Sorption and diffusion of helium in garnet: Implications for volatile tracing and dating

We present data on He sorption in garnet obtained by loading garnet samples with He at 250 +/- 10 bar in the temperature range 800-990 degrees C. For this temperature range we find a three-fold decrease in He sorption from 2.1 . 10(-5) (800 degrees C) to 0.72 . 10(-5) cm(3) STPg(-1) bar(-1) (990 degrees C). We use the sorption data of partially saturated samples to obtain the activation energy (E(a) = 660 +/- (190)(120) kJ/mol) and frequency factor (D-0 = 10(15.6(+8,-5.2)) cm(2)/s) for He diffusion in garnet. The results of our study suggest that garnet is probably one of the most retentive silicate minerals for He, which is in agreement with its very low ion porosity. The high retentivity results in a high closure temperature (T-c) for the (U + Th)-He system. For cooling rates as low as O.1-1 degrees C/Ma, T-c lies between 590 degrees and 630 degrees C. Accordingly, (U + Th)-He dating of garnet may provide information about the medium to high temperature cooling path of a rock at the lower end of the temperature range of currently used isotopic systems in garnet. In addition, the high retentivity for He makes garnet a potentially useful mineral in the field of exposure age dating. Currently, it is the only-known common silicate mineral occurring in upper crustal rocks which probably retains cosmogenic noble gases under all climatic conditions. For geochemical studies using noble gases as tracers for geofluids, garnet is a perfect container if temperatures never exceed 600 degrees C; that is, all originally trapped noble gases are quantitatively retained.