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

¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125

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, Advanced Photon Source, Argonne National Laboratory, 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 is found within the rim region of olivine crystals in contact with melt pockets from the Tissint meteorite, a recent Martian shergottite fall, and in other highly shocked meteorites. The typical sequence of phase assemblages traversing across a Tissint melt pocket into olivine is: quenched melt Mg-bearing wüstite + vitreous Mg-Fe metasilicate ahrensite or ringwoodite amorphous orthosilicate (or highly-deformed olivine) olivine. This sequence is mainly the result of temperature rather than pressure differences across the hot spot and is consistent with peak pressures of ~20 GPa in the olivine “wall rock” of the melt pocket, not more than 10–15 GPa in the melt pocket and an intermediate pressure for the Mg-bearing wüstite + metasilicate partial-melt zone. Alternative scenarios involving vitrified silicate-perovskite imply wall rock pressures above 40 GPa and are at odds with expected thermodynamic relations in the MgO-FeO_x-SiO₂ system. Ahrensite forms through the shock-mediated, solid-state transformation of Fe-rich olivine near the grain boundaries of olivine megacrysts and with crystal size decreasing away from the nearby melt pocket.

The grain size distribution, lack of crystallite-preferred orientation, and conservation of olivine Fe-Mg zoning across the ahrensite-ringwoodite region are strongly indicative of a solid-state transformation of olivine to silicate-spinel. The observed incongruent growth of silicate-spinel grains is very different from previous reports of ringwoodite lamellae in olivine near shock-melt veins in Tissint and other meteorites of different origin. Lack of Mg-Fe interdiffusion between ringwoodite and ahrensite, and constraints on the temperature regime during shock, indicate shock duration on the order of 1 to 100 ms, which is also consistent with the observed widths of Mg-bearing wüstite-metasilicate melt zones.