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Anomalous Cosmogenic He Production and Elevation Scaling In Available online at www.sciencedirect.com Earth and Planetary Science Letters 265 (2008) 287–301 www.elsevier.com/locate/epsl Anomalous cosmogenic 3He production and elevation scaling in the high Himalaya ⁎ William H. Amidon a, , Kenneth A. Farley a, Douglas W. Burbank b, Beth Pratt-Sitaula c a California Institute of Technology, GPS Division, 1200 E. California Blvd., Pasadena, CA 91125, United States b University of California Santa Barbara, Department of Earth Science, Webb Hall, Santa Barbara, CA 93106, United States c Central Washington University, Department of Geological Sciences, 400 E. University Way, Ellensburg, WA 98926, United States Received 12 July 2007; received in revised form 10 October 2007; accepted 11 October 2007 Available online 22 October 2007 Editor: R.W. Carlson Abstract The production rate of cosmogenic 3He in apatite, zircon, kyanite and garnet was obtained by cross-calibration against 10Be in co-existing quartz in glacial moraine boulders from the Nepalese Himalaya. The boulders have 10Be ages between 6 and 16 kyr and span elevations from 3200 to 4800 m. In all of these minerals 3He correlates with 10Be and is dominantly cosmogenic in origin. After modest correction for non-cosmogenic components, 3He/10Be 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 garnet. 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/10Be 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 10Be are distinct from each other at least at this location over the last ∼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. © 2007 Elsevier B.V. All rights reserved. Keywords: cosmogenic; 3He; 6Li; lithium; helium; apatite; zircon; kyanite; garnet; Nepal; Himalayas; paleoelevation 1. Introduction decades (Lal and Peters, 1967; Bierman, 1994; Gosse and Phillips, 2001). Although 3He is not as widely Due to its role in determining ages and erosion rates applied as 10Be or 26Al, 3He occupies a unique niche in of surfaces in the landscape, cosmogenic nuclide the family of cosmogenic isotopes for several reasons. It analysis has grown in popularity over the last several has a higher production rate relative to its detection limit than other cosmogenic isotopes, and can thus be used to ⁎ Corresponding author. Tel.: +1 626 395 2190. date very small samples or young surfaces. It is E-mail address: [email protected] (W.H. Amidon). produced by spallation from nearly all target elements, 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.10.022 288 W.H. Amidon et al. / Earth and Planetary Science Letters 265 (2008) 287–301 so can potentially be applied to many different mineral calibrated against 10Be in glacial moraine boulders from phases. Because it is stable, 3He is potentially useful for the Nepalese Himalaya (Gayer et al., 2004). In addition, estimating erosion rates on extremely old surfaces, for Kober et al. (2005) provide estimates of element- determining exposure ages of paleo-surfaces, and for specific 3He production rates based on a combination of estimating catchment-scale erosion rates from ancient field calibration and neutron bombardment experiments. sediments. In addition, cosmogenic 3He dating poten- These estimates are useful for predicting production tially provides a faster and simpler alternative to rates in minerals that have not been directly calibrated. cosmogenic radionuclide dating because it does not However, due to complicating variables such as Li involve intensive preparation chemistry and measure- content, grain size, elevation, and lithology, further ment on an accelerator mass spectrometer. calibration studies are needed before robust and widely Most previous studies using 3He have targeted applicable production rates are established. olivine and pyroxene phenocrysts, and numerous Here we calibrate the production rate of cosmogenic production rate determinations for olivine have been 3He in zircon, apatite, kyanite, and garnet against 10Be made on basalt flows of known age, yielding sea-level in quartz in a suite of glacial moraine boulders in the high-latitude (SLHL) rates between ∼ 100 and Nepalese Himalaya. Our approach and sampling locality 150 atoms g− 1 yr− 1 (Kurz et al., 1990; Cerling and are similar to the study of