V41B-3070
Diffusion of SiO2 in Rhyolitic Melt

Thursday, 17 December 2015
Poster Hall (Moscone South)
Yi Yu1, Youxue Zhang1 and Yuping Yang2, (1)University of Michigan Ann Arbor, Ann Arbor, MI, United States, (2)Chinese Academy of Geological Sciences, Institute of Mineral Resources, Beijing, China
Abstract:
SiO2 is the most major component in silicate melts, and the diffusion of SiO2 plays a controlling role in growth or dissolution of quartz from or into silicate melts. The diffusivity of SiOis small and highly dependent on melt compositions (Lesher and Walker, 1986; Koyaguchi, 1989), making it difficult to extract high-quality Si diffusivity data.

We conducted quartz dissolution experiments in rhyolitic melt (0.1wt% H2O, 73 wt% SiO2) at 1300-1600 °C. We also have preliminary data on quartz dissolution in basaltic melt at 1300 °C , one quartz dissolution experiment (Zhang et al., 1989) in andesitic melt at 1300 °C , and five cassiterite dissolution experiments (Yang et al, in review) in various hydrous rhyolitic melts (containing 0.1-5.9 wt% H2O, and 74-77 wt% SiO2) at 900-1100 °C. All experiments were conducted at 0.5 GPa using piston cylinder apparatus. All data were combined to examine the dependence of DSiO2 on melt compositions. Though in individual experiments lnDSiO2 is a linear function of SiO2 concentration as shown in literature, the combined data show that lnDSiO2 decreases linearly with XSi+Al, where XSi+Al is defined as cation mole fraction of Si+Al in melts. By fitting concentration profiles at different temperatures using DSiO2 = Dea(1-XSi+Al), the results show that the parameter a is roughly composition-independent across all experiments and is linear to 1/T:

a = 2.603(±0.451) + 35282(±627)/Tr2= 0.996.

D0 is the extrapolated effective binary diffusivity of SiO2 in pure silica melt (XSi+Al = 1 roughly corresponds to XSi = 1 in quartz dissolution experiments). For quartz dissolution experiments in rhyolitic melt, the dependence of D0 on is:

lnD0 = -14.041(±1.915) - 34719(±3125)/Tr2= 0.875

For cassiterite dissolution, lnD0 values do not follow the above trend because the dominant SnO concentration gradient can affect interdiffusion between SiO2 and other components. That is, XSi+Al alone is not enough to account for how D0 depends on other components. The effect of H2O on both a and D0 is roughly accounted for by simply including H as a cation when calculating XSi+Al. Namely, adding H2O equals to lowering XSi+Al, e.g. at 1300°C, every addition of 1 wt% H2O (i.e. lowering XSi+Al by ~0.05) would increase DSiO2 by ~1.2 lnD units for a melt with initial XSi+Al = 0.85 on anhydrous basis.