Diffusion in complex systems

Abstract

Diffusion is an important process in the crust and mantle of the Earth and has a large bearing on geochemical processes. The results of three experimental studies that examine complex diffusion behavior related to Earth systems are reported. First, lithium diffusion and the associated diffusive isotopic fractionation in olivine is examined. Lithium is found to diffuse by two mechanisms, interstitial and vacancy, and partition between the two sites. The magnitude of diffusive isotopic fractionation is found to be large and is best described by a beta factor of 0.36 - 0.42. Second, diffusive fractionation of lithium isotopes in a synthetic harzburgite rock is investigated. Large isotopic fractionation is found. The experiments are best represented by a model that considers both the grain boundaries and grains and shows that the grain boundaries are the most likely place of isotopic fractionation. Third, the effective diffusivity of manganese is examined in a polycrystalline rock consisting of quartz and minor enstatite. The effective diffusivity of manganese is found to decrease as the proportion of enstatite increases in the polycrystalline rock. The experimental results are modeled using numerous approaches used in the past for diffusion in heterogeneous media. A numerical model that closely matches the physical aspects of the experimental matrices is also explored. The results show that the presence of a phase, in which the diffusant is compatible, in the polycrystalline rock will decrease the effective diffusivity of an element through the rock. This must be accounted for when considering long range diffusion through most of the rocks in the crust and mantle.