Cosmogenic 3He in Himalayan garnets indicating an altitude dependence of the 3He/10Be production ratio

To investigate the potential of using cosmogenic helium in garnet as a dating tool, He-3(c) and Be-10(c) have been measured within garnets and coexisting quartz, respectively, sampled in the Himalayan range at various elevations. Comparison and correlation of He-3(c) and Be-10(c) concentrations demonstrate that cosmogenic He-3 is well retained in Himalayan garnets and can thus be used in this mineral to quantify precisely earth's surface processes. This, in the case of recently cooled orogenic samples which contain limited amounts of non-cosmogenic helium. For these high-elevation samples, the exposure ages derived from He-3(c) using classical scaling factors lead however to an overestimation of He-3(c) ages, compared to Be-10(c) ages (0.1 to 28 ky), by up to a factor of 2. Additionally, over the samples altitude range (3000-4622 m), the theoretically invariant polar He-3(c)/Be-10(c) production ratio (similar to22.5), increases with increasing elevation, challenging classical models of production rate evolution. To explain this observation, we propose a new He-3(c) production mechanism, based on the altitudinal dependence of the energy spectrum of cosmic rays, and according to which neutrons or protons resulting from a first spallation reaction within the rock (tertiary particles) may have sufficient energy to induce an additional in situ spallation reaction producing a second H-3 or He-3, but insufficient energy to produce Be-10, the probability of such a mechanism increasing with altitude. According to these observations, we propose for future and ongoing studies to use the empirically determined attenuation length of 121 g/cm(2) for helium production at high altitude. (C) 2004 Elsevier B.V All rights reserved.