Heterogeneous distribution of 26Al at the birth of the Solar System: Evidence from corundum-bearing refractory inclusions in carbonaceous chondrites

• Published in: Geochimica et cosmochimica acta 2013-06, Vol.110, p.190-215
• Main Authors: Makide, Kentaro, Nagashima, Kazuhide, Krot, Alexander N., Huss, Gary R., Hutcheon, Ian D., Hellebrand, Eric, Petaev, Michail I.
• Format: Article
• Language: English
• Subjects: acid tolerance; Carbonaceous chondrites; Corundum; Disks; Grains; heat treatment; Inclusions; mass spectrometry; mineralogy; mixing; oxygen; Refractories; Reservoirs; Solar system; wind
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2013.01.028

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