SOME REMARKS ON THE MULTIPLE TIME SCALES OF DIFFUSION IN MINERALS AND MELTS (Invited Presentation)

Diffusion is an important mass transport mechanism for many petrologic and geochemical processes, such as crystal growth and dissolution, magma contamination and mixing, and diffusive re-equilibration. The purpose of this study is to demonstrate, through both laboratory experiments and simple diffusion modeling, that there are multiple time scale of diffusion of cations in minerals and melts. This happens because the petrologic systems are multicomponent, different components diffuse at different rates, and different components are often coupled through both kinetic and thermodynamic interactions. In the first example, we consider REE diffusion between an alkali basalt and a basaltic andesite in an laboratory experiment in which abundances of some of the REE in the alkali basalt are higher than those in the basaltic andesite, whereas the rest of the REE are lower in the alkali basalt than in the basaltic andesite. We will show that diffusion rates of REE are higher when they diffuse along SiO₂ or MgO concentration gradients, but lower when they diffuse against the major element concentration gradient. In the second example, we consider REE diffusion in olivine, again in the presence of major element concentration gradients in the olivine. We show the importance of diffusive coupling between REE and the major elements and present a simple model to explain our observations. In the third example, we consider some field applications, comparing closure temperatures derived from REE and major elements in olivine in mafic and ultramafic samples.