The nature of shock-induced calcite (CaCO _3) devolatilization in an open system investigated using a two-stage light gas gun

摘要

The shock-induced decarbonation of non-porous calcite was investigated in an open system over a wide range of peak shock pressures using a two-stage light gas gun and a quadrupole mass spectrometer. We developed a new experimental technique that avoids chemical contamination from the acceleration gas used in the gun. High-speed imaging and spectroscopic observations were conducted simultaneously to investigate the validity of the experimental procedure. The decarbonation efficiency along the Hugoniot curve changed at around 50GPa-the approximate incipient decarbonation pressure predicted by the previous theoretical studies. Decarbonation, albeit at a low efficiency, was detected at pressures below the 50GPa threshold, as observed in previous experimental studies, possibly as a result of energy localization due to a process like shear banding. A simple theoretical model for shock-induced decarbonation during isentropic release was constructed based on the entropy method and the lever rule, assuming the experimental conditions. The predicted amount of released CO _2 as a function of the peak shock pressure agreed well with the experimental results at pressures exceeding 50GPa, strongly suggesting that the amount of shock-induced CO _2 gas was determined only by the shock-induced entropy gain and by the entropies for incipient and complete decarbonation at the ambient pressure. In future experiments, the new method will be used for quantitative measurements of the chemical composition of impact-induced gases derived from other solid materials. The proposed method is useful in determining the peak shock pressure required for vaporization/devolatilization of geologic materials and for estimating the final chemical composition of impact-induced vapor clouds.