CO2- and Ca-rich Fluids Drive Dolomite Formation During Hydrothermal Alteration of Peridotite

Abstract:

We present an experimental study investigating reaction pathways during the interaction of CO₂-rich aqueous fluids with mantle peridotite, which have major implications for geochemical budgets and microbial life in oceanic lithosphere. Powdered harzburgite was reacted with a Ca-enriched fluid in a flexible-cell hydrothermal apparatus at 300°C and 35 MPa for 1.7 years. A CO₂-rich fluid was subsequently injected and allowed to react for 8 months to examine the formation of carbonates under reducing conditions. Fluids were sampled throughout the experiment to monitor changes in fluid chemistry, and the secondary mineralogy was analyzed at the end of the experiment. Fluid speciation and mineral analyses suggest that initial serpentinization of harzburgite led to the precipitation of serpentine, brucite, magnetite, chlorite, calcite and Ni-sulfides. Fluids during this stage were characterized by low concentrations of dissolved Si, Mg and CO₂, alkaline pH_(25°C), and high concentrations of dissolved Ca, consistent with buffering by serpentine-brucite-diopside-calcite equilibria. H_2(aq) concentrations increased during the first 10 months of reaction (due to magnetite formation), but subsequently plateaued, suggesting that serpentinization approached completion prior to CO₂ injection. The introduction of CO₂ resulted in acidic pH_(25°C), substantial decreases in H_2(aq) concentrations, and increases in dissolved SiO₂ and Mg²⁺ concentrations. Dolomite and high-Mg calcite appear to have formed at the expense of olivine, calcite and likely brucite. However, petrographic observations suggest that Mg-calcite was only a transient phase and was ultimately destabilized in favor of dolomite. Replacement textures with carbonate in mesh centers are strikingly similar to those found in dolomite-altered abyssal serpentinites from the Atlantis Massif. While magnesite precipitation seems possible in ridge environments, high CO_2(aq) and Ca²⁺ activities in serpentinization systems appear to favor the precipitation of dolomite over calcite and magnesite. Exposure of actively serpentinizing peridotite to CO₂-rich aqueous fluids may therefore represent an important mechanism for dolomite formation and explain the low abundance of magnesite in mid-ocean ridge serpentinites.