Abstract Details
(2020) U-Pb Geochronology and Geochemical Characteristics of the Oritate Travertine, Toyama Prefecture, Japan
Fukuyama M & Ogasawara M
https://doi.org/10.46427/gold2020.769
06n: Room 2, Wednesday 24th June 23:39 - 23:42
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Submitted by Yuri Amelin on Wednesday 24th June 12:27
Waters from which continental carbonate minerals crystallise can be in strong disequilibrium of U and Th decay chain. In particular, a significant excesses 234U relative to 238U is common. In your U-Pb dating, do you assume the system to be at secular equilibrium at the time of formation? How do you deal with the possibility of disequilibrium?
Thank you for your comments. Yes, we should consider excess U in the decay chain as in the case of natural aqueous systems, U is slightly soluble, but Th is highly insoluble. We monitor 232Th as for U-Th-Pb dating measurement, we could not detect 232Th above detection limit of our measurement. Thus we assume at the time of carbonate deposition there was no 234Th which is an intermediate isotope of 238U chain. However the system could return near secular equilibrium by seven or ten times of the half-life of 234Th (about 500 kyr). As our carbonate is 60 Ma, we consider that our sample is in near secular equilibrium and degree of the disequilibrium is below our detection limit.
Waters from which continental carbonate minerals crystallise can be in strong disequilibrium of U and Th decay chain. In particular, a significant excesses 234U relative to 238U is common. In your U-Pb dating, do you assume the system to be at secular equilibrium at the time of formation? How do you deal with the possibility of disequilibrium?
Thank you for your comments. Yes, we should consider excess U in the decay chain as in the case of natural aqueous systems, U is slightly soluble, but Th is highly insoluble. We monitor 232Th as for U-Th-Pb dating measurement, we could not detect 232Th above detection limit of our measurement. Thus we assume at the time of carbonate deposition there was no 234Th which is an intermediate isotope of 238U chain. However the system could return near secular equilibrium by seven or ten times of the half-life of 234Th (about 500 kyr). As our carbonate is 60 Ma, we consider that our sample is in near secular equilibrium and degree of the disequilibrium is below our detection limit.
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