2:00 PM - 2:15 PM
[PPS06-14] Contemporaneous formation of condensed and igneous Ca-Al-rich inclusions in the solar nebula inferred from 26Al−26Mg systematics
Ca-Al-rich inclusions (CAIs) in meteorites, the oldest objects formed in our Solar System [1], are composed of high-temperature condensate minerals from the solar nebular gas [e.g. 2]. Most of CAIs are thought to have contained detectable amounts of live 26Al, a short-lived radionuclide with a half-life of ~0.7 Myr, at their formation [3]. Recent high-precision 26Al−26Mg mineral isochron studies using secondary ion mass spectrometry (SIMS) offer detailed distributions of initial 26Al/27Al values, (26Al/27Al)0, for individual CAIs [e.g. 4, 5]. The data of [4, 5] show that high-temperature nebular condensates exhibit extremely narrow ranges of (26Al/27Al)0 ~ 5.2 × 10−5, while igneous CAIs range from ~5.2 to ~4.2 × 10−5, corresponding to a formation age spread of ~0.2 Myr. In this study, we newly obtained 26Al−26Mg mineral isochrons of three high-temperature nebular condensates, one fine-grained, spinel-rich inclusion and two fluffy Type A CAIs, from the reduced CV chondrites Efremovka and Vigarano by in situ Al−Mg isotope measurements using a SIMS instrument (CAMECA ims-1280HR) installed at Hokkaido University.
The obtained 26Al−26Mg mineral isochron for the fine-grained, spinel-rich inclusion gives an initial 26Al/27Al value, (26Al/27Al)0, of (5.20 ± 0.17) × 10−5. This is essentially identical to the canonical (26Al/27Al)0 determined by whole-rock 26Al−26Mg isochron studies for CAIs in the CV chondrites [7, 8]. On the other hand, inferred (26Al/27Al)0 for two fluffy Type A CAIs, (4.64 ± 0.12) × 10−5 and (4.37 ± 0.12) × 10−5, are significantly lower than the canonical value. The range of (26Al/27Al)0 of the observed nebular condensates, from (5.20 ± 0.17) to (4.37 ± 0.12) × 10−5, corresponds to a formation age spread of 0.18 ± 0.04 Myr, similar to ~0.2 Myr for the igneous CAIs [4, 5]. These data indicate that the nebular condensates formed contemporaneously with the igneous CAIs during ~0.2 Myr from the canonical age. Our findings demonstrate that both condensation and melting of minerals in a hot nebular gas occurred contemporaneously to form various types of CAIs and continued for at least ~0.2 Myr after the birth of the Solar System.
[1] Connelly et al. (2012) Science 338, 651−655. [2] Grossman (1972) GCA 86, 597–619. [3] MacPherson et al. (1995) Meteoritics 30, 365–386. [4] MacPherson et al. (2012) EPSL 331−332, 43−54. [5] MacPherson et al. (2017) GCA 201, 65−82. [6] Kawasaki et al. (2018) GCA 221, 318−341. [7] Jacobsen et al. (2008) EPSL 272, 353−364. [8] Larsen et al. (2011) ApJL 735, L37−L43.
The obtained 26Al−26Mg mineral isochron for the fine-grained, spinel-rich inclusion gives an initial 26Al/27Al value, (26Al/27Al)0, of (5.20 ± 0.17) × 10−5. This is essentially identical to the canonical (26Al/27Al)0 determined by whole-rock 26Al−26Mg isochron studies for CAIs in the CV chondrites [7, 8]. On the other hand, inferred (26Al/27Al)0 for two fluffy Type A CAIs, (4.64 ± 0.12) × 10−5 and (4.37 ± 0.12) × 10−5, are significantly lower than the canonical value. The range of (26Al/27Al)0 of the observed nebular condensates, from (5.20 ± 0.17) to (4.37 ± 0.12) × 10−5, corresponds to a formation age spread of 0.18 ± 0.04 Myr, similar to ~0.2 Myr for the igneous CAIs [4, 5]. These data indicate that the nebular condensates formed contemporaneously with the igneous CAIs during ~0.2 Myr from the canonical age. Our findings demonstrate that both condensation and melting of minerals in a hot nebular gas occurred contemporaneously to form various types of CAIs and continued for at least ~0.2 Myr after the birth of the Solar System.
[1] Connelly et al. (2012) Science 338, 651−655. [2] Grossman (1972) GCA 86, 597–619. [3] MacPherson et al. (1995) Meteoritics 30, 365–386. [4] MacPherson et al. (2012) EPSL 331−332, 43−54. [5] MacPherson et al. (2017) GCA 201, 65−82. [6] Kawasaki et al. (2018) GCA 221, 318−341. [7] Jacobsen et al. (2008) EPSL 272, 353−364. [8] Larsen et al. (2011) ApJL 735, L37−L43.