VOLUME SELF-DIFFUSION OF OXYGEN IN BIOTITE, MUSCOVITE, AND PHLOGOPITE MICAS

The kinetics of oxygen self-diffusion have been measured in natural samples of biotite, muscovite, and phlogopite micas under hydrothermal conditions using both bulk exchange and profiling analyses. All experiments were run at 1000 bars (100 MPa) water pressure except those designed to measure water pressure dependence, which ranged from 200 to 2000 bars. The Arrhenius relations obtained for layer parallel diffusive transport in the three micas by bulk exchange experiments, fit to an infinite cylinder model, are [GRAPHICS] The results of ion microprobe (SIMS) depth profiling perpendicular to the layers (parallel to c-axis) yield diffusion coefficients 3 to 4 orders of magnitude lower than for transport parallel to the layers. A single layer-parallel step profiling experiment yields a diffusion coefficient identical, within uncertainty, to those obtained from bulk exchange experiments. A slight dependence of diffusion rate on water pressure may obtain in phlogopite; however, the difference is approximately within the uncertainty of the measurement. Diffusion rates ares imilar, within about a half an order of magnitude, for a range of biotite composition from annite4 to annite63, as well as for muscovite. Comparison of oxygen diffusion rates in biotite from this study with Ar, K, and rb diffusion from published data supports the idea that water is the oxygen-bearing species in hydrothermal diffusion experiments. The implication of these results for natural systems is that the micas, expecially biotite, will continue to exchange oxygen isotopes via volume diffusion down to temperatures as low as 300-degrees-C. The rapid diffusion kinetics may explain biotite oxygen isotope disequilibrium, which is commonly observed even in rocks where other mineral phases preserve equilibrium compositions.