Experimental calibration of iron and multiple sulfur isotope fractionation processes relevant to mid-ocean ridge hydrothermal systems

Abstract

Mid-ocean ridge hydrothermal systems experience a multitude of physiochemical processes, which control the fate of metals leached from the crust as a consequence of changes in pressure, temperature, and chemical conditions along the fluid flow pathway in the subseafloor. The processes of phase separation, conductive cooling, and mixing of hydrothermal fluids with seawater, for example, can all induce changes in the saturation state of sulfides, effectively enriching metals as seafloor sulfide deposits. However, these specific processes are not well understood with respect to the nature of isotopic partitioning of metals and of sulfur between minerals and constituent aqueous species. Furthermore, recent advances in mass-spectrometry now allow the perspective of non-traditional isotope systems, such as iron (Fe) and low-abundance isotopes of sulfur (S), capable of examination. Thus, this set of studies focuses on three main processes, 1) rapid precipitation and 2) recrystallization of pyrite, and 3) phase separation of metal-bearing fluids, which have been attributed to enrichment of metal sulfides at the seafloor and by controlling the composition of hydrothermal fluids emitting from the oceanic crust. Chapters 1 and 2 focus on Fe and multiple S isotope systematics associated with pyrite and dissolved constituent aqueous species at 300-350�C and 500 bars during rapid precipitation and recrystallization. This experimental dataset provides important calibrations to the Fe and S isotope systems that were either lacking or inaccurate, consequently providing important constraints on theoretical predictions and the Fe and S isotope fractionations observed at mid-ocean ridge hydrothermal systems. Correspondingly, the complimentary Fe and S isotope data are applied to the East Pacific Rise 9-10�N mid-ocean ridge hydrothermal system, where the natural data suggests that pyrite is typically is in disequilibrium with high temperature vent fluids and is indicative of forming from FeS precursors. The focus of Chapter 3 is on the observed Fe isotope fractionation between vapor and liquid phases during phase separation of a Fe- and NaCl-bearing solution at pressure and temperature conditions indicative of volcanic activity in the shallow subseafloor along the mid-ocean ridge system. These data provide insight on how mass transport of Fe and the associated isotope fractionation between phases is a function of differences in speciation of Fe-Cl complexes during isothermal decompression, where the vapor phase is isotopically enriched in the heavy isotopes of Fe and consists of the neutral FeCl2 complex while the chloride rich liquid phase is depleted and is dominated by the FeCl42- complex in solution.