Diffusion Kinetics of Selected Trivalent Ions and Hf in Garnet: Experimental Studies and Applications

University Of Arizona, Tucson AZ

Investigators

Abstract

Diffusion is a fundamental atomic process that underlies a variety of behavior of solids at relatively high temperatures, and thus is a subject of major interest in a number of fields in science and engineering. Although late in coming, there is now a major interest in the study of diffusion properties of rock-forming minerals as the modeling of diffusion-controlled properties have provided important constraints on the time scales of evolution of mountain belts, rate of subduction of tectonic plates, dynamical processes within the Earth's interior (e.g. melting relations and magma generation, convection), residence time scales of melts in magma chambers prior to eruptions that hold the potential of predicting volcanic activities etc. The proposed research is focused (a) on the experimental determination of diffusion kinetic properties of selected cations, namely Sm3+, Lu3+, Y3+ and Hf4+, in natural garnets as function of the important physical and chemical factors that influence diffusion process, and (b) applications of the data to the quantitative modeling of the time scales of a large spectrum of geological processes. Integration of these data with the results of earlier studies by the team on the diffusion properties of divalent (2+) cations in garnet would constitute a major step towards developing predictive models of diffusion in solids that is of fundamental importance to the general field of solid state diffusion. Depending on the sample and other details, the experimentally induced diffusion profiles would be analyzed by SIMS (secondary ion mass spectrometry) depth profiling in an Ion-probe, step scanning in a nanoSIMS or step/beam-scanning in our recently acquired electron microprobe, which is especially suited for trace element analysis. There has been a long standing need in the Earth sciences community for experimental data on the pressure dependence of 3+ and 4+ cations in garnets as the geological processes of interest involving these cations take place at high pressures. The experimental data to be obtained would fill this major gap. As the diffusivity of Y3+ seems to be significantly slower than those of the major divalent cations (Fe, Mg, Mn, Ca), for which reliable data are now available (except for Ca), simultaneous modeling of the concentration profiles of Y3+ and the divalent cations in garnet, using computer programs to be developed in the course of this study, would impose tight constraints on the time scales of a wide range of metamorphic and magmatic processes. Garnets in metamorphic rocks have been extensively dated by Sm-Nd and Lu-Hf geochronological systems, usually with discrepant results for the same rock. Unlike the Sm-Nd system, there seems to be a large difference between the diffusivities of parent nuclide, Lu3+, and the daughter nuclide, Hf4+. This property and the relatively much slower diffusivity of Hf4+, as our preliminary experimental data seem to suggest, have interesting implications on the development of garnet-whole rock Lu-Hf isochrons that need to be considered carefully, and would be addressed in the proposed research, for the proper interpretation of ages and 176Lu/177Hf initial ratios, which are derived from the isochrons in natural samples. Additionally, analysis of the discrepant Sm-Nd and Lu-Hf ages of garnets in metamorphic rocks in terms of the diffusion kinetic properties of the species would help constrain the growth rates of garnets in natural samples, which is a very important parameter for modeling tectono-metamorphic processes.