Mathematics – Logic
Scientific paper
Feb 1985
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985gecoa..49..337f&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 49, Issue 2, pp.337-347
Mathematics
Logic
10
Scientific paper
Iodine-129 ( t 1/2 , 16 My) is a naturally-occurring tracer which can be used to study hydrologic and geologic processes on time scales up to 100 My. The pre-bomb atmospheric ratio 129 I / I should have been constant in time and space and is the starting value in ground-water recharge. Subsequent ratio changes in ground water should be determined by isotope contributions from three sources: recharge water, iodine leached from the formation, and in situ uranium fission. The mathematical expression of these time-dependent processes forms the basis for dating and tracing applications. The expected behavior of natural 129 I in ground water is compared to results from field studies. Ground-water samples from the Great Artesian Basin, Australia, provide an estimate of the atmospheric equilibrium ratio, 6 × 10 -13 . Down-gradient changes in water up to 1 My old suggest that subsurface production can be significant. The usefulness of 129 I as an indicator of brine source and age is verified in brines collected in and around Louisiana salt domes. The method leads to ages of 7 and 9 My for two brine pockets trapped within Jurassic salt, and 32 to >40 My for oil-field brines in Miocene sands adjacent to the domes. The results demonstrate the capability of tandem accelerator mass spectrometry to measure as few as 10 7 atoms of 129 I in 10 mg I with 10% precision. Although such atom sensitivity can be achieved otherwise, the accelerator technique is necessary to measure naturally low pre-1945 cosmogenic isotope ratios.
Airey P. L.
Bentley H.
Elmore David
Fabryka-Martin J.
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