¹Antonio S. Buono, ¹David Walker
¹Lamont Doherty Earth Observatory, Department of Earth and Environmental Sciences, Columbia University, Palisades, New York, USA

The Fe-FeS system maintains a eutectic temperature of 990 ± 10 °C to at least 8 GPa if starting materials and pressure media are rigorously dehydrated. Literature reports of pressure-induced freezing point depression of the eutectic for the Fe-FeS system are not confirmed. Modest addition of oxygen alone is confirmed to cause negligible freezing point depression at 6 GPa. Addition of H alone causes a progressive decrease in the eutectic temperature with P in the Fe-FeS-H system to below 965 °C at 6 GPa to below 950 °C at 8 GPa. It is our hypothesis that moisture contamination in unrigorously dried experiments may be an H source for freezing point depression. O released from H2O disproportionation reacts with Fe and is sequestered as ferropericlase along the sample capsules walls, leaving the H to escape the system and/or enter the Fe-FeS mixture. The observed occurrence of ferropericlase on undried MgO capsule margins is otherwise difficult to explain, because an alternate source for the oxygen in the ferropericlase layer is difficult to identify. This study questions the use of pressure-depressed Fe-S eutectic temperatures and suggests that the lower eutectic temperatures sometimes reported are achieved by moving into the ternary Fe-S-H system. These results adjust slightly the constraints on eutectic temperatures allowed for partly solidified cores on small planets. H substantially diminishes the temperature extent of the melting interval in Fe-S by reducing the melting points of the crystalline phases more than it depresses the eutectic.

Reference
Buono AS, Walker D (2014) H, not O or pressure, causes eutectic T depression in the Fe-FeS System to 8 GPa. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12372]

Published by arrangement with John Wiley&Sons

¹Sebastian Sturm,¹Thomas Kenkmann, ²Malte Willmes, ³Gisela Pösges, ⁴Harald Hiesinger
¹Institute of Earth and Environmental Sciences—Geology, Albert-Ludwigs-Universität Freiburg (ALU), Freiburg, Germany
²Research School of Earth Sciences, Australian National University (ANU), Acton, Canberra, Australia
³Rieskrater Museum, Nördlingen, Germany
⁴Institut für Planetologie, Westfälische Wilhelms-Universität Münster (WWU), Münster, Germany

The Ries crater is a well-preserved, complex impact crater that has been extensively used in the study of impact crater formation processes across the solar system. However, its geologic structure, especially the megablock zone, still poses questions regarding crater formation mechanics. The megablock zone, located between the inner crystalline ring and outer, morphologic crater rim, consists of allochthonous crystalline and sedimentary blocks, Bunte Breccia deposits, patches of suevite, and parautochthonous sedimentary blocks that slumped into the crater during crater modification. Our remote sensing detection method in combination with a shallow drilling campaign and geoelectric measurements at two selected megablocks proved successful in finding new megablock structures (>25 m mean diameter) within the upper approximately 1.5 m of the subsurface in the megablock zone. We analyzed 1777 megablocks of the megablock zone, 81 of which are new discoveries. In our statistical analysis, we also included 2318 ejecta blocks >25 m beyond the crater rim. Parautochthonous megablocks show an increase in total area and size toward the final crater rim. The sizes of allochthonous megablocks generally decrease with increasing radial range, but inside the megablock zone, the coverage with postimpact sediments obscures this trend. The size-frequency distribution of all megablocks obeys a power-law distribution with an exponent between approximately −1.7 and −2.3. We estimated a total volume of 95 km3 of Bunte Breccia and 47 km3 of megablocks. Ejecta volume calculations and a palinspastic restoration of the extension within the megablock zone indicate that the transient cavity diameter was probably 14–15 km.

Reference
Sturm S, Kenkmann T, Willmes M, Pösges G, Hiesinger H (2014) The distribution of megablocks in the Ries crater, Germany: Remote sensing, field investigation, and statistical analyses. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12408]
Published by arrangement with John Wiley&Sons

¹Sherry K. Fieber-Beyer, ¹Michael J. Gaffey, ²William F. Bottke, ¹Paul S. Hardersen
¹Department of Space Studies, University Stop 9008, University of North Dakota, 58202
²Southwest Research Institute and NASA Lunar Science Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, USA

The research is an integrated effort beginning with telescopic observations and extending through detailed mineralogical characterizations to provide constraints on the albedo, diameter, composition, and meteorite affinity of near-Earth object-potentially hazardous asteroid (NEO-PHA 2007 LE). Results of the analysis indicate a diameter of 0.56 kilometers (km) and an albedo of 0.08. 2007 LE exhibits a 1-μm absorption feature without a discernible Band II feature. Compositional analysis of 2007 LE reveal Fs17 and Fa19 values, which are consistent with the Fa and Fs values for the H-type ordinary chondrites (Fs14.5-18 and Fa16-20) and of asteroid (6) Hebe (Fs17 and Fa15). Spectroscopically, 2007 LE does not appear like the average H-chondrite spectra, exhibiting a reddened spectrum and subdued absorption feature. Further investigation of the meteorite classes yielded a black chondrite, Rose City, which is both similar in mineralogy and spectrally to PHA 2007 LE. Dynamical analysis could not directly link the fall of the Rose City meteorite to 2007 LE. As it stands, 2007 LE and Rose City have a compositional link, and both could come from the same parent body/possible family, one known source of the H chondrites is (6) Hebe.

Reference
Fieber-Beyer SL, Gaffey MJ, Bottke WF, Hardersen PS (2014) Potentially Hazardous Asteroid 2007 LE: Compositional Link to the Black Chondrite Rose City and Asteroid (6) Hebe. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2014.12.021]

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