Doctor of Philosophy
Planetary Science and Exploration
Osinski, Gordon R.
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.
Summary for Lay Audience
When a meteor strikes the surface of the Earth at hypervelocity, a shockwave is produced that travels through the ground compressing rocks and minerals as it passes. Upon decompression, the rock is heated depending on the amount of shock pressure it experienced. Both the intense pressure and temperature will permanently modify the rocks and minerals to the point where scientists can identify these products as impact in origin. One such product is melt that has cooled to become melt rock. Typical crystalline rock like granite or basalt will commonly produce melt rock, whereas carbonate rocks such as limestone are more complicated. Because of its chemical makeup (CaCO3 for calcite in limestone), carbonates will release CO2 gas when heated at atmospheric pressure and leave behind lime—they do not melt. However, the large amount of residual pressure from the shockwave, and lithostatic pressure if the rock is at depth, will make it so CO2 does not release and that the carbonate rock becomes melted. Evidence for this is present in the most recent drill core at the Chicxulub crater, M0077A. Several carbonate varieties were characterized from this core with two suggesting a large amount of melt rock, although this melt rock is scattered throughout as small fragments and melt lenses.
When carbonate minerals experience shock, but not enough to melt, their crystal structure will become compressed. This compression can be measured by the degree at which X-rays diffract when interacting with the strained lattice structure of the crystal. Carbonates from both the Crooked Creek and Jebel Waqf as Suwwan impact structures show an elevated degree of mineral strain that can only be explained by the intense pressure produced by a hypervelocity impact event. Additionally, the amount of mineral strain appears to decrease with increasing distance from the centre of the crater, such as would happen with the degradation of the initial shockwave. This further proves that the X-ray diffraction method can be used to diagnose a hypervelocity impact origin for carbonate rocks that have been struck by a meteorite.
Garroni, Nicolas D., "The Fate of Carbonate Rocks During Hypervelocity Impacts: Case Studies from Three Impact Structures on Earth" (2023). Electronic Thesis and Dissertation Repository. 9129.