Water Storage and Early Hydrous Melting of the Martian Mantle

Details Water Storage and Early Hydrous Melting of the Martian Mantle Water Storage and Early Hydrous Melting of the Martian Mantle

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p14a..07p&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P14A-07

Physics

[3630] Mineralogy And Petrology / Experimental Mineralogy And Petrology, [6225] Planetary Sciences: Solar System Objects / Mars, [8147] Tectonophysics / Planetary Interiors

Scientific paper

We present an experimental investigation of a water-saturated analogue of the Martian mantle at low temperature (700-920°C) and high pressure (4-7GPa) using a multi-anvil apparatus. The results of this study are used to explore the role of water in the early chemical differentiation of the planet, 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. Results show that the wet solidus reaction, located at ~800°C, remains at that temperature between 4GPa and 7GPa. The Martian primitive mantle can store significant amounts of water in hydrous minerals stable near the solidus. Humite minerals and phase E represent the most abundant hydrated minerals stable under pressure. The amount of water that can be stored in the mantle and mobilized during melting ranges from 1 to up to 4wt% at the wet solidus. Hydrous melt has also been analyzed in an experiment at 920°C and 5.2GPa and is roughly andesitic, consistent with the findings of others that partial melting of peridotite produces high silica melts. Based on our experimental data and considering both impact and radioactive heat sources, we propose a thermal model of Mars accretion. We assume that Mars formed very rapidly (3.6 Myr or less according to recent studies) and accreted initially from a mix of chondrites (85%H, 11%CV, 4%CI) that contain a bulk water content of 1.1 wt.% H2O. Because Mars accreted quickly and early in solar system history, 26Al decay played an important role in the thermal evolution of the planet. We found that at 20% of its present mass (corresponding to ~60% of its size), the planet is cool enough to retain the water stored in hydrous minerals. At 30% (~70% of its size), melting starts at -but is not limited to- a shallow depth (1-3GPa) and water can still be bound in crystalline solids. The critical stage is at 50% (~80% of its size), where Mars is now above the wet and dry solidi with most of its interior melted. Water allows melting to occur earlier in the accretion process and the presence of water promotes the formation of a significant amount of melt, contrasting with dry accretion scenarios. Interestingly, the 50 % accretion step matches with the time estimated for core formation by recent Hf/W isotopic studies [1]. Therefore, we suggest that water may have promoted early core formation on Mars and rapidly extended melting over a large portion of Mars interior. [1] Dauphas, N., Pourmand, A., 2011. Hf-W-Th evidence for rapid growth of Mars and its status as a planetary embryo. Nature, 473, doi:10.1038/nature10077.

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