UC Santa Cruz

Fractional crystallization of intermediate depth (~160-260 km), impact-induced magma oceans on Mars are examined. Residual liquids become denser than normal Martian mantle. Fractional crystallization near 3 GPa establishes inverted density distributions that can generate melt descent within the mantle. Liquid compositions produced by ~45-80 wt% crystallization become dense enough to descend to the core-mantle boundary (CMB) and could form a stably stratified thermochemical boundary layer (TCBL). If this layer crystallizes, its mineralogy would be dominated by either garnet and ferropericlase, or stishovite and ringwoodite. Although the size of Mars’ core remains uncertain, the addition of such a thermal boundary layer would impede stabilization of (Mg, Fe)SiO3-perovskite at the base of the mantle. A TCBL would both elevate the inferred temperature of the core and inhibit heat flow out of the core, with a potentially causal relation with the current lack of an internally generated Martian magnetic field. A comprehensive analysis of experimental data and theoretical simulations on the partial molar volume of water in silicate melt indicates that finite strain theory successfully describes the compression of the H2O component dissolved in silicate melt at high pressures and temperatures. However, because of the high compressibility of the water component, a fourth order equation of state fit is required to accurately simulate experimental results on water’s volume in silicate melts at deep upper mantle, transition zone and lower mantle pressures. Data from previous shock compression experiments on hydrous minerals in which melting occurs along the Hugoniot are used to provide an experimental constraint on the partial molar volume of water in silicate melt at deep mantle temperatures and pressures. The equation of state of the water component indicates that, depending on elastic averaging technique, the amount of water that could be present in neutrally or negatively buoyant mafic/ultramafic melts above the 410 km seismic discontinuity is upper-bounded at 5.6 wt%: smaller than previously inferred, and consistent with melt being confined to a narrow depth range above the 410 km discontinuity. If melt is predominantly distributed along grain boundaries in low aspect ratio films, extents of melting as low as 2% could produce observed seismic velocity reductions. The ability of the lowermost mantle to contain negatively buoyant hydrous liquids hinges on the trade-off between iron content and hydration: at these depths, substantially higher degrees of hydration could be present within partial melts. The crystal structure and bonding environment of K2Ca(CO3)2 bütschliite were probed under isothermal compression via Raman spectroscopy to 95 GPa and single crystal and powder x-ray diffraction to 12 and 68 GPa, respectively. A second order Birch-Murnaghan equation of state fit to the x-ray data yields a bulk modulus, K_0=46.9 GPa with an imposed value of K_0^'= 4 for the ambient pressure-phase. Compression of bütschliite is highly anisotropic, with contraction along the c-axis accounting for most of the volume change. Bütschliite undergoes a phase transition to a monoclinic C2/m structure at around 6 GPa, mirroring polymorphism within isostructural borates. A fit to the compression data of the monoclinic phase yields V_0=322.2 Å^3, K_0=24.8 GPa and K_0^'=4.0 using a third order fit; the ability to access different compression mechanisms gives rise to a more compressible material than the low-pressure phase. In particular, compression of the C2/m phase involves interlayer displacement and twisting of the [CO3] units, and an increase in coordination number of the K+ ion. Three more phase transitions, at ~28, 34, and 37 GPa occur based on the Raman spectra and powder diffraction data: these give rise to new [CO3] bonding environments within the structure. Raman spectra of Na2Ca2(CO3)3 shortite and K2Ca(CO3)2 bütschliite were measured to 715°C and 740°C respectively, under vacuum. Shortite converts to nyerereite and calcite at 535°C. This assemblage is stable to ~700°C, where nyerereite begins to decompose. Bütschliite converts to isochemical fairchildite at 570°C. Fairchildite is stable to 665°C, where it decomposes to an assemblage of K2CO3 and CaO. The spectra of both fairchildite and nyerereite exhibit features consistent with extensive disordering.