Evaluation of the applicability of acid leaching for the 238U–230Th internal isochron method

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書誌事項

公開日
2015-03
資源種別
journal article
権利情報
  • https://www.elsevier.com/tdm/userlicense/1.0/
  • https://www.elsevier.com/legal/tdmrep-license
DOI
  • 10.1016/j.chemgeo.2014.12.025
公開者
Elsevier BV

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説明

Abstract The 238 U– 230 Th internal isochron method has great potential for determining eruption ages of young volcanic materials if the separated groundmass phases have a sufficiently wide range of U/Th ratios. We examined the fractionation behavior of U and Th in aged (i.e., > 0.5 Ma) basaltic samples by 6 M hydrochloric acid leaching to evaluate the applicability of acid-leaching treatment for the 238 U– 230 Th internal isochron method. Acid leaching of aged basaltic rocks in 238 U– 234 U– 230 Th secular equilibrium at the bulk-rock scale results in 230 Th– 238 U and 234 U– 238 U radioactive disequilibria for both leachates and residues. These radioactive disequilibria can be explained by redistribution of 234 Th (parent of 234 U) and 230 Th between acid-soluble and acid-resistant phases due to α-recoil. The number of 230 Th atoms redistributed by α-recoil can be calculated by using a mass conservation equation for 234 U atoms and by the relative amount of recoiled 230 Th and 234 Th, the latter proportional to the kinetic energy of the recoiled nuclide. When the fraction of daughter nuclide 234 U remaining in either the residue or leachate, after α-recoil redistribution of 238 U, is large enough (> 95%), the corrected ( 230 Th/ 238 U) values of leachate and residue show radioactive equilibria. This result demonstrates that preferential fractionation between U and Th does not occur during acid leaching for basaltic samples if there is no selective etching of the α-recoil track. This study implies that acid-leaching can be used in conjunction with the 238 U– 230 Th internal isochron method for dating young volcanic rocks by evaluating the degree of the α-recoil redistribution of 234 U.

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