Thermodynamic and kinetic controls on phase stability and incorporation of water in larnite (β-Ca2SiO4): implications for calcium silicate inclusions in diamonds

Abstract

Larnite (β-Ca₂SiO₄) has previously been reported as an inclusion in sub-lithospheric diamonds and is generally interpreted as a retrograde reaction product of calcium silicate perovskite. In this study, we review the controls on the stability of the Ca₂SiO₄ polymorphs and show that phosphorus is likely essential for the preservation of β-Ca₂SiO₄. We also report a detailed study of the solubility of water and its incorporation mechanisms in γ-Ca₂SiO₄ and phosphorus-doped β-Ca₂SiO₄ using FTIR spectroscopy on high-pressure experiments quenched from 4–9.5 GPa and 1000–1200 °C combined with ab initio calculations. The experimentally determined water solubilities are in the range of 107–178 ppm. Our FTIR spectra and ab initio calculations indicate that for phosphorus-free γ-Ca₂SiO₄ the incorporation mechanism involves protonated Si and Ca1 vacancies. For phosphorus-bearing β-Ca₂SiO₄, our preferred incorporation mechanism involves one Si⁴⁺ ion replaced by one P⁵⁺ ion with a single protonated Ca2 vacancy. The low water solubility observed here for larnite implies that if primary calcium silicate perovskite inclusions trap high water concentrations during diamond growth from a volatile-rich fluid, measurements of the concentration of water in larnite will not provide a useful record of the initial volatile concentration. Instead, water would be hosted in other retrograde reaction products, possibly including exsolved fluids.