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

Chi Ma1, Oliver Tschauner2, John R. Beckett1, Yang Liu3, George R. Rossman1, Stanislav V. Sinogeikin4, Jesse Smith4, Lawrence A. Taylor5

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

High 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-FeOx-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.


Ahrensite in a section of the Tissint meteorite