¹Shaofan Che,¹Adrian J.Brearley
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.05.055]
¹Department of Earth and Planetary Sciences, MSC03-2040, University of New Mexico, Albuquerque, NM 87131-0001, USA
Copyright Elsevier

Fine-grained, spinel-rich inclusions (FGIs) are abundant in CV3 chondrites and exhibit textures and compositions that are consistent with a condensation origin. We have conducted a systematic investigation of FGIs from two reduced CV3 chondrites, Leoville and Efremovka, which has revealed a number of microscale variations in the primary mineralogies and textures of nodules, and provided further insights into the origins of FGIs. Nodules in individual FGIs vary in size and exhibit variations in their mineralogical zonation, resulting in significant heterogeneity within each FGI. In individual FGIs, nodules with a small size (typically <10 μm) commonly form clusters, whereas larger nodules (often >20 μm) are either embedded in the mass of small nodules or occur as shells surrounding clusters of small nodules. The size difference is associated with a difference in mineralogy: small nodules typically contain single or a few spinel/melilite grains as cores, while the spinel/melilite cores of large nodules are polycrystalline and more compact. Transmission Electron Microscope observations show that the nodules have complex microstructures, including the presence of fine-grained spinel, the close association of fine-grained Al-Ti-diopside with spinel, and a crystallographic orientation relationship between adjacent clinoenstatite and diopside grains.

Our microstructural observations indicate that disequilibrium condensation played an important role in the formation of FGIs, consistent with some previous studies. Specifically, the presence of spinel-cored and melilite-dominant nodules, as well as the different occurrences of spinel (in the cores and on the periphery), suggest that formation of these nodules occurred under disequilibrium conditions, which may be caused by physical isolation of condensates.

Nodules in FGIs show textural and compositional similarities with other types of non-igneous CAIs: hibonite-spinel inclusions and fluffy Type A CAIs. We suggest that mineralogically-distinct nodules are micrometer-sized counterparts of different types of non-igneous CAIs and record an evolutionary condensation sequence in the solar nebula. It is likely that different nodules in individual FGIs formed in the same gaseous reservoir, but at different times. The mechanism of physical isolation of condensates probably controlled the accretion behavior of nodules with different mineralogies and sizes, resulting in the observed distribution patterns of nodules. On the other hand, some mineralogically-zoned FGIs, with a Mg-rich core and a Ca-rich mantle, can be better explained by condensation, followed by transport of the inclusions to a different region of the protoplanetary disk.

^1,3Ke Zhu(朱柯),¹Frédéric Moynier,²Martin Schiller,³Harry Becker,⁴Jean-Alix Barrat,^1,2Martin Bizzarro
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.05.053]
¹Université de Paris, Institut de Physique du Globe de Paris, CNRS, 75005, Paris France
²Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, Øster Voldgade 5–7, Copenhagen DK-1350, Denmark
³Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstr. 74-100, 12249 Berlin, Germany
⁴Univ. Brest, CNRS, UMR 6539 (Laboratoire des Sciences de l’Environnement Marin), LIA BeBEST, Institut Universitaire Européen de la Mer (IUEM), Place Nicolas Copernic, 29280 Plouzané, France
Copyright Elsevier

Enstatite achondrites (including aubrites) are the only differentiated meteorites that have similar isotope compositions to the Earth-Moon system for most of the elements. However, the origin and differentiation of enstatite achondrites and their parent bodies remain poorly understood. Here, we report high-precision mass-independent and mass-dependent Cr isotope data for 10 enstatite achondrites, including eight aubrites, Itqiy and one enstatite-rich clast in Almahatta Sitta, to further constrain the origin and evolution of their parent bodies. The ε⁵⁴Cr (per 10,000 deviation of the mass bias corrected ⁵⁴Cr/⁵²Cr ratio from a terrestrial standard) systematics define three groups: main-group aubrites with ε⁵⁴Cr = 0.06 ± 0.12 (2SD, N =7) that is similar to the enstatite chondrites and the Earth-Moon system, Shallowater aubrite with ε⁵⁴Cr = -0.12 ± 0.04 and Itqiy-type meteorites with ε⁵⁴Cr = -0.26 ± 0.03 (2SD, N =2). This shows that there were at least three enstatite achondrite parent bodies in the Solar System. This is confirmed by their distinguished mass-dependent Cr isotope compositions (δ⁵³Cr values): 0.24 ± 0.03 ‰, 0.10 ± 0.03 ‰ and -0.03± 0.03 ‰ for main-group, Shallowater and Itqiy parent bodies, respectively. Aubrites are isotopically heavier than chondrites (δ⁵³Cr =-0.12 ± 0.04 ‰), which likely results from the formation of an isotopically light sulfur-rich core. We also obtained the abundance of the radiogenic ⁵³Cr (produced by the radioactive decay of ⁵³Mn, T_1/2= 3.7 million years). The radiogenic ε⁵³Cr excesses correlate with the ⁵⁵Mn/⁵²Cr ratios for aubrites (except Shallowater and Bustee) and also the Cr stable isotope compositions (δ⁵³Cr values). We show that these correlations represent mixing lines that also hold chronological significance since they are controlled by the crystallization of sulfides and silicates, which mostly reflect the main-group aubrite parent body differentiation at 4562.5 ± 1.1 Ma (i.e., 4.8 ± 1.1 Ma after Solar System formation). Furthermore, the intercept of these lines with the ordinate axis which represent the initial ε⁵³Cr value of main-group aubrites (0.50 ± 0.16, 2σ) is much higher than the average ε⁵³Cr value of enstatite chondrites (0.15 ± 0.10, 2SD), suggesting an early sulfur-rich core formation that effectively increased the Mn/Cr ratio of the silicate fraction of the main-group aubrite parent body.