Evidence of impact melting and post-impact decomposition of sedimentary target rocks from the Steen River impact structure, Alberta, Canada

1,2Erin L.Walton,3Nicholas E.Timms,4Tyler E.Hauck,2Ebberly A.MacLagan,2Christopher D.K.Herd
Earth and Planetary Science Letters 515, 173-186 Link to Article [https://doi.org/10.1016/j.epsl.2019.03.015]
1Department of Physical Sciences, MacEwan University, City Centre Campus, 10700 104 Ave, Edmonton, AB, T5J 4S2, Canada
2Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Science Building, Edmonton, AB, T6G 2E3, Canada
3Department of Applied Geology, The Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA, Australia
4Alberta Geological Survey – Alberta Energy Regulator, 402 Twin Atria Building, 4999 98 Ave, Edmonton, AB, T6B 2X3, Canada
Copyright Elsevier

Hypervelocity bolide impacts deliver vast amounts of energy to the Earth’s near surface. This crustal process almost universally includes sedimentary target rocks; however, their response to impact is poorly understood, in part because of complexities due to layering, pore space and the presence of volatiles that are difficult to model. The response of carbonates to bolide impact remains contentious, yet whether they melt or decompose and liberate gases by the reaction CaCO3(s) → CaO(s) + CO2(g)↑, has significant implications for post-impact climatic effects. We report on previously unknown carbonate impact melts at the Steen River impact structure, Canada, and the first description of naturally shocked barite, BaSO4. Carbonate melts are preserved as groundmass-supported calcite-rich clasts, sampled from an up to 164 m thick, continuous sequence of crater-fill polymict breccias. Electron microscopy reveals fluidal- and ocellar-textured calcite and barite, intimately associated with silicate melt, consistent with these phases being in the liquid state at the same time. Raman spectroscopy and electron backscatter diffraction (EBSD) mapping confirm the presence of high-pressure phases – reidite and coesite – within some Steen River carbonate melt-bearing breccias. These minerals attest to the strong shock provenance of the breccia and provide constraints on their shock history. Preservation of reidite lamellae in zircon indicates a shock pressure >30 GPa <60 GPa and temperatures <1473 K. In addition to melting, we present compelling evidence for widespread (70–100%) decomposition of carbonate target rocks, mixed as lithic clasts into hot impact breccias. In this context, decomposition occurs strictly post-impact due to thermal equilibration-related heating. We demonstrate that this mechanism for CO2 outgassing is likely more widespread than previously recognized. The presence of andradite-grossular garnet serve as mineralogical markers of decomposition, analogous to limestone-replacing skarn deposits. Ca-rich garnet may therefore prove an important indicator mineral for post-shock decomposition of carbonate-bearing target rocks at other craters. These findings significantly advance our understanding of how sedimentary rocks respond to hypervelocity impact, and have wide-reaching implications for estimating the amount and timing of climatically-active volatile release due to impact events.

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