Numerical modelling of impact melt production in porous rocks

Melting of rocks by impact is a fundamental geologic process that results from shock wave compression and subsequent release from high pressure. Shock-melted material is found in many terrestrial craters; however, most impact craters in sedimentary targets, which are typically porous, have been reported as containing significantly less melt than craters in nonporous crystalline targets. To investigate the effect of porosity on shock melting we present a numerical modelling approach that combines a porosity compaction model with an equation of state for the solid component. To demonstrate that our model accurately calculates shock pressures in porous materials we compare these results with Hugoniot data for porous quartzite (Coconino sandstone) and calcite. Our results show that the presence of porosity significantly reduces the critical pressure required for melting, which enhances melt production. We show that this effect is reduced but not totally diminished by faster shock wave decay in porous materials, so that, all other things being equal, impact melt production is greater when target porosity is higher. We conducted a series of simulations of impacts into targets of different material composition (quartzite, calcite, dunite) and porosity of up to 50% and varied the impact velocity between 6 and 50 km/s. We found a significant increase in the production of impact melt with increasing velocity (in agreement with several previous studies) and quantify the net increase in impact melt due to target porosity. For the nonporous case our results agree well with estimated melt volumes at terrestrial impact craters over a range in size (7–90 km in diameter). In porous targets our results contradict the widely cited observation of a melt deficiency in sedimentary target craters. However, our model results agree well with recently revised estimates of melt volumes at terrestrial craters formed in sedimentary, or mixed sedimentary and crystalline targets. This further strengthens the view that substantial volumes of impact melt should be produced during impacts into porous sedimentary targets and that the apparent paucity of impact melt in some sedimentary target craters may be due to difficulties in recognizing impact melts derived from sedimentary rocks.