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

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.