17:15 〜 18:45
[PPS07-P10] Thermal Histories of Two CAIs in the CV3 Allende: Evidence from Mg Zoning in Anorthite and Diffusion Model
キーワード：炭素質コンドライト、CAIs、カソードルミネッセンス、拡散モデル、熱の履歴
Introduction: Ca-Al-rich inclusions (CAIs) in the Allende CV3 chondrite have undergone at least two thermal events: (1) high-temperature CAI formation in the solar nebula [1,2]; (2) a lower temperature re-heating during metamorphism of the Allende parent body [3,4]. In this study, we identify Mg-zoning in primary anorthite in two type B CAIs from Allende using cathodoluminescence (CL) [5]. We compare the Mg-zoning with model diffusion curves to constrain the duration of thermal events at both high (CAI formation in the nebula) and low (parent body metamorphism) temperature events.
Methods: The CAIs from Allende were characterized using microscope observations, a Hitachi (S-3400N) scanning electron microscope (SEM) with a MonoCL4 CL detector for cathodoluminescence (CL) analysis, and a JEOL JXA-8900 electron probe micro-analyzer (EPMA) for quantitative analyses of anorthite. Diffusion calculations were conducted using MATLAB. For diffusion coefficient calculations, we used log10D0 = -6.45 (for D [m2/s]) and Qd = 271 [kJ/mol] based on experiments by [6] on An93 plagioclase feldspar.
Results: CAI 3655A is a type B1, with a melilite-rich mantle, and fassaite, spinel and primary anorthite concentrated in the CAI core, whereas 4022 is a type B2, with a more homogeneous distribution of minerals (see [7] for details). Primary anorthite crystals with distinct zoning patterns were identified in both CAIs.
Primary anorthite in CAI 3655A has a CL-bright core and CL-dark rim, with a sharp angular boundary dividing core from rim (Fig. 1). The CL-bright domain is Mg-poor (mean MgO = 0.10 wt%) compared to the CL-dark domain (mean MgO = 0.19 wt%; see Fig. 2). As in 3655A, the CL-bright domain of primary anorthite in 4022 is Mg-poor (mean MgO = 0.07 wt%) compared to the CL-dark domain (mean MgO = 0.16 wt%) and the boundary between CL-bright and CL-dark is sharp. However, in contrast to 3655A, the core is CL-dark and rim is CL-bright, and the boundary has a curved shape indicating some resorption of the CL-dark core.
Diffusion of Mg between the Mg-poor and Mg-bearing domains in the primary anorthite crystals was modeled using the parameters from [6] indicated above, temperatures of 1500K (igneous CAI crystallization [1,2]) and 500°C (773K, Allende parent body metamorphism [3,4]), assuming an initial discontinuous boundary. For the igneous condition, the boundary starts to become more diffuse after approximately one hour at 1500K (Fig. 3). For the metamorphic temperature condition, the boundary starts to become more diffuse after approximately 105 years at 773K (Fig. 4).
Discussion: The sharp angular boundary between CL domains in primary anorthite in CAI 3655A implies that the CL-bright core did not undergo partial melting during formation of the CL-dark rim. In contrast, the resorbed boundary of the CL-dark core of anorthite in CAI 4022 suggests that some partial melting of the core occurred prior to formation of the CL-bright rim. The higher volatility of Mg relative to Al and Ca suggests that the Mg-bearing, CL-dark domains formed at slightly lower temperatures than the CL-bright domains. If so, the CL-dark rim in 3655A could have formed after a cooling step, whereas the CL-bright rim in 4022 could have occurred after a reheating event that was short enough so that parts of the core were preserved. Based on diffusion modeling, cooling histories after formation of the anorthite rims were rapid (less than ~ one hour at 1500K) in both CAIs, and metamorphic temperatures of the Allende parent body could not have exceeded 500°C for longer than 105 years.
