Anorthite: thermal equation of state to high pressures

Abstract

We present shock-wave (Hugoniot) data on single-crystal and porous anorthite (CaAl_2Si_2O_8) to pressures of 120 GPa. These data are inverted to yield values of the Grüneisen parameter (γ), adiabatic bulk modulus (K_s) and coefficient of thermal expansion (α) over a broad range of pressures and temperatures which in turn are used to reduce the raw Hugoniot data and construct an experimentally-based, high-pressure thermal equation of state for anorthite. We find surprisingly high values of γ which decrease from about 2.2 to 1.2 over the density range 3.4 to 5.0 Mg m^(−3). Our data clearly indicate that whereas the zeroth order anharmonic (quasi-harmonic) properties such as γ and α decrease upon compression of a single phase, these properties apparently increase dramatically (200 per cent or more) in going from a low to a high pressure phase. The results for anorthite also support the hypothesis that higher-order anharmonic contributions to the thermal properties decrease more rapidly upon compression than the lowest order anharmonicities. We find an initial density p_0 ~ 3.4 Mg m^(−3) for the 'high-pressure phase' portion of the Hugoniot, with an initial value of Ks essentially identical to that of anorthite at zero pressure (90 GPa). This is surprising in light of recently documented candidate high-pressure assemblages for anorthite with significantly higher densities, and it raises the question of the non-equilibrium nature of Hugoniot data. By correcting the properties of anorthite to lower mantle conditions we find that although the density of anorthite is comparable to that of the lowermost mantle, its bulk modulus is considerably less, hence making enrichment in the mantle implausible except perhaps near its base.

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