Cosmogenic neon in ultramafic nodules from Asia and in quartzite from Antarctica

Abstract

We have performed systematic analyses of both cosmogenic 3He ( 3He c) and cosmogenic 21Ne ( 21Ne c) in ultramafic xenoliths from Central Asia and in a quartz sample from Antarctica. Five xenoliths, which show no or insignificant 21Ne c excesses, were used to estimate the initial 4He/ 3He ratio of 90,470 in the subcontinental lithospheric mantle under the Baikal extension zone. Seven xenoliths show large 21Ne/ 22Ne anomalies ranging up to 0.204 and 4He/ 3He down to 31,000, due to the presence of cosmogenic 21Ne and 3He. The ( 3He/ 21Ne) c ratio is 1.41 ± 0.22 in the xenoliths and 2.76 in the quartzite. This difference is due to the dependence of the 21Ne c production rate on the elemental composition of the target material. We estimated the 3He c and 21Ne c production rates at different locations worldwide and calculated the 3He c and 21Ne c 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 3He c and 21Ne c 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 3He c diffusive losses from the quartz mineral grains, even at low temperatures, due to the relatively high diffusion coefficient for cosmogenic 3He.