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., > 0.5 Ma) basaltic samples by 6 M 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.