Experimental characterization of diffusive and Soret isotopic fractionation of sulfur in a reduced, anhydrous basaltic melt

Abstract

The diffusivity of isotopes of a given element in magmas can be significantly different, leading to observable effects on isotope ratios. Similarly, the Soret effect, or thermal diffusion, can also lead to considerable isotopic fractionations. Seeking to characterize isotope effects accompanying sulfur diffusion in basaltic melts, we conducted experiments involving (separately) both chemical and thermal diffusion at 1350–1500 °C and 1–1.5 GPa. In contrast to the isotopes of most elements investigated previously (Ca, Li, Mg, Fe, Cl), no significant isotope fractionations beyond analytical precision were found for sulfur isotope system (32S, 33S, 34S, and 36S) in response to thermal or chemical concentration gradients. Analysis of our run products by secondary ion mass spectrometry (SIMS) reveals a near-unity minimum diffusivity ratio for 34S/32S (β = 0) in anhydrous basaltic melt at conditions where sulfide species predominate. These results suggest that S isotope fractionations by diffusive processes are not important in basaltic melts, and therefore need not be considered when interpreting S isotope signatures. However, we also report potential Soret fractionations as large as approximately 1‰ for δ33S, 2.1‰ for δ34S, 3.8‰ for δ36S, and 40 ppm for elemental S, with enrichments of S and its heavier isotopes at the colder ends of the thermal gradients.