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We report the mineralogy, petrography, and in situ measured 26 Al– 26 Mg systematics in chondrules from the least metamorphosed CV3 (Vigarano-type) chondrites, Kaba and Yamato (Y) 980145. Two Y 980145 chondrules measured show no resolvable excesses in 26 Mg ( 26 Mg ∗ ), a decay product of a short-lived ( t 1/2 ∼0.7 Ma) radionuclide 26 Al. Plagioclase in one of the chondrules is replaced by nepheline, indicative of thermal metamorphism. The lack of 26 Mg ∗ in the Y 980145 chondrules is most likely due to disturbance of their 26 Al– 26 Mg systematics during the metamorphism. Although Kaba experienced extensive metasomatic alteration (<300 °C), it largely avoided subsequent thermal metamorphism, and the 26 Al– 26 Mg systematics of its chondrules appear to be undisturbed. All eight Kaba chondrules measured show 26 Mg ∗ , corresponding to the initial 26 Al/ 27 Al ratios [( 26 Al/ 27 Al) 0 ] ranging from (2.9 ± 1.7) × 10 −6 to (6.3 ± 2.7) × 10 −6 . If CV parent asteroid accreted rapidly after chondrule formation, the inferred ( 26 Al/ 27 Al) 0 ratios in Kaba chondrules provide an upper limit on 26 Al available in this asteroid at the time of its accretion. The estimated initial abundance of 26 Al in the CV asteroid is too low to melt it and contradicts the existence of a molten core in this body suggested from the paleomagnetic records of Allende [Carporzen et al. (2011) Magnetic evidence for a partially differentiated carbonaceous chondrite parent body. Proc. Natl. Acad. Sci. USA 108 , 6386–6389] and Kaba [Gattacceca et al. (2013) More evidence for a partially differentiated CV parent body from the meteorite Kaba. Lunar Planet. Sci. 44 , abstract#1721]. [ABSTRACT FROM AUTHOR]

High precision secondary ion mass-spectrometry (SIMS) analyses of kirschsteinite (CaFeSiO 4 ) in the reduced CV3 chondrites Vigarano and Efremovka yield well resolved 53 Cr excesses that correlate with 55 Mn/ 52 Cr, demonstrating in situ decay of the extinct short-lived radionuclide 53 Mn. To ensure proper correction for relative sensitivities between 55 Mn + and 52 Cr + ions, we synthesized kirschsteinite doped with Mn and Cr to measure the relative sensitivity factor. The inferred initial ratio ( 53 Mn/ 55 Mn) 0 in chondritic kirschsteinite is (3.71 ± 0.50) × 10 –6 . When anchored to 53 Mn– 53 Cr relative and U-corrected 207 Pb– 206 Pb absolute ages of the D’Orbigny angrite, this ratio corresponds to kirschsteinite formation 3.2 - 0.7 + 08 Ma after CV Ca-, Al-rich inclusions. The kirschsteinite data are consistent within error with the data for aqueously-formed fayalite from the Asuka 881317 CV3 chondrite as reported by Doyle et al. (2015), supporting the idea that Ca–Fe silicates in CV3 chondrites are cogenetic with fayalite (and magnetite) and formed during metasomatic alteration on the CV3 parent body. Concentrically-zoned crystals of kirschsteinite and hedenbergite indicate that they initially formed as near end-member compositions that became more Mg-rich with time, possibly as a result of an increase in temperature. [ABSTRACT FROM AUTHOR]

Abstract: We report on the mineralogy, petrology, and in situ oxygen- and magnesium-isotope measurements using secondary ion mass spectrometry of 10 corundum-bearing calcium–aluminum-rich inclusions (CAIs) from the Adelaide (ungrouped), Murray and Murchison (CM) carbonaceous chondrites. We also measured in situ oxygen-isotope compositions of several isolated corundum grains in the matrices of Murray and Murchison. Most of the corundum-bearing objects studied are uniformly 16O-rich [Δ17O values range from −17‰ to −28‰ (2σ =±2.5‰) (Δ17Oavr =−23±5‰)], suggesting that they formed in a 16O-rich gas of approximately solar composition and largely avoided subsequent thermal processing in an 16O-poor gaseous reservoir. There is a large spread of the initial 26Al/27Al ratio [(26Al/27Al)0] in the corundum-bearing CAIs. Two Adelaide CAIs show no resolvable excess of radiogenic 26Mg (δ26Mg∗): the inferred (26Al/27Al)0 are (0.6±2.0)×10−6 and (−0.9±1.2)×10−6, respectively. Slopes of the model 26Al–26Mg isochrons in five CAIs from Murray and Murchison are (4.4±0.2)×10−5, (3.3±0.3)×10−5, (4.1±0.3)×10−5, (3.9±0.4)×10−5, and (4.0±2.0)×10−6, respectively. These values are lower than the canonical (26Al/27Al)0 ratio of (5.23±0.13)×10−5 inferred from the whole-rock magnesium-isotope measurements of the CV CAIs, but similar to the (26Al/27Al)0 ratio of (4.1±0.2)×10−5 in the corundum-bearing CAI F5 from Murray. Five other previously studied corundum-bearing CAIs from Acfer 094 (ungrouped) and CM carbonaceous chondrites showed no resolvable δ26Mg∗. We conclude that the corundum-bearing CAIs, as well as the solar corundum grains from matrices and acid-resistant residues of unequilibrated ordinary and carbonaceous chondrites, recorded heterogeneous distribution of 26Al in the Solar System during an epoch of CAI formation. The 26Al-rich and 26Al-poor corundum-bearing CAIs and solar corundum grains represent different generations of refractory objects formed during this epoch. As a result, its duration cannot be inferred from 26Al–26Mg systematics of CAIs. Oxygen-isotope composition of the protoplanetary disk was probably heterogeneous during this time reflecting either initial differences in oxygen isotopic compositions of the solid and gaseous reservoirs in the early Solar System or rapid isotopic evolution of these reservoirs in the protoplanetary disk with time. We suggest that 26Al was injected into the protosolar molecular cloud core, possibly by a wind from a neighboring massive star or by or a low-mass asymptotic giant branch star, prior to formation of CAIs and refractory grains, and was subsequently homogenized through the protoplanetary disk by radial mixing. [Copyright &y& Elsevier]