Large effect of water on Fe–Mg interdiffusion in garnet

Abstract

The effect of water on the kinetics of Fe–Mg interdiffusion in garnet was investigated at 3 GPa and 1373–1673 K using diffusion couples of pyrope and almandine aggregates, over a wide range of water content (CH2O) from <7 up to ∼1260 wt. ppm. Diffusion profiles were measured by electron microprobe, and the obtained data were fitted by the Boltzmann–Matano equation. Our results show that Fe–Mg interdiffusion coefficient (DFe–Mg) is nearly independent of the composition of garnet over a wide range of Fe/Mg ratios (with XFe ranging from 0 to 1). The determined DFe–Mg can be described by the following Arrhenius relation: DFe–Mg(m2/s)=D0CH2Orexp⁡(−ΔH/RT), where log⁡(D0)=−1.70±0.50, r=1.38±0.06, and H=310±33 kJ/mol. The exponent r of 1.38 implies the incorporation mechanism of water is hydrogen associated with metal and partly silicon vacancies to form neutral point defect complexes. While at the same time, it also suggests a profound role of water in enhancing the Fe–Mg interdiffusion in garnet. With increasing water content from 100 to 1260 wt. ppm H2O, the Fe–Mg inter-diffusivity is enhanced by about two orders of magnitude, comparable to the effect of temperature increased by about 300°C under dry condition. Such large effect of water on the kinetics of garnet is expected to have important implications for evaluating the closure temperature of available geospeedometry and geothermometry.