Oxygen- and magnesium-isotope compositions of calcium–aluminum-rich inclusions from Rumuruti (R) chondrites

Abstract

We report oxygen- and magnesium-isotope compositions of Ca,Al-rich inclusions (CAIs) from several Rumuruti (R) chondrites measured in situ using a Cameca ims-1280 ion microprobe. On a three-isotope oxygen diagram, δ 17O vs. δ 18O, compositions of individual minerals in most R CAIs analyzed fall along a slope-1 line. Based on the variations of Δ 17O values (Δ 17O = δ 17O − 0.52 × δ 18O) within individual inclusions, the R CAIs are divided into (i) 16O-rich (Δ 17O ∼ −23–26‰), (ii) uniformly 16O-depleted (Δ 17O ∼ −2‰), and (iii) isotopically heterogeneous (Δ 17O ranges from −25‰ to +5‰). One of the hibonite-rich CAIs, H030/L, has an intermediate Δ 17O value of −12‰ and a highly fractionated composition (δ 18O ∼ +47‰). We infer that like most CAIs in other chondrite groups, the R CAIs formed in an 16O-rich gaseous reservoir. The uniformly 16O-depleted and isotopically heterogeneous CAIs subsequently experienced oxygen-isotope exchange during remelting in an 16O-depleted nebular gas, possibly during R chondrite chondrule formation, and/or during fluid-assisted thermal metamorphism on the R chondrite parent asteroid. Three hibonite-bearing CAIs and one spinel–plagioclase-rich inclusion were analyzed for magnesium-isotope compositions. The CAI with the highly fractionated oxygen isotopes, H030/L, shows a resolvable excess of 26Mg ( 26Mg ∗) corresponding to an initial 26Al/ 27Al ratio of ∼7 × 10 −7. Three other CAIs show no resolvable excess of 26Mg ( 26Mg ∗). The absence of 26Mg ∗ in the spinel–plagioclase-rich CAI from a metamorphosed R chondrite NWA 753 (R3.9) could have resulted from metamorphic resetting. Two other hibonite-bearing CAIs occur in the R chondrites (NWA 1476 and NWA 2446), which appear to have experienced only minor degrees of thermal metamorphism. These inclusions could have formed from precursors with lower than canonical 26Al/ 27Al ratio.