Advances in Shock Compression of Mantle Minerals and Implications

Synthesis and consolidation of polycrystalline, 3 mm diameter, shock wave targets of the high-pressure mantle polymorphs Mg$_{2}$SiO$_{4}$ wadsleyite and MgSiO$_{3}$ perovskite was carried out in multi-anvil high-pressure modules at Caltech and Bayreuth, respectively. Hugoniot equation of state measurements on these samples, together with data on lower-density isochemical materials, constrain the thermal equations of state and melting temperatures of the simplified lower mantle assemblages MgSiO$_{3 }$(perovskite and post-perovskite) and MgSiO$_{3}$(perovskite) + MgO. Evaluation of V($\partial $P/$\partial $E)$_{v}$ for the high-pressure melts of both compositions yield Gr\"{u}neisen parameters that \textbf{\textit{increase}} by about a factor of three upon two-fold compression. This result predicts a large temperature increase with depth along a model lower mantle magma ocean adiabat, and is consistent with the deep residual magma ocean model of Labrosse et al. (2007). Possibly, the high density, partially molten layer at the base of the mantle discovered by seismology is a very thin, highly enriched (K,U,Th and Xe) remnant of an ancient magma ocean.