[en] High precision Mg isotope measurements have been performed to determine radiogenic ²⁶Mg of CAI minerals by secondary ion mass spectrometry using Faraday cup multi-collection system. Terrestrial samples of spinel and augite, and synthetic glasses of melilite and fassaite, were prepared to correct instrumental mass fractionation. Reproducibility of Mg isotope measurements for each standard were limited to ∼0.4 per mille (2σ). On the other hand, the standard errors for one spot are ∼0.05 per mille (2σ). The poor reproducibility resulted from variations of instrumental mass fractionation among individual measurement spots. We propose novel calculation method of radiogenic ²⁶Mg considering instrumental and natural mass fractionation for each mineral. The overall measurement error of radiogenic ²⁶Mg of the minerals can be calculated less than 0.2 per mille (2σ). This provides that the time resolution of early solar system chronometer was improved up to 100K year for ∼20 μm scale objects formed in the early solar system

[en] Oxygen is one of the major rock-forming elements in the solar system and the third most abundant element of the Sun. Oxygen isotopic composition of the Sun, however, is not known due to a poor resolution of astronomical spectroscopic measurements. Several Δ¹⁷O values have been proposed for the composition of the Sun based on (1) the oxygen isotopic measurements of the solar wind implanted into metallic particles in lunar soil (< -20 per mille by Hashizume and Chaussidon and ∼ +26 per mille by Ireland et al.), (2) the solar wind returned by the Genesis spacecraft (-27 per mille ± 6 per mille by McKeegan et al.), and (3) the mineralogically pristine calcium-aluminum-rich inclusions (CAIs) (-23.3 per mille ± 1.9 per mille by Makide et al. and -35 per mille by Gounelle et al.). CAIs are the oldest solar system solids, and are believed to have formed by evaporation, condensation, and melting processes in hot nebular region(s) when the Sun was infalling (Class 0) or evolved (Class 1) protostar. Corundum (Al₂O₃) is thermodynamically the first condensate from a cooling gas of solar composition. Corundum-bearing CAIs, however, are exceptionally rare, suggesting either continuous reaction of the corundum condensates with a cooling nebular gas and their replacement by hibonite (CaAl₁₂O₁₉) or their destruction by melting together with less refractory condensates during formation of igneous CAIs. In contrast to the corundum-bearing CAIs, isolated micrometer-sized corundum grains are common in the acid-resistant residues from unmetamorphosed chondrites. These grains could have avoided multistage reprocessing during CAI formation and, therefore, can potentially provide constraints on the initial oxygen isotopic composition of the solar nebula, and, hence, of the Sun. Here we report oxygen isotopic compositions of ∼60 micrometer-sized corundum grains in the acid-resistant residues from unequilibrated ordinary chondrites (Semarkona (LL3.0), Bishunpur (LL3.1), Roosevelt County 075 (H3.2)) and unmetamorphosed carbonaceous chondrites (Orgueil (CI1), Murray (CM2), and Alan Hills A77307 (CO3.0)) measured with a Cameca ims-1280 ion microprobe. All corundum grains, except two, are ¹⁶O-rich (Δ¹⁷O = -22.7 per mille ± 8.5 per mille, 2σ), and compositionally similar to the mineralogically pristine CAIs from the CR carbonaceous chondrites (-23.3 per mille ± 1.9 per mille, 2σ), and solar wind returned by the Genesis spacecraft (-27 per mille ± 6 per mille, 2σ). One corundum grain is highly ¹⁷O-enriched (δ¹⁷O ∼ +60 per mille, δ¹⁸O ∼ -40 per mille) and is probably of the presolar origin; the origin of another ¹⁷O-rich grain (δ¹⁷O ∼ -15 per mille , δ¹⁸O ∼ -35 per mille) is unclear. We conclude that the ¹⁶O-rich corundum grains in the acid-resistant residues from unequilibrated ordinary and unmetamorphosed carbonaceous chondrites recorded initial oxygen isotopic composition of the solar nebula, and, hence, of the Sun. Our inferred oxygen isotopic composition of the Sun is inconsistent with the more extreme ¹⁶O-rich value (Δ¹⁷O ∼ -35 per mille) proposed by Gounelle et al. on the basis of two extremely ¹⁶O-rich CAIs from the CH/CB-like chondrite Isheyevo and with the ¹⁶O-poor value observed as a component of the solar wind implanted into the metallic particles in lunar soil (Ireland et al.).

[en] It is believed that ²⁶Al, a short-lived (t_1/2 = 0.73 Ma) and now extinct radionuclide, was uniformly distributed in the nascent solar system (SS) with the initial ²⁶Al/²⁷Al ratio of ∼5.2 x 10^-5, suggesting an external, stellar origin rather than local, solar source. However, the stellar source of ²⁶Al and the manner in which it was injected into the SS remain controversial: the ²⁶Al could have been produced by an asymptotic giant branch star, a supernova, or a Wolf-Rayet star and injected either into the protosolar molecular cloud, protosolar cloud core, or protoplanetary disk. Corundum (Al₂O₃) is predicted to be the first condensate from a cooling gas of solar composition. Here we show that micron-sized corundum condensates from ¹⁶O-rich (Δ¹⁷O ∼ -25 per mille ) gas of solar composition recorded heterogeneous distribution of ²⁶Al at the birth of the SS: the inferred initial ²⁶Al/²⁷Al ratio ranges from ∼6.5x10^-5 to <2x10^-6; 52% of corundum grains measured are ²⁶Al-poor. Abundant ²⁶Al-poor, ¹⁶O-rich refractory objects include grossite- and hibonite-rich calcium-aluminum-rich inclusions (CAIs) in CH (high metal abundance and high iron concentration) chondrites, platy hibonite crystals in CM (Mighei-like) chondrites, and CAIs with fractionation and unidentified nuclear effects CAIs chondrites. Considering the apparently early and short duration (<0.3 Ma) of condensation of refractory ¹⁶O-rich solids in the SS, we infer that ²⁶Al was injected into the collapsing protosolar molecular cloud and later homogenized in the protoplanetary disk. The apparent lack of correlation between ²⁶Al abundance and O-isotope composition of corundum grains constrains the stellar source of ²⁶Al in the SS.