¹Derek W. G. Sears, ¹Alexander Sehlke, ^2,3Jon M. Friedrich, ⁴Mark L. Rivers, ²Denton S. Ebel
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13183]
¹NASA Ames Research Center/BAER Institute, Mountain View, California, USA
²American Museum of Natural History, New York, New York, USA
³Department of Chemistry, Fordham University, Bronx, New York, USA
⁴Center for Advanced Radiation Sources, University of Chicago, Argonne, Illinois, USA
Published by arrangement with John Wiley & Sons

The radiation record of extraterrestrial rocks provides important insights into their thermal and radiation history. For meteorites this relates to their orbits, thermal history, terrestrial age, preatmospheric size and shape, and possibly cosmic ray exposure age. For meteorites from the Moon and Mars, the radiation record allows insights into transit times. For Martian surface samples, the radiation record enables estimates of their sedimentary age. Despite this, there is a growing tendency to artificially expose these samples to large radiation doses by the use of X‐ray computed tomography (CT) imaging, often as part of their initial examination. In order to understand the effect of synchrotron microCT on meteorites, we placed samples of the Bruderheim L6 chondrite in the CT imaging port of the Advanced Photon Source at the Argonne National Laboratory, Argonne, Illinois. Monoenergetic X‐ray beams of 25 and 46 keV and a high flux broad spectrum beam were used. The synchrotron CT procedure exposed the samples to radiation doses significantly higher than the natural doses observed for meteorites (1670 to ~10,000 Gyr, compared to ~1000 Gyr for natural samples). It is clear that CT imaging, whether using a laboratory system as in our previous report or using the synchrotron source, makes measurement of the natural radiation record of the samples impossible. Samples should not be placed in a CT scanner without due consideration of the loss of unique information for these valuable extraterrestrial samples.

¹Julie Castillo‐Rogez, ^2,3Marc Neveu, ⁴Harry Y. McSween, ⁵Roger R. Fu, ⁶Michael J. Toplis, ⁷Thomas Prettyman
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13181]
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
²School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
³NASA Postdoctoral Management Program Fellow, NASA Headquarters, Washington, District of Columbia, USA
⁴Department of Earth and Planetary Sciences, The University of Tennessee in Knoxville, Knoxville, Tennessee, USA
⁵Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA
⁶IRAP, Université de Toulouse, CNRS, UPS, Toulouse, France
⁷Planetary Science Institute, Tucson, Arizona, USA
Published by arrangement with John Wiley & Sons

Inspired by the recent results of the Dawn mission, thermodynamic models of rock alteration and brine evaporation have been used to help understand the conditions under which water–rock interaction took place within the dwarf planet Ceres. This analysis constrains Ceres’s early history and offers a framework within which future observations may be interpreted. A broad range of alteration conditions have been simulated using the Geochemist’s Workbench and PHREEQC software, associated with the FREZCHEM model that constrains the consequences of freezing the liquid phase in equilibrium with the observed mineralogical assemblage. Comparison of the modeling results with observed surface mineralogy at Ceres indicates advanced alteration under a relatively high fugacity of hydrogen, a conclusion that is consistent with predictions for, and observations of, large ice‐rich bodies. The simulations suggest production of methane that could help regulate the redox environment and possibly form clathrate hydrates upon freezing of the early ocean. The detection of localized occurrences of natrite (sodium carbonate) at the surface of Ceres provides key constraints on the composition of fluids that are necessarily alkaline. In addition, the combined hydrothermal and freezing simulations suggest that hydrohalite may be abundant in Ceres’s subsurface, similar to Earth’s polar regions. The global homogeneity of Ceres’s surface, made of material formed at depth, suggests a large‐scale formation mechanism, while local heterogeneities associated with impact craters and landslides suggest that some form of sodium carbonate and other salts are accessible in the shallow subsurface.

^1,2Bo-Magnus Elfers, ^1,2Peter Sprung, ^1,2,3Markus Pfeifer, ^1,2Frank Wombacher, ⁴Stefan T.M.Peters, ^1,2CarstenMünker
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.08.009]
¹Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Straße 49b, 50674 Köln, Germany
²Steinmann-Institut, Poppelsdorfer Schloss, Meckenheimer Allee 169, 53115 Bonn, Germany
³School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ, United Kingdom
⁴Geowissenschaftliches Zentrum der Georg-August-Universität Göttingen, Goldschmidtstraße 1, 37077 Göttingen, Germany
Copyright Elsevier

The stepwise acid digestion of primitive chondritic meteorites allows the identification of nucleosynthetic isotope anomalies that are otherwise hidden on the bulk rock scale. Here, we present combined Hf and W isotope data for acid leachates, residues, and bulk rock aliquots of several primitive chondrites that include highly precise analyses of the heavy p-process isotopes 174Hf and 180W. Including data for these two p-process isotopes enables, for the first time, the clear-cut discrimination between s- and r-process contributions to the Hf and W isotope inventory. Our analyses reveal Hf and W isotopic homogeneity at the bulk rock scale, but significant Hf and W isotope anomalies that are complementary between acid leachates and residues. Since both r- to p-process isotope ratios are invariant in leachates and residues, the observed anomalies can unambiguously be tied to variable contributions of carrier phases enriched in s-process nuclides, as previously inferred for, i.e., Mo and Ru in leaching experiments. Hafnium and W isotope anomalies co-vary in leachate and residue fractions from CM chondrites, whereas CO and CV chondrites are characterized by distinctly larger Hf isotope anomalies compared to W. This observation is most likely explained by more efficient homogenization of s-process W carrier(s) or, alternatively, by local redistribution of anomalous W into secondary less resistant phases during parent body and/or nebular processing. This implies the presence of different s-nuclide carrier phases for Hf and W. Several carriers of s-process-material appear to have been selectively dissolved by our leaching protocol, while contributions from r- and p-process Hf and W carrier phases appear invariant, possibly due to the generally more labile nature of their carrier phases during solar nebula and/or parent body processing.