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 comp osition Alm(38)Pyr(50)Gr(10)Sp(2) and Alm(73)Pyr(21)Gr(5)Sp(1) have be en 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 specia lly prepared surface of a garnet single crystal. Diffusion anneals at 1 atmosphere (101325 Pa) were carried out at a controlled f(O2) of sim ilar to 10(-17.5) bars maintained by a flowing gas mix of CO-CO2. Anne aling conditions were carefully chosen to avoid decomposition of garne t by redox reactions. High pressure anneals were carried out in a mult ianvil apparatus. Induced diffusion profiles (0.1-0.6 mu m) were measu red 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 o f Cygan and Lasaga (1985) if the diffusion coefficients are assumed to be proportional to f(O2)(1/6). Such an f(O2) dependence, however, mak es 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 temperatu re 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. Th e measurements at low temperatures either eliminate the need for, or g reatly reduce the uncertainty of, extrapolation of laboratory data for modelling metamorphic processes. The high pressure results combined w ith 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). T his 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 diffusio n as high as 8 cm(3)/mol may be estimated using the present data in co mbination with earlier results. These data suggest that at a temperatu re of 1300 degrees C, Mg tracer diffusion rates in garnets will decrea se by an order of magnitude for every 100 km depth. The pressure effec t will be stronger at lower temperatures. For calculations involving d iffusion coefficients of garnets at high pressures (e.g. mantle xenoli ths, eclogites) the pressure dependence of diffusivity must be taken i nto account.