Shock-Wave Properties and High-Pressure Equations of State of Geophysically Important Materials

Citation

Boslough, Mark Bruce (1984) Shock-Wave Properties and High-Pressure Equations of State of Geophysically Important Materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/C9GZ-3121. https://resolver.caltech.edu/CaltechETD:etd-09262002-154053

Abstract

Shock wave (Hugoniot), shock temperature, and release data are presented for several geophysically important, refractory materials. A sensitive multichannel optical pyrometer was developed to measure shock temperatures (2500 to 5600°K at pressures from 48 to 117 GPa) in anorthite (CaAl₂Si₂O₈) glass. Shock temperatures of 3750 to 6000°K at pressures from 140 to 182 GPa were measured in calcium oxide (CaO). Temperature data were used to constrain the energetics of the B1-B2 phase transition at 70 GPa in CaO, and to construct a finite-strain equation of state for CaO consistent with previous Hugoniot data.

The new CaO equation of state was used with equation of state parameters of other oxides to construct a theoretical mixed oxide Hugoniot of anorthite, which is in agreement with new Hugoniot data above about 50 GPa, determined using new experimental techniques developed in this study. The mixed oxide model, however, overestimates the shock temperatures, and does not accurately predict measured release paths. Both shock temperature and release data for anorthite indicate that several high pressure phase regions of stability exist above 50 GPa. A similar mixed oxide Hugoniot was constructed for lunar gabbroic anorthosite, and agrees with two new Hugoniot points at 120 GPa. Release data from lunar gabbroic anorthosite shocked to 120 GPa give evidence for shock vaporization.

Because the densities and bulk properties of CaO and the high pressure phase or phases of anorthite are so close to those determined seismologically for the lower mantle, the amount of these materials present in the lower mantle is not well constrained. The possibility of significant enrichment of the lower mantle in these refractory materials, as predicted by inhomogeneous accretion models, is still open.

A simple model is developed to explain the measured time dependences of radiated light in the shock temperature experiments, and constrain the absorption coefficient of the shocked material. The absorption coefficient is found to be an increasing function of shock pressure in shocked anorthite glass.

Hugoniot and release paths were determined using electromagnetic particle velocity gauges for San Gabriel anorthosite and San Marcos Gabbro shocked to peak stresses between 5 and 11 GPa. The data indicate a loss of shear strength in both rocks, and a partial phase transition of the anorthosite to a denser phase. This implies that estimates of shock wave attenuation in these materials based on elastic-plastic models are too high, and previously calculated amounts of internal energy gained by surface materials from impact or explosion events have been underestimated.

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