¹Mark R. Boyd,¹Julia A. Cartwright,²Jaspreet Singh,²Paul A.J. Bagot,^3,4Charlotte L. Bays,³Queenie H.S. Chan,^3,5Matthew J. Genge,²Michael P. Moody
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.08.023]
¹Department of Geological Sciences, University of Alabama, Tuscaloosa, AL 35487, USA
²Department of Materials, University of Oxford, Oxford, OX1 3PH, UK
³Planetary Materials Group, Natural History Museum London, SW7 5BD, UK
⁴Department of Earth Sciences, Royal Holloway, University of London, Egham, TW20 0EX, UK
⁵Department of Earth Science and Engineering, Imperial College London, London, SW7 2AZ, UK
Copyright Elsevier

Micrometeorites (MMs) recovered from the Earth’s surface may have undergone significant changes prior to their collection, including weathering while residing in the terrestrial environment. These alteration processes, such as the precipitation of hydrous phases, may overprint both primary and atmospheric entry features, obscuring pre-existing material properties. In addition, weathering exerts a prominent control on the geochemical interactions, such as species mobility, between extraterrestrial material and the terrestrial environment, particularly in Antarctica. In this study, we have characterised the textural and compositional consequences of weathering on an unmelted, fine-grained, Antarctic MM, which includes the mapping of nanometre-scale features using atom probe tomography. In particular, we investigate geochemical behaviour across textural boundaries in the MM and observe nanoscale elemental heterogeneity within complex alteration assemblages. In one sample region, a compositional boundary is highlighted by distinct elemental differences, consistent with a weathering encrustation of mixed mineralogies, while analyses in other targeted regions show evidence for nanoscale elemental networks, as well as a grain boundary adjacent to a carbon-rich region. We discuss our findings in the context of terrestrial weathering as a dominant cause for the nanoscale features observed. Weathering processes responsible for these features include the leaching of extraterrestrial material, precipitation of secondary alteration products with associated layering, and the influence of mechanical stress on pre-existing weaknesses. From these results, we derive a weathering sequence to explain the formation of the alteration product assemblage and highlight the controls on MM geochemistry in the terrestrial environment. Our observations show that nanoscale carbonaceous material may be preserved in oxy-hydroxides under icy conditions, which can also act as tracers for local environmental changes.

^1,2Lisa Maria Eckart,¹Jon K. Hillier,¹Frank Postberg,³Simone Marchi,⁴Zoltan Sternovsky
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14060]
¹Freie Universität Berlin, Berlin, Germany
²ETH Zürich, Zürich, Switzerland
³Southwest Research Institute, Boulder, Colorado, USA
⁴University of Colorado Boulder, Boulder, Colorado, USA
Published by arrangement with John Wiley & Sons

Linking meteorites to their asteroid parent bodies remains an outstanding issue. Space-based dust characterization using impact ionization mass spectrometry is a proven technique for the compositional analysis of individual cosmic dust grains. Here we investigate the feasibility of determining asteroid compositions via cation mass spectrometric analyses of their dust ejecta clouds during low (7–9 km s−1) velocity spacecraft flybys. At these speeds, the dust grain mass spectra are dominated by easily ionized elements and molecular species. Using known bulk mineral volume abundances, we show that it is feasible to discriminate the common meteorite classes of carbonaceous chondrites, ordinary chondrites, and howardite–eucrite–diogenite achondrites, as well as their subtypes, relying solely on the detection of elements with ionization efficiencies of ≤700 or ≤800 kJ mol−1, applicable to low (~7 km s−1) and intermediate (~9 km s−1) flyby speed scenarios, respectively. Including the detection of water ion groups enables greater discrimination between certain meteorite types, and flyby speeds ≥10 km s−1 enhance the diagnostic capabilities of this technique still further. Although additional terrestrial calibration is required, this technique may allow more unequivocal asteroid-meteorite connections to be determined by spacecraft flybys, emphasizing the utility of dust instruments on future asteroid missions.