[P-PS07] Formation and evolution of planetary materials in the Solar System

[PPS07-07] Oxygen and Al−Mg isotopic constraints on the cooling rate, age, and precursor of partial melting of an Allende Type B CAI, Golfball

*Noriyuki Kawasaki¹, Shoichi Itoh², Naoya Sakamoto¹, Steven B. Simon³, Daiki Yamamoto⁴, Hisayoshi Yurimoto¹ (1.Hokkaido University, 2.Kyoto University, 3.University of New Mexico, 4.JAXA)

Coarse-grained, igneous Ca-Al-rich inclusions (CAIs) in CV chondrites formed through multiple melting events (e.g., MacPherson and Davis, 1993; Yurimoto et al., 1998; Simon et al., 2005; Kawasaki et al., 2018). We conducted in situ O-isotope analysis and Al−Mg systematics by secondary ion mass spectrometry of relict and later-crystallized minerals from multiple melting events in an Allende Type B CAI, Golfball. Golfball has a type B CAI bulk composition (Simon and Grossman, 2004) and a unique structure (Simon et al., 2005): a fassaite-rich mantle enclosing a melilite-rich core. Blocky melilite crystals in the core often contain irregularly shaped Al-rich (Åk₅₋₁₂) grains enclosed in strongly zoned (Åk₁₅₋₇₀) overgrowths. Since the Al-rich melilite grains could not have formed from a melt of Golfball (Beckett et al., 1999), they are relict survived later melting events (Simon et al., 2005). The O-isotopic compositions of the blocky melilite crystals plot along the carbonaceous chondrite anhydrous mineral line, ranging between Δ¹⁷O ~ −14‰ and −5‰. The Al-rich relict melilite grains and their overgrowths exhibit the same O-isotopic compositions, while the O-isotopic variations for the melilite are spatially distributed. We found O-isotopic variations across several melilite crystals that gradually change within few tens of micrometers. The O-isotopic compositions of the melt could not have been homogenized during the partial melting in that scale and thus O-isotope exchange with the nebular gas was insufficient. Considering a timescale of O self-diffusion in the melt (Yamamoto et al., 2018), a cooling rate of partial melting would be > ~10⁴ K/h. Al−Mg isotope data of relict and later-crystallized minerals perfectly plot on a straight line on an Al−Mg evolutionary diagram. A mineral isochron for Golfball gives initial ²⁶Al/²⁷Al of (4.42 ± 0.14) × 10^–5 and initial ²⁶Mg/²⁴Mg of −0.020 ± 0.047‰. The O-isotopic compositions and initial values for Golfball are consistent with those for fluffy Type A (Kawasaki et al., 2019) and fine-grained CAIs (Kawasaki et al., 2020), implying its precursor contains fluffy Type A and/or fine-grained CAIs. The multiple melting events for Golfball occurred in very short order after the formation of the precursor.