Shock recovery experiments on dolomite and thermodynamical calculations of impact induced decarbonation

Abstract

We have studied experimentally shocked dolomites and calcites by scanning electron microscopy (SEM), analytical transmission electron microscopy (ATEM), X ray diffractometry (XRD) using Rietveld refinements, and mass spectrometry analysis of the abundance ratios of the stable isotopes of carbon and oxygen. The shock‐recovery experiments have been perfomed by the multiple reverberation technique on natural dolomite rocks at 60 GPa, using steel devices and high‐explosive driver‐flyer plates as plane shock wave generators. Modified assemblies with grooves and holes were built in order to facilitate the escape of CO2, in case the conditions of breakdown of the carbonates into oxides and CO2 would be reached during the shock or postshock history of the samples. In contrast with the results from previous studies, almost no evidence for outgassing, expressed, for example, by the identification of CaO or MgO, could be observed. Consequently, no isotope fractionation occured in the shocked samples. This result is consistent with the calculations of peak‐shock and postshock temperatures, as well as with the examination of outgassing conditions of carbonates, calculated in this study up to 80 GPa. We have shown that in a direct shock, outgassing in air of nonporous dolomites and calcites should occur in impacts at 55–65 GPa and 35–45 GPa, respectively. The effect of porosity, which strongly lowers these values, has been estimated. The experimental setup for shock‐recovery experiments is shown to be an important parameter: reverberated shocks lead to lower peak‐shock and postshock temperatures than direct shocks at the same pressure; differences among experimental setups might explain part of the discrepancies between previous studies. In this study, the only surviving shock‐induced phenomenon is pulverization leading to grain sizes smaller than in the starting material. The decrease in grain size has been quantified via a structure refinement by Rietveld analysis of X ray powder patterns, which also allows the estimation of the lattice strains. A future pressure scale of shock effects in carbonates could probably be based on such parameters.