10:45 AM - 12:15 PM
[PPS08-P08] 26Al–26Mg chronology of high-temperature condensate minerals in a fine-grained, Ca-Al-rich inclusion from reduced CV chondrite
Keywords:CAI, Al-Mg, early solar system, SIMS
Fine-grained, Ca-Al-rich inclusions (FGIs) are aggregates of high-temperature condensate minerals (Krot et al., 2004). Al−Mg mineral isochron studies using secondary ion mass spectrometry (SIMS) indicate FGIs in CV chondrites have variations in initial 26Al/27Al ratio, (26Al/27Al)0, from ~5.2 × 10–5 to ~3.4 × 10–5 (MacPherson et al., 2010, 2020; Kawasaki et al., 2020), corresponding to a formation age spread of ~0.5 Myr under the assumption of homogeneous distribution of 26Al in the forming region.
In previous studies (Kawasaki et al., 2020; Wada et al., 2020), we conducted Al−Mg isotope analyses of hibonite, spinel, and melilite in a hibonite-rich FGI, HKD01, from Northwest Africa 8613 reduced CV chondrite. The mineral isochron for melilite and spinel gives (26Al/27Al)0 of (4.81 ± 0.09) × 10–5, while a regression line for the data of hibonite gives (26Al/27Al)0 of (4.55 ± 0.05) × 10–5. In contrast, the textural relationships (Wada et al., 2020) and equilibrium condensation calculation (Yoneda and Grossman, 1995) indicate that hibonite should have formed prior to the melilite and spinel condensation. Thus, if the lower (26Al/27Al)0 inferred for hibonite than those for spinel and melilite is correct, these data and observations would provide evidence for a heterogeneous distribution of 26Al during the formation of HKD01 FGI. Alternatively, Wada et al. (2020) suggested that relative sensitivity factor (RSF) for Al/Mg of SIMS for the CAI hibonite with 27Al/24Mg > ~320 may be different from that for the Madagascar hibonite with 27Al/24Mg ~28 that was used as the standard.
To address the above issue of RSF of hibonite and elucidate the (26Al/27Al)0 of high-temperature condensate minerals in the HKD01 FGI, we synthesized hibonite standards with different 27Al/24Mg ratios (~30, ~100, and ~400) and measured RSFs for 27Al/24Mg. We also measured hibonite crystals in HKD01 under the same analytical conditions for direct comparison with spinel and melilite data. The analytical conditions with SIMS follow those described in Kawasaki et al. (2019, 2020, 2021) and Wada et al. (2020).
The synthetic hibonite are polycrystalline samples with their sizes typically less than a few μm. The three synthetic samples show stoichiometric compositions of hibonite and homogeneous 27Al/24Mg ratios; 27Al/24Mg = 31.71 ± 0.05 (named as Hib30), 27Al/24Mg = 100.71 ± 0.12 (Hib100), and 27Al/24Mg = 386.8 ± 1.0 (Hib400). RSFs of 27Al/24Mg for the hibonite samples were found to vary with their chemical compositions. The RSFs for Hib100 and Hib400 are identical within the analytical error of 0.3–0.6%, while the RSF for Hib30 is significantly lower, by 2–3% level, than the RSFs for Hib100 and Hib400. Our results indicate that appropriate hibonite standards are essential to obtain accurate chorological data.
The obtained mineral isochron of hibonite in the FGI, using the data corrected with these synthesized standards, gives (26Al/27Al)0 of (4.73 ± 0.09) × 10–5, which is identical to that previously obtained from the mineral isochron of spinel and melilite in the same FGI, (4.81 ± 0.09) × 10–5 (Kawasaki et al., 2020). Since the hibonite formed before the melilite and spinel, a maximum age difference between them is estimated to be ~0.02 Myr from the two mineral isochrons, (4.812 ± 0.089) × 10–5 from the hibonite and (4.729 ± 0.090) × 10–5 from the spinel and melilite, indicating that the major constituent minerals of the HKD01 FGI formed within ~0.02 Myr in the earliest Solar System. This duration is much shorter than the inferred formation age spread of ~0.5 Myr for FGIs (Kawasaki et al., 2020). The HKD01 FGI formed in such a shorter period would escape subsequent remelting and significant gas-solid reaction, and was rapidly transported from the CAI-forming region to the accretion region of the parent body.
