Cosmogenic 3He in Himalayan garnets indicating an altitude dependence of the 3He/10Be production ratio
ABSTRACT To investigate the potential of using cosmogenic helium in garnet as a dating tool, 3Hec and 10Bec have been measured within garnets and coexisting quartz, respectively, sampled in the Himalayan range at various elevations. Comparison and correlation of 3Hec and 10Bec concentrations demonstrate that cosmogenic 3He 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 3Hec using classical scaling factors lead however to an overestimation of 3Hec ages, compared to 10Bec 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 3Hec/10Bec production ratio (∼22.5), increases with increasing elevation, challenging classical models of production rate evolution. To explain this observation, we propose a new 3Hec 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 3H or 3He, but insufficient energy to produce 10Be, 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/cm2 for helium production at high altitude.
Article: Monsoonal forcing of Holocene glacier fluctuations in Ganesh Himal (Central Nepal) constrained by cosmogenic 3He exposure ages of garnets[show abstract] [hide abstract]
ABSTRACT: In the Himalayas, the late Pleistocene glacier oscillations have produced spectacular glacial landforms. Detailed reconstructions of the chronology and extent of these oscillations are essential to document the sensitivity of the Himalayan glaciers to past and future climatic changes. In this paper, we present a new cosmogenic helium 3 (3Hec) dating on garnets, that were sampled on moraine blocks and ice-scoured surfaces in a small glaciated valley of the Central Nepal (the Mailun valley), and that provided a detailed chronology of Himalayan glacier fluctuations during the Holocene. Soon after the Younger Dryas, the glacier of the Mailun valley underwent a significant retreat around 10 ka. This retreat was followed by relative stability of the extent of the glacier between ∼ 8.5 and ∼ 7.5 ka. A second phase of rapid retreat occurred at ∼ 7 ka, but rapidly slowed down at ∼ 5–6 ka. Finally, a last phase of re-advance occurred between 0 and 1 ka. The interpretation of the Equilibrium Line Altitude (ELA) variation, deduced from this chronology (for the Holocene period) and carbon 14 (14C) dating (for the Pleistocene period), shows that the early history of the Mailun valley deglaciation (late Pleistocene) is in good agreement with the global paleoclimatic records. The main extent of the glacier and the major ice volume drop are in phase with the global Last Glacial Maximum (25–17 ka) and with the major worldwide temperature increase following the Younger Dryas, respectively, indicating that the Mailun glacier was primarily driven by temperature oscillations during the late Pleistocene. In contrast, the glacier chronology during the Holocene suggests that the Mailun glacier was modulated by the variation in annual precipitation, and is asynchronous relative to most glaciers of the Northern Hemisphere. The significant sensitivity of the Himalayan glaciers to precipitation might explain the striking lack of synchronism of the Himalayan glaciers both along and across the Himalayan arc.Earth and Planetary Science Letters. 01/2006;
Article: In situ cosmogenic Be-10 and Ne-21 in sanidine and in situ cosmogenic He-3 in Fe-Ti-oxide minerals[show abstract] [hide abstract]
ABSTRACT: We report concentrations of in situ cosmogenic Be-10 and Ne-21 from coexisting quartz and sanidine separates and of cosmogenic He-3 in coexisting Fe-Ti-oxide minerals from ignimbritic successions of northern Chile (Oxaya and Lauca ignimbrites). New mineral-isotope pairs such as sanidine and Fe-Ti-oxide minerals are helpful in quantitative geomorphology for geological settings where the lithology lacks the commonly used minerals quartz, pyroxene or olivine. Production rates in sanidine and Fe-Ti-oxide minerals were determined by normalizing nuclide concentrations to established production rates in quartz. The experimentally determined production rates are compared to model production rates calculated with new cross-sections for He-3, Ne-21, and Be-10 production from the individual target elements. The mean experimental Ne-21 production rate for five sanidine samples is 30.4 +/- 3.7 atoms g(-1) yr(-1) (30.4 +/- 5.4 atoms g(-1) yr(-1) including the uncertainty of the Ne-21 production rate in quartz, (PNeqtz)-Ne-21). This is in excellent agreement with the modelled value of 28.3 atoms g(-1) yr(-1), which has an estimated uncertainty of 20%. The Ne-21 production rate in sanidine is thus about 50% higher than that in quartz. The cosmogenic neon in sanidine is entirely released in low temperature steps (400-600 degrees C) and no signs of an interfering nucleogenic neon component were observed. This is in stark contrast to quartz and makes sanidine an attractive mineral for terrestrial cosmogenic neon studies. He-3 diffuses out of the sanidine structure. Preliminary results also indicate that sanidine is well suited for Be-10 studies. The mean experimental Be-10 production rate from two sanidine samples is 4.45 +/- 0.38 atoms g(-1) yr(-1) (4.45 +/- 0.42 atoms g(-1) yr-1 including uncertainty of (PBeqtz)-Be-10), very close to the modelled value of 4.55 atoms g(-1) yr(-1), which has an estimated uncertainty of 20%. We emphasize that Be-10 and Ne-21 production in sanidine is composition dependent. Therefore, major element analyses should be carried out and production rates calculated on a sample by sample basis.Fe-Ti-oxide minerals retain He-3 quantitatively. Experimentally derived production rates are in excellent agreement with new values derived from physical modelling. The mean experimental He-3 production rate for five Fe-Ti-oxide minerals samples is 120 +/- 11 atoms g(-1) yr(-1) (120 +/- 12 atoms g(-1) yr(-1) including the uncertainties of (PNeqtz)-Ne-21), compared to a mean modelled value of 124 atoms g(-1) yr(-1), which has an estimated uncertainty of 20%. The variable chemical and structural composition of the solid solution lines of Fe-Ti-oxide minerals has little effect on the total He-3 production rate. CosmogeniC Ne-21 is not produced in significant quantities in Fe-Ti-oxide minerals due to the absence of suitable target elements. (c) 2005 Elsevier B.V. All rights reserved.236:404-418.