^1,2Phillip Gopon,²Michael J. Spicuzza,³Thomas F. Kelly,³David Reinhard,³Ty J. Prosa,²John Fournelle
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12899]
¹Department of Earth Science, University of Oxford, Oxford, OX1 3AN, UK
²Department of Geoscience, University of Wisconsin, Madison, Wisconsin, USA
³CAMECA Instruments Inc., Madison, Wisconsin, USA
Published by arrangement with John Wiley & Sons

The lunar regolith contains a variety of chemically reduced phases of interest to planetary scientists and the most common, metallic iron, is generally ascribed to space weathering processes (Lucey et al. 2006). Reports of silicon metal and iron silicides, phases indicative of extremely reducing conditions, in lunar samples are rare (Anand et al. 2004; Spicuzza et al. 2011). Additional examples of Fe-silicides have been identified in a survey of particles from Apollo 16 sample 61501,22. Herein is demonstrated the utility of low keV electron probe microanalysis (EPMA), using the Fe Ll X-ray line, to analyze these submicron phases, and the necessity of accounting for carbon contamination. We document four Fe-Si and Si0 minerals in lunar regolith return material. The new Fe-Si samples have a composition close to (Fe,Ni)3Si, whereas those associated with Si0 are close to FeSi2 and Fe3Si7. Atom probe tomography of (Fe,Ni)3Si shows trace levels of C (60 ppma and nanodomains enriched in C, Ni, P, Cr, and Sr). These reduced minerals require orders of magnitude lower oxygen fugacity and more reducing conditions than required to form Fe0. Documenting the similarities and differences in these samples is important to constrain their formation processes. These phases potentially formed at high temperatures resulting from a meteorite impact. Whether carbon played a role in achieving the lower oxygen fugacities—and there is evidence of nearby carbonaceous chondritic material—it remains to be proven that carbon was the necessary component for the unique existence of these Si0 and iron silicide minerals.

^1,2Myriam Telus, ²Gary R. Huss, ²Kazuhide Nagashima, ²Ryan C. Ogliore, ³Shogo Tachibana
Geochimica et Cosmochmica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.013]
¹Geology and Geophysics, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
²Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
³Department of Natural History Sciences, Hokkaido University, N10 W8, Sapporo 060-0810, Japan
Copyright Elsevier

The initial 60Fe/56Fe ratio of chondrules from unequilibrated ordinary chondrites (UOCs) can potentially help constrain the stellar source of short-lived radionuclides and develop the 60Fe-60Ni (t1/2=2.6 Ma) system for early solar system chronology. However, progress with the 60Fe-60Ni system has been hindered by discrepancies between initial ratios inferred from bulk and in situ Fe-Ni analyses. Telus et al. (2016) show that discrepancies between these different techniques stem from late-stage open-system Fe-Ni mobilization. Here, we report in situ analyses of the Fe-Ni isotopic composition of ferromagnesian silicates in chondrules from UOCs using the ion microprobe. Of the 24 chondrules analyzed for this study, a few chondrules have clearly resolved excesses in 60Ni of up to 70‰; however, the correlations with the Fe/Ni ratios are weak. Although complications from Fe-Ni redistribution make it difficult to interpret the data, we show that the initial 60Fe/56Fe ratio for UOC chondrules is between 5×10-8 and 3.0×10-7. This is consistent with a late supernova source for 60Fe, but self-enrichment of the molecular cloud is another possible mechanism for incorporating 60Fe in the solar system. Discrepancies between bulk and in situ analyses remain, but likely stem from late-stage open-system Fe-Ni mobilization.

¹V. Vinogradoff, ²C. Le Guillou, ³S. Bernarda, L. Binet, ⁴P. Cartigny, ⁵A.J. Brearley, ¹L. Remusat
Geochmica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.009]
¹Institut de minéralogie, de physique des matériaux et de cosmochimie (IMPMC), Sorbonne Universités, UMR CNRS 7590, MNHN, UPMC, UMR IRD 206, CP 52, 57 rue Cuvier, 75005 Paris, France
²Unité matériaux et transformation (UMET), CNRS UMR 8207, Université Lille 1, France
³PSL Research University, Chimie-ParisTech, Institut de Recherche de Chimie-Paris, 11 rue Pierre et Marie Curie, 75005 Paris, France
⁴Institut de physique du globe de Paris (IPGP), Sorbonne Paris Cité, Université Paris Diderot, UMR CNRS 7154, 1 rue Jussieu, 75238 Paris cedex 05, France
⁵Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA
Copyright Elsevier

Unravelling the origin of organic compounds that were accreted into asteroids requires better constraining the impact of asteroidal hydrothermal alteration on their isotopic signatures, molecular structures, and spatial distribution. Here, we conducted a multi-scale/multi-technique comparative study of the organic matter (OM) from two CM chondrites (that originate from the same parent body or from identical parent bodies that accreted the same mixture of precursors) and underwent a different degree of hydrothermal alteration: Paris (a weakly altered CM chondrite – CM 2.8) and Murchison (a more altered one – CM 2.5). The Paris insoluble organic matter (IOM) shows a higher aliphatic/aromatic carbon ratio, a higher radical abundance and a lower oxygen content than the Murchison IOM. Analysis of the OM in situ shows that two texturally distinct populations of organic compounds are present within the Paris matrix: sub-micrometric individual OM particles and diffuse OM finely distributed within phyllosilicates and amorphous silicates. These results indicate that hydrothermal alteration on the CM parent body induced aromatization and oxidation of the IOM, as well as a decrease in radical and nitrogen contents. Some of these observations were also reported by studies of variably altered fragment of Tagish Lake (C2), although the hydrothermal alteration of the OM in Tagish Lake was apparently much more severe. Finally, comparison with data available in the literature suggests that the parent bodies of other chondrite petrologic groups could have accreted a mixture of organic precursors different from that accreted by the parent body of CMs.