Olivine dissolution in basaltic melt

The main purpose of this work is to understand and quantify diffusive and convective olivine dissolution in basaltic melt. Crystal dissolution and growth in a magma chamber is often accompanied by the descent or ascent of the crystal in the chamber due to gravity. The motion induces convection that enhances mass transport. Such convective dissolution and growth rates have not been quantified before. MgO diffusivity in the melt (D-MgO), MgO concentration of the interface melt (C-0) and the effective thickness of the compositional boundary layer (6) are necessary parameters to model the convective dissolution. Experiments of non-convective olivine dissolution in a basaltic melt were conducted at 1271-1480 degrees C and 0.47-1.42 GPa in a piston-cylinder apparatus. At specific temperature and pressure conditions, multiple experiments of different durations show that the interface melt reaches near-saturation within 2 min. Therefore, diffusion, not interface reaction, is the rate-controlling step for non-convective olivine dissolution in basaltic melt. The compositional profile length and olivine dissolution distance are proportional to the square root of experimental duration, consistent with diffusive dissolution. D-MgO and C-0 are obtained from the experimental results. D-MgO displays Arrhenian dependence on temperature, but the pressure dependence is small and not resolved. C-0 increases with increasing temperature and decreases with increasing pressure. Comparison with literature data shows that D-MgO depends strongly on the initial melt composition, while C-0 does not. 6 is estimated from fluid dynamics. D-MgO/delta, which characterizes the kinetic and dynamic aspects of convective crystal dissolution, is parameterized as a function of temperature, pressure, and olivine composition. Convective olivine dissolution rate in basaltic melt can be conveniently calculated from the model results. Application to convective crystal growth and xenolith digestion is discussed. (C) 2008 Elsevier Ltd. All rights reserved.