0) and desulfurization leading to a general loss of PGE and data patterns that are consistent overall with low abundances of relatively radiogenic Os in arc peridotites globally (Brandon et al., 1996; Widom et al., 2003; Saha et al., 2005). Kharchinsky pyroxenites, which are relatively undeformed and only weakly metasomatized, have similarly fractionated PGE patterns with enrichments in Pt (5.7–46 ppb) and Pd (0.5–2.7 ppb) but with Os, Ir, and Ru abundances < 0.23 ppb. The patterns resemble arc basalts and may reflect preferential mobility of Pt, Pd, and Re in slab fluids. High Pt and Pt/Pd up to 33 in the pyroxenites indicate that a Pt-rich alloy with high Pt/Pd is present in the early (olivine-rich) cumulate assemblage. Several Kharchinsky samples (peridotites and pyroxenites) are Pt-enriched with high Pt/Os and Pt/Re > 20 that will produce rapid ingrowth of 186Os relative to 187Os. Deep recycling and storage of such subduction-modified rocks might preclude the need for core-mantle exchange to explain 186Os-enrichments in komatiites and Hawaiian picrites originating in the deep mantle (Brandon et al., 1998; Brandon and Walker, 2005).