Unconfined shock experiments: A pilot study into the shock-induced melting and devolatilization of calcite

We shocked calcite in an unconfined environment by launching small marble cylinders at 0.8-5.5 km s(-1) into aluminum or copper plates, producing shock stresses between 5 and 79 GPa. The resulting 5-20 mm craters contained intimately mixed clastic and molten projectile residues over the entire pressure range, with melting commencing already at 5 GPa. Stoichiometrically pure calcite melts were not observed as all melts contained target metal. Some of these residues were distinctly depleted in CO2 and some contained even tiny CaO crystals, thus illustrating partial to complete loss of CO2. We interpret a thin seam of finely crystalline calcite to be the product of back reactions between CaO and CO2. The amount of carbonate residue in these craters, especially those at low velocities (<2 km s(-1)), is dramatically less than that of silicate impactors in similar cratering experiments, and we suggest that this is due to substantial outgassing of CO2. Similarly, the volume of carbonate melts relative to the volume of limestone or dolomite in many terrestrial crater structures seems insignificant as well, as is the volume of carbonate melt compared to the volume of impact melts derived from silicates. These volume considerations suggest that volatilization of CO2 is the dominant process in carbonate-containing targets. Because we have difficulties in explaining naturally occurring calcite melts by shock processes in dolomite-dominated targets, we speculate-essentially via process of elimination-that such carbonate melt blebs might be condensation products from an impact-produced vapor cloud.