Arc magmas oxidized by water dissociation and hydrogen incorporation in orthopyroxene

Abstract

Elevated H 2 O concentrations and oxygen fugacities are two fundamental properties that distinguish magmas formed in subduction zones from new crust generated at mid ocean ridges. However, the mechanism of magma oxidation, and how it relates to the increase in H 2 O remains unclear. In this study, we use infrared spectroscopy of mantle wedge orthopyroxene to trace the temporal and spatial evolution of oxygen fugacity during transport of hydrous arc melts towards the crust. A positive correlation between equilibrium oxygen fugacity and orthopyroxene H 2 O concentrations for the peridotite samples studied allowed the assignment of specific, commonly-observed absorption bands to redox-sensitive crystallographic defects. H 2 O content associated with these redox-sensitive defects increases in concentration across individual crystals, uniquely preserving the time-dependent transition from reduced to oxidised conditions during the migration of hydrous melts through the mantle wedge. A separate, but related process of reaction with H 2 occurring primarily during the earliest stages of melt-mantle reaction may be fundamental in generating the oxidised nature of hydrous melts. Our study proposes that the oxidised nature of arc magmas may not be a primary feature, but is instead acquired progressively as hydrous primary melts react with the surrounding mantle.

Uncovering and quantifying the subduction zone sulfur cycle from the slab perspective

“…However, our finding of negligible sulfate in the slab fluids indicates that slab SO 4 2− was unlikely to be the main oxidizing agent during South Tianshan Ocean subduction. In such environments, the high fO 2 of sub-arc mantle may instead result from addition of slab H 2 O and CO 2 (refs 63,64 ), instead of oxidized sulfur species. Processes including incorporation of H 2 into orthopyroxene 63 and the formation of diamond 64 and CH4 (ref.…”

“…Therefore, the negativeto-positive shift in the δ 34 S composition of melts must happen as part of the partial melting processes, such that significant sulfur isotope fractionation accompanies the melt oxidization. For example, 32 S may be scavenged into surrounding mantle to form sulfides while H 2 is incorporated into orthopyroxene 63 , producing 34 S-rich sulfate in oxidizing melt and finally isotopically heavier arc magmas. Thus, further studies will be necessary to assess the processes that may lead to the positive δ 34 S compositions in arc magmas.…”

Reduced methane-bearing fluids as a source for diamond

“…Considering the redox conditions of our experiments, reactions 2 and 3 should be more relevant where diamond forms upon removal of H 2 . One way for this to happen is for H to become sequestered in orthopyroxene and olivine via the equilibria proposed by Tollan and Herman 44 for orthopyroxene: and by Tollan et al 45 for olivine: The [] in equilibrium (4) denotes a lattice vacancy in orthopyroxene. The formation of OH in olivine during the experiments is documented by the FTIR spectra presented in Fig.…”

Unraveling the Effects of Melt–Mantle Interactions on the Gold Fertility of Magmas

“…Hence, the formation of oxidized magmas can efficiently promote the titration of sulfur and ore metals from the mantle, ultimately increasing the ore metal endowment of the overlying crust (Sillitoe, 2008;Richards, 2015). Furthermore, recent studies have provided evidence that restricted fertile blocks of the SCLM may act as fertile source regions from where ore metals were tapped by partial melting, fluxed melting, or by interaction with ascending melts (Griffin et al, 2013;Groves and Santosh, 2015;Groves et al, 2016;Tassara et al, 2017). However, the impact and extent of silicate melt-SCLM interaction on the composition and oxidation state of ascending melts that ultimately reach the crust to form ore deposits, and the mechanisms of ore metals extraction from the SCLM remain largely unexplored.…”