¹Andrew W. Beck, ¹David J. Lawrence, ¹Patrick N. Peplowski, ¹Christina E. Viviano-Beck, ²Thomas H. Prettyman, ³Timothy J. McCoy, ⁴Harry Y. McSween Jr, ²Naoyuki Yamashita
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2017.01.008]
¹The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, 20723, USA
²Planetary Science Institute, Tucson, Arizona, 85719, USA
³Department of Mineral Sciences, Smithsonian Institution, Washington, District of Columbia, 20560, USA
⁴Department of Earth and Planetary Sciences, Knoxville, Tennessee, 37996-1410, USA
Copyright Elsevier

We use data collected by the Dawn Gamma-Ray and Neutron Detector (GRaND) at Vesta to map compositions corresponding to nearly pure igneous lithologies in the howardite, eucrite, diogenite (HED) meteorite clan (samples likely from Vesta). At the ∼300-km spatial scale of GRaND measurements, basaltic eucrite occurs on only 3% of the surface, whereas cumulate eucrite and orthopyroxenitic diogenite are not detected. The basaltic eucrite region is generally coincident with an area of the surface with thick regolith, elevated H, and moderate crater density, and may represent the best compositional sample of primordial vestan crust. We observe an absence of pure orthopyroxenitic diogenite terrains in the Rheasilvia basin and its ejecta, an observation corroborated by VIR (0.1%), which suggests the south-polar crust was a polymict mixture of igneous lithologies (howardite) at the time of the Rheasilvia impact, or was a thick basaltic eucrite crust with heterogeneously distributed orthopyroxenitic diogenite plutons. The most dominant igneous composition detected (11% of the surface) corresponds to one of the least-abundant igneous lithologies in the HED meteorite collection, the Yamato Type B diogenites (plagioclase-bearing pyroxenites). The distribution of Type B diogenite is spatially correlated with post-Rheasilvia craters in the north-polar region that are in close proximity to the Rheasilvia basin antipode. This suggests that north-polar Type B plutonism may have been associated with the Rheasilvia impact event. We propose that this was either through 1) uplift of pre-existing plutons at the antipode through focusing of Rheasilvia impact stress, or 2) Rheasilvia impact antipodal crustal melting, creating magmas that underwent fractionation to produce Type B plutons.

^1,2Jon M. Friedrich, ³Alex Ruzicka, ^5,6Robert J. Macke, ⁴James O. Thostenson, ⁴Rebecca A. Rudolph, ⁵Mark L. Rivers, ^2,7,8Denton S. Ebel
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.12.039]
¹Department of Chemistry, Fordham University, Bronx, NY 10458, USA
²Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
³Cascadia Meteorite Laboratory, Portland State University, Department of Geology, Portland, OR 97207-0751, USA
⁴Microscopy and Imaging Facility, American Museum of Natural History, New York, NY 10024, USA
⁵Center for Advanced Radiation Sources, University of Chicago, Argonne, IL 60439, USA
⁶Vatican Observatory, V-00120 Vatican City-State
⁷Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA
⁸Graduate Center of the City University of New York, New York 10016, USA
Copyright Elsevier

Collisions and attendant shock compaction must have been important for the accretion and lithification of planetesimals, including the parent bodies of chondrites, but the conditions under which these occurred are not well constrained. A simple model for the compaction of chondrites predicts that shock intensity as recorded by shock stage should be related to porosity and grain fabric. To test this model, we studied sixteen ordinary chondrites of different groups (H, L, LL) using X-ray computed microtomography (μCT) to measure porosity and metal fabric, ideal gas pycnometry and 3D laser scanning to determine porosity, and optical microscopy (OM) to determine shock stage. These included a subsample of six chondrites previously studied using transmission electron microscopy (TEM) to characterize microstructures in olivine. Combining with previous data, results support the simple model in general, but not for chondrites with low shock-porosity-foliation (low-SPF chondrites). These include Kernouvé (H6), Portales Valley (H6/7), Butsura (H6), Park (L6), GRO 85209 (L6), Estacado (H6), MIL 99301 (LL6), Spade (H6), and Queen’s Mercy (H6), among others. The data for these meteorites are best explained by high ambient heat during or after shock. Low-SPF chondrites tend to have older 40Ar/39Ar ages (∼4435-4526 Ma) than other, non-low-SPF type 6 chondrites in this study. We conclude that the H, L, and LL asteroids all were shock-compacted at an early stage while warm, with collisions occurring during metamorphic heating of the parent bodies. Results ultimately bear on whether chondrite parent bodies have internal structures more akin to a metamorphosed onion shell or metamorphosed rubble pile, and on the nature of accretion and lithification processes for planetesimals.