Water solubility in mantle Ca-silicate phases – A high-pressure experimental and first-principles calculations study

Abstract:

Larnite (β–Ca₂SiO₄) inclusions in diamond, usually coexisting with breyite (walstromite-structured CaSiO₃) and titanite-structured CaSi₂O₅, are found in super-deep diamonds that are believed to have a transition zone or lower mantle origin. These calcium silicate inclusions are thought to represent retrograde transformation products from the major lower mantle phase CaSiO₃-perovskite. Estimates of water solubility in CaSiO₃-perovskite are contradictory and range from 1 wt.% to near zero [1, 2]. Furthermore, our understanding of the solubility of water in the retrograde transformation products such as larnite and breyite is very poor. This information is necessary to determine whether measured water contents in naturally-occurring larnite and breyite inclusions in super-deep diamond represents a solubility limit imposed by crystal structure during retrogression or the original water content of the CaSiO₃-perovskite within the diamond forming region of the lower mantle.

In this study we focus on determining the water content of larnite using high-pressure/temperature experiments and FTIR measurements of single crystals from run products. Larnite shows narrow hydroxyl (OH^-) absorption bands around 3550 cm^-1 and the maximum solubility measured is less than 100 ppm H₂O. To understand the nature of hydrous defects in larnite we also performed first-principle simulations. We calculated OH defect geometries, formation energies and vibrational frequencies of various hydrous defects in larnite to determine the most favourable hydrogen incorporation mechanism. Based on this study and our previous work, the water solubility of both larnite and breyite appears to be restricted to less than 100 ppm. This could be much less than the solubility in CaSiO₃-perovskite [1]. Therefore, the measured water contents of these retrograde reaction products in diamond inclusions is unlikely to reflect the original water content of CaSiO₃-perovskite from a hydrous deep mantle reservoir.

[1] Chen et al. (2020) Phys. Earth Planet. Inter. 299, 106412.

[2] Ross et al. (2003) Am. Mineral 88, 1452-1459.