IMPROVING THE SiO2 EQUATION OF STATE WITH SHOCK AND POST-SHOCK TEMPERATURES

Introduction: Planet formation and evolution involves high energy impacts capable of melting and vaporizing silicate mantles [1, 2]. SiO2 is an important end-member phase and reference material. At present, researchers lack a wide-ranging equation of state model for SiO2 that accurately captures the temperatures on the shock Hugoniot and post-shock states. The quartz and fused silica (amorphous SiO2) equations of state (EOS) can be improved with additional lab data, particularly in situ shock and post-shock temperatures in the region where these materials undergo shock melting. Along with their utility as compositional endmember minerals, these materials are often used in a variety of shock experiments as windows and standards for impedance matching and thermal emission [3, 4]. Thus, improving the laboratory measurements and modeled data for these materials provides better standard references. Previous studies using gas guns [5, 6] and laserdriven shocks [3] sampled this region, but little data is available in the superheating region of the Hugoniot and liquid region along the vapor curve. Additional data in this region provides insight to both the transition of SiO2 into the liquid phase in a shocked state as well as the onset of melting and vaporization upon release. The analytic equations of state code package (ANEOS) is frequently used by the planetary science community as it is capable of spanning the substantial temperature and pressure range achieved in natural impact phenomenon [7]. The code package has multiple features that enable modeling of solids, liquids, gases and plasmas. For most natural materials, the code package cannot accurately model the entire pressuretemperature range needed. As a result, each developer must make decisions about which features to use in the code package and which regions to fit more accurately. These decisions lead to a set of material parameters for use with a specific version of the ANEOS code that are constrained by data in some regions of phase space. Melosh [7] made updates to ANEOS using SiO2 where a Mie-type potential is used for the solid phase and molecular clusters are used for the vapor phase (M-ANEOS). At present, the available ANEOS models for silica have significant discrepancies in the melt region and liquidvapor phase boundary compared to laboratory observations. Figure 1 shows currently available ANEOS model Hugoniot and vapor domes for SiO2 alongside lab data [6, 11, 8]. This study focuses on taking shock and post-shock temperatures of quartz and fused silica in the pressure range where these materials undergo superheating and melting, approximately 55-130 GPa using multiple pyrometry systems. Here, we describe our shock pyrometry experiments on fused silica as well as plans for improving the model equation of state.