The Fate of Carbonate Rocks During Hypervelocity Impacts: Case Studies from Three Impact Structures on Earth
Abstract
Approximately 28% of all hypervelocity impact structures discovered on Earth exist in a carbonate-dominated target sequence. Despite decades of research, how carbonate rocks and minerals react to shock metamorphism is still poorly understood. In this contribution, three impact structures on Earth were studied to determine the effects of shock metamorphism on carbonate minerals: Chicxulub, Crooked Creek and Jebel Waqf as Suwwan.
At Chicxulub, carbonates from the impact-melt bearing breccia of drill core, M0077A were characterized petrographically and geochemically. Calcite was the only carbonate mineral present and is abundant throughout the impact breccia in five distinct varieties: limestone clasts (Type A); clasts with clay altered rims (Type B); fine-crystalline matrix (Type C); coarse-crystalline void-filling (Type D); and flow-textured (Type E). Wavelength dispersive spectroscopy shows all calcite varieties are >98% pure with slight, yet distinct differences in MgO and MnO. Type B is a product of quenched carbonate melt via molten fuel-coolant interaction, whereas Type E has not been quenched and is the largest known accumulation of carbonate melt rock from a hypervelocity impact. Two stages of hydrothermal calcite permeated the impact melt-bearing breccia: a high-temperature, early calcite (Type C); and a low-temperature, late calcite (Type D).
Dolomite-dominated carbonates from Crooked Creek and calcite-dominated carbonates from Jebel Waqf as Suwwan impact structures were sampled at increasing distances from their centres and analyzed for mineral strain using X-ray diffraction (XRD). Strain was observed to decrease with increasing distance from the centre as would be expected of an attenuating shockwave, with a range of 0.025–0.122% for dolomite at Crooked Creek (~1-6 GPa) and 0.027–0.174% for calcite at Jebel Waqf as Suwwan. However, the decrease in strain is not uniform and can be explained by (1) uneven displacement and fault stacking during crater modification, (2) shock impedance variation from rock heterogeneities, and (3) user and software error. Both studies provide additional evidence promoting the effectiveness of XRD as a tool for identifying shock metamorphism in carbonate target rocks, with Jebel Waqf as Suwwan being the first in-depth XRD study of calcite-dominated target rocks.