Shock Wave Studies of Mineral and Melt Physics at Deep Mantle Pressures

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Study of the deep interior of the Earth is essential to understanding the long-term evolution of our planet, including the habitability and natural hazards of the surface where we live. We are particularly interested in the melting of the deep interior, and its consequences for the chemical evolution of the mantle. The extreme conditions that characterize great depth, particularly high pressure and temperature, and the limits of direct observations at such depths, however, require the application of specialized experimental techniques. One family of techniques uses large stationary guns to collide mineral and metal targets together at high speed and so generate very high pressures for a short time, a method referred to as shock wave research. The measurements that we can make with these experiments are unique, but also complementary to results obtained by other experimental and theoretical techniques. We will dedicate the Caltech shock wave laboratory over the next three years to the study of melting and the thermodynamic properties of minerals and melts under lower mantle conditions. The ultimate goal is development of accurate models of phase equilibria and physical properties of simplified mantle compositions that can be compared to geophysical observables or inserted into dynamic models of processes that may occur in the deep mantle today, such as formation of mantle plumes, or that may have occurred early in Earth history, such as magma ocean crystallization. With constraints on the plausible temperature, composition, and relative buoyancy of partially molten regions that might constitute the ultra-low velocity zones seismically observed at the base of the mantle, we will place limits on the dynamical and chemical consequences of the existence of such zones. Likewise, knowledge of the liquidus mineralogy and the relative buoyancy of liquids and solids helps to define how an early terrestrial whole-mantle magma ocean might have evolved by crystallization-differentiation and so the initial state of solid mantle evolution.Our experimental approach will be simultaneous determination of shock velocity, sound speed, and shock temperature in transparent materials, principally MgO and MgSiO3 shocked into lower mantle phase assemblages. Determination of all these quantities together provides optimal constraints on melting curves and thermal equation of state parameters of solids and liquids. We focus this effort on materials of geophysical interest where these quantities remain most uncertain due to limitations of existing data and experimental capabilities. We have demonstrated through sound-speed measurements on porous MgO that we are able to shock melt this material and define the location of its highly uncertain melting curve. We will continue this program using different porosities and pre-heated single crystals both to obtain more constraints on the melting curve and to define the unknown thermophysical properties of MgO liquid. To our existing database of shock results in the MgSiO3 system, we propose to add a series of shock velocity, sound speed, and shock temperature measurements on single crystal enstatite and on preheated MgSiO3 liquid in order to remove parameter trade-offs in the thermodynamic description of this liquid composition.