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

The production rate of cosmogenic He-3 in apatite, zircon, kyanite and garnet was obtained by cross-calibration against Be-10 in co-existing quartz in glacial moraine boulders from the Nepalese Himalaya. The boulders have Be-10 ages between 6 and 16 kyr and span elevations from 3200 to 4800 m. In all of these minerals He-3 correlates with Be-10 and is dominantly cosmogenic in origin. After modest correction for non-cosmogenic components, He-3/Be-10 systematics imply apparent sea-level high-latitude (SLHL) apparent production rates for He-3 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 He-3/Be-10 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 He-3 via thermal neutron capture on Li-6, 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 He-3 and Be-10 are distinct from each other at least at this location over the last similar to 10 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. (c) 2007 Elsevier B.V. All rights reserved.