Kinetics of Sr and Nd exchange in silicate liquids: Theory, experiments, and applications to uphill diffusion, isotopic equilibration, and irreversible mixing of magmas
Authors: Lesher C. E. 
Published: February 2004

Diffusion coefficients that govern chemical and isotopic exchange of Sr and Nd were determined from compositional profiles developed between juxtaposed anhydrous basaltic and rhyolitic liquids. Analysis of simple diffusion couples involving isotopically enriched and normal tholeiitic basalt and metaluminous rhyolite recover Sr and Nd self‐diffusion coefficients (D*) in the end‐member compositions of contrasting polymerization. Self‐diffusion of Sr is 7 times faster in basaltic melt than rhyolitic melt at 1255°C and 1 GPa, while self‐diffusion of Nd is more than 1 order of magnitude greater in basalt than rhyolite. Also, at these conditions, DSr* is a factor of 3 greater than DNd* in basalt and an order of magnitude greater than DNd* in rhyolite. Variations in DSr* and DNd* with bulk composition and temperature between 1255°–1465°C are described by ln DSr* = −16.25 ‐ (16348/T) + 8.02(0.73 ‐ XSiO2) and ln DNd* = −13.73 ‐ (23875/T) + 14.43(0.73 ‐ XSiO2), where the units of D* are square meters per second, T is temperature in kelvins, and XSiO2 is weight fraction silica. The results of a Boltzmann‐Matano analysis of 87Sr/∑Sr and 144Nd/∑Nd profiles of complex diffusion couples composed of isotopically normal basalt and enriched rhyolite yield diffusion coefficients for intermediate bulk compositions in agreement with interpolated values given by the relationships above. An important feature of the interdiffusion of basaltic and rhyolitic liquids is the equilibration of isotopic composition in advance of chemical homogenization. This behavior is best displayed by Sr in the present experiments and predicted for Nd. In addition, Sr, Nd, and Al show initial periods of uphill diffusion, while other melt components diffuse down their respective concentration gradients leading to chemical homogenization. Despite uphill diffusive effects, the rate of chemical homogenization is slow and approximated by the effective binary diffusion coefficient for silica, that is, ln DSiO2B‐M = −6.70 ‐ (31195/T) + 12.28(0.52 ‐ XSiO2). Chemical fluxes of Sr and Nd are calculated using Darken's (1948) equation, that is, Dic = Di*[1 + (d ln γi/d ln xi)], relating the chemical diffusion coefficient for species i(Dic) defined by Pick's law to the self‐diffusion coefficient (Di*) for the species and the variation in the species’ activity coefficient, γi, with concentration xi along the diffusional pathway. Parameters of the Darken equation are provided in this study and from the literature. Simulations of interdiffusion of basalt and rhyolite quantitatively reproduce salient features of the present experiments including (1) the initial stage of Sr and Nd uphill diffusion into silica‐poor regions of couples followed by back diffusion into silica‐rich regions, (2) strong asymmetry in the evolving Sr and Nd isotopic profiles, and (3) equilibration of isotopic composition in advance of bulk chemical composition. These results are considered in a magmatic context, where intimate blending of magmas during mixing is frustrated by large rheological contrasts and/or insufficient exposure time. Time‐dependent diffusional exchange between mingling magmas leads to covariations in chemical and isotopic compositions that differ markedly from the expectations of bulk mixing. Examples presented offer alternative interpretations for the compositional relationships found among magmatic rocks of hybrid origin.