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 sa mples with He at 250 +/- 10 bar in the temperature range 800-990 degre es C. For this temperature range we find a three-fold decrease in He s orption 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 sat urated samples to obtain the activation energy (E(a) = 660 +/- (190)(1 20) 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 r esults 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 m ay provide information about the medium to high temperature cooling pa th of a rock at the lower end of the temperature range of currently us ed isotopic systems in garnet. In addition, the high retentivity for H e makes garnet a potentially useful mineral in the field of exposure a ge dating. Currently, it is the only-known common silicate mineral occ urring in upper crustal rocks which probably retains cosmogenic noble gases under all climatic conditions. For geochemical studies using nob le gases as tracers for geofluids, garnet is a perfect container if te mperatures never exceed 600 degrees C; that is, all originally trapped noble gases are quantitatively retained.