11:15 〜 11:30
[SMP28-09] ひすい輝石分解によるシンプレクタイト形成過程の熱力学的モデル化
キーワード：ひすい輝石岩、ひすい輝石、シンプレクタイト、非平衡組織、ラメラ間隔
Symplectites are characteristic textures commonly observed in medium- to high-grade metamorphic rocks under subsolidus conditions, defined by vermicular fine-grained minerals aggregates. The process of textural formation are dirved by reducing total Gibbs free energy of system which are widely oversebed in natural system as self-organization mechacnism.
Our study focuses on the coarse-grained symplectites in retrograde jadeitite from the North Motagua Mélange, Guatemala, which were formed by the partial breakdown of jadeite (Tsujimori et al., 2004). These symplectites can be classified as two types: one involving water (albite + analcime) and the other isochemically formed (albite + nepheline) symplectites. A notable feature of these symplectites is their relatively coarse grain size despite forming at relatively low temperatures (P = 0.6–1.2 GPa, T = 300–450 °C).
Based on textural characterization and mathematical approaches, we primarily examined the quantitative formation mechanism of albite + nepheline symplectites. Particularly, high-resolution FE-SEM observations of a cube-shaped, three-sided polished sample reveal a layered structure within the symplectite. Subsequently, to explain the thickness and spacing of the zonal structure, we apply the model proposed by Ashworth and Chambers (2000). In their model, the texture formation speed is expressed as:
u = (-ΔrG -2/λ*fΩσ)/(λ2/δMdiff+1/Mif)
where ΔrG is Gibbs energy change of reaction; fΩ is volume factor; σ is interfacial energy; δ is width of reaction front; Mdiff is the mobility of diffusion, Mif is the mobility of interface. We assume that the actual spacing λ is established at the condition of maximum speed (∂u/∂λ = 0). This assumption is justified by the principle that self-organization occurs in such a way as to maximize energy dissipation(Gadies et al., 2017). The formulation of λ is quite complicated, but with the appropriate approximation it gives:
λ=-(fΩσMdiffδ/ΔGMif)1/3
By applying this approach, we were notably able to successfully determine the reaction rates of the layered symplectite based on its layer thickness and spacing.
It is well known that the decomposition of a single solid phase into two solid phases exhibits grain boundary morphologies similar to eutectoid reactions in alloys. By incorporating the physics of alloy decomposition, we were able to simulate the dynamic process of phase separation using phase-field modeling.
By applying this approach, we successfully determined the reaction rates of layered symplectite based on its layer thickness and spacing Moreover, by integrating the principles of alloy decomposition, we simulated the phase separation dynamics of layered symplectite using phase-field modeling.
Our study quantitatively constrained its reaction rates using the model proposed by Ashworth and Chamber (2000). Furthermore, our findings suggest that the eutectoid reaction model for alloys can be extended to symplectite structures in metamorphic rocks, providing new insights into the self-organizing mechanisms that drive complex texture formation.
Tsujimori, T., Liou, J. G., & Coleman, R. G. (2004). A pictorial introduction to coarse-grained symplectites in low-temperature jadeitite from Guatemala. The Journal of the Geological Society of Japan, 110(9), XVII-XVIII.
Ashworth, J. R., & Chambers, A. D. (2000). Symplectic reaction in olivine and the controls of intergrowth spacing in symplectites. Journal of Petrology, 41(2), 285-304.
Gaidies, F., Milke, R., Heinrich, W., & Abart, R. (2017). Metamorphic mineral reactions: Porphyroblast, corona and symplectite growth.
Our study focuses on the coarse-grained symplectites in retrograde jadeitite from the North Motagua Mélange, Guatemala, which were formed by the partial breakdown of jadeite (Tsujimori et al., 2004). These symplectites can be classified as two types: one involving water (albite + analcime) and the other isochemically formed (albite + nepheline) symplectites. A notable feature of these symplectites is their relatively coarse grain size despite forming at relatively low temperatures (P = 0.6–1.2 GPa, T = 300–450 °C).
Based on textural characterization and mathematical approaches, we primarily examined the quantitative formation mechanism of albite + nepheline symplectites. Particularly, high-resolution FE-SEM observations of a cube-shaped, three-sided polished sample reveal a layered structure within the symplectite. Subsequently, to explain the thickness and spacing of the zonal structure, we apply the model proposed by Ashworth and Chambers (2000). In their model, the texture formation speed is expressed as:
u = (-ΔrG -2/λ*fΩσ)/(λ2/δMdiff+1/Mif)
where ΔrG is Gibbs energy change of reaction; fΩ is volume factor; σ is interfacial energy; δ is width of reaction front; Mdiff is the mobility of diffusion, Mif is the mobility of interface. We assume that the actual spacing λ is established at the condition of maximum speed (∂u/∂λ = 0). This assumption is justified by the principle that self-organization occurs in such a way as to maximize energy dissipation(Gadies et al., 2017). The formulation of λ is quite complicated, but with the appropriate approximation it gives:
λ=-(fΩσMdiffδ/ΔGMif)1/3
By applying this approach, we were notably able to successfully determine the reaction rates of the layered symplectite based on its layer thickness and spacing.
It is well known that the decomposition of a single solid phase into two solid phases exhibits grain boundary morphologies similar to eutectoid reactions in alloys. By incorporating the physics of alloy decomposition, we were able to simulate the dynamic process of phase separation using phase-field modeling.
By applying this approach, we successfully determined the reaction rates of layered symplectite based on its layer thickness and spacing Moreover, by integrating the principles of alloy decomposition, we simulated the phase separation dynamics of layered symplectite using phase-field modeling.
Our study quantitatively constrained its reaction rates using the model proposed by Ashworth and Chamber (2000). Furthermore, our findings suggest that the eutectoid reaction model for alloys can be extended to symplectite structures in metamorphic rocks, providing new insights into the self-organizing mechanisms that drive complex texture formation.
Tsujimori, T., Liou, J. G., & Coleman, R. G. (2004). A pictorial introduction to coarse-grained symplectites in low-temperature jadeitite from Guatemala. The Journal of the Geological Society of Japan, 110(9), XVII-XVIII.
Ashworth, J. R., & Chambers, A. D. (2000). Symplectic reaction in olivine and the controls of intergrowth spacing in symplectites. Journal of Petrology, 41(2), 285-304.
Gaidies, F., Milke, R., Heinrich, W., & Abart, R. (2017). Metamorphic mineral reactions: Porphyroblast, corona and symplectite growth.