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

Original language | English |
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Pages (from-to) | 255-264 |

Number of pages | 10 |

Journal | Chemical Geology |

Volume | 396 |

DOIs | |

Publication status | Published - Mar 9 2015 |

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### Keywords

- U-Th internal isochron
- Acid leaching
- α-Recoil

### ASJC Scopus subject areas

- Geochemistry and Petrology
- Geology

### Cite this

*Chemical Geology*,

*396*, 255-264. https://doi.org/10.1016/j.chemgeo.2014.12.025