¹Christopher D. K. Herd
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13935]
¹Department of Earth and Atmospheric Sciences, University of Alberta, 1–26 Earth Sciences Building, Edmonton, Alberta, T6G 2E3 Canada
Published by arrangement with John Wiley & Sons

The petrogenesis of the Northwest Africa (NWA) 7635 Martian meteorite involved the entrainment of xenocrystic olivine grains into a relatively magnesian and oxidized melt, followed by a redox-dependent reaction between olivine and melt that resulted in the crystallization of orthopyroxene and magnetite. Subsequent crystallization of the melt began with augite, plagioclase, and magnetite phenocrysts, and was followed by crystallization of augite, plagioclase, magnetite, ilmenite, and pyrrhotite in the groundmass, which took place under more rapid conditions of cooling, as reflected in the groundmass grain size. The petrogenetic history of NWA 7635 is similar in many ways to that of NWA 8159; this observation, coupled with similarities in geochemical and isotopic characteristics from other studies, suggests that the parent melts of the two rocks—as represented by all minerals except the xenocrystic olivine—were one and the same. The main distinctions between the two rocks are that their parent melts entrained xenocrystic olivine of different composition, and the cooling rate of the groundmass of NWA 7635 was more rapid than that of NWA 8159. The conclusion that the redox reaction took place between olivine and melt is in contrast to other work that suggests the reaction took place in the subsolidus, and has implications for the nature of the reaction in both NWA 7635 and NWA 8159.

^1,3Ming-Chang Liu,²Marc Chaussidon,¹Nozomi Matsuda
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.12.001]
¹Department of Earth, Planetary, and Space Sciences, UCLA, Los Angeles, CA, USA
²Université de Paris Cité , Institut de Physique du Globe de Paris, CNRS, 1 rue Jussieu, Paris 75005 France
³Lawrence Livermore National Laboratory, Livermore, CA, USA
Copyright Elsevier

The origin of solar 11B/10B ratio ∼4 remains an open question. It has been thought that a significant portion of boron in the Solar System was derived from continuous spallation nucleosynthesis during interactions between Galactic Cosmic Rays and C-N-O nuclei in the interstellar medium. However, because GCR-produced boron is characterized by 11B/10B ∼2.5, an endmember with 11B/10B > 4 is required to account for the solar 11B/10B ratio. Two leading hypotheses for the sources of 11B-rich components include low energy spallation in the Sun’s parental molecular cloud and the neutrino process during the supernova explosions. In this study, lithium and boron elemental and isotopic compositions of seven porphyritic olivine chondrules and one isolated olivine crystal from four meteorites (Allende, Yamato 81020, Asuka 12236, and QUE 97008) were determined in-situ to help constrain which of the two nucleosynthetic mechanisms has most likely supplied the forming Solar System with extra 11B. Apparent isotopic variations in Li/Si, B/Si, δ7Li and δ11B were found in chondrule olivine crystals, but only three chondrules exhibit statistically resolved δ11B heterogeneities. Using these three chondrules as a basis for discussion, we evaluated the processes that could potentially cause elemental and isotopic variations of Li and B. We argue that the data can be best understood in the context of condensation of lithium-boron-rich material onto chondrule precursors in (an) initially heterogeneous gaseous reservoir(s), which could be realized if the Solar System derived 11B-rich components from a supernova or supernovae.