Post-shock temperatures in minerals

An experimental technique has been developed for the measurement of post-shock temperatures in a wide variety of materials, including those of geophysical interest such as silicates. The technique uses an infrared radiation detector to determine the brightness temperature of samples shocked to pressures in the range 5 to ∼ 30 GPa; in these experiments measurements have been made in two wavelength ranges (4.5 to 5.75 μm; 7 to 14 μm). Reproducible results, with the temperatures in the two wavelength bands generally in excellent agreement, have been obtained for aluminium-2024 (10.5 to 33 GPa; 125 to 260°C), stainless steel-304 (11.5 to 50 GPa; 80 to 350°C), crystalline quartz (5.0 to 21.5 GPa; 80 to 250°C), forsterite (7.5 to 28.0 GPa; ∼ 30 to 160°C) and Bamble bronzite (6.0 to 26.0 GPa; ∼ 30 to 225°C). These results are generally much higher at low pressures (where they may even be in excess of the calculated shock temperatures) than the values calculated assuming a hydrodynamic rheology and isentropic release parallel to the Hugoniot but tend towards them at higher pressures. In aluminium-2024, the theoretical post-shock temperatures, assuming a fluid-like rheology, are 35 to 218°C, for the pressure range 10.5 to 33 GPa. However, the results are in considerably better agreement with values calculated assuming elasto-plastic behaviour (80 to 270°C) which probably also causes the high measured temperatures for stainless steel. In forsterite the measured values ranged from 65°C at 9.6 GPa (there was no detectable rise at 7.5 GPa) to 156° at 28.0 GPa, whereas the ‘hydrodynamic values’ were 30 to 120°C. Values obtained for quartz were in excellent agreement with those calculated by Mashimo et al. using release adiabat data. It is concluded that release adiabat data should be used, wherever available, for calculations of residual temperature, and that adequate descriptions of the shock and release processes in minerals need to be more complex than generally assumed