¹Alex M. Ruzicka,¹Melinda Hutson,^2,3Jon M. Friedrich,⁴Mark L. Rivers,^3,5Michael K. Weisberg,³Denton S. Ebel,⁶Karen Ziegler,⁷Douglas Rumble III,^2,8Alyssa A. Dolan
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12901]
¹Cascadia Meteorite Laboratory, Department of Geology, Portland State University, Portland, Oregon, USA
²Department of Chemistry, Fordham University, Bronx, New York, USA
³Department of Earth and Planetary Sciences, American Museum of Natural History, New York City, New York, USA
⁴Consortium for Advanced Radiation Sources, University of Chicago, Argonne, Illinois, USA
⁵Department of Physical Sciences, Kingsborough College and Graduate School of the City University of New York, Brooklyn, New York, USA
⁶Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, USA
⁷Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C., USA
⁸Georgetown Law Center, Washington, D.C., USA
Published by arrangement with John Wiley & Sons

Miller Range 07273 is a chondritic melt breccia that contains clasts of equilibrated ordinary chondrite set in a fine-grained (<5 μm), largely crystalline, igneous matrix. Data indicate that MIL was derived from the H chondrite parent asteroid, although it has an oxygen isotope composition that approaches but falls outside of the established H group. MIL also is distinctive in having low porosity, cone-like shapes for coarse metal grains, unusual internal textures and compositions for coarse metal, a matrix composed chiefly of clinoenstatite and omphacitic pigeonite, and troilite veining most common in coarse olivine and orthopyroxene. These features can be explained by a model involving impact into a porous target that produced brief but intense heating at high pressure, a sudden pressure drop, and a slower drop in temperature. Olivine and orthopyroxene in chondrule clasts were the least melted and the most deformed, whereas matrix and troilite melted completely and crystallized to nearly strain-free minerals. Coarse metal was largely but incompletely liquefied, and matrix silicates formed by the breakdown during melting of albitic feldspar and some olivine to form pyroxene at high pressure (>3 GPa, possibly to ~15–19 GPa) and temperature (>1350 °C, possibly to ≥2000 °C). The higher pressures and temperatures would have involved back-reaction of high-pressure polymorphs to pyroxene and olivine upon cooling. Silicates outside of melt matrix have compositions that were relatively unchanged owing to brief heating duration.