¹Jens Barosch,^1,2Dominik C. Hezel,³Yves Marrocchi,³Andrey Gurenko,^1,4Christoph Lenting
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13496]
¹Department of Geology and Mineralogy, University of Cologne, Zülpicher Str. 49b, 50674 Köln, Germany
²Department of Mineralogy, Natural History Museum, Cromwell Road, London, SW7 5BD UK
³CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre‐lès‐Nancy, 54501 France
⁴Institute of Geoscience, University of Bonn, Meckenheimer Allee 169, 53111 Bonn, Germany
Published by arrangement with John Wiley & Sons

We found a large (~2 mm) compound object in the primitive Yamato 793408 (H3.2‐an) chondrite. It consists mostly of microcrystalline material, similar to chondrule mesostasis, that hosts an intact barred olivine (BO) chondrule. The object contains euhedral pyroxene and large individual olivine grains. Some olivine cores are indicative of refractory forsterites with very low Fe‐ and high Ca, Al‐concentrations, although no ¹⁶O enrichment. The entire object is most likely a new and unique type, as no similar compound object has been described so far. We propose that it represents an intermediate stage between compound chondrules and macrochondrules, and formed from the collision between chondrules at low velocities (below 1 m s⁻¹) at high temperatures (around 1550 °C). The macrochondrule also trapped and preserved a smaller BO chondrule. This object appears to be the first direct evidence for a genetic link between compound chondrules and macrochondrules. In accordance with previous suggestions and studies, compound chondrules and macrochondrules likely formed by the same mechanism of chondrule collisions, and each represents different formation conditions, such as ambient temperature and collision speed.

^1,2T.V.Kizovski et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.05.030]
¹Centre for Applied Planetary Mineralogy, Department of Natural History, Royal Ontario Museum, 100 Queen’s Park, Toronto, Ontario, M5S 2C6, Canada
²Department of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, Ontario, M5S 3B1, Canada
Copyright Elsevier

Northwest Africa (NWA) 7042 is an intermediate, permafic shergottite consisting of two generations of olivine (early zoned olivine Fo41-76, and late-stage fayalitic olivine Fo46-56), complexly zoned pyroxene (En35-64Fs22-46Wo5-34), shock-melted or maskelynitized feldspar (An5-30Ab16-61Or1-47), and accessory merrillite, apatite, ilmenite, titanomagnetite, Fe-Cr-Ti spinels, pyrrhotite, and baddeleyite. The zoned olivine grains have been pervasively modified, containing conspicuous brown Mg-rich cores surrounded by colorless, unaltered Fe-rich overgrowth rims. This textural relationship suggests that the cores were altered at magmatic temperatures prior to crystallization of the rims on Mars. Launch-generated shock veins in NWA 7042 also crosscut and displace several of the altered olivine grains indicating that alteration occurred before ejection of the meteorite. While this type of olivine alteration is rare in shergottites, it is similar to deuterically altered olivine in basalts and gabbros on Earth, caused by residual water-rich magmatic fluids. Transmission electron microscopy analysis of the olivine alteration did not reveal the high-temperature phases expected from this process; however, NWA 7042 has also been subjected to extensive terrestrial weathering which may explain their absence. The potential presence of deuterically altered olivine in NWA 7042 has significant implications, as it is the third martian meteorite where deuteric alteration of olivine has been observed (the others being NWA 10416, and ALH 77005). The different mantle sources for the parental melts of these three meteorites would suggest many, if not all martian mantle reservoirs have the potential to produce water-rich magmas.