Water storage and early hydrous melting of the Martian mantle

Abstract

We report an experimental investigation of the near-solidus phase equilibria of a water-saturated analog of the Martian mantle. Experiments were performed at low temperatures (700–920°C) and high pressure (4–7GPa) using multi-anvil apparatus and piston cylinder device (4GPa). The results of this study are used to explore the role of water during early melting and chemical differentiation of Mars, and to further our understanding of the near-solidus behavior in planetary mantle compositions at high pressure. Water has a significant effect on the temperature of melting and, therefore, on accretion and subsequent differentiation processes. Experiments locate the wet solidus at ∼800°C, and is isothermal between 4GPa and 7GPa. The Martian primitive mantle can store significant amounts of water in hydrous minerals stable near the solidus. Humite-group minerals and phase E represent the most abundant hydrous minerals stable in the 4–7GPa pressure range. The amount of water that can be stored in the mantle and mobilized during melting ranges from 1 to up to 4wt% near the wet solidus. We discuss thermal models of Mars accretion where the planet formed very rapidly and early on in solar system history. We incorporate the time constraint of Dauphas and Pourmand (2011) that Mars had accreted to 50% of its present mass in 1.8Myr and include the effects of 26Al radioactive decay and heat supplied by rapid accretion. When accretion has reached 30% of Mars current mass (∼70% of its present size), melting starts, and extends from 100 to 720km depth. Below this melt layer, water can still be bound in crystalline solids. The critical stage is at 50% accretion (∼80% of its size), where Mars is above the wet and dry solidi with most of its interior melted. This is earlier in the accretion process than what would be predicted from dry melting. We suggest that water may have promoted early core formation on Mars and rapidly extended melting over a large portion of Mars interior.