^1,2H. M. Sapers, ^1,2,3G. R. Osinski, ^1,2R. L. Flemming, ¹E. Buitenhuis, ¹N. R. Banerjee, ^1,2L. L. Tornabene, ¹S. Blain, ¹J. Hainge
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12796]
¹Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada
²Centre for Planetary Science and Exploration, University of Western Ontario, London, Ontario, Canada
³Department of Physics & Astronomy, University of Western Ontario, London, Ontario, Canada
Published by arrangement with John Wiley & Sons

The ~15 Ma, 26 km diameter Ries impact structure in south-central Germany was one of the first terrestrial impact structures where evidence of impact-associated hydrothermal alteration was recognized. Previous studies suggested that pervasive, high-temperature hydrothermal activity was restricted to the area within the “inner ring” (i.e., the crater-fill impactite units). Here we present mineralogical evidence for localized hydrothermal activity in the ejecta beyond the crater rim in two previously unstudied settings: a pervasively altered lens of suevite ejecta directly overlying the Bunte Breccia at the Aumühle quarry; and suevite ejecta at depth overlain by ~20 m of lacustrine sediments sampled by the Wörnitzostheim 1965 drill core. A comprehensive set of X-ray diffraction analyses indicates five distinct alteration regimes (1) surficial ambient weathering characterized by smectite and a minor illitic component; (2) locally restricted hydrothermal activity characterized by an illitic component and minor smectite; (3) hydrothermal activity at depth characterized by smectite, a minor illitic component, and calcite; (4) hydrothermal activity at depth characterized by smectite, a minor illitic component, calcite, zeolites, and clinochlore; and (5) pervasive hydrothermal activity at depth characterized by smectite, a minor illitic component, and minor clinochlore. These data spatially extend the Ries postimpact hydrothermal system suggesting a much more extensive, complex, and dynamic system than previously thought. Constraining the mineralogical alteration regimes at the Ries impact structure may also further our understanding of impact-associated phyllosilicate formation on Mars with implications for climate models and habitability.

¹A. Basu Sarbadhikari,²E. V. S. S. K. Babu,²T. Vijaya Kumar
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12790]
¹Physical Research Laboratory, Ahmedabad, 380009, India
²National Geophysical Research Institute (Council of Scientific and Industrial Research), Hyderabad, India
Published by agreement with John Wiley & Sons

Melting of Martian mantle, formation, and evolution of primary magma from the depleted mantle were previously modeled from experimental petrology and geochemical studies of Martian meteorites. Based on in situ major and trace element study of a range of olivine-hosted melt inclusions in various stages of crystallization of Tissint, a depleted olivine–phyric shergottite, we further constrain different stages of depletion and enrichment in the depleted mantle source of the shergottite suite. Two types of melt inclusions were petrographically recognized. Type I melt inclusions occur in the megacrystic olivine core (Fo76-70), while type II melt inclusions are hosted by the outer mantle of the olivine (Fo66-55). REE-plot indicates type I melt inclusions, which are unique because they represent the most depleted trace element data from the parent magmas of all the depleted shergottites, are an order of magnitude depleted compared to the type II melt inclusions. The absolute REE content of type II displays parallel trend but somewhat lower value than the Tissint whole-rock. Model calculations indicate two-stage mantle melting events followed by enrichment through mixing with a hypothetical residual melt from solidifying magma ocean. This resulted in ~10 times enrichment of incompatible trace elements from parent magma stage to the remaining melt after 45% crystallization, simulating the whole-rock of Tissint. We rule out any assimilation due to crustal recycling into the upper mantle, as proposed by a recent study. Rather, we propose the presence of Al, Ca, Na, P, and REE-rich layer at the shallower upper mantle above the depleted mantle source region during the geologic evolution of Mars.

¹Raphael J. Baumgartner, ¹Marco L. Fiorentini, ^2,3David Baratoux, ⁴Ludovic Ferrière, ⁵Marek Locmelis, ⁶Andrew Tomkins, ⁷Kerim A. Sener
Meteoritics &Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12795]
¹School of Earth and Environment, Centre for Exploration Targeting, ARC Centre of Excellence for Core to Crust Fluid Systems, The University of Western Australia, Crawley, Australia
²Géosciences Environnement Toulouse, CNRS, IRD and University of Toulouse, Toulouse, France
³Institut Fondamental d’Afrique Noire Cheikh Anta Diop, Dakar, Senegal
⁴Natural History Museum Vienna, Vienna, Austria
⁵Department of Geosciences and Geological and Petroleum Engineering, Missouri University of Science & Technology, Rolla, Missouri, USA
⁶School of Geosciences, Monash University, Melbourne, Australia
⁷Matrix Exploration Pty Ltd, Armadale, Australia
Published by arrangement with John Wiley & Sons

