5:15 PM - 6:45 PM
[PPS08-P11] Evidence for the heterogeneous distribution of Al-26 in the CAI-forming region of the earliest solar system
Keywords:CAI, chondrule, Al–Mg system, SIMS
Introduction: 26Al is a short-lived radionuclide that decays to 26Mg with a half-life of 0.705 Myr [1] and the initial 26Al abundances of the solar system materials have been used for a relative chronology for the early solar system. Al–Mg bulk isochron studies [2, 3] suggest that 26Al abundances were once homogenized where “canonical” CAIs formed, while the presence of 26Al-poor refractory objects [4] and age discrepancies between Pb–Pb and Al–Mg systematics for angrites [5] and chondrules [6] suggest the heterogeneous distribution of 26Al in the early solar system. Here we present direct evidence for the heterogeneous distribution of 26Al in the early solar system inferred from a unique CAI-chondrule compound object.
Methods: Mineralogical and petrological observations for the CAI-chondrule compound object were performed using SEM-EDS system. We measured Al–Mg isotopic compositions of minerals in the object using a SIMS instrument (Cameca ims-1280HR).
Results and Discussion: The compound object has a rounded shape with ~1 mm diameter and is petrographically divided into two regions; a CAI-like region is surrounded by a chondrule-like region. The CAI-like region is mainly composed of spinel, anorthite, olivine and Al-bearing low-Ca pyroxene. Small amounts of hibonite, perovskite and Si-rich mesostasis are also present. The chondrule-like region is mainly composed of olivine, low-Ca pyroxene and anorthite. Anorthite, low-Ca pyroxene and olivine exhibit systematically different chemical compositions in the different regions. Anorthite is Ca-rich and Na-poor (An#: > 98, Na2O: < ~0.4 wt%) in the CAI-like region compared to the chondrule-like region (An#: ~92–99, Na2O: ~0.3–1.6 wt%). Low-Ca pyroxene is enriched in Al (Al2O3: ~10–14 wt%) and Ti and depleted in Ca in the CAI-like region relative to the chondrule-like region (Al2O3: ~2–9 wt%). The Mg# of olivine is ~98.8–99.1 for the CAI-like region and ~97.4–98.7 for the chondrule-like region. The boundary between the two regions is texturally continuous, suggesting that the compound object experienced remelting and crystallization after the accumulation.
We obtained Al–Mg mineral isochrons of the two regions. The isochron for the CAI-like region gives an initial 26Al/27Al of (–0.4 ± 0.5) × 10–6 and that for the chondrule-like region gives (3.3 ± 0.9) × 10–6. The significantly lower initial 26Al/27Al for the CAI-like region than for the coexisting chondrule-like region indicates that a precursor of the CAI-like region is most likely formed before the incorporation of 26Al into the solar protoplanetary disk. Our data demonstrate that 26Al was heterogeneously distributed in the CAI-forming region.
References: [1] Norris et al. (1983) JGR 88, B331–B333. [2] Jacobsen et al. (2008) EPSL 272, 353–364. [3] Larsen et al. (2011) ApJL 735, L37–43. [4] Krot et al. (2012) MaPS 47, 1948–1979. [5] Schiller et al. (2015) EPSL 420, 45–54. [6] Bollard et al. (2019) GCA 260, 62–83.
Methods: Mineralogical and petrological observations for the CAI-chondrule compound object were performed using SEM-EDS system. We measured Al–Mg isotopic compositions of minerals in the object using a SIMS instrument (Cameca ims-1280HR).
Results and Discussion: The compound object has a rounded shape with ~1 mm diameter and is petrographically divided into two regions; a CAI-like region is surrounded by a chondrule-like region. The CAI-like region is mainly composed of spinel, anorthite, olivine and Al-bearing low-Ca pyroxene. Small amounts of hibonite, perovskite and Si-rich mesostasis are also present. The chondrule-like region is mainly composed of olivine, low-Ca pyroxene and anorthite. Anorthite, low-Ca pyroxene and olivine exhibit systematically different chemical compositions in the different regions. Anorthite is Ca-rich and Na-poor (An#: > 98, Na2O: < ~0.4 wt%) in the CAI-like region compared to the chondrule-like region (An#: ~92–99, Na2O: ~0.3–1.6 wt%). Low-Ca pyroxene is enriched in Al (Al2O3: ~10–14 wt%) and Ti and depleted in Ca in the CAI-like region relative to the chondrule-like region (Al2O3: ~2–9 wt%). The Mg# of olivine is ~98.8–99.1 for the CAI-like region and ~97.4–98.7 for the chondrule-like region. The boundary between the two regions is texturally continuous, suggesting that the compound object experienced remelting and crystallization after the accumulation.
We obtained Al–Mg mineral isochrons of the two regions. The isochron for the CAI-like region gives an initial 26Al/27Al of (–0.4 ± 0.5) × 10–6 and that for the chondrule-like region gives (3.3 ± 0.9) × 10–6. The significantly lower initial 26Al/27Al for the CAI-like region than for the coexisting chondrule-like region indicates that a precursor of the CAI-like region is most likely formed before the incorporation of 26Al into the solar protoplanetary disk. Our data demonstrate that 26Al was heterogeneously distributed in the CAI-forming region.
References: [1] Norris et al. (1983) JGR 88, B331–B333. [2] Jacobsen et al. (2008) EPSL 272, 353–364. [3] Larsen et al. (2011) ApJL 735, L37–43. [4] Krot et al. (2012) MaPS 47, 1948–1979. [5] Schiller et al. (2015) EPSL 420, 45–54. [6] Bollard et al. (2019) GCA 260, 62–83.