Calcite Phase Transitions and Melting Under Shock Loading and Release Using Ultrafast X-Ray Diffraction

Calcite is the most abundant carbonate phase in the Earth's crust and has been extensively examined under both dynamic and static compression. Characterizing the dynamic response of calcite is critical for interpretation of shock metamorphism in samples from terrestrial impact sites and identifying the role of any impact devolatilization of CO₂ in the evolution of the atmosphere. There is also significant interest in the behavior of carbonates at lower mantle conditions. Constraining the high-pressure-temperature (P-T) phase stability of carbonates is fundamental for understanding the global carbon cycle , as well as carbon storage in the deep Earth. CaCO₃ exhibits complex polymorphism at elevated P-T conditions. It adopts the rhombohedral calcite structure at ambient P-T (CaCO₃-I) and upon 300-K compression undergoes a series of structural transitions below 20 GPa into CaCO₃-II, CaCO₃-III, and CaCO₃-VI. Early shock - wave experiments identified a phase transition near 15 GPa, but lattice-level structural information was not obtained. Based on thermodynamic considerations, this high-pressure phase was not consistent with the CaCO₃-I, -II, or -III phases known at the time. More recent laser-heated diamond - anvil - cell studies show evidence of disordering as well as the formation of new phases including aragonite-II and CaCO₃-VII, although the high-P-T phase diagram remains a topic of considerable interest and continued debate.

We have carried out an ultrafast X-ray diffraction (XRD) study of shock-compressed polycrystalline calcite (Solenhofen limestone) using the Matter in Extreme Conditions beamline of the Linac Coherent Light Source. Two high-powered lasers were used to generate ablation-driven compression waves in the calcite sample and in-situ XRD patterns were collected at Hugoniot stresses of ~15 GPa, ~25 GPa, and ~60 GPa, yielding P-T states comparable to mantle conditions. X-ray probe times extended up to 20 ns after release, enabling the study of both the high-pressure crystal structure as well as any structural modifications on release. Our results demonstrate a crystallographic phase transformation above 20 GPa, followed by melting by 60 GPa. In all cases, the sample reverted back to CaCO₃-I on release, with no evidence for significant CaO formation associated with devolatilization.