Kinetics of Quartz Dissolution in Natural Silicate Melts and Dependence of SiO2 Diffusivity on Melt Composition

Quartz is a major mineral in silicic rocks, and a common phenocryst in rhyolite. An understanding of the kinetics of quartz dissolution and growth may provide insight into magma crystallization and constraints on magma dynamics and cooling rates. We have carried out quartz dissolution experiments in rhyolitic (0.1 wt % H2O, ∼73 wt % SiO2) and basaltic (∼0.35 wt % H2O, and ∼50 wt % SiO2) melts at 1300–1600 °C and 0.5 GPa using piston cylinder apparatus. The experiments constrain the interface melt compositions at quartz saturation up to 1600 °C, which depend on whether the initial melt is rhyolite or basalt. The data on silica concentration at quartz saturation in each melt are modeled and will be important for future improvement of thermodynamic models of silicate melts. In addition, the experiments provide data on SiO2 diffusivity, which plays major roles in the kinetics and dynamics of various igneous processes, including magma mixing. SiO2 diffusivity depends on melt composition, consistent with previous results. Combined with other data from our lab and literature, we show that ln DSiO2 decreases linearly with X (= Si + Al cation mole fraction) in rhyolitic to andesitic to basaltic melts, instead of just the SiO2 concentration. The effect of H2O is also captured by X when H2O is included in the cation mole fraction calculation. Each SiO2 diffusion profile during quartz dissolution can be fit well by assuming DSiO2 = DX =1 eβ(1–X). Using data from our experiments, SiO2 diffusivity during quartz dissolution in rhyolitic, andesitic, to basaltic melts can be expressed as the following Arrhenius relation with compositional dependence, DSiO2quartz dissolution = exp­(−14.168 + 2.758­(1 – X) – [(35003 – 38829­(1 – X))/T], where DSiO2 is in m2/s and T is in K. The 1σ standard deviation and maximum deviation of the above equation in predicting ln DSiO2 are 0.32 (or 0.14 log10 D units) and 0.95 (or 0.41 log10 D units). Because SiO2 diffusivity depends on SiO2 concentration, no theory is currently available to predict diffusive quartz dissolution rate. We develop a method by adopting the formulation for the case of constant D but replacing the constant D by an effective D (Deff), L = 2α­(Defft)1/2, where α is solved from the composition of the dissolving crystal, the interface melt and the far-field melt. Using experimental data, the effective diffusivity during mineral dissolution may be related to diffusivity in the farfield (Dfarfield) and interface (Dinterface) melts as follows: ln­(Deff/Dfarfield) = (0.6996 + 0.0327Y)Y, where Y = ln­(Dinterface/Dfarfield). The method is applied successfully to treat diffusive and convective quartz dissolution rates.