a Particle Accelerator Based Study of Major and Trace Element Diffusion in Minerals

In this work, the techniques of Rutherford Backscattering (RBS), nuclear reaction analysis, and ion implantation are used to measure diffusion in minerals, and the results applied to obtain a better understanding of geological problems. In the first section, a technique to measure trace -element diffusion is developed in which ion implantation is used to introduce the diffusant and RBS is used to measure diffusion profiles. With this method, Pb diffusion in fused silica, apatite, zircon, and titanite and Sr diffusion in apatite are measured. Results for apatite are consistent with dry diffusion measurements made with more traditional geologic techniques. Results for zircon and titanite indicate that measured diffusivities are affected by radiation damage induced by ion implantation. Evidence of this is the high diffusivities and low activation energy measured for Pb diffusion in zircon, and the appearance of a "kink" in the Arrhenius plot for Pb diffusion in titanite. The temperature for this transition corresponds to the point at which rapid annealing of radiation damage occurs. Radiation damage and its annealing in these materials appears to be related to bond type and rigidity. The diffusion results for Pb and Sr diffusion in apatite and Pb diffusion in titanite are then considered in a geological context. Closure temperatures for each of these mineral-element systems, calculated with the measured diffusion parameters, agree well with other determinations. Other calculations based on the diffusion parameters indicate that some apatite crystals may retain Sr isotopic information reflecting their source region even after thermal events of 50000 to several million years. In the second part, oxygen diffusion in iron-bearing olivine is measured with the reaction ^ {18}O(p,alpha) ^{15}N. These results show a positive dependence of oxygen diffusion on oxygen fugacity, indicating that this process proceeds by an interstitial or countervacancy mechanism. Results are also considered in the context of solid-state creep processes in the upper mantle. While they are consistent with oxygen diffusion as a rate-limiting step in creep, the transport of slower diffusing Si may instead be the significant factor.