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

Abstract

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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