J. M. Paque^1,*, S. R. Sutton², S. B. Simon², J. R. Beckett¹, D. S. Burnett¹, L. Grossman^2,3, H. Yurimoto⁴, S. Itoh⁴, H. C. Connolly Jr.^5,6,7,8

¹Division of Geological and Planetary Sciences, Caltech, Pasadena, California, USA
²Department of Geophysical Sciences, The University of Chicago, Chicago, Illinois, USA
³Enrico Fermi Institute, The University of Chicago, Chicago, Illinois, USA
⁴Department of Natural History Science, Hokkaido University, Sapporo, Hokkaido, Japan
⁵Department of Physical Sciences, Kingsborough Community College of the City University of New York, Brooklyn, New York, USA
⁶Department of Earth and Environmental Sciences, The Graduate Center of the City University of New York, New York, New York, USA
⁷Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
8Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA

Ti valence measurements in MgAl₂O₄ spinel from calcium-aluminum-rich inclusions (CAIs) by X-ray absorption near-edge structure (XANES) spectroscopy show that many spinels have predominantly tetravalent Ti, regardless of host phases. The average spinel in Allende type B1 inclusion TS34 has 87% Ti⁺⁴. Most spinels in fluffy type A (FTA) inclusions also have high Ti valence. In contrast, the rims of some spinels in TS34 and spinel grain cores in two Vigarano type B inclusions have larger amounts of trivalent titanium. Spinels from TS34 have approximately equal amounts of divalent and trivalent vanadium. Based on experiments conducted on CAI-like compositions over a range of redox conditions, both clinopyroxene and spinel should be Ti⁺³-rich if they equilibrated with CAI liquids under near-solar oxygen fugacities. In igneous inclusions, the seeming paradox of high-valence spinels coexisting with low-valence clinopyroxene can be explained either by transient oxidizing conditions accompanying low-pressure evaporation or by equilibration of spinel with relict Ti⁺⁴-rich phases (e.g., perovskite) prior to or during melting. Ion probe analyses of large spinel grains in TS34 show that they are enriched in heavy Mg, with an average Δ²⁵Mg of 4.25 ± 0.028‰, consistent with formation of the spinel from an evaporating liquid. Δ²⁵Mg shows small, but significant, variation, both within individual spinels and between spinel and adjacent melilite hosts. The Δ²⁵Mg data are most simply explained by the low-pressure evaporation model, but this model has difficulty explaining the high Ti⁺⁴ concentrations in spinel.

Reference
Paque JM, Sutton SR, Simon SB, Beckett JR, Burnett DS, Grossman L, Yurimoto H, Itoh S and Connolly HC (in press) XANES and Mg isotopic analyses of spinels in Ca-Al-rich inclusions: Evidence for formation under oxidizing conditions. Meteoritics & Planetary Science
[doi:10.1111/maps.12216]
Published by arrangement with John Wiley & Sons

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Mariana V. Maziviero^1,*, Marcos A. R. Vasconcelos¹, Alvaro P. Crosta¹, Ana M. Góes², Wolf U. Reimold^3,4, Cleyton de C. Carneiro¹

¹Department of Geology and Natural Resources, Institute of Geosciences, University of Campinas, Campinas, São Paulo, Brazil
²Department of Sedimentary and Environmental Geology, Institute of Geosciences, University of São Paulo, São Paulo, Brazil
³Museum für Naturkunde, Leibniz Institute for Research on Evolution and Biodiversity, Berlin, Germany
⁴Humboldt-Universität zu Berlin, Berlin, Germany

Riachão, located at S7°42′/W46°38′ in Maranhão State, northeastern Brazil, is a complex impact structure of about 4.1 km diameter, formed in Pennsylvanian to Permian sedimentary rocks of the Parnaíba Basin sequence. Although its impact origin was already proposed in the 1970s, information on its geology and shock features is still scarce in the literature. We present here the main geomorphological and geological characteristics of the Riachão impact structure obtained by integrated geophysical and remote sensing analysis, as well as geological field work and petrographic analysis. The identified lithostratigraphic units consist of different levels of the Pedra de Fogo Formation and, possibly, the Piauí Formation. Our petrographic analysis confirms the presence of shock-diagnostic planar microdeformation structures in quartz grains of sandstone from the central uplift as evidence for an impact origin of the Riachão structure. The absence of crater-filling impact breccias and melt rocks, shatter cones, as well as the restricted occurrence of microscopic shock effects, suggests that intense and relatively deep erosion has occurred since crater formation.

Reference
Maziviero MV, Vasconcelos MAR, Crosta A., Góes AM, Reimold WU and de C Carneiro C (in press) Geology and impact features of Riachão structure, northern Brazil. Meteoritics & Planetary Science
[doi:10.1111/maps.12213]
Published by arrangement with John Wiley & Sons

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B.C. Johnson^a and H.J. Melosh^a,b

^aDepartment of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907
^bDepartment of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907

We present a model that describes the formation of melt droplets, melt fragments, and accretionary impact lapilli during a hypervelocity impact. Using the iSALE hydrocode, coupled to the ANEOS equation of state for silica, we create high-resolution two-dimensional impact models to track the motion of impact ejecta. We then estimate the size of the ejecta products using simple analytical expressions and information derived from our hydrocode models. Ultimately, our model makes predictions of how the size of the ejecta products depends on impactor size, impact velocity, and ejection velocity. In general, we find that larger impactor sizes result in larger ejecta products and higher ejection velocities result in smaller ejecta product sizes. We find that a 10 km diameter impactor striking at a velocity of 20 km/s creates millimeter scale melt droplets comparable to the melt droplets found in the Chicxulub ejecta curtain layer. Our model also predicts that melt droplets, melt fragments, and accretionary impact lapilli should be found together in well preserved ejecta curtain layers and that all three ejecta products can form even on airless bodies that lack significant volatile content. This prediction agrees with observations of ejecta from the Sudbury and Chicxulub impacts as well as the presence of accretionary impact lapilli in lunar breccia.

Reference
Johnson BC and Melosh HJ (in press) Formation of melt droplets, melt fragments, and accretionary impact lapilli during a hypervelocity impact. Icarus
[doi:10.1016/j.icarus.2013.10.022]
Copyright Elsevier

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