No detectable redox exchange between sulfur and iron during rapid cooling of basalts

Abstract

The valence states of iron and sulfur in mantle-derived melts influence and respond to the composition of the mantle as well as the subsequent evolution of those melts in Earth's crust. Characterizing the valence states of iron and sulfur in silicate melts that have quenched to form glasses is therefore critical to understanding the composition of the mantle and the magmatic processes that take place before and during volcanic eruptions. Glasses quenched from silicate melts offer the opportunity to precisely measure sulfur and iron valence states, but whether the transition from melt to glass affects the valence state of sulfur and iron is uncertain. Here, we use the glassy margins of two pillow basalts, one from a mid-ocean ridge and one from a back-arc basin, as a natural experiment to test the effect of quench rate on sulfur and iron valence states. We report micro-X-ray Absorption Near Edge Structure (XANES) measurements of S6+/ΣS (S6+/[S6++S2−]) and Fe3+/ΣFe (Fe3+/[Fe2++Fe3+]) along transects from the rapidly quenched rims of the pillows down into the slowly cooled crystal-rich interiors. The range of average quench rates estimated for our selected analysis areas range from 1.2×105°C/sec at the pillow margin to 1.1°C/sec in the interior and span quench rates experienced by natural and experimental glasses. On average, S6+/ΣS = 0.09±0.01 and Fe3+/ΣFe = 0.147±0.002 in the mid-ocean ridge sample and S6+/ΣS = 0.21±0.01and Fe3+/ΣFe = 0.153±0.004 in the back-arc sample, where the quoted uncertainty is the 1σ standard deviation of n= 9-16 analyses. We find that the S6+/ΣS and Fe3+/ΣFe values along transects in each sample show no systematic changes with depth, and by proxy, no systematic changes with quench rate. In each pillow, all measured S6+/ΣS and Fe3+/ΣFe ratios are within the absolute uncertainty of each technique. We propose that these observations are consistent with either sluggish reaction kinetics between sulfur and iron, or an insignificant temperature effect on sulfur-iron redox exchange equilibria. These observations demonstrate that sulfur and iron valence states in glasses with varying quench rates can be directly compared, and that changes in sulfur and iron valence during quench are below the limits of detection with XANES.