Browsing by Author "Timms, Nick E."
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- ItemConstraining the shock conditions experienced by Haughton crystalline basement rocks: a combined Raman spectroscopy and electron backscatter diffraction study of Anomaly Hill zircons(2020) Walton, Erin L.; Jurak, Haley A. M.; Timms, Nick E.; Osinski, Gordon R.Haughton is a 23-km diameter impact structure on Devon Island, Canada [1, 2]. The target stratigraphy comprised ~1880 m of Lower Paleozoic sedimentary rocks, unconformably overlying the Precambrian Canadian Shield . Near the centre of the structure, is a location characterized by negative gravimetric and positive magnetic anomalies, known as “Anomaly Hill” . Highlyshocked, pumice-like lithic clasts are abundant at this locale, and include gneissic and carbonaterich clasts [4, 5]. In this study, we examined 20 zircon grains from a single crystalline clast collected at Anomaly Hill, to reveal microstructures at the micrometer to nanometer scale. Earlier work on Haughton zircons  did not incorporate EBSD, and so, is missing a wealth of information to facilitate the identification of key microstructures including FRIGN (former reidite in granular neoblasts) zircon, non-FRIGN granular textures, neoblasts versus sub-grain rotation formation of subdomains, and various dissociation textures, as described in [7, 8]. The goal of our study is to constrain the shock conditions experienced by crystalline basement rocks at Haughton using zircon, a mineral that is increasingly recognized as a sensitive shock indicator.
- ItemEvidence of impact melting and post-impact decomposition of sedimentary target rocks from the Steen River impact structure, Alberta, Canada(2019) Walton, Erin L.; Timms, Nick E.; Hauck, Tyler E.; MacLagan, Ebberly A.; Herd, Christopher D. K.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.
- ItemShock conditions experienced by Haughton crystalline basement rocks: A combined raman spectroscopy and electron backscatter diffraction study of a sample from Anomaly Hill(2019) Walton, Erin L.; Timms, Nick E.; Jurak, Haley A. M.; Osinski, Gordon R.Haughton is a 23-million year old impact structure with an apparent crater diameter of 23-km, formed in mixed target rocks of the Canadian High Arctic on Devon Island . At the time of impact, the target stratigraphy comprised 1880 meters of Lower Paleozoic sedimentary rocks unconformably overlying granulite-facies tonalitic and granitic gneisses of the Precambrian Canadian Shield. A location near the center of the structure, characterized by strong negative gravimetric and positive magnetic anomalies, has been coined “Anomaly Hill” . Highly shocked lithic clasts resembling pumice are particularly abundant at this locale, including carbonate-rich and gneiss clasts [4, 5]. In this study, a hand specimen from Anomaly Hill was investigated using an array of advanced analytical techniques. The goal is to constrain shock conditions experienced by Haughton crystalline basement rocks and their post-shock evolution.