Mass transfer of Fe during the serpentinization of olivine by SiO2 rich fluid at 300°C, 500 bars: Perspectives from mineral dissolution/precipitation rates and Fe isotope systematics

Abstract:

High temperature (~300°C) hydrothermal alteration of peridotites can produce an alteration assemblage abundant in Fe-bearing serpentine and magnetite without the presence of brucite. This is particularly so in systems with SiO₂-rich fluids derived from the hydration of orthopyroxene in basaltic intrusions and gabbros [1]. Few experimental studies have investigated the effects of aSiO_2(aq) on the rate of olivine serpentinization and none that have examined the Fe isotopic composition of olivine hydrolysis products. Thus, this study addresses these problems by using flexible gold cell hydrothermal equipment to react olivine (Fo₉₀) and talc with a NaCl-bearing fluid at 300 °C and 500 bars for ~90 days; providing time-series solution chemistry data coupled with Fe isotope, magnetic susceptibility, and Mössbauer measurements of reactant olivine and the serpentinization product. Talc is used to elevate the aSiO_2(aq)above the serpentine-brucite buffer, effectively preventing brucite formation and allowing only the formation of Fe-bearing serpentine and magnetite from olivine alteration.

Initial time series solution chemistry data indicate that the net rate of the serpentinization of olivine and talc dissolution is such that the experimental system is poised between the serpentine-brucite and serpentine-talc stability fields, with little H₂ generated by the oxidation of Fe²⁺ upon formation of Fe-serpentine and magnetite. However, as the talc Si-source becomes effectively titrated, the continued hydration of olivine decreases the aSiO_2(aq) towards the serpentine-brucite stability field concurrent with an increasing rate of H₂ generation. This chemical transition likely reflects an enhanced rate of magnetite formation upon a decrease in the relative stability of Fe-serpentine. Fe isotope data indicate a slight enrichment trend in δ⁵⁶Fe versus Fe³⁺/ΣFe of the altered mineral phases, magnetite > Fe-serpentine > olivine, although the observed inter-mineral fractionations are small, <0.1 ‰. These experimental data are consistent with observations of natural Fe isotope data derived from hydrothermally altered peridotites [2] while providing requisite quantitative constraints to understand better their origin and evolution.

[1] Bach et al., 2006 (GRL)

[2] Craddock et al., 2014 (EPSL)