cosmogenic 3He in garnet Craig, 1994; Licciardi et al., 1999; Dunai and Wijbrans, performed by Gayer et al. (2004). Our sample suite also 2000; Ackert et al., 2003; Licciardi et al., 2006). Recent allows us to assess Gayer et al.'s observations of efforts have explored extending 3He dating by establish- anomalous production rates and altitude scaling of ing production rates and the non-cosmogenic back- cosmogenic 3He in Himalayan garnets, and a recently ground in additional mineral phases found in more proposed explanation that these anomalies arise from diverse lithologies. For example, production rates of nuclear reactions on lithium (Dunai et al., 2007). 3He in apatite, zircon, titanite and Fe–Ti oxides were Natural samples have multiple sources of 3He in determined by cross-calibration against cosmogenic addition to the sought-after cosmogenic spallation 21Ne in Andean tuffs (Kober et al., 2005; Farley et al., component. With the knowledge of the Li concentration 2006). Similarly, the production rate in garnet was of the analyzed phases, the composition of the whole Fig. 1. Map of field area showing sample sites (white circles), towns (black squares), and major summits (black triangles). Major structural features are shown following Searle and Godin (2003), and delineate the Tethyan Sedimentary Series (north of the South Tibetan Detachment), from the Greater Himalayan Series (south of the Deurali-Chame detachment). The shaded relief map is derived from SRTM 90 m data. W.H. Amidon et al. / Earth and Planetary Science Letters 265 (2008) 287–301 289 rock, and appropriate models, we can isolate cosmo- leucocratic granite, which typically contains ∼32% genic 3He from these other components. SLHL quartz, ∼37% plagioclase (An 2-21), ∼21% K-feldspar, production rates are then estimated by multiplying the ∼7% muscovite, and ∼3% biotite (Deniel et al., 1987). cosmogenic 3He/10Be ratio by the known SLHL 10Be Useable quantities of zircon were recovered only production rate. This approach eliminates the need to from the quartzitic and gneissic boulders, whereas assume negligible surface erosion or burial, but requires apatites were recovered from all lithologies. Kyanites that cosmogenic isotope production rates scale identi- and garnet occurred only in a subset of the gneisses, and cally with elevation and latitude. Unless otherwise were hand-picked from the 250b500 μm size fraction stated, 3He production in this paper refers to both direct (Table 3). Garnets show a narrow compositional range, production, and production via 3H. averaging about 70% almandine, and 18% pyrope (Supplementary Table 2). Although the retentivity of 2. Geologic setting and samples 3He in kyanite has never been demonstrated, it is a member of the nesosilicate family and is structurally Field sampling and 10Be analysis of glacial moraine similar to other retentive nesosilicates such as olivine, boulders were performed by Pratt-Sitaula (2004). She zircon and garnet. sampled three types of glacial moraine boulders: Beryllium concentrations in quartz range from 0.32 to quartzite, gneiss, and granite. Quartzitic moraine 1.3 Mat g−1, interpreted as moraine ages from 5.2 to boulders were sampled entirely above 4000 m and 16.3 kyr (Pratt-Sitaula, 2004). The preliminary ages listed contain low but variable concentrations of illite, sericite, in Tables 1–3 were calculated using the spreadsheet of and other clay minerals, as well as trace amounts of Pigati and Lifton (2004), and an SLHL production rate of rutile, pyrite, and zircon. The gneissic moraine boulders 5.1 atoms g−1 yr−1. Topographic shielding was calculated were sampled between 3215 and 3960 m, and are following Farber et al. (2005), and snow shielding as derived from Formation I of the Greater Himalayan described in Pratt-Sitaula (2004), values for both are listed Series (Fig. 1). These amphibolite grade gneisses in Supplementary Table 1. The wide range of 10Be typically contain varying amounts of quartz, K-feldspar, concentrations, moraine ages, and elevations makes this plagioclase (albite to labradorite), muscovite and biotite. an ideal sample suite with which to assess factors con- The Manaslu granite is a generally medium-grained trolling the cosmogenic 3He production rate. Interestingly, Table 1
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