The Martian meteorites comprise mantle-derived mafic to ultramafic rocks that formed in shallow intrusions and/or lava flows. This study reports the first in situ platinum-group element data on chromite and ulvöspinel from a series of dunitic chassignites and olivine-phyric shergottites, determined using laser-ablation ICP-MS. As recent studies have shown that Ru has strongly contrasting affinities for coexisting sulfide and spinel phases, the precise in situ analysis of this element in spinel can provide important insights into the sulfide saturation history of Martian mantle-derived melts. The new data reveal distinctive differences between the two meteorite groups. Chromite from the chassignites Northwest Africa 2737 (NWA 2737) and Chassigny contained detectable concentrations of Ru (up to ~160 ppb Ru) in solid solution, whereas chromite and ulvöspinel from the olivine-phyric shergottites Yamato-980459 (Y-980459), Tissint, and Dhofar 019 displayed Ru concentrations consistently below detection limit (<42 ppb). The relatively elevated Ru signatures of chromite from the chassignites suggest a Ru-rich (~1–4 ppb) parental melt for this meteorite group, which presumably did not experience segregation of immiscible sulfide liquids over the interval of mantle melting, melt ascent, and chromite crystallization. The relatively Ru-depleted signature of chromite and ulvöspinel from the olivine-phyric shergottites may be the consequence of relatively lower Ru contents (<1 ppb) in the parental melts, and/or the presence of sulfides during the crystallization of the spinel phases. The results of this study illustrate the significance of platinum-group element in situ analysis on spinel phases to decipher the sulfide saturation history of magmatic systems.

¹Michael S.P. Kelley,, ²Charles E. Woodward, ²Robert D. Gehrz, ³William T. Reach, ⁴David E. Harker
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2016.11.029]
¹Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
²Minnesota Institute for Astrophysics, School of Physics and Astronomy, University of Minnesota, 116 Church Street S. E., Minneapolis, MN 55455, USA
³Universities Space Research Corporation, Stratospheric Observatory for Infrared Astronomy, MS 232-11, NASA Ames Research Center, Moffett Field, CA 94035, USA
⁴Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0424, USA
Copyright Elsevier

Comet nuclei and D-type asteroids have several similarities at optical and near-IR wavelengths, including near-featureless red reflectance spectra, and low albedos. Mineral identifications based on these characteristics are fraught with degeneracies, although some general trends can be identified. In contrast, spectral emissivity features in the mid-infrared provide important compositional information that might not otherwise be achievable. Jovian Trojan D-type asteroids have emissivity features strikingly similar to comet comae, suggesting that they have the same compositions and that the surfaces of the Trojans are highly porous. However, a direct comparison between a comet and asteroid surface has not been possible due to the paucity of spectra of comet nuclei at mid-infrared wavelengths. We present 5–35 µm thermal emission spectra of comets 10P/Tempel 2, and 49P/Arend-Rigaux observed with the Infrared Spectrograph on the Spitzer Space Telescope. Our analysis reveals no evidence for a coma or tail at the time of observation, suggesting the spectra are dominated by the comet nucleus. We fit each spectrum with the near-Earth asteroid thermal model (NEATM) and find sizes in agreement with previous values. However, the NEATM beaming parameters of the nuclei, 0.74 to 0.83, are systematically lower than the Jupiter-family comet population mean of 1.03± 0.11, derived from 16- and 22-µm photometry. We suggest this may be either an artifact of the spectral reduction, or the consequence of an emissivity low near 16 µm. When the spectra are normalized by the NEATM model, a weak 10-µm silicate plateau is evident, with a shape similar to those seen in mid-infrared spectra of D-type asteroids. A silicate plateau is also evident in previously published Spitzer spectra of the nucleus of comet 9P/Tempel 1. We compare, in detail, these comet nucleus emission features to those seen in spectra of the Jovian Trojan D-types (624) Hektor, (911) Agamemnon, and (1172) Aneas, as well as those seen in the spectra of seven comet comae. The comet comae present silicate features with two distinct shapes, either trapezoidal, or more rounded, the latter apparently due to enhanced emission near 8 to 8.5 µm. The surfaces of Tempel 2, Arend-Rigaux, and Hektor best agree with the comae that present trapezoidal features, furthering the hypothesis that the surfaces of these targets must have high porosities in order to exhibit a spectrum similar to a comet coma. An emissivity minimum at 15 µm, present in the spectra of Tempel 2, Arend-Rigaux, Hektor, and Agamemnon, is also described, the origin of which remains unidentified. The compositional similarity between D-type asteroids and comets is discussed, and our data supports the hypothesis that they have similar origins in the early Solar System.