CA DIFFUSION IN GARNET: A TALE OF TWO SITES

Ca diffusion in aluminosilicate garnets remains poorly understood in spite of its significance. A number of studies have addressed this topic through experiments and modeling. Some of the same studies measured or modeled diffusion rates for other elements such as Fe, Mn and Mg in garnets and reached a broad consensus. However, this has not been the case for Ca diffusion. To address this problem, we have carried out diffusion couple experiments at high P-T conditions (25 kbar and 1260, 1300 and 1320°C) in a piston cylinder apparatus with very specific compositional gradients. Fe and Mg contents (expressed as cations per formula unit) were different on two sides of the couple whereas Ca contents were practically the same. In addition, garnets on one side of the couple had significant Cr. After diffusion anneals, it was found that Ca profiles remained flat, although diffusive coupling would have been expected to generate strong uphill diffusion in these cases. At the same time, concentration profiles of Cr were observed. As Cr is typically present in garnets as the uvarovite component, diffusion of Cr is also expected to generate coupled fluxes of Ca. It was found that compositional profiles of grossularite, andradite as well as uvarovite components had been generated during the diffusion anneals, but overall Ca concentration showed a flat profile. These unique observations provide a number of insights into the nature and application of Ca diffusion in garnets: (i) Activation energies of Ca diffusion are likely to be the combination of activation energies for diffusive jumps in the dodecahedral and the octahedral sites. As a result, these are expected to be higher than the activation energies for diffusion of other dodecahedral cations (when the garnet contains only small amounts of Fe³⁺). (ii) The dependence of Ca diffusion rates on oxygen fugacity is expected to be complex. (iii) The higher activation energy of diffusion could account for the fact that at lower temperatures, diffusion rates of Ca are much slower than those of Fe, Mn and Mg, even if the differences between the diffusion rates of these elements at experimental conditions (typically > 1100 °C) are within a factor of two or three of each other. This last aspect can explain many observed features of compositional zoning in natural garnets.