Mineralogical and oxygen isotopic study of a new ultrarefractory inclusion in the Northwest Africa 3118 CV3 chondrite

1Yong Xiong,1,2Ai‐Cheng Zhang,3Noriyuki Kawasaki,4Chi Ma, 5Naoya Sakamoto,5Jia‐Ni Chen,6Li‐Xin Gu,3,5Hisayoshi Yurimoto
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13575]
1State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing, 210023 China
2CAS Center for Excellence in Comparative Planetology, Hefei, China
3Department of Natural History Sciences, Hokkaido University, Sapporo, 060‐0810 Japan
4Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, 91125 USA
5Isotope Imaging Laboratory, Creative Research Institution Sousei, Hokkaido University, Sapporo, 001‐0021 Japan
6Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China
Published by arrangement with John Wiley & Sons

Calcium‐aluminum‐rich inclusions (CAIs) are the first solid materials formed in the solar nebula. Among them, ultrarefractory inclusions are very rare. In this study, we report on the mineralogical features and oxygen isotopic compositions of minerals in a new ultrarefractory inclusion CAI 007 from the CV3 chondrite Northwest Africa (NWA) 3118. The CAI 007 inclusion is porous and has a layered (core–mantle–rim) texture. The core is dominant in area and mainly consists of Y‐rich perovskite and Zr‐rich davisite, with minor refractory metal nuggets, Zr,Sc‐rich oxide minerals (calzirtite and tazheranite), and Fe‐rich spinel. The calzirtite and tazheranite are closely intergrown, probably derived from a precursor phase due to thermal metamorphism on the parent body. The refractory metal nuggets either exhibit thin exsolution lamellae of Fe,Ni‐dominant alloy in Os,Ir‐dominant alloy or are composed of Os,Ir,Ru,Fe‐alloy and Fe,Ni,Ir‐alloy with troilite, scheelite, gypsum, and molybdenite. The later four phases are apparently secondary minerals. The Zr,Sc,Y‐rich core is surrounded by a discontinuous layer of closely intergrown hibonite and spinel. The CAIs are rimmed by Fe‐rich spinel and Al‐rich diopside. Perovskite has high concentrations of the most refractory rare earth elements (REEs) but is relatively depleted in the moderately refractory and volatile REEs, consistent with the ultrarefractory REE pattern. Based on this unusual Zr,Sc,Y‐rich mineral assemblage, the layered distribution in CAI 007, and the REE concentrations in perovskite, we suggest that CAI 007 is an ultrarefractory inclusion of condensation origin. In CAI 007, hibonite, spinel, and probably Al‐rich diopside are 16O‐rich (Δ17O ~–22‰) whereas perovskite and davisite are 16O‐poor (Δ17O ~–3‰). Such oxygen isotope heterogeneity suggests that the UR inclusion formed in the various degrees of 16O‐rich nebular setting or was originally 16O‐rich and then experienced oxygen isotope exchange with 16O‐poor fluid on the CV3 chondrite parent body.


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