Highly siderophile element behavior in the subduction zone mantle: Constraints from data on the Yap Trench peridotites

The highly siderophile elements (HSEs) in peridotites from the Yap Trench are investigated to understand their behavior in the subduction zone mantle, which is characterized by high melt depletion and metasomatism. Results reveal that the abundance of HSEs in Yap Trench peridotites is extremely low compared with that in oceanic peridotites from the mid-ocean ridge and other subduction-zone environments, indicating the extensive extraction of the sulfide melt. Chondrite-normalized HSE patterns are heterogeneous and exhibit strong fractionation of platinum-group elements (PGEs) and slight enrichment of Re and Au. The Ru/Ir (1.64–16.12), Os/Ir (0.15–7.19), and Pt/Pd (0.53–13.17) ratios in most of the studied samples are higher than those in CI chondrite (Ru/Ir: 1.49; Os/Ir: 1.06; and Pt/Pd: 1.67). These suprachondritic PGE ratios result from the exhaustion of the base metal sulfide (BMS) and the formation of platinum-group minerals and alloys, such as Os–Ru-rich sulfides and PtFe alloys, that are selectively enriched with certain HSEs. Mantle metasomatism appears to have no systematic influence on PGEs, although it may increase the abundance of Re and Au in the Yap Trench peridotites. Moreover, compared to abyssal peridotites, subduction-related peridotites have fractionated PGE patterns, particularly for the iridium group, suggesting that subduction conditions may promote the fractionation of the PGEs. However, suprachondritic PGE ratios are generally observed in subduction-related peridotites with a low Al2O3 content (<1 wt%) instead of subduction-related fertile samples. This suggests that highly depleted and subduction-related conditions are required for the fractionation of PGEs. We recommend that the high oxygen fugacity, hydrous melting, and high degree melting of the subduction-zone mantle may result in the exhaustion of the BMS in highly depleted peridotites, promoting the formation of platinum-group minerals and HSE-rich alloys, in which the HSEs are significantly fractionated.