Ahrensite, γ-Fe2SiO4, a new shock-metamorphic mineral from the Tissint meteorite: implications for the Tissint shock event on Mars

1Chi Ma, 2Oliver Tschauner, 1John R. Beckett, 3Yang Liu, 1George R. Rossman, 4Stanislav V. Sinogeikin, 4Jesse S. Smith, 5Lawrence A. Taylor
1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
2High Pressure Science and Engineering Center and Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154, USA
3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA
4HPCAT, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, Illinois 60439, USA
5Planetary Geosciences Institute, Department of Earth and Planetary Science, University of Tennessee, Knoxville, Tennessee 37996, USA

Ahrensite (IMA 2013-028), γ-Fe2SiO4, is the natural Fe-analog of the silicate-spinel ringwoodite (γ-Mg2SiO4). It occurs in the Tissint Martian meteorite, where it forms through the transformation of the fayalite-rich rims of olivine megacrysts or Fe-rich microphenocrysts in contact with shock melt pockets. We report the first comprehensive set of crystallographic, spectroscopic, and quantitative chemical analysis of type ahrensite, and show that concentrations of ferric iron and inversion in the type material of this newly approved mineral are negligible. We also report the occurrence of nanocrystalline ringwoodite in strained olivine and establish correlations between grain size and distance from melt pockets. The ahrensite and ringwoodite crystals show no preferred orientation, consistent with random nucleation and incoherent growth within a highly strained matrix of olivine. Grain sizes of ahrensite immediately adjacent to melt pockets are consistent with growth during a shock of moderate duration (1-10 ms).

Reference
Ma C, Tschauner O, Beckett JR, Liu Y, Rossman GR, Sinogeikin SV, Smith JS, Taylor LA (2016) Ahrensite, γ-Fe2SiO4, a new shock-metamorphic mineral from the Tissint meteorite: implications for the Tissint shock event on Mars. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.04.042]
Copyright Elsevier

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