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Shock conditions recorded in NWA 8159 martian augite basalt with implications for the impact cratering history on Mars

Sharp, Thomas G. and Walton, Erin L. and Hu, Jinping and Agee, Carl (2019) Shock conditions recorded in NWA 8159 martian augite basalt with implications for the impact cratering history on Mars. Geochimica et Cosmochimica Acta, 246 . pp. 197-212. ISSN 0016-7037.

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NWA 8159 is an augite-rich martian basalt, formed by cooling of a relatively evolved, Ca-rich, Ti-poor and LREE-depleted lava, under relatively oxidizing conditions, during the early Amazonian. In addition to its distinct igneous petrogenesis and high fO_2, NWA 8159 is also set apart from most martian shergottites with respect to the low degree of shock metamorphism required to preserve crystalline igneous plagioclase (An_(50–65)). In this study, mineral transformations within and adjacent to shock veins in NWA 8159 were investigated using scanning electron microscopy, Raman spectroscopy and transmission electron microscopy to better constrain the unusal shock history of this meteorite. The transformation of olivine to ahrensite (Fe-ringwoodite) along shock vein margins, and tissintite and coesiteformed from igneous mineral (labradorite and silica) grains entrained as clasts within shock veins has been documented in this study. We report on a previously unidentified mineral assemblage of Ca-Na-majoritic garnet, sodic-clinopyroxene and stishovite crystallized from shock melt. This mineral assemblage indicates a crystallization pressure of approximately 16 GPa, which is within the range of previous shock pressure estimates for this meteorite (15–23 GPa). The presence of a majoritic garnet-bearing assemblage throughout veins up to 0.6 mm wide indicates that the sample remained at high-pressure throughout the melt vein quench. Based on thermal models, the sample must have remained at high pressure for ∼100 ms. This shock duration is an order of magnitude longer than those experienced by more highly shocked shergottites such as Tissint or Zagami (>30 GPa; 10–20 ms) and would seem to imply a relatively large impact event. Recent numerical models demonstrate that a range of shock pressures and durations are realized by rocks within the ejected spall zone of a hypervelocity impact. The shock conditions experienced by NWA 8159 therefore do not require an impact event distinct from other shergottites. Rather, our findings suggest that this meteorite originated from near the martian surface at the edge of the impact site. The shock history of NWA 8159 provides a picture of Mars consistent with that derived from remote observation; that of a random cratering process that samples a geologically long-lived and complex planet.

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Walton, Erin L.0000-0003-1301-5300
Additional Information:© 2018 Elsevier Ltd. Received 28 March 2018, Accepted 9 November 2018, Available online 24 November 2018. We thank Andrew Locock and Martin von Dollen at the University of Alberta for expertise in EMP and sample preparation, respectively. Financial support for this project was provided by NSERC Discovery Grant RES00007057 awarded to ELW and NASA Cosmochemistry Grant NNH08ZDA001B-Cos awarded to TGS. JH is supported by NASA Solar System Workings grant 80NSSC18K0532. The authors would like to thank three anonymous reviewers who provided input that improved the overall quality of this manuscript. We acknowledge the use of facilities within the Eyring Materials Center at Arizona State University supported in part by NNCI-ECCS-1542160.
Funding AgencyGrant Number
Natural Sciences and Engineering Research Council of Canada (NSERC)RES00007057
Subject Keywords:Shock veins; martian meteorites; majoritic garnet; impact metamorphism; high-pressure phases; impact cratering
Record Number:CaltechAUTHORS:20181126-100419186
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Official Citation:Thomas G. Sharp, Erin L. Walton, Jinping Hu, Carl Agee, Shock conditions recorded in NWA 8159 martian augite basalt with implications for the impact cratering history on Mars, Geochimica et Cosmochimica Acta, Volume 246, 2019, Pages 197-212, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:91159
Deposited By: Tony Diaz
Deposited On:26 Nov 2018 18:13
Last Modified:13 Dec 2018 17:49

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