Experimental Investigations of the Deep Earth's Mantle Melting Properties

Abstract:

Studying the melting properties of the mantle is fundamental to understand the global dynamics of our planet. Already in the early stages of its history, Earth experienced extensive melting events leading to the formation of a magma ocean, due to the massive energy released by large-scale meteoritic impacts, radiogenic decay and core-mantle gravitational segregation. Partial melting however still plays a key role in today’s mantle’s dynamics. Seismic observations have in fact highlighted the presence of anomalies in the upper mantle, atop the 410 km discontinuity. These regions, called ultra-low velocity zones, show a significant reduction in shear wave velocity, compatible with the presence of partial melting. We studied the melting properties of the mantle using in situ X-ray diffraction experiments and electrical conductivity measurements at high pressure and high temperature. Both technics are very sensitive to the presence of liquid in the sample. They provide accurate information not only on the evolution with pressure of the solidus temperature, but also on the degree of partial melting with increasing temperature. Our samples consisted of a synthetic CI chondrite glass, a “proxy” composition for the primitive mantle after core segregation. Pressures up to 25 GPa (upper/lower mantle limit, at 670 km depth) were generated by the multi-anvil press. Our results suggest that previous studies overestimated by ~300 K the solidus temperature, probably due to a lack of resolution in determining low degrees of partial melting in their experimental procedure, compared to the in situexperiments presented here. Our results imply that partial melting could happen more frequently than initially expected, particularly in mantle regions where incompatible elements and volatiles are concentrated.