Browsing by Author "Sharp, Thomas G."

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    A previously unrecognized example of the shock-induced breakdown of biotite to garnet from the Steen River impact structure, Canada
    (2016) Walton, Erin L.; Sharp, Thomas G.; Hu, Jinping; Tschauner, Oliver
    Shock metamorphic effects from quartz and feldspar have been studied in detail, and to a lesser extent olivine and pyroxene. Less is known about how shock is manifest in other minerals such as double chain inosilicates, phyllosilicates, carbonates and sulfates. In this study, we investigate shock in crystalline basement rocks from the Steen River impact structure (SRIS), a 25-km diameter complex crater in NW Alberta, Canada.
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    Dwell time at high pressure of meteorites during impact ejection from Mars
    (2020) Bowling, Timothy J.; Johnson, Brandon C.; Wiggins, Sean E.; Walton, Erin L.; Melosh, Henry Jay; Sharp, Thomas G.
    Martian meteorites are currently the only rock samples from Mars available for direct study in terrestrial laboratories. Linking individual specimens back to their source terrains is a major scientific priority, and constraining the size of the impact craters from which each sample was ejected is a critical step in achieving this goal. During ejection from the surface of Mars by hypervelocity impacts, these meteorites were briefly compressed to high temperatures and pressures. The period of time that these meteorites spent at high pressure during ejection, or the ‘dwell time’, has been used to infer the size of the crater from which they were ejected. This inference requires assumptions that relate shock duration to impactor size, and the relation used by many authors is neither physically motivated nor accurate. Using the iSALE2D shock physics code we simulate vertical impacts at high resolution to investigate the dwell time that basaltic rocks from Mars (shergottites) spend at high pressure and temperature during ejection. Future simulation of oblique impacts will lead to more accurate dwell time estimates. Ultimately, we find that dwell time is insensitive to changes in impact velocity but for a given impact, dwell times are longer for material originating from greater depth and material that experiences higher shock pressures. Using our results, we provide scaling laws for estimating impactor size. During the formation of craters 1.9, 14, and 104 km in diameter, material capable of escaping Mars will have mean dwell times of 1, 10, and 100 ms, respectively.
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    Heterogeneous mineral assemblages in Martian meteorite Tissint as a result of a recent small impact event on Mars
    (2014) Walton, Erin L.; Sharp, Thomas G.; Hu, Jinping; Filiberto, Justin R.
    The microtexture and mineralogy of shock melts in the Tissint martian meteorite were investigated using scanning electron microscopy, Raman spectroscopy, transmission electron microscopy and synchrotron micro X-ray diffraction to understand shock conditions and duration. Distinct mineral assemblages occur within and adjacent to the shock melts as a function of the thickness and hence cooling history. The matrix of thin veins and pockets of shock melt consists of clinopyroxene + ringwoodite + or - stishovite embedded in glass with minor Fe-sulfide. The margins of host rock olivine in contact with the melt, as well as entrained olivine fragments, are now amorphosed silicate perovskite + magnesiowuestite or clinopyroxene + magnesiowuestite. The pressure stabilities of these mineral assemblages are approximately 15 GPa and >19 GPa, respectively. The approximately 200-mu m-wide margin of a thicker, mm-size (up to 1.4 mm) shock melt vein contains clinopyroxene + olivine, with central regions comprising glass + vesicles + Fe-sulfide spheres. Fragments of host rock within the melt are polycrystalline olivine (after olivine) and tissintite + glass (after plagioclase). From these mineral assemblages the crystallization pressure at the vein edge was as high as 14 GPa. The interior crystallized at ambient pressure. The shock melts in Tissint quench-crystallized during and after release from the peak shock pressure; crystallization pressures and those determined from olivine dissociation therefore represent the minimum shock loading. Shock deformation in host rock minerals and complete transformation of plagioclase to maskelynite suggest the peak shock pressure experienced by Tissint > or = 29-30 GPa. These pressure estimates support our assessment that the peak shock pressure in Tissint was significantly higher than the minimum 19 GPa required to transform olivine to silicate perovskite plus magnesiowuestite. Small volumes of shock melt (<100 mu m) quench rapidly (0.01 s), whereas thermal equilibration will occur within 1.2 s in larger volumes of melt (1 mm (super 2) ). The apparent variation in shock pressure recorded by variable mineral assemblages within and around shock melts in Tissint is consistent with a shock pulse on the order of 10-20 ms combined with a longer duration of post-shock cooling and complex thermal history. This implies that the impact on Mars that shocked and ejected Tissint at approximately 1 Ma was not exceptionally large.
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    Investigating the response of biotite to impact metamorphism: examples from the Steen River impact structure, Canada
    (2018) Walton, Erin L.; Sharp, Thomas G.; Hu, Jinping; Tschauner, Oliver
    Impact metamorphic effects from quartz and feldspar and to a lesser extent olivine and pyroxene have been studied in detail. Comparatively, studies documenting shock effects in other minerals, such as double chain inosilicates, phyllosilicates, carbonates, and sulfates, are lacking. In this study, we investigate impact metamorphism recorded in crystalline basement rocks from the Steen River impact structure (SRIS), a 25 km diameter complex crater in NW Alberta, Canada. An array of advanced analytical techniques was used to characterize the breakdown of biotite in two distinct settings: along the margins of localized regions of shock melting and within granitic target rocks entrained as clasts in a breccia. In response to elevated temperature gradients along shock vein margins, biotite transformed at high pressure to an almandine-Ca/Fe majorite-rich garnet with a density of 4.2 g cm−3. The shock-produced garnets are poikilitic, with oxide and silicate glass inclusions. Areas interstitial to garnets are vesiculated, in support of models for the formation of shock veins via oscillatory slip, with deformation continuing during pressure release. Biotite within granitic clasts entrained within the hot breccia matrix thermally decomposed at ambient pressure to produce a fine-grained mineral assemblage of orthopyroxene + sanidine + titanomagnetite. These minerals are aligned to the (001) cleavage plane of the original crystal. In this and previous work, the transformation of an inosilicate (pargasite) and a phyllosilicate (biotite) to form garnet, an easily identifiable, robust mineral, has been documented. We contend that in deeply eroded astroblemes, high-pressure minerals that form within or in the environs of shock veins may serve as one of the possibly few surviving indicators of impact metamorphism.
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    Shock metamorphism in Northwest Africa 8159, Tissint and Elephant Moraine A79001: implications for thermal histories and geochronology
    (2016) Sharp, Thomas G.; Hu, Jinping; Walton, Erin L.
    Shock metamorphic effects in Martian meteorites provide a record of recent impact events on Mars. The key to interpreting this record is understanding the shock conditions and relating these to impact processes. Martian meteorites have experienced shock effects, ranging from nearly unshocked (S1) to very highly shocked (S6). The shock effects observed are typically heterogeneous and local shock effects depend on pressure, deformation and thermal histories. In this study we examine shock pressures and thermal histories of three Martian meteorites and discuss implications for shock resetting of isotope systems. Each sample experienced a different P-Tt path, resulting in different textures and mineral assemblages.