Experimental study and modeling of fluid reaction paths in the quartz–kyanite ± muscovite–water system at 0.7 GPa in the 350–550 °C range: Implications for Al selective transfer during metamorphism

In order to quantify Al transfer in response to fluid–mineral equilibration under evolving metamorphic conditions, isobaric (0.7 GPa) experiments were conducted in the 350–550 °C range. Disequilibrium was induced (1) by holding initially pure water and natural minerals (kyanite + quartz ± muscovite enclosed in a perforated inner capsule) under isothermal conditions and (2) by stepwise temperature variations. In all experiments, secondary Al-bearing phases crystallized in the external tube of a “tube-in-tube” setup (SEM characterization); they are interpreted as witnesses of the evolution of the fluid composition (fluid reaction path). These reaction paths and the subsequent amount of secondary crystallizations were modeled using thermodynamic data from SUPCRT92 and estimates of both starting-mineral dissolution rates and elemental diffusion coefficients from the literature. A major result is that the amount of aluminum transferred to secondary phases is a thousand times larger than the calculated Al concentration in the fluid. Although the crystallization of Al-bearing phases was expected as a response to a temperature decrease, the stepwise temperature increase (20 °C/day) also led to aluminum transfer towards secondary phases. In the course of re-equilibration, the fluid first becomes saturated with respect to aluminosilicates and then reaches silica saturation, due to the low solubility of Al-minerals. Consequently, aluminosilicates partly recrystallize in response to a temperature increase. Crystallization of secondary Al-phases in the external tube implies that aqueous aluminum was efficiently transported from the inner capsule, even in the pure Al2O3-SiO2-H2O system. Therefore, mass balance calculations considering a constant Al reference frame, i.e., postulating Al immobility, should be regarded with caution.