COSMOGENIC NEON IN ULTRAMAFIC NODULES FROM ASIA AND IN QUARTZITE FROM ANTARCTICA

We have performed systematic analyses of both cosmogenic He-3 (He-3c) and cosmogenic Ne-21 (Ne-21c) in ultramafic xenoliths from Central Asia and in a quartz sample from Antarctica. Five xenoliths, which show no or insignificant Ne-21c excesses, were used to estimate the initial He-4/He-3 ratio of 90,470 in the subcontinental lithospheric mantle under the Baikal extension zone. Seven xenoliths show large Ne-21/Ne-22 anomalies ranging up to 0.204 and He-4/He-3 down to 31,000, due to the presence of cosmogenic Ne-21 and He-3. The (He-3/Ne-21c) ratio is 1.41 +/- 0.22 in the xenoliths and 2.76 in the quartzite. This difference is due to the dependence of the Ne-21c production rate on the elemental composition of the target material. We estimated the He-3c and Ne-21c production rates at different locations worldwide and calculated the He-3c and Ne-21c exposure ages. These ages range between 7100 and 28,000 years for the xenoliths, and we determined their relative positions within the volcanic tuff layer. The mean He-3c and Ne-21c exposure ages of the quartz sample are 1.35 +/- 0.07 and 2.21 +/- 0.12 Ma, respectively. This difference is most probably related to He-3c diffusive losses from the quartz mineral grains, even at low temperatures, due to the relatively high diffusion coefficient for cosmogenic He-3.