Water solubility in coesite at realistic temperatures of subduction zones

Abstract

To constrain the water solubility of coesite (Coe) at typical temperatures of subduction zones, a series of Coe coexisting with an aqueous fluid was synthesized at 3–6 GPa and at 600–800 °C. Most experiments were performed in the system SiO2-H2O, and some were performed with small amounts of boron addition. With very long heating durations (120–336 h), all these experiments successfully produced Coe crystals of large grain size, ranging from ∼100 to 1300 μm. For every experimental product, multiple unpolarized FTIR spectra were collected on randomly-selected Coe crystals. We have found that type-I hydrogarnet substitution (Si4+(Si2) + 4O2− = [4]□(Si2) + 4OH−) is the major water-incorporation mechanism whereas B-related defect (H+ + B3+ ↔ Si4+) makes no much contribution. The water solubility of Coe, ranging from 3(1) to 47(12) wt ppm, positively correlates with both P and T. It can be well described by the empirical equation cH2O = −49(17) + 6.0(21) × P + 0.06(2) × T (cH2O representing water content in wt ppm, P pressure in GPa and T temperature in °C) and by the thermodynamic expression [Formula presented] (cOH representing water content in H/106 Si, fH2O water fugacity in GPa, R the gas constant, P pressure in GPa, T temperature in K, ∆S1 bar reaction entropy as 18.5(509) J/mol/K, ∆H1 bar reaction enthalpy as −10.7(516) kJ/mol, and ∆Vsolid the volume change of Coe during hydroxylation as 23.6(42) cm3/mol). Consequently, the water solubility of Coe in subduction zones should be ∼0–123 wt ppm. When Coe with deep origin becomes metastable (i.e., approaching the P-T locus of the Coe-quartz (Qz) reaction), its water content is likely less than ∼10 wt ppm. This trace water may be quickly lost along with further exhumation process, and metastable Coe becomes completely dry, as observed in the Coe discovered in all exhumed ultrahigh-pressure (UHP) metamorphic rocks. Since structurally-bonded water substantially speeds up the Coe-to-Qz phase transition, zero water in metastable Coe may be the key to the preservation of Coe in the UHP metamorphic rocks.