In previous studies (Kawasaki et al., 2020; Wada et al., 2020), we conducted Al−Mg isotope analyses of hibonite, spinel, and melilite in a hibonite-rich FGI, HKD01, from Northwest Africa 8613 reduced CV chondrite. The mineral isochron for melilite and spinel gives (26Al/27Al)0 of (4.81 ± 0.09) × 10–5, while a regression line for the data of hibonite gives (26Al/27Al)0 of (4.55 ± 0.05) × 10–5. In contrast, the textural relationships (Wada et al., 2020) and equilibrium condensation calculation (Yoneda and Grossman, 1995) indicate that hibonite should have formed prior to the melilite and spinel condensation. Thus, if the lower (26Al/27Al)0 inferred for hibonite than those for spinel and melilite is correct, these data and observations would provide evidence for a heterogeneous distribution of 26Al during the formation of HKD01 FGI. Alternatively, Wada et al. (2020) suggested that relative sensitivity factor (RSF) for Al/Mg of SIMS for the CAI hibonite with 27Al/24Mg > ~320 may be different from that for the Madagascar hibonite with 27Al/24Mg ~28 that was used as the standard.
To address the above issue of RSF of hibonite and elucidate the (26Al/27Al)0 of high-temperature condensate minerals in the HKD01 FGI, we synthesized hibonite standards with different 27Al/24Mg ratios (~30, ~100, and ~400) and measured RSFs for 27Al/24Mg. We also measured hibonite crystals in HKD01 under the same analytical conditions for direct comparison with spinel and melilite data. The analytical conditions with SIMS follow those described in Kawasaki et al. (2019, 2020, 2021) and Wada et al. (2020).
The synthetic hibonite are polycrystalline samples with their sizes typically less than a few μm. The three synthetic samples show stoichiometric compositions of hibonite and homogeneous 27Al/24Mg ratios; 27Al/24Mg = 31.71 ± 0.05 (named as Hib30), 27Al/24Mg = 100.71 ± 0.12 (Hib100), and 27Al/24Mg = 386.8 ± 1.0 (Hib400). RSFs of 27Al/24Mg for the hibonite samples were found to vary with their chemical compositions. The RSFs for Hib100 and Hib400 are identical within the analytical error of 0.3–0.6%, while the RSF for Hib30 is significantly lower, by 2–3% level, than the RSFs for Hib100 and Hib400. Our results indicate that appropriate hibonite standards are essential to obtain accurate chorological data.
The obtained mineral isochron of hibonite in the FGI, using the data corrected with these synthesized standards, gives (26Al/27Al)0 of (4.73 ± 0.09) × 10–5, which is identical to that previously obtained from the mineral isochron of spinel and melilite in the same FGI, (4.81 ± 0.09) × 10–5 (Kawasaki et al., 2020). Since the hibonite formed before the melilite and spinel, a maximum age difference between them is estimated to be ~0.02 Myr from the two mineral isochrons, (4.812 ± 0.089) × 10–5 from the hibonite and (4.729 ± 0.090) × 10–5 from the spinel and melilite, indicating that the major constituent minerals of the HKD01 FGI formed within ~0.02 Myr in the earliest Solar System. This duration is much shorter than the inferred formation age spread of ~0.5 Myr for FGIs (Kawasaki et al., 2020). The HKD01 FGI formed in such a shorter period would escape subsequent remelting and significant gas-solid reaction, and was rapidly transported from the CAI-forming region to the accretion region of the parent body.