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Shock-induced P-T conditions and formation mechanism of akimotoite-pyroxene glass assemblages in the Grove Mountains (GRV) 052082 (L6) meteorite

Feng Lu, Masaaki Miyahara, Toshiro Nagase, Eiji Ohtani, Hu Sen, Ahmed El Goresy and Lin Yangting
Shock-induced P-T conditions and formation mechanism of akimotoite-pyroxene glass assemblages in the Grove Mountains (GRV) 052082 (L6) meteorite
American Mineralogist (June 2017) 102 (6): 1254-1262

Abstract

Akimotoite [(Mg,Fe)SiO (sub 3) -ilmenite] was encountered in shock-induced melt veins of Grove Mountains (GRV) 052082, a highly equilibrated low iron ordinary chondritic meteorite (L6). Coexistence of ringwoodite, majorite, and majorite-pyrope solid solution indicates the shock pressure at 18-23 GPa and temperature of 2000-2300 degrees C during the natural dynamic event. Most low-Ca pyroxene clasts entrained in the melt veins have been partially or entirely transformed into akimotoite-pyroxene glass assemblages, which contain micrometer-sized areas with various brightness in the backscattered electron images, different from the chemically homogeneous grains in the host-rock (Fs (sub 20.5-21.3) ). The transmission electron microscopy study of a focused ion beam (FIB) slice from the heterogeneous areas shows that the assemblages are composed of FeO-depleted and heterogeneous akimotoite (Fs (sub 6-19) ) crystals (100 nm up to 400 nm in size) scattered in FeO-enriched and relatively homogeneous pyroxene glass (Fs (sub 31-39) ). All analyses of the akimotoite-pyroxene glass assemblages plot on a fractionation line in FeO-MgO diagram, with the host-rock pyroxene at the middle between the compositions of FeO-depleted akimotoite and the FeO-enriched pyroxene glass. These observations are different from previous reports of almost identical compositions of akimotoite, bridgmanite [(Mg,Fe)SiO (sub 3) -perovskite], or pyroxene glass to the host rock pyroxene (Chen et al. 2004; Ferroir et al. 2008; Ohtani et al. 2004; Tomioka and Fujino 1997), which is consistent with solid-state transformation from pyroxene to akimotoite and preexisting bridgmanite that could be vitrified. The observed fractionation trend and the granular shapes of akimotoite suggest crystallization from liquid produced by shock melting of the host-rock pyroxene, and the pyroxene glass matrix was probably quenched from the residual melt. However, this interpretation is inconsistent with the static experiments that expect crystallization of majorite [(Mg,Fe)SiO (sub 3) -garnet], instead of akimotoite, from pyroxene liquid (Sawamoto 1987). Our discovery raises the issue on formation mechanisms of the high-pressure polymorphs of pyroxene and places additional constraints on the post-shock high-pressure and high-temperature conditions of asteroids.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 102
Serial Issue: 6
Title: Shock-induced P-T conditions and formation mechanism of akimotoite-pyroxene glass assemblages in the Grove Mountains (GRV) 052082 (L6) meteorite
Affiliation: Chinese Academy of Sciences, Institute of Geology and Geophysics, Beijing, China
Pages: 1254-1262
Published: 201706
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 58
Accession Number: 2017-052117
Categories: Petrology of meteorites and tektites
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: Tohoku University, JPN, JapanHiroshima University, JPN, JapanUniversitaet Bayreuth, DEU, Germany
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201728
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