Redox equilibria of iron in low- to high-silica melts: A simple model and its applications to C-H-O-S degassing

Abstract

Delineating the redox variations of magmatic processes relies on quantitative models based on the redox equilibria of multi-valent elements, such as Fe and S, in the melt and/or minerals. Much effort has been made to quantify the redox equilibrium between ferrous and ferric iron (Fe2+ and Fe3+) in silicate melts; however, existing Fe-redox models display large, systematic uncertainties for low- and high-SiO2 melts. Here we present a new Fe-redox model that can predict the oxygen fugacity (ƒO2) and Fe3+/Fe2+ ratios for magmatic liquids with 0 – 79 wt% SiO2 at variable temperatures and pressures. Our model calibration is based on an effective equilibrium constant using a comprehensive compilation of experimental data (N = 1000; 1 bar – 7 GPa; 800 – 1750 °C). Taking the recently published experiments (N = 69; 1 bar – 24 GPa; 900 – 2600 °C; SiO2 = 39 – 79 wt%) as independent tests, we show that our model can be extrapolated to silicate melts at higher pressure and temperature. Applications of this model to representative melts demonstrate that adiabatic transport from the source to surface does not appear to vary the ƒO2 of mid-ocean ridge basalt and rhyolite melts but can increase the ƒO2 of alkali ocean island basalt (OIB) melts and peridotite magma ocean liquids by ∼2 – 3 log units. Notably, the adiabatic ƒO2 profile of alkali OIB indicates an auto-oxidation process controlled by Fe redox equilibria, decoupling the ƒO2 of OIBs and their mantle sources. For an improved understanding of the redox evolution during magma degassing, we calibrate a new predictive model for S6+/ΣS ratios in silicate melts. With the newly developed Fe- and S-redox models, we then construct a new Magma And Gas Equilibrium Calculator (MAGEC) with improved user-settings that can simulate S degassing in dynamically evolving gas-melt coupled magmatic systems with a broad range of compositions. Using olivine-hosted melt inclusion data from silica-undersaturated lavas, we demonstrate that the new MAGEC has predictive capacities of redox equilibria and S distribution during magmatic degassing, making it a useful tool for a quantitative understanding of volatile behaviors in various magmatic systems relevant to planetary differentiation and crustal formation.