THE EFFECTS OF U-SERIES DISEQUILIBRIUM ON (U-TH)/HE CHRONOMETRY OF less than 1 MA SECONDARY FE-OXIDES

Secondary Fe-oxides are common precipitates of hydrothermal fluids present in or migrating through the shallow crust. Fe-oxides are targeted for (U-Th)/He investigation to constrain the timing and rates of geologic processes, including paleofluid circulation, fault slip, and/or exhumation. Recently, (U-Th)/He dating has been applied to Quaternary Fe-oxides, which is feasible because of high He productivity of U and Th decay chains paired with the precision of modern mass spectrometers. The standard equation used to calculate (U-Th)/He dates may not capture the true age of Quaternary samples because this equation assumes U and Th decay chains have maintained secular equilibrium throughout a sample’s history. Secular equilibrium requires the activity of all radioisotopes in a decay chain to be equal, a condition generally reached in closed systems after ~5 half-lives of the longest-lived intermediate daughter. ²³⁴U and ²³⁰Th are intermediate daughters of the ²³⁸U decay chain and are most relevant to secular equilibrium because they have the longest half-lives (246 and 75 kyr, respectively) of all intermediate daughters within U and Th decay chains. Parent fluids of Fe-oxides may not be in secular equilibrium with respect to the ²³⁸U decay series owing to differences in solubility between U and Th. Initial disequilibrium results in an excess and/or deficiency of radiogenic He that, in the absence of He diffusive loss, is maintained throughout the lifetime of a sample. Here, we apply a numerical approximation of the Bateman equation, which mathematically describes the abundances of isotopes in a decay chain, to explore the competing effects of initial ²³⁴U and ²³⁰Th disequilibrium. For example, the effect of disequilibrium on previously reported Pleistocene (~0.8-0.4 Ma) Fe-oxide (U-Th)/He dates ranges from a few percent to ~20 %, depending on the assumed initial abundance of ²³⁴U and ²³⁰Th. The modeled disequilibrium correction is comparable to the typical sample standard deviation of ~5-15%, which is influenced by grain size variation, aliquot F_T correction, and dating multiple mineralization events. Ongoing work will characterize the ²³⁴U and ²³⁰Th contents of hematite aliquots from previously dated samples to benchmark disequilibrium corrections.