5:15 PM - 6:45 PM
[PPS08-P16] Formation process of compact Type A CAI from CV chondrite: melilite condensation after the igneous process
Keywords:CAI, condensation, Al-Mg, SIMS
Ca-Al-rich inclusions (CAIs) in meteorites are the oldest objects formed in the Solar System, and are divided into coarse-grained, igneous CAIs and condensed CAIs. Igneous CAIs are expected to have experienced the melting of precursor solids and crystallization from the melt droplets. On the other hand, condensed CAIs are suggested to be formed by condensation from a solar nebular gas. CAIs contained live-26Al, a short-lived radionuclide with a half-life of 0.705 Myr, and 26Al–26Mg systematics has been applied to the relative chronology of the early Solar System. High-precision Al–Mg isotopic studies using secondary ion mass spectrometry (SIMS) revealed the detailed distribution of the initial 26Al/27Al ratio, (26Al/27Al)0, for condensed and igneous CAIs. The ranges of (26Al/27Al)0 are consistent with each other (~5.2 × 10−5 to ~3.4 × 10−5), suggesting that the contemporaneous formation of the condensed and igneous CAIs occurred during ~0.4 Myr at the very beginning of the Solar System.
Compact Type A (CTA) CAIs which are igneous CAIs, have åkermanite (åk) -poor melilite mantle. It is reported that the mantle mainly consists of reverse zoned melilite crystals, and is inferred to be formed by condensation. However, the mechanism and timing of the mantle formation are still ambiguous. Therefore, although the contemporaneous formation of the condensed and igneous CAIs has been revealed, it remains uncertain the age difference between the igneous and condensation processes that a single CAI experienced. In this study, we revealed the formation mechanisms of the CTA mantle in more detail through comprehensive studies of petrographic observations, in situ O-isotope analysis, and Al–Mg isotope measurements at Hokkaido University (Cameca ims-1280HR) for KU-N-02 CTA from the Northwest Africa (NWA) 7865 reduced CV3 chondrite. KU-N-02 CTA is composed of igneous core mantled by the åk-poor layer. The core shows a typical texture of igneous compact Type A CAIs. The mineralogy, petrology, and O isotopic compositions of core minerals were described in detail by Suzumura et al. (2021). The formation mechanisms and Al–Mg systematics in KU-N-02 CTA derived the comprehensive signatures of the CTA which experienced both melting and condensation processes.
The mantle consists of spinel, åk-poor melilite, and perovskite. Individual mantle melilite crystals show reverse zoning towards the crystal grain boundary, in contrast to core melilite crystals showing normal zoning. The O isotopic compositions of the minerals in KU-N-02 plot along the carbonaceous chondrite anhydrous mineral line on a three O-isotope diagram. The mantle and core spinel crystals are uniformly 16O-rich (Δ17O ~ −23‰). The mantle melilite crystals exhibit variable O isotopic compositions ranging between Δ17O ~ −2‰ and −9‰, in contrast to the uniformly 16O-poor (Δ17O ~ −2‰) core melilite. The mantle melilite crystals also exhibit variable δ25Mg values (δ25MgDSM-3 ~ −2% to +3‰) compared to nearly constant δ25Mg values for the core melilite (δ25MgDSM-3 ~ +2 to +3‰). These petrographic and isotopic features indicate that the mantle minerals are likely to have formed by condensation from the solar nebular gas after core formation. The Al–Mg mineral isochrons of the core and mantle give initial 26Al/27Al ratios of (4.66 ± 0.15) × 10−5 and (4.74 ± 0.14) × 10−5, respectively. The age difference between the core and mantle formation is estimated to be within ~0.05 Myr, implying that both melting and condensation processes in the variable O isotopically solar nebular environments occurred in a short time period (< 0.05 Myr) during the single CAI formation.
Compact Type A (CTA) CAIs which are igneous CAIs, have åkermanite (åk) -poor melilite mantle. It is reported that the mantle mainly consists of reverse zoned melilite crystals, and is inferred to be formed by condensation. However, the mechanism and timing of the mantle formation are still ambiguous. Therefore, although the contemporaneous formation of the condensed and igneous CAIs has been revealed, it remains uncertain the age difference between the igneous and condensation processes that a single CAI experienced. In this study, we revealed the formation mechanisms of the CTA mantle in more detail through comprehensive studies of petrographic observations, in situ O-isotope analysis, and Al–Mg isotope measurements at Hokkaido University (Cameca ims-1280HR) for KU-N-02 CTA from the Northwest Africa (NWA) 7865 reduced CV3 chondrite. KU-N-02 CTA is composed of igneous core mantled by the åk-poor layer. The core shows a typical texture of igneous compact Type A CAIs. The mineralogy, petrology, and O isotopic compositions of core minerals were described in detail by Suzumura et al. (2021). The formation mechanisms and Al–Mg systematics in KU-N-02 CTA derived the comprehensive signatures of the CTA which experienced both melting and condensation processes.
The mantle consists of spinel, åk-poor melilite, and perovskite. Individual mantle melilite crystals show reverse zoning towards the crystal grain boundary, in contrast to core melilite crystals showing normal zoning. The O isotopic compositions of the minerals in KU-N-02 plot along the carbonaceous chondrite anhydrous mineral line on a three O-isotope diagram. The mantle and core spinel crystals are uniformly 16O-rich (Δ17O ~ −23‰). The mantle melilite crystals exhibit variable O isotopic compositions ranging between Δ17O ~ −2‰ and −9‰, in contrast to the uniformly 16O-poor (Δ17O ~ −2‰) core melilite. The mantle melilite crystals also exhibit variable δ25Mg values (δ25MgDSM-3 ~ −2% to +3‰) compared to nearly constant δ25Mg values for the core melilite (δ25MgDSM-3 ~ +2 to +3‰). These petrographic and isotopic features indicate that the mantle minerals are likely to have formed by condensation from the solar nebular gas after core formation. The Al–Mg mineral isochrons of the core and mantle give initial 26Al/27Al ratios of (4.66 ± 0.15) × 10−5 and (4.74 ± 0.14) × 10−5, respectively. The age difference between the core and mantle formation is estimated to be within ~0.05 Myr, implying that both melting and condensation processes in the variable O isotopically solar nebular environments occurred in a short time period (< 0.05 Myr) during the single CAI formation.