Water Storage in Hydrous Minerals in the Shallow Martian Mantle

In this paper we investigate the possibility of storing water in the shallow martian mantle under water-saturated fluid absent conditions for different bulk silicate mars (BSM) compositions. We performed phase equilibria experiments on two BSM compositions with comparable Mg number for pressure between 2 and 4 GPa, temperatures between 950 to 1150°C, and for a water content of 0.3 % wt. The amphibole stability fields derived from both compositions are consistent with previous results obtained for Earth-like composition: (i) the water-saturated fluid-absent conditions expand the stability field towards higher pressures and temperature compared to fluid present conditions for both compositions tested; and (ii) the martian alkali-rich composition tend to stabilize the amphiboles at higher temperatures, as it is the case for alkali-rich Earth-like compositions. We used these results to recalculate the water storage capacity of the early martian mantle: by taking into account the possibility of amphibole crystallization in the upper mantle, the storage capacity of water in the martian mantle increases by 40 to 1000 Earth’s ocean mass (400 m to 1.5 km Global Equivalent Layer) depending on the bulk composition, water content and temperature profile of the martian mantle.