Geochemistry

Mantle (geology)

Andesites

Volcano

Volcanic rock

Saturation (graph theory)

Basalt

Geology

Phenocryst

Igneous differentiation

Organic chemistry

Magma

Pyrite

Mathematics

Combinatorics

Basaltic andesite

Chemistry

Pyrrhotite

Sulfide

Mineral redox buffer

Magma chamber

Andesite

Lithos

Magmatic sulfide saturation and dissolution are increasingly thought to play important roles in controlling the behavior of chalcophile metals in arc-related magma. However, the mechanisms of these processes during magma evolution in the Okinawa Trough (OT) are still poorly known. Here, we firstly report magmatic sulfides characterized by variable oxidation and dissolution in basaltic andesites from the southern OT. Pyrrhotite (Po) is the main sulfide mineral identified in the clinopyroxene phenocrysts and groundmass in these rocks. Thermodynamic modeling suggests that the oxygen fugacity (fO2), temperature (T) and pressure (P) in equilibrium with Po are ~∆FMQ + 1.5, ~1050 °C, and ~ 3 kbar, respectively. The positive δ34SV-CDT values (1.57 ± 0.35‰) of magmatic sulfides and the extremely nonradiogenic Sr and radiogenic HfNd isotope compositions of the basaltic andesites (mean of 87Sr/86Sr = 0.703816, εHf = 17.72 and εNd = 6.19) indicate that the incorporation of external crustal sulfur is not the reason for sulfide saturation in the oxidized magma. Instead, sulfide saturation is driven by a decrease in the sulfur content at sulfide saturation (SCSS) due to changes in melt chemistry and dropping temperatures along with a concurrent increase in S contents during magmatic differentiation. The S content in the glasses (~362 ppm) is far lower than the SCSS for the basaltic andesitic magma in the magma chamber (~1 wt%), revealing that extensive S degassing occurred during final magma ascent and decompression. The associated drop in S fugacity (fS2) and increase in fO2 induced resorption and oxidation of those sulfides, which were not shielded in the clinopyroxenes, to Fe-oxides and released their Cu and S back into the melt. These elements were then likely partitioned into the vapor phase, resulting in the formation of a metal-rich magmatic fluid emitted to the ocean during magma ascent and degassing. Therefore, magmatic sulfide saturation and dissolution play vital roles in controlling the behavior of chalcophile elements during magmatic differentiation and degassing.

Magmatic sulfide saturation and dissolution in the basaltic andesitic magma from the Yaeyama Central Graben, southern Okinawa Trough