Competing effects of mantle melting, magma crystallization and degassing on sulfur, chalcophile, and siderophile elements in volcanic glasses from Shatsky Rise and Ojin Rise seamounts

Oceanic large igneous provinces (LIPs), such as Shatsky Rise in the western Pacific, commonly exhibit variable and locally elevated abundances of chalcophile and siderophile elements (CSEs), reflecting the conditions of mantle melting and subsequent magma differentiation. However, the processes and conditions controlling these enrichments remain poorly understood. To unravel these processes, we analyzed major, volatile, and trace elements, including the strongly chalcophile and siderophile trace and ultra-trace elements Cu, Ag, Se, Re, Pt, Au, and Bi, in 48 glass samples from Shatsky Rise and the adjacent Ojin Rise seamounts. We also determined Fe 3+ /ΣFe ratios in 19 glasses and measured major element and sulfur contents in plagioclase-hosted melt inclusions from the Tamu Massif, the oldest in the Shatsky Rise. Parental Shatsky Rise magmas are strongly enriched in CSEs relative to MORB. They are inferred to have formed at f O 2 of FMQ to FMQ − 1 and to have evolved by fractional crystallization, resulting in auto-oxidation to as high as FMQ + 1 in the most evolved melts. The CSE patterns of the normal-type Shatsky Rise magmas are consistent with extensive (22–25%) mantle plume melting that exhausted sulfide from the mantle residue. In contrast, parental Ojin magmas resemble MORB and became sulfide-saturated during the early stages of fractional crystallization. They were likely generated by more moderate degrees of mantle melting, leaving a sulfide-bearing residue at f O 2 below FMQ. All Shatsky Rise melts remained sulfide-undersaturated during fractional crystallization down to ~7 wt% MgO. At lower MgO, the glasses define two contrasting evolutionary trends: sulfur-rich, CSE-depleted, sulfide-saturated compositions from the Ori Massif and Ojin Rise seamounts, and sulfur-poor, CSE-enriched, slightly more oxidized, sulfide-undersaturated compositions from the Tamu Massif. Ori Massif magmas record little or no sulfur degassing during crystallization and eruption and remain to be sulfide-saturated. In contrast, Tamu magmas experienced substantial pre-eruptive sulfur loss within an open-vent magmatic system, where prolonged recycling and mixing of degassed and undegassed magmas progressively depleted the magma in sulfur. These results highlight the fundamental role of shallow magmatic processes in controlling CSEs behavior in oceanic plateau magmas and determine their potential for the formation of magmatic sulfide deposits. Sulfide undersaturation during shallow evolution diminishes the ore-forming potential of LIP magmas, whereas late-stage sulfide saturation favors formation of Au–Cu–PGE-rich but Ni-poor magmatic sulfides.