Abstract:
The accuracy of 21Ne surface exposure ages depends critically on the correction for a trapped Ne component. Commonly, the amount of cosmogenic Ne used to calculate 21Ne exposure ages is considered to be the Ne excess relative to a trapped component of atmospheric composition (21Ne/20Ne=0.00296). Here, we document a trapped Ne component in quartz samples from a series of river terraces at the northern margin of Tibet [Hetzel et al., Nature 417 (2002) 428-432], which has a non-atmospheric 21Ne/20Ne ratio varying from 0.00299 to 0.00398. Vacuum crushing of amalgamated samples, each derived from 30-80 quartz clasts, revealed that the non-atmospheric trapped component is present in fluid inclusions. It has probably been incorporated into the quartz crystals during their growth in veins at low-grade metamorphic conditions. Only if the amount of cosmogenic 21Ne is determined relative to this trapped component are the calculated 21Ne exposure ages consistent with the relative ages of the tectonically induced terraces and in agreement with independent 10Be and 26Al exposure ages of the fluvial terraces. A significant in situ production of nucleogenic 21Ne by the reaction 18O(α,n)21Ne in the Paleozoic quartz minerals is ruled out by extremely low U contents of the order of 1-5 ppb. All ages have been corrected for an inherited cosmogenic component that results from cosmic ray exposure during erosion of the host rock and transport of the clasts to the terraces. The origin of the trapped component from crustal fluids is supported by high 40Ar/36Ar ratios of 2000-6000 and slightly elevated 136Xe/132Xe ratios relative to air, which can be explained by radioactive decay of 40K and spontaneous fission of U in the crust.
