Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p42c..08w&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P42C-08
Mathematics
Logic
[3672] Mineralogy And Petrology / Planetary Mineralogy And Petrology, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [6207] Planetary Sciences: Solar System Objects / Comparative Planetology, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
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 landers and rovers sent to Mars to characterize the geology and astrobiology. This paper addresses the field measurements and sample analyses of an analog impact crater to prepare for current and future landers/rovers to interpret impact ejecta deposits on Mars, but can be applied to other surfaces as well where basalt makes up the upper crust. Glasses produced by shock, whether they are diaplectic maskelynite at 20-40 GPa, flowing and vesiculated glasses at higher shock pressures, or impact melts, are amorphous materials that need to be accounted for during rover/lander investigations. Also of interest with respect to Mars are the alteration of impactites from different 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 the existence of altered basalt protoliths (pre-impact alteration) now vitrified as in-situ breccia clasts or float. Field investigations at *any* of the confirmed ~180 terrestrial impact structures are beneficial to study products from the impact process. However, very few have well-preserved ejecta and a majority are emplaced into quartz sandstones, limestones, or Proterozoic granites/metagranites not analogous to the surfaces of the rocky planets. Lonar Crater, India is a young (~570 ka), ~1.8 km impact site emplaced in Deccan basalt - an excellent analog material for Mars with ~45-50% labradorite and ~35% augite/pigeonite. A few other, larger impact sites in Brazil are emplaced into basalt, but are >10 Ma; whereas valuable inner-crater basaltic impact breccias exist at these sites, there are no ejecta remaining. Lonar Crater has a well-preserved ejecta blanket with two layers of ejecta: a lower, ~8 m thick lithic breccia that represents overturned "throw out", and a ~1 m suevite "fall out" layer consisting of basalt clasts shocked a range of shock pressures in a matrix of finely pulverized basalt with local spherules and lapilli [Beal et al., this AGU abstract volume]. There are two aspects of studies of Lonar Crater that will be described: fieldwork and sample analyses. Fieldwork is documented as detailed descriptions, including photography, of outcrops of pristine ejecta along with occurrences of reworked ejecta. Sample analyses of shocked basalt are measured from a wide range of instrumentation and compared to unshocked 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 are, including the determination of the state of alteration of the protolith and constraints on the amount of shock pressure, and 2) those that mirror spectral and instrumental analyses sent to other planets, such as TIR, VNIR, and Mossbauer spectrometers, LIBS to proxy MSL ChemCam, and, again, XRD, but to proxy MSL CheMin. Whole rock and SEM geochemistry is carefully interpreted; whereas shock does not change the elemental chemistry of minerals, alteration trends are noted, and geochemistry is necessary to determine the protolith of several glasses.
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