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[PPS08-02] Al−Mg systematics for compact type A CAI from Northwest Africa 7865
Keywords:Ca-Al-rich Inclusions, Al-Mg systemtics, SIMS
Ca-Al-rich inclusions (CAIs) in meteorites are the oldest objects formed in the Solar System (Connelly et al., 2012). The oxygen isotopic compositions of constituent minerals in CAIs plot along the CCAM line on an oxygen three-isotope diagram and the oxygen isotopic distributions among minerals are heterogeneous. These variations are suggested to be derived from 16O-rich and 16O-poor reservoirs (e. g., Yurimoto et al, 2008; Krot 2019). Most CAIs contained 26Al, a short-lived radionuclide with a half-life of 0.705 Myr, at the time of their formation (e.g., MacPherson et al. 1995). The Al-Mg systematics can be applied to infer the relative chronology of the early Solar System.
Coarse-grained, igneous CAIs, such as compact Type A (CTA) and Type B, experienced melting of precursor solids and crystallization from a melt. The oxygen isotopic evolution model of these igneous CAIs melt during crystallization has been proposed based on petrographic observation and in situ oxygen isotope analysis (Kawasaki et al., 2018; Suzumura et al., 2021). The Al-Mg systematics of relict minerals and later crystallized minerals from igneous CAIs has been determined (e.g., Kawasaki et al., 2021). In this study, For KU-N-02 CTA from NWA 7865 CVred 3.1 chondrite which revealed the entire crystallization history, we determined the Al-Mg systematics relict minerals and later crystallized minerals in CTA by SIMS (Hokkaido Univ. Cameca ims-1280HR).
KU-N-02 CTA mainly consists of melilite and fassaite (blocky), which crystallized from melt after precursor formation, and relict spinel. Inside the single melilite crystals, round fassaite grains (~50 µm) are included. These round fassaite grains were relict that survived the partial melting event (Suzumura et al., 2021). The Al-Mg isotope data for melilite and blocky fassaite perfectly plot on a straight line. If we define an 26Al-26Mg mineral isochron, the isochron gives initial values of (26Al/27Al)0 = (4.68 ± 0.15) × 10–5 (Fig. 1, solid line). If we assume that 26Al is homogeneously distributed in the CAI forming region, a relative age for the partial melting process is calculated to be 11 ± 3 Myr from the canonical age (Jacobsen et al., 2008; Larsen et al., 2011). The Al–Mg isotope data for relict minerals, such as spinel and round fassaite, plot within the error on this isochron defined by melilite and blocky fassaite. If we assume that relict minerals have initial Mg isotopic composition (Larsen et al., 2011), the model isochron gives initial values of (26Al/27Al)0 = (5.16 ± 0.17) × 10–5 (Fig. 1, dotted line). The initial 26Al/27Al value of model isochron is consistent with those of whole-rock CAI isochron (~5.2 × 10–5; Jacobsen et al., 2008; Larsen et al., 2011). If Al/Mg chemical fractionation for KU-N-02 precursor occurred at the formation time of canonical CAIs (26Al/27Al ~5.2 × 10–5), the Mg-isotopic composition of KU-N-02 precursor would have evolved to have (δ26Mg*)0 = 0.10 ± 0.07 ‰ at 26Al/27Al = (4.68 ± 0.15) × 10–5. This value is consistent with the inferred 26Mg excess (0.041 ± 0.036 ‰) from mineral isochron within the error. This result suggests that KU-N-02 precursor formed contemporaneously with canonical CAIs, and the partial melting process occurred at 11 ± 3 Myr after the precursor formation.
Coarse-grained, igneous CAIs, such as compact Type A (CTA) and Type B, experienced melting of precursor solids and crystallization from a melt. The oxygen isotopic evolution model of these igneous CAIs melt during crystallization has been proposed based on petrographic observation and in situ oxygen isotope analysis (Kawasaki et al., 2018; Suzumura et al., 2021). The Al-Mg systematics of relict minerals and later crystallized minerals from igneous CAIs has been determined (e.g., Kawasaki et al., 2021). In this study, For KU-N-02 CTA from NWA 7865 CVred 3.1 chondrite which revealed the entire crystallization history, we determined the Al-Mg systematics relict minerals and later crystallized minerals in CTA by SIMS (Hokkaido Univ. Cameca ims-1280HR).
KU-N-02 CTA mainly consists of melilite and fassaite (blocky), which crystallized from melt after precursor formation, and relict spinel. Inside the single melilite crystals, round fassaite grains (~50 µm) are included. These round fassaite grains were relict that survived the partial melting event (Suzumura et al., 2021). The Al-Mg isotope data for melilite and blocky fassaite perfectly plot on a straight line. If we define an 26Al-26Mg mineral isochron, the isochron gives initial values of (26Al/27Al)0 = (4.68 ± 0.15) × 10–5 (Fig. 1, solid line). If we assume that 26Al is homogeneously distributed in the CAI forming region, a relative age for the partial melting process is calculated to be 11 ± 3 Myr from the canonical age (Jacobsen et al., 2008; Larsen et al., 2011). The Al–Mg isotope data for relict minerals, such as spinel and round fassaite, plot within the error on this isochron defined by melilite and blocky fassaite. If we assume that relict minerals have initial Mg isotopic composition (Larsen et al., 2011), the model isochron gives initial values of (26Al/27Al)0 = (5.16 ± 0.17) × 10–5 (Fig. 1, dotted line). The initial 26Al/27Al value of model isochron is consistent with those of whole-rock CAI isochron (~5.2 × 10–5; Jacobsen et al., 2008; Larsen et al., 2011). If Al/Mg chemical fractionation for KU-N-02 precursor occurred at the formation time of canonical CAIs (26Al/27Al ~5.2 × 10–5), the Mg-isotopic composition of KU-N-02 precursor would have evolved to have (δ26Mg*)0 = 0.10 ± 0.07 ‰ at 26Al/27Al = (4.68 ± 0.15) × 10–5. This value is consistent with the inferred 26Mg excess (0.041 ± 0.036 ‰) from mineral isochron within the error. This result suggests that KU-N-02 precursor formed contemporaneously with canonical CAIs, and the partial melting process occurred at 11 ± 3 Myr after the precursor formation.