Presentation Information
[PPS12-16]Mg isotopic evolution of type B CAI-like evaporation residues: An experimental study
*Daiki Yamamoto1, Noriyuki Kawasaki2, Yasuaki Tsuruoka3, Shogo Tachibana3, Hisayoshi Yurimoto2 (1.Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu Univ. , 2.Department of Natural History Sciences, Hokkaido University, 3.Department of Earth and Planetary Science, The University of Tokyo)
Keywords:
Calcium-aluminum-rich inclusions (CAI),Melt evaporation,Crystallization,Mg isotopes,Protosolar disk
Coarse-grained igneous calcium-aluminum-rich inclusions (CAIs) are evaporation residues formed from melts in a hydrogen-dominant protosolar disk gas (e.g., Grossman et al., 2002; Kamibayashi et al., 2021). The evaporation and crystallization behaviors of CAIs are recorded in Mg isotopic compositions of the residues. Previous studies reported systematic enrichments in heavier Mg isotopes in marginal melilite of igneous CAIs, either toward or inward from the inclusion surface (e.g., Simon et al., 2005; Bullock et al., 2013). However, such isotopic profiles from natural CAIs have not yet been directly compared with those obtained from experiments. Here, we present Mg isotopic data for melilite and fassaite in experimental samples produced from two plausible type B CAI precursors (CAIχ and CAIδ; melilite liquidus of ~1400 and 1310°C, respectively) under variable PH2 and cooling rate conditions to better constrain the thermal history of type B CAIs.
Mg isotopic compositions (Δ25Mg) of the experimental residues were measured with SIMS (Cameca ims-1280HR) at Hokkaido University. These samples had been heated at 1420°C under hydrogen gas pressures (PH2) of 10–6–10–4 bar for 1 h and subsequently cooled to 1100°C at rates (Rc) of 5 and 50°C h–1. We also conducted a crystallization experiment with the slowest cooling rate at PH2 = 10–5 bar using a CAIχ sample. This additional CAIχ sample was heated at 1420°C for 1 h, cooled to 1350°C with Rc = 1°C h–1, and then cooled to 1100°C with Rc = 50°C h–1.
For both compositions, continuous melilite mantles characteristic of type B1 CAIs formed at PH2 = 10–4 bar with Rc = 5–50°C h–1 and at PH2= 10–5 bar with Rc = 5°C h–1, whereas samples heated at PH2 = 10–5 bar with Rc = 50°C h–1 and at PH2 = 10–6 bar with Rc = 5–50°C h–1exhibited partial or no melilite mantle characteristic of type B2 CAIs (Kamibayashi et al., 2021; Kamibayashi, 2022). The slowest-cooled CAIc sample also exhibited a type B1-like texture.
CAIδ: At PH2 = 10–6 bar with Rc = 5°C h–1, melilite showed Δ25Mg enrichment of ~3.5–5‰, largely independent of distance from the surface. At PH2 = 10–5–10–4 bar, with Rc = 50°C h–1, interior Δ25Mg values of melilite were nearly constant, separated from localized surface enrichment by an abrupt change, whereas the samples with Rc = 5°C h–1 showed both inward and outward Δ25Mg gradients near the margins.
CAIχ: At PH2 = 10–6 bar with Rc = 5°C h–1, melilite exhibited larger Δ25Mg variations (~3–7‰). At PH2 = 10–4 bar with Rc = 5–50°C h–1and PH2 = 10–5 bar with Rc = 50°C h–1, Δ25Mg values of melilite systematically decreased inward from the surface and became uniform abruptly beyond ~100–200 μm. At PH2 = 10–5 bar with Rc = 5°C h–1, melilite showed elevated surface Δ25Mg enrichments followed by further inward increases.
