Mg tracer diffusion in aluminosilicate garnets at 750-850 degrees C, 1 atm and 1300 degrees C, 8.5 GPa

Tracer diffusion coefficients of Mg in natural aluminosilicate of composition Alm(38)Pyr(50)Gr(10)Sp(2) and Alm(73)Pyr(21)Gr(5)Sp(1) have been measured at 1 bar, 750-850 degrees C and at 8.5 GPa, 1300 degrees C by chemically depositing a salt layer enriched in Mg-26 on the specially prepared surface of a garnet single crystal. Diffusion anneals at 1 atmosphere (101325 Pa) were carried out at a controlled f(O2) of similar to 10(-17.5) bars maintained by a flowing gas mix of CO-CO2. Annealing conditions were carefully chosen to avoid decomposition of garnet by redox reactions. High pressure anneals were carried out in a multianvil apparatus. Induced diffusion profiles (0.1-0.6 mu m) were measured by an ion-microprobe with SIMS attachment. Diffusion coefficients at 1 atmosphere are in excellent agreement with extrapolation of data from high P-T experiments (Loomis et al. 1985; Chakraborty and Ganguly 1992) and also with the low temperature (750-900 degrees C) dataset of Cygan and Lasaga (1985) if the diffusion coefficients are assumed to be proportional to f(O2)(1/6). Such an f(O2) dependence, however, makes this dataset inconsistent with the recent dataset of Schwandt et al. (1995) on garnets of composition (Alm(15)Pyr(72)Gr(13)Sp(0)) unless a strong compositional dependence of Mg tracer diffusivity for Mg-rich garnets is invoked. The present experimental results show that such a compositional dependence is weak to non-existent for garnets with >38 mole percent almandine component. It is emphasized that the temperature dependence of diffusion coefficients at constant oxygen fugacities (activation energy approximate to 54 kcal/mol) are different from that along an oxygen fugacity buffer (activation energy approximate to 64.5 kcal/mol), as already pointed out by Chakraborty and Ganguly (1991). This distinction is of importance for modelling natural processes. The measurements at low temperatures either eliminate the need for, or greatly reduce the uncertainty of, extrapolation of laboratory data for modelling metamorphic processes. The high pressure results combined with those from Chakraborty and Ganguly (1992) and Loomis et al. (1985) indicate that pressure dependence of Mg tracer diffusivity in garnets is much stronger than that in forsterite (Chakraborty et al. 1994). This difference in pressure dependence of diffusivity may be caused by the difference in compressibility of the coordination polyhedra of Mg between olivines and garnets. Activation volumes of Mg tracer diffusion as high as 8 cm(3)/mol may be estimated using the present data in combination with earlier results. These data suggest that at a temperature of 1300 degrees C, Mg tracer diffusion rates in garnets will decrease by an order of magnitude for every 100 km depth. The pressure effect will be stronger at lower temperatures. For calculations involving diffusion coefficients of garnets at high pressures (e.g. mantle xenoliths, eclogites) the pressure dependence of diffusivity must be taken into account.