Textural and compositional evidence for in situ crystallization of palisade bodies in coarse‐grained Ca‐Al‐rich inclusions

1,2Mingming Zhang,1,2Yangting Lin,3Ingo Leya,4 Guoqiang Tang,4Yu Liu
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13260]
1Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China
2University of Chinese Academy of Sciences, Beijing, 100049 China
3Physical Institute, Space Sciences and Planetology, University of Bern, 3012 Bern, Switzerland
4State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China
Published by arrangement with John Wiley & Sons

Palisade bodies, mineral assemblages with spinel shells, in coarse‐grained Ca‐, Al‐rich inclusions (CAIs) have been considered either as exotic “mini‐CAIs” captured by their host inclusions (Wark and Lovering 1982) or as in situ crystallization products of a bubble‐rich melt (Simon and Grossman 1997). In order to clarify their origins, we conducted a comprehensive study of palisade bodies in an Allende Type B CAI (BBA‐7), using electron backscatter diffraction (EBSD), micro‐computed tomography (Micro‐CT), electron probe microanalysis (EPMA), and secondary ion mass spectrometry (SIMS). New observations support the in situ crystallization mechanism: early/residual melt infiltrated into spinel‐shelled bubbles and crystallized inside. Evidence includes (1) continuous crystallography of anorthite from the interior of the palisade body to the surrounding host; (2) partial consolidation of two individual palisade bodies revealed by micro‐CT; (3) a palisade body was entirely enclosed in a large anorthite crystal, and the anorthite within the palisade body shows the same crystallographic orientation as the anorthite host; and (4) identical chemical and oxygen isotopic compositions of the constituent minerals between the palisade bodies and the surrounding host. Oxygen isotopic compositions of the major minerals in BBA‐7 are bimodal‐distributed. Spinel and fassaite are uniformly 16O‐rich with ∆17O = −23.3 ± 1.5‰ (2SD), and melilite and anorthite are homogeneously 16O‐poor with ∆17O = −3.2 ± 0.7‰ (2SD). The latter ∆17O value overlaps with that of the Allende matrix (∆17O ~ −2.87‰) (Clayton and Mayeda 1999), which could be explained by secondary alteration with a 16O‐poor fluid in the parent body. The mobility of fluid could be facilitated by the high porosity (1.56–2.56 vol%) and connectivity (~0.17–0.55 vol%) of this inclusion.

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