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

The ²³⁸U-²³⁰Th 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., >0.5Ma) basaltic samples by 6M hydrochloric acid leaching to evaluate the applicability of acid-leaching treatment for the ²³⁸U-²³⁰Th internal isochron method. Acid leaching of aged basaltic rocks in ²³⁸U-²³⁴U-²³⁰Th secular equilibrium at the bulk-rock scale results in ²³⁰Th-²³⁸U and ²³⁴U-²³⁸U radioactive disequilibria for both leachates and residues. These radioactive disequilibria can be explained by redistribution of ²³⁴Th (parent of ²³⁴U) and ²³⁰Th between acid-soluble and acid-resistant phases due to α-recoil. The number of ²³⁰Th atoms redistributed by α-recoil can be calculated by using a mass conservation equation for ²³⁴U atoms and by the relative amount of recoiled ²³⁰Th and ²³⁴Th, the latter proportional to the kinetic energy of the recoiled nuclide. When the fraction of daughter nuclide ²³⁴U remaining in either the residue or leachate, after α-recoil redistribution of ²³⁸U, is large enough (>95%), the corrected (²³⁰Th/²³⁸U) 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 ²³⁸U-²³⁰Th internal isochron method for dating young volcanic rocks by evaluating the degree of the α-recoil redistribution of ²³⁴U.