¹Hugues Leroux, ¹Damien Jacob, ²Maya Marinova, ^3,4Roger H. Hewins, ^3,4,5Brigitte Zanda, ³Sylvain Pont, ⁶Jean-Pierre Lorand, ⁷Munir Humayun
¹Unité Matériaux et Transformations, University of Lille & CNRS, Villeneuve d’Ascq, France
²Institut Chevreul, University of Lille & CNRS, Villeneuve d’Ascq, France
³Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Université, Muséum National d’Histoire Naturelle, UPMC Université Paris 06, IRD & CNRS, Paris, France
⁴Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
⁵Institut de Mécanique Céleste et de Calcul des Ephémérides, Observatoire de Paris, Paris Cedex, France
⁶Laboratoire de Planétologie et Géodynamique, Université de Nantes, Nantes, France
⁷Department of Earth, Ocean & Atmospheric Science and National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida, USA

Northwest Africa (NWA) 7533 is a Martian regolith breccia. This meteorite (and its pairings) offers a good opportunity to study (near-) surface processes that occurred on early Mars. Here, we have conducted a transmission electron microscope study of medium- and coarse-grained (a few tens to hundreds of micrometers) Ca-rich pyroxene clasts in order to define their thermal and shock histories. The pyroxene grains have a high-temperature (magmatic) origin as revealed by the well-developed pigeonite–augite exsolution microstructure. Exsolution lamella characteristics (composition, thickness, and spacing) indicate a moderately slow cooling. Some of the pyroxene clasts display evidence for local decomposition into magnetite and silica at the submicron scale. This phase decomposition may have occurred at high temperature and occurred at high oxygen fugacity at least 2–3 log units above the QFM buffer, after the formation of the exsolution lamellae. This corresponds to oxidizing conditions well above typical Martian magmatic conditions. These oxidizing conditions seem to have prevailed early and throughout most of the history of NWA 7533. The shock microstructure consists of (100) mechanical twins which have accommodated plastic deformation. Other pyroxene shock indicators are absent. Compared with SNC meteorites that all suffered significant shock metamorphism, NWA 7533 appears only mildly shocked. The twin microstructure is similar from one clast to another, suggesting that the impact which generated the (100) twins involved the compacted breccia and that the pyroxene clasts were unshocked when they were incorporated into the NWA 7533 breccia.

Reference
Leroux H, Jacob D, Marinova M, Hewins RH, Zanda B, Pont S, Lorand J-P, Humayun M (2016) Exsolution and shock microstructures of igneous pyroxene clasts in the Northwest Africa 7533 Martian meteorite. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12637]
Published by arrangement with John Wiley & Sons

^1,2Arshad Ali, ¹Iffat Jabeen, ³David Gregory, ⁴Robert Verish,¹Neil R. Banerjee
¹Department of Earth Sciences & Centre for Planetary Science and Exploration, Western University, London, Ontario, Canada
²Earth Sciences Research Centre (ESRC), Sultan Qaboos University, Muscat, Sultanate of Oman
³St. Thomas, Ontario, Canada
⁴Meteorite-Recovery Lab, Escondido, California, USA

We report precise triple oxygen isotope data of bulk materials and separated fractions of several Shergotty–Nakhla–Chassigny (SNC) meteorites using enhanced laser-assisted fluorination technique. This study shows that SNCs have remarkably identical Δ17O and a narrow range in δ18O values suggesting that these meteorites have assimilated negligibly small surface materials (<5%), which is undetectable in the oxygen isotope compositions reported here. Also, fractionation factors in coexisting silicate mineral pairs (px-ol and mask-ol) further demonstrate isotopic equilibrium at magmatic temperatures. We present a mass-dependent fractionation line for bulk materials with a slope of 0.526 ± 0.016 (1SE) comparable to the slope obtained in an earlier study (0.526 ± 0.013; Franchi et al. 1999). We also present a new Martian fractionation line for SNCs constructed from separated fractions (i.e., pyroxene, olivine, and maskelynite) with a slope of 0.532 ± 0.009 (1SE). The identical fractionation lines run above and parallel to our terrestrial fractionation line with Δ17O = 0.318 ± 0.016‰ (SD) for bulk materials and 0.316 ± 0.009‰ (SD) for separated fractions. The conformity in slopes and Δ17O between bulk materials and separated fractions confirm oxygen isotope homogeneity in the Martian mantle though recent studies suggest that the Martian lithosphere may potentially have multiple oxygen isotope reservoirs.

Reference
Ali A, Jabeen I, Gregory D, Verish R, Banerjee NR (2016) New triple oxygen isotope data of bulk and separated fractions from SNC meteorites: Evidence for mantle homogeneity of Mars. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12640]
Published by arrangement with John Wiley & Sons