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Discovering research value in field and sample data of terrestrial impact sites and impactites

Wednesday, June 4, 2014
Shawn
Wright

The primary geologic processes on Mars are basaltic volcanism, sedimentation, impact cratering, and alteration. All potentially create amorphous materials and complex mineralogies, and these must be measured by rovers sent to Mars to characterize the geology. This talk addresses the field measurements and sample analyses of an analog impact crater to interpret rover and perhaps orbital data.  Two geologic histories are compared: 1.) the alteration of impactites (glasses) of a range of shock pressures (“post-impact alteration”), which likely increase the rate of alteration and affects the order of alteration where compared to pristine, igneous minerals, and 2.) the existence of altered basalt protoliths (“pre-impact alteration” or the precursor to #1) now vitrified as in-situ breccia clasts or float.  This comparison has implications for the discoveries of alteration minerals found solely in Martian ejecta blankets with remote VNIR data. Lonar Crater, India is a young (~570 ka), ~1.8 km impact site emplaced in ~65 Ma Deccan basalt, which is an excellent analog material for Mars with ~45-50% labradorite and ~35% augite/pigeonite *before* lower flows were altered and then shocked. Pre-impact stratigraphy was not complex: 3 flows of fresh basalt overlying 3 flows of aqueously-altered basalt, and both are found as impact breccia clasts in a ~8 m thick lithic (unshocked, “throw out”) and ~1 m suevite (all ranges of shock pressure, “fall out”) ejecta. Two aspects of studies of Lonar Crater will be described: fieldwork and sample analyses. Fieldwork demonstrates that underlying, altered basalt (by groundwater/aqueous alteration) is only exposed in the ejecta due to impact.  Otherwise it would be at depth. Sample analyses of shocked basalt are measured from a wide range of instrumentation and compared to unshocked (both fresh and altered) basalt. There are two goals of sample analyses performed for Lonar Crater samples: 1) those that characterize the mineralogy and geochemistry (petrography, XRD, SEM) for detailed descriptions of what the samples were and are, i.e. the determination of the state of alteration of the protolith and constraints on the amount of shock pressure received, and 2) those that mirror spectral and instrumental analyses sent toMars, such as TIR, VNIR, and Mossbauer spectrometers, LIBS to proxy MSL ChemCam, and, again, XRD, but to proxy MSL CheMin.

Second and third impact sites will be discussed concerning 1.) shatter cones and breccias in Proterozoic metagranites from the Santa Fe, NM impact structure; and 2.) impact mechanics as the Campo del Cielo, Argentina crater field (4 ka and interestingly described by natives) can be used to bridge the gap between observations, modeling, and impact experiments if the impact event is modelled as 22 experiments of various meteorite masses into a loess soil target at low (~8°) impact angles producing 4 explosion craters and 18 penetration funnels.

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