Kinetics of anorthite dissolution in basaltic melt

Abstract

We report convection-free anorthite dissolution experiments in a basaltic melt at 1280–1500°C and 0.5GPa on two different crystallographic surfaces, (121¯) and (3¯02) to investigate dissolution kinetics. The anisotropy of the anorthite dissolution rate along these two surfaces is negligible. Time series experiments at ∼1280°C show that anorthite dissolution is mainly controlled by diffusion in the melt within experimental uncertainty. Analytical solutions were used to model the dissolution and diffusion processes, and to obtain the diffusivities and the saturation concentrations of the equilibrium-determining component (Al2O3) for anorthite dissolution into the basaltic melt. For the first time, we are able to show the physical and chemical characteristics of quench growth effect on the near-interface melt using high spatial resolution (0.3μm) EDS analyses. For anorthite (An#⩾90) saturation in a melt with 39–53wt% SiO2 and ⩽0.4wt% H2O, the concentration of Al2O3 in wt% depends on temperature as follows:lnCs,Al2O3=8.032(±0.192)-7882(±287)T,where T is temperature in K, and errors are given at 1σ level. Al2O3 diffusivity in basaltic melt during plagioclase dissolution can be expressed as:lnDAl2O3EBD,plag diss=-13.69(±1.57)-19,313(±2485)T,where D is in m2/s, and the activation energy is 161±21kJ/mol. These results are applied to model the convective dissolution of anorthite in basaltic melts. The model indicates that though anorthite crystals can survive for a longer time compared to olivine crystals of the same size in the same melt, the former would rise for a much smaller distance compared to the olivine sinking distance, due to the smaller density contrast between anorthite and melt.