UC Davis

Shock processing via impacts is a common occurrence during planet formation and evolution. Characterizing how rocks and minerals respond to extreme conditions is critical to understand and model realistic impact events.First, we review SiO2 at high pressure, including static compression and ab initio calculations of the melt curve and solid phase boundaries, and a particular focus on shock wave data. The first shock temperature measurements on α-quartz and fused silica displayed unusual drops in temperature with increasing pressure. The temperature excursions were interpreted as metastable superheating of the solid phase followed by a drop in temperature to the equilibrium melt curve. For several decades, this superheating interpretation has been accepted and inferred to occur in other materials. Upon compilation of available data, we propose a new interpretation of the melt curve and the SiO2 Hugoniot, focusing on the correlation of shock data with solid phase changes and recent static melt curve experiments. Finally, we suggest superheating does not occur and this feature can be explained by changes in the solid phases and physical properties of the liquid along the melt curve. We propose that the shock Hugoniots provides direct information about the melt curve in agreement with static measurements, including regions with negative slopes in temperature with increasing pressure. Next, we performed a series of gas gun experiments on quartz and fused silica, one of the major components of silicates and a common material on the surfaces of evolved bodies and major component of rocky bodies. We probed the thermodynamic region where these materials undergo melting with a focus on measuring shock and release temperatures. We found that the thermal histories of these samples provide insight on the phase transitions of these materials as well as support for proposed changes to the SiO2 melt curve and understanding of superheating or lack thereof. Finally, we performed a series of experiments on the pyroxenes enstatite (MgSiO3) and bronzite ((Mg,Fe)SiO3 and a pyrolitic (olivine + pyroxene) composition glass at the Sandia Z Machine. These experiments reach higher pressures than are available to gas gun platforms. Z experiments reach pressures similar to peak giant impact conditions and planetary interiors. We focused on defining the principal Hugoniots with temperatures. These materials are incredibly useful for understanding planet formation and impact modeling as pyroxene is the most common silicate in the mantle and pyrolite represents the composition of Earth’s mantle or a quenched magma ocean. Additionally, these materials are colored and contain iron, which have non-ideal optical properties for typical pyrometry diagnostics. With rigorous consideration of wavelength-dependent absorption, we determined temperatures and compared to similar experiments on the OMEGA EP laser platform that uses an alternative pyrometry diagnostic.