SHOCK RECOVERY EXPERIMENTS ON DOLOMITE AND THERMODYNAMICAL CALCULATIONS OF IMPACT-INDUCED DECARBONATION

We have studied experimentally shocked dolomites and calcites by scann ing electron microscopy (SEM), analytical transmission electron micros copy (ATEM), X ray diffractometry (XRD) using Rietveld refinements, an d mass spectrometry analysis of the abundance ratios of the stable iso topes of carbon and oxygen, The shock-recovery experiments have been p erfomed by the multiple reverberation technique on natural dolomite ro cks at 60 GPa, using steel devices and high-explosive driver-flyer pla tes as plane shock wave generators, Modified assemblies with grooves a nd holes were built in order to facilitate the escape of CO2, in case the conditions of breakdown of the carbonates into oxides and CO2 woul d be reached during the shock or postshock history of the samples. In contrast with the results from previous studies, almost no evidence fo r 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 calculatio ns of peak-shock and postshock temperatures, as well as with the exami nation of outgassing conditions of carbonates, calculated in this stud y up to 80 GPa. We have shown that in a direct shock, outgassing in ai r of nonporous dolomites and calcites should occur in impacts at 55-65 GPa and 35-45 GPa, respectively. The effect of porosity, which strong ly lowers these values, has been estimated. The experimental setup for shock-recovery experiments is shown to be an important parameter: rev erberated shocks lead to lower peak-shock and postshock temperatures t han direct shocks at the same pressure; differences among experimental setups might explain part of the discrepancies between previous studi es. In this study, the only surviving shock-induced phenomenon is pulv erization leading to grain sizes smaller than in the starting material , The decrease in grain size has been quantified via a structure refin ement by Rietveld analysis of X ray powder patterns, which also allows the estimation of the lattice strains. A future pressure scale of sho ck effects in carbonates could probably be based on such parameters.