2001 Fall Meeting          
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Cite abstracts as Eos Trans. AGU, 82(47),
Fall Meet. Suppl., Abstract xxxxx-xx, 2001
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marchant

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HR: 1330h
AN: H42D-0391
TI: Magmatic He-3 in Ferrar Dolerite: Implications for Cosmogenic He-3 Surface Exposure Dating in the Dry Valleys of East Antarctica
AU: * Phillips, W M
EM: wmp@geo.ed.ac.uk
AF: Department of Geography, University of Edinburgh, Edinburgh, EH8 9XP United Kingdom
AU: Landis, G P
AF: U.S. Geological Survey, Denver Federal Center, Denver, CO 80225 United States
AU: Marchant, D R
AF: Department of Earth Sciences, Boston University, Boston, MA 02215 United States
AU: Lewis, A R
AF: Department of Earth Sciences, Boston University, Boston, MA 02215 United States
AU: Mills-Herring, L M
AF: U.S. Geological Survey, Denver Federal Center, Denver, CO 80225 United States
AU: Margerison, H R
AF: Department of Geography, University of Edinburgh, Edinburgh, EH8 9XP United Kingdom
AB: Sills of Jurassic Ferrar Dolerite are common in the Transantarctic Mountains. Cosmogenic He-3 surface exposure dating of glacial deposits using Ferrar clinopyroxene has become an important tool in Antarctic paleoclimatology. Here we report preliminary evidence for magmatic helium in Ferrar clinopyroxene from the Mullins Valley rock glacier, Quartermain Mountains, East Antarctica (S. 77.88\deg, E. 160.58\deg). Magmatic helium has not previously been recognized in Ferrar clinopyroxene, and all He-3 released by sample fusion has been ascribed to cosmogenic production. However, our evidence indicates that a correction for non-cosmogenic He-3 is needed for at least some samples. The Mullins Valley rock glacier is fed by a small, cold-based alpine glacier. Rockfall onto the alpine glacier becomes concentrated by sublimation, creating a dolerite-rich debris cap. Glacial ice, which may contain ancient atmospheric records, is preserved beneath the debris. We sampled 13 surface dolerite boulders from the head to the terminus of the rock glacier. Magmatic helium was identified by crushing clinopyroxene grains under vacuum 3 samples, yielding gas from broken inclusions with He-3/He-4 ratios of 0.11 to 0.45 R/Ra. Fusion ratios for these samples are 0.24 to 1.77 R/Ra. The magmatic component was also quantified by replicate fusion analyses of aliquots from the same mineral separates. These yielded He-3 and He-4 concentrations that plot along linear mixing lines. Since cosmogenic He-3 is the same in these replicates, this behavior must be due to trapped contributions. Thus, He-3 concentrations seem controlled by both cosmogenic production and irregularly distributed gas-rich inclusions. Radiogenic He-4 is also present. Magmatic and radiogenic He creates large He-3 exposure age errors in some samples. For example, two samples near the head of the glacier had apparent ages of 280 ka and 400 ka. After correction, these exposure ages were reduced to about 9 ka and 20 ka. Corrected ages from the rock glacier terminus are about 200 ka.
DE: 1035 Geochronology
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1827 Glaciology (1863)
DE: 1863 Snow and ice (1827)
DE: 9310 Antarctica
SC: H
MN: 2001 AGU Fall Meeting


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