A vacancy‐rich, partially inverted spinelloid silicate, (Mg,Fe,Si)2(Si,□)O4, as a major matrix phase in shock melt veins of the Tenham and Suizhou L6 chondrites

1Chi Ma,2Oliver Tschauner,3,4Luca Bindi,1John R. Beckett,5,6Xiande Xie
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13349]
1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, 91125 USA
2Department of Geoscience, University of Nevada, Las Vegas, Nevada, 89154 USA
3Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via G. La Pira 4, I‐50121 Firenze, Italy
4CNR, Istituto di Geoscienze e Georisorse, Sezione di Firenze, Via G. La Pira 4, I‐50121 Firenze, Florence, Italy
5Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640 China
6Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Chinese Academy of Sciences, Guangzhou, 510640 China
Published by arrangement with John Wiley & Sons

A new high‐pressure silicate, (Mg,Fe,Si)2(Si,□)O4 with a tetragonal spinelloid structure, was discovered within shock melt veins in the Tenham and Suizhou meteorites, two highly shocked L6 ordinary chondrites. Relative to ringwoodite, this phase exhibits an inversion of Si coupled with intrinsic vacancies and a consequent reduction of symmetry. Most notably, the spinelloid makes up about 30–40 vol% of the matrix of shock veins with the remainder composed of a vitrified (Mg,Fe)SiO3 phase (in Tenham) or (Mg,Fe)SiO3‐rich clinopyroxene (in Suizhou); these phase assemblages constitute the bulk of the matrix in the shock veins. Previous assessments of the melt matrices concluded that majorite and akimotoite were the major phases. Our contrasting result requires revision of inferred conditions during shock melt cooling of the Tenham and Suizhou meteorites, revealing in particular a much higher quench rate (at least 5 × 103 K s−1) for veins of 100–500 μm diameter, thus overriding formation of the stable phase assemblage majoritic garnet plus periclase.

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