¹Thomas J. Lapen, ¹Minako Righter, ^1,2Rasmus Andreasen, ³Anthony J. Irving, ^4,5Aaron M. Satkoski, ^4,5Brian L. Beard, ⁶Kunihiko Nishiizumi, ⁷A. J. Timothy Jull,^8,9Marc W. Caffee
Science Advances 3, e1600922 Link to Article [DOI: 10.1126/sciadv.1600922]
¹Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204–5007, USA.
²Department of Geoscience, Aarhus University, Aarhus, Denmark.
³Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195–1310, USA.
⁴Department of Geoscience, University of Wisconsin–Madison, Madison, WI 53706–1692, USA.
⁵NASA Astrobiology Institute, University of Wisconsin–Madison, Madison, WI 53706, USA.
⁶Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA, USA.
⁷Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA.
⁸Department of Physics, Purdue University, West Lafayette, IN 47907–2036, USA.
⁹Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907–2051, USA

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¹A. Lindskog, ^2,3M. M. Costa, ^2,4C.M.Ø. Rasmussen, ^2,3J. N. Connelly, ¹M. E. Eriksson
Nature Communications 8, 14066 Link to Article [doi:10.1038/ncomms14066]
¹Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden
²Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5–7, DK-1350 Copenhagen K, Denmark
³Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5–7, DK-1350 Copenhagen K, Denmark
⁴Center for Macroecology, Evolution and Climate, University of Copenhagen, Denmark

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^1,2S.R. Sutton, ^3,4C.A. Goodrich, ²S. Wirick
Geochimica et Cosmochmica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.01.036]
¹Dept. of Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA
²Center for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637, USA
³Planetary Science Institute, 1700 E. Ft. Lowell, Tucson, AZ 85719, USA
⁴Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston TX 77058
Copyright Elsevier

Titanium, Cr, and V valences were determined by applying micro-X-ray Absorption Near Edge Structure (micro-XANES) spectroscopy methods to individual grains of olivine and pyroxene in the ungrouped achondrite NWA 7325 and ureilite Y-791538, as well as to plagioclase in NWA 7325. The advantages of applying multiple, multivalent-element-based oxybarometers to individual grains are (1) the ability to cover the entire oxygen fugacity (fO2) range encountered in nature, and (2) the increased reliability from consistent results for semi-independent fO2 proxies. fO2 values were inferred from each mineral valence determination after correcting with available laboratory-experiment-derived, valence-specific partition coefficients to obtain melt valences and then calibrating with the fO2 values of the relevant equal species proportions points suggested for igneous (primarily basaltic) systems.

The resulting olivine and pyroxene valences are highly reduced and similar in the two meteorites with substantial fractions of Cr2+, Ti3+ and V2+. The exception is Cr in NWA 7325 pyroxene which is much more oxidized than the Cr in its olivine. Chromium and Ti in plagioclase in NWA 7325 is relatively oxidized (V valence not determined). The anomalously oxidized Cr in NWA 7325 pyroxene may be due to a secondary reheating event that oxidized Cr in the pyroxene without similarly oxidizing Ti and V. Such a separation of the redox couples may be an effect of re-equilibration kinetics, where the valence of Cr would be more rapidly modified. These valences yielded similar mean fO2s for the two meteorites; IW-3.1 ± 0.2 for NWA 7325 and IW-2.8 ± 0.2 for Y-791538, consistent with an origin of NWA 7325 in either Mercury or an asteroid that experienced redox conditions similar to those on the ureilite parent body.

¹Shiyong Liao, ²Weibiao Hsu
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.01.037]
¹Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Nanjing, China
²Space Science Institute, Macau University of Science and Technology, Macau
Copyright Elsevier

Studies of petrology, mineralogy and geochronology of eucrites are keys to reconstruct the thermal and impact history of 4 Vesta, the proposed parent body for HED meteorites. Here we report the petrography, mineralogy and geochemistry of NWA 8009, a newly found eucritic impact-melt breccia, and present SIMS U-Pb ages of zircon and phosphates. NWA 8009 consists of coarse- and fine-grained lithic and mineral clasts set in fine-grained recrystallized matrix. It was derived from a protolith of non-cumulate eucrite. Evidence for intense shock metamorphism observed in NWA 8009 includes mosaicism, deformed exsolution lamellae and partial melting of pyroxene, melting and incipient flow of plagioclase, planar fractures and granular textures of zircon. These shock effects indicate NWA 8009 was subjected to an impact metamorphism with peak pressure of ∼50–60 Gpa and post-shock temperature of ∼1160–1200 °C. NWA 8009 is among the most intensely shocked HEDs reported yet. After the impact, the sample was buried near the surface in target rocks and experienced rapid cooling (∼23°C/h) and annealing, resulting in recrystallization of the matrix and devitrification of plagioclase and silica glasses. U-Pb isotopic system of apatite within plagioclase groundmass of lithic clasts is completely reset and constrains the timing of impact at 4143 ± 61 Ma, providing a new robust impact age on Vesta. Combined with the presence of synchronous impact resetting events, especially those recorded by Lu-Hf, Sm-Nd, and Pb-Pb isotopic systems, we identified a period of high impacts flux at ca. 4.1–4.2 Ga on Vesta. This impact flux occurred coincident with the uptick at ca. 4.1–4.2 Ga in impact age spectra of the moon, probably reflects widespread intense bombardment throughout the inner solar system at ca. 4.1–4.2 Ga. Based on evidence from zircon chemical zoning, petrographic occurrences, as well as the distinctive Zr/Hf ratios, we suggested that zircons in NWA 8009 have had a metamorphic, instead of magmatic origin. They mainly crystallized from melts produced by partial melting of mesostasis area due to reheating event during early global thermal metamorphism, rather than by Zr release from Zr-rich minerals. The U-Pb isotopic system in zircons was not disturbed by subsequent impacts, the weighted-mean 207Pb/206Pb age of 4560 ± 8 Ma represents the timing of zircon growth during thermal metamorphism. Zircons from NWA 8009 and other eucrites may share a common origin during metamorphic growth events, and constraining the global thermal metamorphism on Vesta at ca. 4.55 Ga. The main heat sources responsible for global metamorphism in basaltic crust of Vesta might be heating from the hot interior, especially heat flow related to magmatism, rather than impact.