Other
Scientific paper
May 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989gecoa..53.1025s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 53, Issue 5, pp.1025-1034
Other
3
Scientific paper
Whole rock samples of hydrothermally altered Biscuit Basin rhyolite from Yellowstone drill cores Y-7 and Y-8 were analyzed for 226 Ra and 230 Th to determine the extent of radioactive disequilibrium and its relation to the rates and mechanisms of element transport in the shallow portion of an active hydrothermal system. The ( 226 Ra/ 230 Th) activity ratios range from 0.73 to 1.46 and are generally correlated with Th-normalized Ba concentrations (Ba N ). Values of ( 226 Ra/ 230 Th) and Ba N > 1 were found in samples containing large modal fractions of clinoptilolite; whereas values of ( 226 Ra/ 230 Th) and Ba N < 1 were found in samples containing large modal fractions of mordenite. Composition clinoptilolite and mordenite in these samples are consistent with ion exchange equilibrium between zeolites and coexisting thermal waters. Average K d mineral-water Ba values are 1.0·10 5 mL/g for clinoptilolite and 1.4·10 4 mL/g for mordenite. Apparent diffustvities through matrix porosity estimated for Ra and Ba range from ~10 -12 to ~10 -10 cm 2 s -1 in thoroughly zeolitic rhyolite; these rates of diffusion are too low to account for the observed distance scale of ( 226 Ra/ 230 Th) disequilibrium. The correlated values of ( 226 Ra / 230 Th) disequilibrium and Ba N represent zeolite-water ion exchange equilibrium that is caused by porous flow of water through the rock matrix and by the relatively rapid diffusion of cations within the zeolite lattices. A water flux of at least ~2.5 (cm water 3 /cm rock 3 ) yr -1 is required to produce measurable ( 226 Ra/ 230 Th) disequilibrium, whereas at least~23 (cm water 3 /cm rock 3 ) yr -1 is r for the sample exhibiting the most extreme ( 226 Ra/ 230 Th) disequilibrium; these fluxes are much higher than those that can be inferred from net mass transfers of stable species. The zeolite-water ion exchange process appears to have been operating for at least 8000 yr in the environment of the Y-7 and Y-8 drill holes.
Binz Carl M.
Böhlke J. K.
Sturchio Neil C.
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