11:00 AM - 11:15 AM
[SMP24-07] Quantification and modelling of symplectitic breakdown texture after jadeite: An irreversible thermodynamic approach with steady-state diffusion
Keywords:jadeitite, jadeite, symplectite, non-equilibrium texture, Onsager's phenomenological coefficient
Symplectitic mineral intergrowth, characterized by vermicular-like, complex, and fine-grained mineral assemblages, is a non-equilibrium rock texture typically found in regional high-temperature metamorphic rocks. Under subsolidus conditions, these characteristic textures are typically formed through a time-dependent self-organization process aimed at minimizing the Gibbs free energy of the entire thermodynamic system under consideration. In this study, we investigated coarse-grained symplectites in a partially retrograded jadeitite from the North Motagua Melange, Guatemala, which formed under high-pressure and relatively low-temperature (P = ~0.6–1.2 GPa, T = ~300–450 °C: Harlow, 1994) conditions. Jadeite breakdown textures in the investigated jadeitite exhibits a layered symplectitic structure with varying mineral assemblages, transitioning from the jadeite interface towards the exterior: jadeite|albite + nepheline (symplectite)|albite + analcime (symplectite)|analcime.
To quantitatively interpret the layered features of symplectites, we initially conducted mass balance calculations assuming a closed system. This approach, however, led to the incorrect conclusion that analcime is depleted under these conditions, contradicting the formation of coronal textures via the decomposition of jadeite and water. This inconsistency prompted a shift in our approach towards hypothesizing an open system for our analysis. In this revised scenario, we adopted the Fisher-Joesten model, following the methodology of Ashworth and Birdi. This model accounts for local equilibrium diffusion processes within thermodynamic frameworks, as described by Onsager's phenomenological coefficients, Lij; it was estimated that the ratios LSiSi/LAlAl > 10, LSiSi/LNaNa > 10. Implementing the observed mineral ratio of albite/nepheline = 0.47:0.53 in the symplectites as a constraint, we deduced that ratios LSiSi/LAlAl = LSiSi/LNaNa = 1000 are consistent with the observed mineral ratios. Furthermore, we formulated chemical reaction equations: [Jd] + 0.72[H2O] + 0.04[Na2O] -> 0.06[Ne] + 0.14[Ab] + 0.72[Anl] + 0.06[SiO2], which effectively explain both the overall modal ratio and the chemical component flux across each contact layer, illustrating the interface-to-exterior H2O chemical potential gradient. Additionally, considering a geological context, the interaction of jadeitite lens with the surrounding antigorite serpentinite likely contributes the high LSiSi values we obtained. We concluded that the formation of layered symplectitic mineral assemblages in the investigated jadeitite is significantly influenced by fluid infiltration to during jadeite breakdown. Specifically, the influx of H2O from external sources creates a local chemical potential gradient, facilitating the development of layered symplectite features under relatively low-temperature conditions through diffusion processes describable by Onsager coefficients.
To quantitatively interpret the layered features of symplectites, we initially conducted mass balance calculations assuming a closed system. This approach, however, led to the incorrect conclusion that analcime is depleted under these conditions, contradicting the formation of coronal textures via the decomposition of jadeite and water. This inconsistency prompted a shift in our approach towards hypothesizing an open system for our analysis. In this revised scenario, we adopted the Fisher-Joesten model, following the methodology of Ashworth and Birdi. This model accounts for local equilibrium diffusion processes within thermodynamic frameworks, as described by Onsager's phenomenological coefficients, Lij; it was estimated that the ratios LSiSi/LAlAl > 10, LSiSi/LNaNa > 10. Implementing the observed mineral ratio of albite/nepheline = 0.47:0.53 in the symplectites as a constraint, we deduced that ratios LSiSi/LAlAl = LSiSi/LNaNa = 1000 are consistent with the observed mineral ratios. Furthermore, we formulated chemical reaction equations: [Jd] + 0.72[H2O] + 0.04[Na2O] -> 0.06[Ne] + 0.14[Ab] + 0.72[Anl] + 0.06[SiO2], which effectively explain both the overall modal ratio and the chemical component flux across each contact layer, illustrating the interface-to-exterior H2O chemical potential gradient. Additionally, considering a geological context, the interaction of jadeitite lens with the surrounding antigorite serpentinite likely contributes the high LSiSi values we obtained. We concluded that the formation of layered symplectitic mineral assemblages in the investigated jadeitite is significantly influenced by fluid infiltration to during jadeite breakdown. Specifically, the influx of H2O from external sources creates a local chemical potential gradient, facilitating the development of layered symplectite features under relatively low-temperature conditions through diffusion processes describable by Onsager coefficients.