Shock Thermodynamics of Mantle Rocks: Rockport Fayalite

Introduction. In order to address questions related to giant impacts and impact cratering on terrestrial planets, we need robust equations of state (EOS) and thermodynamic data for major mantle minerals (e.g., the olivine series and enstatite) and rocks under a wide range of pressure-temperature conditions. It is important to accurately characterize the amount of impact-induced heating that occurs in order to understand a range of planetary problems, including the mechanics of basin formation, the formation of the Martian crustal dichotomy, the origin of Earth's moon, and the depths of magma oceans on the early Earth during accretion. The long-term goal of this work is to develop comprehensive EOS for the most important mantle minerals for use in impact modeling and to understand the heterogeneous distribution of shock and post-shock temperatures in rocks. Here we present the results from new post-shock temperature experiments on Rockport fayalite and comparisons to previous post-shock measurements on rocks. Experimental methods. Planar shock experiments were performed in the Harvard Shock Compression Laboratory with a 40-mm single stage powder gun that can achieve shock pressures up to about 50 GPa in mantle materials. Simultaneous measurements of free surface velocity and thermal emission were made with a co-aligned velocity interferometer (VISAR) and absolutely calibrated multi-channel pyrometery system as in previous work [9]. The post-shock temperature measurements were made at 0.65, 0.81, 1.8, 2.3, 3.5 and 4.8 μm. Additionally, empirical estimates of wavelength-dependent emissivity were made by heating a sample in a rough vaccum in order to convert the postshock thermal emission to true temperature. The fayalite rock specimens, from Rockport MA [2], have a mean density of 4.32±0.03 g/cm and lon-­‐ gitudinal wave speed of 6.7±0.2 km/s. A hand specimen from the Harvard Museum of Natural History was cored and cut into nominally 32-mm diameter discs. The fayalite and stainless steel 304 driver plate were lapped plane parallel and polished to an optical (<1 μm) finish. Results. In all experiments, we observe multi-wave shock compression profiles in the VISAR data (Figure 1). Multi-wave profiles have not been reported in previous experiments on Rockport fayalite due to different experimental techniques [3,4]. First and second wave arrivals are shown in Figure 2 with previous shock Hugoniot data on Rockport fayalite [4], synthetic polycrystalline fayalite and synthetic single crystal fayalite [3]. Broadly, the fayalite shock data display the characteristic segments for an elastic shock, mixed phase, and high-pressure phases (e.g., stishovite + FeO). This work is the first to record the elastic shock. The shock velocities in the first wave are consistent with extrapolated longitudinal wave speeds for fayalite. The first wave speeds are also consistent with the bulk sound speed of iron end member ringwoodite within its stability field, but it is likely that fayalite is being dynamically driven beyond its equilibrium phase space.