Mathematics – Logic
Scientific paper
Oct 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984gecoa..48.1973b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 48, Issue 10, pp.1973-1985
Mathematics
Logic
11
Scientific paper
Ground waters in a Precambrian granitic batholith at the Whiteshell Nuclear Research Establishment (WNRE) in Pinawa, Manitoba contain between 5 × 10 -5 and 10 -1 cc STP/g H 2 O of radiogenic helium-4 but have relatively uniform 3 He/ 4 He ratios of between 0.6 × 10 -8 and 2.3 × 10 3 . The highest helium samples also contain radiogenic 21,22 Ne produced by ( , n ) or ( n , ) reactions with other isotopes. As much as 1.8 × 10 -9 cc STP / g H 2 O of excess 21 Ne and 3.8 × 10 -9 cc STP / gH 2 O of excess 22 Ne have been measured. Helium and 21 Ne ages of these ground waters, calculated on the basis of known crustal production rates of 4 He and 21 Ne, are unreasonably high (up to 2 × 10 5 years) and incompatible with the 14 C ages and other isotopic and hydrogeologic data. Uranium enrichment in the flow porosity of the granite may dominate 4 He and 21,22 Ne production in this granite and mask the contributions from more typical U and Th concentrations in the rock matrix. At the Chalk River Nuclear Laboratories in Ontario helium concentrations in ground waters in a Precambrian monzonitic gneiss range from 1.5 × 10 -7 to 8.7 × 10 -4 cc STP / g H 2 O with the 3 He/ 4 He ratios ranging from 2.0 × 10 -3 to 1.5 × 10 -7 . The highest helium concentrations may be attributable to the presence of a thick uraniferous pegmatite vein and yield helium ages more than two orders of magnitude higher than the 14 C ages. Application of He age dating equations to ground waters from Precambrian granitic rocks requires knowledge of the nature of uranium and thorium enrichment in the subsurface in order to select appropriate values for porosity and uranium and thorium concentration in the rock.
Bottomley Dennis J.
Clarke W. B.
Ross David J.
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