Anomalous cosmogenic ^3He production and elevation scaling in the high Himalaya

Abstract

The production rate of cosmogenic ^3He in apatite, zircon, kyanite and garnet was obtained by cross-calibration against ^(10)Be in co-existing quartz in glacial moraine boulders from the Nepalese Himalaya. The boulders have ^(10)Be ages between 6 and 16 kyr and span elevations from 3200 to 4800 m. In all of these minerals ^3He correlates with ^(10)Be and is dominantly cosmogenic in origin. After modest correction for non-cosmogenic components, ^3He/^(10)Be systematics imply apparent sea-level high-latitude (SLHL) apparent production rates for ^3He of 226 atoms g^(-1) yr^(-1) in zircon, 254 atoms g^(-1) yr^(-1) in apatite, 177 atoms g^(-1) yr^(-1) in kyanite, and 153 atoms g^(-1) yr^(-1) in gamet. These production rates are unexpectedly high compared with rates measured elsewhere in the world, and also compared with proposed element-specific production rates. For apatite and zircon, the data are sufficient to conclude that the ^3He/^(10)Be ratio increases with elevation. If this reflects different altitudinal scaling between production rates for the two isotopes then the SLHL production rates estimated by our approach are overestimates. We consider several hypotheses to explain these observations, including production of ^3He via thermal neutron capture on ^6Li, altitudinal variations in the energy spectrum of cosmic-ray neutrons, and the effects of snow cover. Because all of these effects are small, we conclude that the altitudinal variations in production rates of cosmogenic ^3He and ^(10)Be are distinct from each other at least at this location over the last last ~10 kyr kyr. This conclusion calls into question commonly adopted geographic scaling laws for at least some cosmogenic nuclides. If confirmed, this distinction may provide a mechanism by which to obtain paleoelevation estimates.