Experimental constraints on iron and sulfur redox equilibria and kinetics in basaltic melt inclusions

Abstract

The oxidation states of dissolved iron (Fe) and sulfur (S) in silicate melts were studied experimentally by heating Hawaiian olivines containing S-bearing melt inclusions (MIs) in a 1 atm gas-mixing furnace. The MIs are closed with respect to S and C, but equilibrate with the oxygen fugacity (ƒO₂) and water fugacity (ƒH₂O) of the furnace. We also conducted experiments using a S-free synthetic Hawaiian MI composition held on Pt-loops. Experiments were run at 1225 °C for 8–96 hr in H₂-CO₂ gas mixtures corresponding to ƒO₂ values that varied over seven orders of magnitude and then drop-quenched into water. Time-series experiments show that MI Fe³⁺/Fe²⁺ and S⁶⁺/S²⁻ ratios reached constant values within ∼24 hr. The H₂O contents of the Pt-loop glasses and the dehydrated MIs overlap and are proportional to the √fH₂O in the furnace gas. The slope of log₁₀(Fe³⁺/Fe²⁺) vs. log₁₀ƒO₂ is 0.25±0.02(2σ) for the Pt-loop glasses and 0.23±0.01(2σ) for the quenched MIs; the slopes of the two sets of experiments are consistent with equilibrium having been reached via the reaction FeO + ¼ O₂ = FeO_1.5. S-bearing quenched MI glasses have Fe³⁺/Fe²⁺ ratios that are systematically low compared to ratios measured in the S-free Pt-loop glasses run at the same T and ƒO₂; this may reflect an effect of S on Fe³⁺/Fe²⁺, although further experiments are needed to explore this possibility. S⁶⁺/S²⁻ ratios in the experimental melt inclusions vary systematically as a function of ƒO₂, and the measured ratios agree with independent calculations of S oxidation state in basaltic melts.

After being held at 1225 °C for 24 or 48 hr, a subset of olivine-hosted MIs were cooled at rates of 1700–40,000 °C/hr and quenched at temperatures between 900 °C and 1150 °C to explore whether Fe and S speciation are modified during cooling. Fe and S speciation in isothermal and cooled MIs held initially at the same ƒO₂ overlap within uncertainty, suggesting that the T dependence of the homogeneous reaction 8Fe³⁺ + S²⁻ = 8Fe²⁺ + S⁶⁺ is small and/or its kinetics are sluggish relative to cooling rates required to quench basaltic liquids to glasses. The centers of unheated, naturally quenched MIs from Papakōlea, Hawaii with syneruptive cooling rates ranging from ∼50 to 12,000 °C/hr were also measured for Fe and S speciation. MIs that experienced the lowest cooling rates have higher Fe³⁺/Fe²⁺ and S⁶⁺/S²⁻ ratios relative to rapidly cooled inclusions of comparable size. The increase in Fe³⁺/Fe²⁺ is due to olivine crystallization on the inclusion walls during cooling and diffusion of the resulting oxidized boundary layer into MI interiors. Diffusive modification is minimized in MIs that have been rapidly cooled and/or are sufficiently large such that the oxidized boundary layer is restricted to a narrow region adjacent to the inclusion walls. Our experiments and their comparison to naturally quenched MIs show that measurements of Fe³⁺/Fe²⁺ and S⁶⁺/S²⁻ ratios in quenched basaltic glasses and in the centers of rapidly cooled and/or large MIs are likely to be representative of the redox state in the melt at magmatic temperatures prior to quenching to glass.

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