¹Marina Martinez,¹Adrian J. Brearley,²Josep M. Trigo‐Rodríguez,¹Jordi Llorca
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13391]
¹Department of Earth & Planetary Sciences, MSC03-2040, University of New Mexico, Albuquerque, New Mexico 87131, USA
²Institute of Space Sciences (CSIC-IEEC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Bellaterra (Barcelona),
Catalonia, Spain
³Institut de Tecniques Energetiques i Centre de Recerca en Nanoenginyeria, Universitat Politecnica de Catalunya, Diagonal 647,
ETSEIB, 08028 Barcelona, Catalonia, Spain
Published by arrangement with John Wiley & Sons

The petrology and mineralogy of shock melt veins in the L6 ordinary chondrite host of Villalbeto de la Peña, a highly shocked, L chondrite polymict breccia, have been investigated in detail using scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and electron probe microanalysis. Entrained olivine, enstatite, diopside, and plagioclase are transformed into ringwoodite, low‐Ca majorite, high‐Ca majorite, and an assemblage of jadeite‐lingunite, respectively, in several shock melt veins and pockets. We have focused on the shock behavior of diopside in a particularly large shock melt vein (10 mm long and up to 4 mm wide) in order to provide additional insights into its high‐pressure polymorphic phase transformation mechanisms. We report the first evidence of diopside undergoing shock‐induced melting, and the occurrence of natural Ca‐majorite formed by solid‐state transformation from diopside. Magnesiowüstite has also been found as veins injected into diopside in the form of nanocrystalline grains that crystallized from a melt and also occurs interstitially between majorite‐pyrope grains in the melt‐vein matrix. In addition, we have observed compositional zoning in majorite‐pyrope grains in the matrix of the shock‐melt vein, which has not been described previously in any shocked meteorite. Collectively, all these different lines of evidence are suggestive of a major shock event with high cooling rates. The minimum peak shock conditions are difficult to constrain, because of the uncertainties in applying experimentally determined high‐pressure phase equilibria to complex natural systems. However, our results suggest that conditions between 16 and 28 GPa and 2000–2200 °C were reached.

¹Chi Ma,²Oliver Tschauner,¹John R. Beckett,³Yang Liu,⁴Eran Greenberg,⁴Vitali B. Prakapenka
American Mineralogist 104,1521-1525 Link to Article [https://doi.org/10.2138/am-2019-6999]

¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
²Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154, U.S.A.
³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, U.S.A.
⁴GSECARS, University of Chicago, Argonne National Laboratory, Chicago, Illinois 60637, U.S.A.
Copyright: The Mineralogical Society of America

Chenmingite (FeCr2O4; IMA 2017-036) is a high-pressure mineral, occurring as micrometer- to submicrometer-sized lamellae within precursor chromite grains along with xieite and Fe,Cr-rich ulvöspinel next to shock-induced melt pockets, from the Tissint martian meteorite. The composition of type chenmingite by electron probe analysis shows an empirical formula of (Fe2+0.75Mg0.23Mn0.02) (Cr1.60Al0.29Fe3+0.06Fe2+0.04Ti0.02)Σ2.01O4. The general and end-member formulas are (Fe,Mg)(Cr,Al)2O4 and FeCr2O4. Synchrotron X-ray diffraction reveals that chenmingite has an orthorhombic Pnma CaFe2O4-type (CF) structure with unit-cell dimensions: a = 9.715(6) Å, b = 2.87(1) Å, c = 9.49(7) Å, V = 264.6(4) Å3, and Z = 4. Both chenmingite and xieite formed by solid-state transformation of precursor chromite under high pressure and high temperature during the Tissint impact event on Mars. The xieite regions are always in contact with melt pockets, whereas chenmingite lamellae only occur within chromite, a few micrometers away from the melt pockets. This arrangement suggests that chenmingite formed under similar pressures as xieite but at lower temperatures, in agreement with experimental studies.