On the Kinetics of Materials of Geophysical Interest

Abstract

Knowledge of the equation of state and phase diagram of magnesium silicates and light iron alloys is important for understanding the thermal evolution and interior structure of terrestrial planets. Dynamic compression techniques are the primary viable methods to create the temperature and pressure conditions that are relevant to Earth and super-Earth (1-10 Earth mass) sized planets. However, due to the kinetic constraints imposed by the timescale of dynamic compression experiments, the nature of the state within the dynamically compressed sample (whether equilibrium or metastable) is uncertain. Here, we present the results of a series of dynamic compression experiments performed on both laser driven compression and plate impact facilities to study the nanosecond to microsecond response of forsterite and iron silicide. In situ x-ray diffraction measurements are used to probe the crystal structure of solid phases and test for the presence of melt, from which we investigate the decomposition of forsterite and iron silicide into compositionally distinct phases at high pressure. For forsterite, we do not observe chemical segregation in the solid phase, however the presence of melt speeds up the kinetics and allows chemical segregation to occur on nanosecond timescales. For iron silicide, our results show a textured solid phase upon shock compression to pressures ranging from 166(14) to 282(24) GPa consistent with cubic and hcp structures in coexistence. Above 313(29) GPa, the intense and textured solid diffraction peaks give way to a diffuse scattering feature and loss of texture, consistent with melting along the Hugoniot.