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Back-transformation of high-pressure minerals in shocked chondrites: Low-pressure mineral evidence for strong shock

Hu, Jinping and Sharp, Thomas G. (2017) Back-transformation of high-pressure minerals in shocked chondrites: Low-pressure mineral evidence for strong shock. Geochimica et Cosmochimica Acta, 215 . pp. 277-294. ISSN 0016-7037. doi:10.1016/j.gca.2017.07.018.

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Post-shock annealing of meteorites can destroy their shock-induced features, particularly high-pressure minerals, and complicate the estimation of impact pressure–temperature conditions. However, distinguishing post-shock annealing features from thermal metamorphism effects can be practically difficult. Here we report results from Mbale, a highly shocked L chondrite, to investigate the mechanisms, kinetics and identification criteria for post-shock annealing of high-pressure signatures. Olivine fragments within shock-melt veins in Mbale occur as chemically heterogeneous nanocrystalline aggregates that contain trace wadsleyite and ringwoodite. Their strong variation in fayalite content provides evidence of iron partitioning during transformation of olivine to wadsleyite, followed by back-transformation to olivine after decompression. Experimental studies of transformation kinetics show that wadsleyite transforms to olivine in seconds at temperatures above ∼1200 K and in hours at temperatures between 900 and 1200 K. Thermal models of shock-melt cooling show that shock veins in Mbale cooled to 1200 K in 1 s. The shock pulse must have been shorter than ∼1 s to provide the high temperature conditions for post-shock back-transformation of wadsleyite. Many highly shocked L chondrites, which have abundant high-pressure minerals, must have experienced relatively long shock durations combined with rapid cooling of shock-melt regions to preserve high-pressure phases. The most highly shocked samples, such as impact melt breccias, lack high-pressure phases because of post-shock back-transformations.

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Additional Information:© 2017 Elsevier Ltd. Received 27 August 2016, Accepted 11 July 2017, Available online 18 July 2017. Associate editor: Pierre Beck. NASA Cosmochemistry Grants NNG06GF09G and NN09AG41G supported this research. We thank Peter Jenniskens and Center for Meteorite Studies of Arizona State University for providing the samples. We gratefully acknowledge the use of facilities with the LeRoy Eyring Center for Solid State Science at Arizona State University. We thank Erin Walton and anonymous reviewers for the constructive comments. Chi Ma and GPS division analytical facilities in California Institute of Technology are thanked for the support on electron microprobe analysis. Synchrotron X-ray diffraction was performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation – Earth Sciences (EAR-1128799) and Department of Energy- GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
Funding AgencyGrant Number
Department of Energy (DOE)DE-FG02-94ER14466
Department of Energy (DOE)DE-AC02-06CH11357
Subject Keywords:Shock metamorphism; Chondrite; Mbale; Back transformation; Post-shock annealing
Record Number:CaltechAUTHORS:20170921-105153434
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:81677
Deposited By: Ruth Sustaita
Deposited On:21 Sep 2017 18:18
Last Modified:15 Nov 2021 19:45

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