Fractionation of highly siderophile elements in forearc magmas: Influence of mantle source and magmatic differentiation

Abstract

Highly siderophile element (HSE) concentrations are presented for gabbroic rocks from the Yap forearc, Western Pacific. These rocks are categorized into cumulate and noncumulate types, exhibiting HSE contents comparable to those found in subduction-related lavas. They are characterized by a platinum-group elements (PGE) pattern showing enrichment in palladium-PGE (PPGE: Rh, Pt, and Pd) relative to iridium-PGE (IPGE: Os, Ir, and Ru). However, some samples exhibit extremely low Pt concentrations, notably lower than typical subduction-related lavas. The Yap cumulate rocks exhibit higher concentrations of IPGE compared to the noncumulate rocks, while they have similar PPGE concentrations. This suggests that IPGE are incorporated into the cumulates through the crystallization of minerals, which may include olivine, sulfide, or alloys. The notably low Cu/Pd ratios observed in the Yap rocks, particularly in the noncumulate rocks, imply that the magmas were likely sulfur-undersaturated during their evolution, which inhibited extensive sulfide crystallization. Therefore, fractionation of PGE in the magma reflects precipitation of IPGE-rich alloys rather than sulfide segregation. Overall, the Yap rocks, particularly the noncumulate rocks, exhibit lower IPGE concentrations than other subduction-related lavas, with some samples showing significantly depleted Pt concentrations, leading to extremely low Pt/Ir and Pt/Pd ratios. This may be attributed to the presence of Pt- and IPGE-rich alloys in the mantle source, as confirmed by the HSE composition of Yap forearc peridotites. The Yap gabbroic rocks and other arc lavas (e.g., from Grenada, Izu-Bonin, and Tonga) are distinct from mid-ocean ridge basalts (MORB) in having lower Cu/Pd ratios, higher total PGE contents, and elevated PPGE/IPGE ratios. These features are attributed to a combination of subduction-zone processes. High oxygen fugacity increases sulfur solubility, inhibiting sulfide saturation and crystallization, which explains the low Cu/Pd ratios. Concurrently, high degrees of mantle melting exhaust sulfides and promote alloy formation in mantle. This liberates PGE into the melt while retaining IPGE in the residue, leading to the observed high PGE contents and fractionated PPGE/IPGE ratios.