Petrographic Constraints on Shock Induced P/t Conditions in Shergottites

Introduction: Martian meteorites (MM) comprise rocks shocked in the range of 5-55 GPa during natural impact events [1]. Together with the suite of experimentally shocked rocks, they are valuable assets for the study of the progressive stages of shock metamorphism. Petrographic investigations of MM, in combination with shock and post shock temperature calculations are presented. This data set is compared with the shock pressures determined by applying the classification schema, which is based on shock recovery experiments and observations in naturally shocked chondritic and terrestrial rocks [1-4]. Method: The approximation (about ±30%) of the shock induced temperature increase in different rock types follows the method described in [1, 5]. The calculations are based on the linear relation of the particle velocity U to shock wave velocity D: D = c + s * U, with c and s being experimentally determined constants [7-8]. The amount of energy deposited in the decompressed rock was calculated and then divided by the specific heat capacity for mafic rocks, Cp, approximated with 1000 J/kg/K. Results: Shock temperature increase (∆Ts) describes the temperature increase during maximum compression, and post shock temperature increase (∆Tps) describes the temperature increase after passage of the shock wave. ∆Tps for plagioclase dominated (anorthosite), olivine dominated (dunite) and pyroxene dominated (pyroxenite) rocks shocked to 0-55 GPa are shown in Fig. 1. ∆Tps rises with shock pressure to a similar degree in olivine and pyroxene, but a steeper increase is observed for plagioclase shocked above 3035 GPa. This is due to the fact that plagioclase undergoes substantial phase transitions from plagioclase to maskelynite and melt, while the crystal structure of olivine and pyroxene remains unchanged in this pressure range. For gabbro the ∆Tps is intermediate and depends on the proportions of olivine, pyroxene, and plagioclase in the rock sample. The temperature-time path for a gabbro-like MM (i.e. shergottites) shocked to ~45 GPa is shown in Fig. 2. At the shock front the temperature discontinuously jumps from the initial temperature of the rock (TsurfaceMars) to Ts of the individual mineral phases (t1). MM are accelerated by the steep pressure gradient in the “spallation zone” building up by interactions of the shock wave emerging spherically from the impact site and the release wave reflected from the “free surface” of the planet. Thus, the pulse lengths in MM depend on their burial depth below the Martian surface and the size of the impacting projectile [5-6]. Mineralogical arguments support a pulse length of ~10 s for Zagami [9]. After adiabatic release the temperatures drop to the Tps (∆Tps + 233°K initial temperature) of the individual mineral phases (t2). The thermal equilibrium between mm sized grains is assumed to be on the order of seconds (t3). A shock heated 40 cm Ø MM takes ~0.5 h for cooling to ambient temperature in space [1].