The response of carbonate minerals and rocks under shock compression is investigated using equation of state, shock metamorphism, and crater morphology studies. Coralline limestone samples from Cactus Crater, a nuclear explosion crater on Runit Island in Eniwetok Atoll, are used in the investigations of shock deformation as well as the crater structural study. Carbonate minerals and rocks shocked in the laboratory to known dynamic stress levels are used to calibrate shock pressures in the Cactus samples.
Very low shock pressure deformation effects are detected in the explosively and laboratory shocked samples by two bulk sample techniques: electron spin resonance (ESR) and powder X-ray diffraction. According to ESR studies on calcite from Cactus Crater samples, peak shock pressures of 4.5±0.5 GPa were experienced by the material beneath the crater. Aragonite peak broadening analyses of powder X-ray diffraction spectra allows differentiation between two modes of material deformation, mosaicism (or reduction of crystallite size) and strain; both of these effects are detected in Cactus and in laboratory shocked samples. According to the X-ray analysis, peak pressures of 3±1.5 GPa were experienced by the Cactus samples. A phase transition model, based on the variation of mosaicism and strain effects with shock pressure, is proposed. According to this model, residual strain in aragonite increases (crystallite size remaining approximately the same) until a threshold pressure of 8 to 10 GPa, corresponding to a phase transition, is reached; release from shock states above this pressure results in a discontinuous decrease in crystallite size and strain.
The diagenetic high to low magnesium calcite transition boundary occurring in the immediate subsurface of Runit Island is used as a stratigraphic tracer to determine structural features beneath Cactus Crater, including the amount of permanent downward displacement, the presence of a 10 m thick breccia lens which is disturbed and extensively mixed in-situ, and a possible central uplift feature. Applying the Bingham plastic model to Cactus Crater gives a yield strength of approximately 1 bar for the shock-wave engulfed limestone rock; this value is similar to the yield strength of many clays, and suggests a partially liquefied state for the water-saturated limestone immediately after passage of the shock wave.
The first aragonite Hugoniot equation of state data are presented. A Hugoniot elastic limit at 2.5±0.8 GPa and a phase transition at 6.5±1.5 GPa are observed. Above 10 GPa, the aragonite and calcite Hugoniots are approximately coincident, suggesting the transformation of both CaCO3 polymorphs to the same high pressure phase. Release adiabats centered at shock pressures above 18 GPa yield pressure-density isentropes which suggest possible dissociation, i.e. CO2 release, during the decompression process. These experimental data disagree with theoretical calculations, which predict incipient vaporization upon release from shock pressures of 55 and 33 GPa on the aragonite and calcite Hugoniots, respectively. Results from release adiabat experiments on calcite agree with the aragonite data and suggest vaporization upon unloading from shock pressures of approximately 37 GPa; a mass balance calculation using the experimental calcite release paths indicates that 45% of the CaCO3 has dissociated upon release to 0.2 GPa pressures.