References: [1] Stolper E. (1982) GCA 46, 2159-2180. [2] Krot A.N. (2019) MaPS 54, 1647-1691. [3] Krot A.N. et al. (2021) Prog. Earth Planet. Sci. 8, #61 (37 pages). [4] Enokido Y. et al. (2023) MaPS 58, 405-420. [5] Hutcheon I.D. et al. (1978) Proc. LPSC 9, 1345-1368. [6] Van Orman J. A. et al. (2014) EPSL 385, 79-88. [7] Fagan T. J. et al. (2007) MaPS 42, 1221-1240.
Methods: The CAIs from Allende were characterized using microscope observations, a Hitachi (S-3400N) scanning electron microscope (SEM) with a MonoCL4 CL detector for cathodoluminescence (CL) analysis, and a JEOL JXA-8900 electron probe micro-analyzer (EPMA) for quantitative analyses of anorthite. Diffusion calculations were conducted using MATLAB. For diffusion coefficient calculations, we used log10D0 = -6.45 (for D [m2/s]) and Qd = 271 [kJ/mol] based on experiments by [6] on An93 plagioclase feldspar.
Results: CAI 3655A is a type B1, with a melilite-rich mantle, and fassaite, spinel and primary anorthite concentrated in the CAI core, whereas 4022 is a type B2, with a more homogeneous distribution of minerals (see [7] for details). Primary anorthite crystals with distinct zoning patterns were identified in both CAIs.
Primary anorthite in CAI 3655A has a CL-bright core and CL-dark rim, with a sharp angular boundary dividing core from rim (Fig. 1). The CL-bright domain is Mg-poor (mean MgO = 0.10 wt%) compared to the CL-dark domain (mean MgO = 0.19 wt%; see Fig. 2). As in 3655A, the CL-bright domain of primary anorthite in 4022 is Mg-poor (mean MgO = 0.07 wt%) compared to the CL-dark domain (mean MgO = 0.16 wt%) and the boundary between CL-bright and CL-dark is sharp. However, in contrast to 3655A, the core is CL-dark and rim is CL-bright, and the boundary has a curved shape indicating some resorption of the CL-dark core.
Diffusion of Mg between the Mg-poor and Mg-bearing domains in the primary anorthite crystals was modeled using the parameters from [6] indicated above, temperatures of 1500K (igneous CAI crystallization [1,2]) and 500°C (773K, Allende parent body metamorphism [3,4]), assuming an initial discontinuous boundary. For the igneous condition, the boundary starts to become more diffuse after approximately one hour at 1500K (Fig. 3). For the metamorphic temperature condition, the boundary starts to become more diffuse after approximately 105 years at 773K (Fig. 4).
Discussion: The sharp angular boundary between CL domains in primary anorthite in CAI 3655A implies that the CL-bright core did not undergo partial melting during formation of the CL-dark rim. In contrast, the resorbed boundary of the CL-dark core of anorthite in CAI 4022 suggests that some partial melting of the core occurred prior to formation of the CL-bright rim. The higher volatility of Mg relative to Al and Ca suggests that the Mg-bearing, CL-dark domains formed at slightly lower temperatures than the CL-bright domains. If so, the CL-dark rim in 3655A could have formed after a cooling step, whereas the CL-bright rim in 4022 could have occurred after a reheating event that was short enough so that parts of the core were preserved. Based on diffusion modeling, cooling histories after formation of the anorthite rims were rapid (less than ~ one hour at 1500K) in both CAIs, and metamorphic temperatures of the Allende parent body could not have exceeded 500°C for longer than 105 years.
References: [1] Stolper E. (1982) GCA 46, 2159-2180. [2] Krot A.N. (2019) MaPS 54, 1647-1691. [3] Krot A.N. et al. (2021) Prog. Earth Planet. Sci. 8, #61 (37 pages). [4] Enokido Y. et al. (2023) MaPS 58, 405-420. [5] Hutcheon I.D. et al. (1978) Proc. LPSC 9, 1345-1368. [6] Van Orman J. A. et al. (2014) EPSL 385, 79-88. [7] Fagan T. J. et al. (2007) MaPS 42, 1221-1240.