In situ determination of magnesite solubility and carbon speciation in water and NaCl solutions under subduction zone conditions

Abstract

The dissolution behavior of carbonates in subduction zone fluids has not been well constrained. In this study we investigated the solubility of magnesite in pure water and NaCl solutions (with up to 19 wt% NaCl) in situ with a hydrothermal diamond anvil cell, and determined carbon speciation in the fluid by Raman spectroscopy. The solubility of magnesite in pure water falls in the range of 0.01–0.05 mol/kg at 0.7–2.4 GPa and 635–940 °C, enhanced strongly by temperature. Least squares fitting of experimental data leads to the following empirical expression for magnesite solubility in pure water: $C_{{\text{MgCO}}_3}=(964510\pm 41362)\exp \left(\frac{-6953\pm 690}{T}\right)\exp \left[\frac{0.0200\pm 0.0098}{T}(P-1)\right]$ where $C_{{\text{MgCO}}_3}$ is given in μg/g, T is the temperature in K, and P is the pressure in bar, respectively. The solubility of MgCO₃ is about an order of magnitude lower than that of CaCO₃ at 0.7–2.4 GPa, 640–940 °C. The solubility enhancement factor by NaCl (m/m° with m and m° being magnesite solubility in NaCl solution and water, respectively) presents as a parabolic trend with the mole fraction of NaCl in the range of 0–0.07, with a maximum amplification of 5.2 at X_NaCl = 0.035, which is different from the continuously increase of solubility with salinity increasing at high salinity conditions in previous studies and suggests the dissolution reaction of magnesite in dilute NaCl solution is different. Despite slight contamination of CH₄ formed by the reaction of the diamond anvils, we were able to identify CO₃²⁻ and HCO₃^- to be the aqueous carbon species, HCO₃^- was predominant over CO₃²⁻ in the range of 200–800 °C and 1.9–3.8 GPa and its proportion was affected by temperature, but not affected by pressure at 400–600 °C. Our experimental data suggest that in the absence of melting, only a small amount of magnesite can be mobilized by the slab-released fluid at subarc depths.