Lack of systematic distance-dependent Δ25Mg enrichment at PH2 = 10–6 bar suggests that Mg isotopic evolution is dominated by Rayleigh fractionation. Larger Δ25Mg variations in CAIχ reflect a longer melilite crystallization interval. Gradual Δ25Mg decreases from the surface can be explained by diffusion, whereas abrupt margin-to-interior changes in Δ25Mg indicate continued melt-gas interaction after mantle formation. Some marginal melilite grains in CAIδ samples treated under slower cooling rates and/or moderate PH2 conditions exhibit overall inward increases in Δ25Mg, similar to typical type B CAIs (Bullock et al., 2013). The sample cooled at 1°C h–1 is the only CAIχ sample showing such inward increases. The overall inward increase in Δ25Mg likely reflects slow melilite growth under slow cooling rates and/or low temperatures, where evaporation-induced melt isotopic fractionation exceeds melilite crystal growth. The less pronounced Mg isotopic gradients relative to natural type B CAIs suggest that typical type B CAIs cooled at rates no faster than ~1°C h–1 during melilite crystallization.
Mg isotopic compositions (Δ25Mg) of the experimental residues were measured with SIMS (Cameca ims-1280HR) at Hokkaido University. These samples had been heated at 1420°C under hydrogen gas pressures (PH2) of 10–6–10–4 bar for 1 h and subsequently cooled to 1100°C at rates (Rc) of 5 and 50°C h–1. We also conducted a crystallization experiment with the slowest cooling rate at PH2 = 10–5 bar using a CAIχ sample. This additional CAIχ sample was heated at 1420°C for 1 h, cooled to 1350°C with Rc = 1°C h–1, and then cooled to 1100°C with Rc = 50°C h–1.
For both compositions, continuous melilite mantles characteristic of type B1 CAIs formed at PH2 = 10–4 bar with Rc = 5–50°C h–1 and at PH2= 10–5 bar with Rc = 5°C h–1, whereas samples heated at PH2 = 10–5 bar with Rc = 50°C h–1 and at PH2 = 10–6 bar with Rc = 5–50°C h–1exhibited partial or no melilite mantle characteristic of type B2 CAIs (Kamibayashi et al., 2021; Kamibayashi, 2022). The slowest-cooled CAIc sample also exhibited a type B1-like texture.
CAIδ: At PH2 = 10–6 bar with Rc = 5°C h–1, melilite showed Δ25Mg enrichment of ~3.5–5‰, largely independent of distance from the surface. At PH2 = 10–5–10–4 bar, with Rc = 50°C h–1, interior Δ25Mg values of melilite were nearly constant, separated from localized surface enrichment by an abrupt change, whereas the samples with Rc = 5°C h–1 showed both inward and outward Δ25Mg gradients near the margins.
CAIχ: At PH2 = 10–6 bar with Rc = 5°C h–1, melilite exhibited larger Δ25Mg variations (~3–7‰). At PH2 = 10–4 bar with Rc = 5–50°C h–1and PH2 = 10–5 bar with Rc = 50°C h–1, Δ25Mg values of melilite systematically decreased inward from the surface and became uniform abruptly beyond ~100–200 μm. At PH2 = 10–5 bar with Rc = 5°C h–1, melilite showed elevated surface Δ25Mg enrichments followed by further inward increases.
Lack of systematic distance-dependent Δ25Mg enrichment at PH2 = 10–6 bar suggests that Mg isotopic evolution is dominated by Rayleigh fractionation. Larger Δ25Mg variations in CAIχ reflect a longer melilite crystallization interval. Gradual Δ25Mg decreases from the surface can be explained by diffusion, whereas abrupt margin-to-interior changes in Δ25Mg indicate continued melt-gas interaction after mantle formation. Some marginal melilite grains in CAIδ samples treated under slower cooling rates and/or moderate PH2 conditions exhibit overall inward increases in Δ25Mg, similar to typical type B CAIs (Bullock et al., 2013). The sample cooled at 1°C h–1 is the only CAIχ sample showing such inward increases. The overall inward increase in Δ25Mg likely reflects slow melilite growth under slow cooling rates and/or low temperatures, where evaporation-induced melt isotopic fractionation exceeds melilite crystal growth. The less pronounced Mg isotopic gradients relative to natural type B CAIs suggest that typical type B CAIs cooled at rates no faster than ~1°C h–1 during melilite crystallization.