Diffusion of cosmogenic 3He in olivine and quartz: implications for surface exposure dating

Abstract

The in situ production of 3He in surface rocks by cosmic ray induced nuclear reactions offers an important geochronological tool. To evaluate helium loss problems in this technique, cosmogenic 3He diffusivities were measured in quartz and olivine by incremental heating at 150–600°C. Arrhenius temperature dependences were observed in both minerals with similar activation energies ( E a = 25 ± 4 kcal/mole, log D 0 = −3.7 ± 0.9 in olivine and E a = 25.2 ± 0.9 , log D 0 = +0.2 ± 0.4 in quartz) and imply very low diffusivities when extrapolated to environmental temperatures (less than 10 −18 cm 2/s in quartz and 10 −22 cm 2/s in olivine at 20°C). These low diffusivities suggest helium loss will not significantly affect cosmogenic helium exposure dating for time scales on the order of 10 6 years in quartz and 10 9 years in olivine, provided large (2 mm) sample grains are used. Exposure ages obtained with smaller grains need to be corrected for diffusive helium loss and equations are provided for this purpose. Cosmogenic 3He diffuses more than 100 times faster than trapped magmatic 3He in olivine or radiogenic 4He in quartz. The mechanisms responsible for these differences have yet to be determined, but may involve both trapping processes at mineral defects and enhanced mobility associated with spallation produced crystal damage. Therefore, it is possible that helium diffusivities may depend on a sample's cosmic ray exposure or radiogenic production history. Whatever the precise mechanism, the higher mobility of 3He suggests that incremental heating will be useful in separating cosmogenic helium from magmatic or radiogenic helium in future studies. Comparison of the diffusivity results with total cosmogenic 3He contents of different quartz size fractions for an Antarctic quartzite suggests that He loss may have occurred more rapidly than predicted by the laboratory measurements. Nonetheless, retention of cosmogenic 3He is quite high (equivalent to more than a million years of exposure) and cosmogenic 3He geochronology is feasible in this and other quartz-containing rocks.