11:45 AM - 12:00 PM
[PPS07-17] Variations in initial 26Al/27Al ratios among fine-grained CAIs in the reduced CV chondrites
Ca-Al-rich inclusions (CAIs) are oldest solids formed in the Solar System [1] and composed of high-temperature condensates from a solar-composition gas [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) revealed detailed distributions of initial 26Al/27Al values, (26Al/27Al)0, for individual CAIs in the reduced CV chondrites [e.g., 4−9]; coarse-grained, igneous CAIs and fluffy Type A CAIs show similar variations in (26Al/27Al)0 respectively, which range from ~5.2 to ~4.2 × 10−5. In this study, we obtained new 26Al−26Mg mineral isochrons of five fine-grained, spinel-rich CAIs (FGIs) from the reduced CV chondrites Efremovka, Vigarano and TIL 07007 by in situ measurements using a SIMS instrument (CAMECA ims-1280HR installed at Hokkaido University). Since FGIs are likely to be condensates from a solar nebular gas, 26Al−26Mg mineral isochrons of them enable a more systematic comparison of (26Al/27Al)0 between CAIs formed by condensation and by melt crystallization than has previously been achieved.
The obtained 26Al−26Mg mineral isochrons for five FGIs give (26Al/27Al)0 of (5.19 ± 0.17) × 10−5, (5.00 ± 0.17) × 10−5, (4.53 ± 0.18) × 10−5, (4.43 ± 0.31) × 10−5, and (3.35 ± 0.21) × 10−5. The (26Al/27Al)0 for two FGIs are essentially identical to the whole-rock CAI value of (26Al/27Al)0 ~ 5.2 × 10–5 [10, 11], while those for other three FGIs are clearly lower than the whole-rock CAI value. The range of (26Al/27Al)0 values for the FGIs, from (5.19 ± 0.17) to (3.35 ± 0.21) × 10−5, corresponds to a formation age spread of 0.44 ± 0.07 Myr. These variations are slightly larger than those for igneous CAIs ranging from ~5.2 to ~4.2 × 10−5 [5, 6]. Our data imply that CAI condensation events continued for, at least, ~0.4 Myr at the very beginning of our Solar System, if 26Al was distributed homogeneously in the forming region. Alternatively, the observed variations would also raise a possibility of heterogeneous distributions of 26Al in the forming region, corresponding to a range over, at least, 3.4 × 10–5 < (26Al/27Al)0 < 5.2 × 10–5.
[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. (2010) ApJL 711, L117−L121. [5] MacPherson et al. (2012) EPSL 331−332, 43−54. [6] MacPherson et al. (2017) GCA 201, 65−82. [7] Kawasaki et al. (2017) GCA 201, 83−102. [8] Kawasaki et al. (2018) GCA 221, 318−341. [9] Kawasaki et al. (2019) EPSL 511, 25−35. [10] Jacobsen et al. (2008) EPSL 272, 353−364. [11] Larsen et al. (2011) ApJL 735, L37−L43.
The obtained 26Al−26Mg mineral isochrons for five FGIs give (26Al/27Al)0 of (5.19 ± 0.17) × 10−5, (5.00 ± 0.17) × 10−5, (4.53 ± 0.18) × 10−5, (4.43 ± 0.31) × 10−5, and (3.35 ± 0.21) × 10−5. The (26Al/27Al)0 for two FGIs are essentially identical to the whole-rock CAI value of (26Al/27Al)0 ~ 5.2 × 10–5 [10, 11], while those for other three FGIs are clearly lower than the whole-rock CAI value. The range of (26Al/27Al)0 values for the FGIs, from (5.19 ± 0.17) to (3.35 ± 0.21) × 10−5, corresponds to a formation age spread of 0.44 ± 0.07 Myr. These variations are slightly larger than those for igneous CAIs ranging from ~5.2 to ~4.2 × 10−5 [5, 6]. Our data imply that CAI condensation events continued for, at least, ~0.4 Myr at the very beginning of our Solar System, if 26Al was distributed homogeneously in the forming region. Alternatively, the observed variations would also raise a possibility of heterogeneous distributions of 26Al in the forming region, corresponding to a range over, at least, 3.4 × 10–5 < (26Al/27Al)0 < 5.2 × 10–5.
[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. (2010) ApJL 711, L117−L121. [5] MacPherson et al. (2012) EPSL 331−332, 43−54. [6] MacPherson et al. (2017) GCA 201, 65−82. [7] Kawasaki et al. (2017) GCA 201, 83−102. [8] Kawasaki et al. (2018) GCA 221, 318−341. [9] Kawasaki et al. (2019) EPSL 511, 25−35. [10] Jacobsen et al. (2008) EPSL 272, 353−364. [11] Larsen et al. (2011) ApJL 735, L37−L43.