DIFFUSIVITY OF STRONTIUM, NEODYMIUM, AND LEAD IN NATURAL RHYOLITE MELT AT 1.0 GPA

Abstract

Chemical diffusivities approximating tracer diffusivities of Sr, Nd, and Pb have been measured in a natural rhyolite melt at 1.0 GPa and temperatures from 1000 to 1450°C using the diffusion couple technique. Diffusivities for all three elements were determined under nominally anhydrous conditions. Lead diffusivities were also determined at # 1 and 2.5 wt% dissolved H2O in the melt. The results are described by the following Arrhenius equations (D in m2/s, activation energy in J/mol, T in Kelvin):D(Sr,dry) = 1.58 x 10-3 e-(264200/RT)D(Nd,dry) = 1.28 x 10-2 e-(330200/RT)D(Pb,dry) = 1.92 x 10-7 e-(148400/RT)D(Pb,#1%) = 1.18 x 10-7 e-(131700/RT)D(Pb,#2.5%) = 2.18 x 10-9 e-(72600/RT)The results are consistent to within an order of magnitude with previously reported Sr, Nd, and Pb tracer diffusivities. Under anhydrous conditions, the relative magnitudes of the diffusion coefficients areDSr # DPb > DNd. DNd determined in this work is similar to previously reported DSi. The implication of these results is that Pb and Sr diffusion-controlled isotopic and chemical exchange can occur at a faster rate than major element chemical exchange, but that Nd exchange occurs at about the same rate as major element exchange. The consequences of this process are illustrated with a model of isotopic exchange in a basaltic-granitic, double-diffusive, convecting magma chamber. The diffusivity of Pb increases with increasing dissolved water more than that of Si increases suggesting that, at least for Pb, isotopic homogenization also occurs faster than chemical homogenization under hydrous conditions.