5:15 PM - 7:15 PM
[SMP29-P02] Numerical calculation of the formation process of fluid inclusions in mineral crystals
Keywords:fluid inclusion, crystal growth, phase-field model, numerical simulation, impurities, mineral
Most minerals form in fluids such as magma or hydrothermal fluids. As mineral crystals form in a fluid, the fluid around the mineral may be trapped within the mineral. This trapped fluid is called fluid inclusions. Fluid inclusions can be regarded as small closed systems that retain the state of the fluid when it is captured, and are useful in that they record information about places that are out of the reach of the human hand and eye [1]. One problem in the analysis of fluid inclusions is that the composition of the fluid inclusions cannot directly infer the composition of the bulk (liquid phase far enough from the interface), i.e., the formation environment of minerals. Impurities that are difficult to be incorporated into minerals are concentrated near the interface between the mineral crystal and the fluid (hereinafter simply referred to as “crystal surface”) as the mineral grows, so the composition of the bulk and the composition near the crystal surface generally do not match. Furthermore, even after the fluid inclusions are incorporated into the mineral, their compositions change due to diffusion of components within the fluid inclusions and crystal growth on the inclusions' walls, so the compositions near the crystal surface and those of the fluid inclusions also do not match. In other words, the composition of the fluid inclusions does not always reflect the environment in which the mineral was formed. The difference between the near-surface composition of a growing mineral crystal and the bulk composition of the fluid has conventionally been studied based on analytical solutions of the fluid composition distribution near the crystal surface [2]. However, this analytical solution did not address interface deformation and fluid uptake, i.e., the formation process of fluid inclusions, and it was not clear what determines its composition.
The purpose of this study is to investigate what determines the composition of fluid inclusions by numerically reproducing its formation process. Assuming a binary dilute solution, the growth process of mineral crystals in the undercooled state was numerically calculated based on the phase-field method [3]. The numerical calculations reproduced the incorporation process of the liquid phase into the solid phase. Fluid inclusions were continuously arranged in rows in the solid phase, which are also observed in minerals in nature. The compositions of the fluid inclusions, the liquid phase near the solid–liquid interface, and the bulk liquid phase were compared and found to be all different. It was also found that the composition of the fluid inclusions after some time has elapsed since their incorporation is correlated with their size. The size dependence of the composition of the fluid inclusions was in good agreement with the theoretical equation for the equilibrium composition, which takes into account the curvature effect (Gibbs–Thomson effect) at the solid–liquid interface. The present results suggest that the fluid inclusions do not necessarily preserve the information at the moment the fluid is incorporated into the solid phase, but are determined by the equilibrium state of the fluid with the surrounding solid phase after incorporation.
References
[1] Sawaki, T. (2003). Journal of Mineralogical and Petrological Sciences, 32, 23–41.
[2] Baker, D. R. (2008). Contributions to Mineralogy and Petrology, 156(3), 377–395.
[3] Kim, S. G., Kim, W. T., and Suzuki, T. (1999). Physical Review E, 60(6), 7186–7197.
The purpose of this study is to investigate what determines the composition of fluid inclusions by numerically reproducing its formation process. Assuming a binary dilute solution, the growth process of mineral crystals in the undercooled state was numerically calculated based on the phase-field method [3]. The numerical calculations reproduced the incorporation process of the liquid phase into the solid phase. Fluid inclusions were continuously arranged in rows in the solid phase, which are also observed in minerals in nature. The compositions of the fluid inclusions, the liquid phase near the solid–liquid interface, and the bulk liquid phase were compared and found to be all different. It was also found that the composition of the fluid inclusions after some time has elapsed since their incorporation is correlated with their size. The size dependence of the composition of the fluid inclusions was in good agreement with the theoretical equation for the equilibrium composition, which takes into account the curvature effect (Gibbs–Thomson effect) at the solid–liquid interface. The present results suggest that the fluid inclusions do not necessarily preserve the information at the moment the fluid is incorporated into the solid phase, but are determined by the equilibrium state of the fluid with the surrounding solid phase after incorporation.
References
[1] Sawaki, T. (2003). Journal of Mineralogical and Petrological Sciences, 32, 23–41.
[2] Baker, D. R. (2008). Contributions to Mineralogy and Petrology, 156(3), 377–395.
[3] Kim, S. G., Kim, W. T., and Suzuki, T. (1999). Physical Review E, 60(6), 7186–7197.