Considering the Complexities of U-Pb Geochronology at the Millennial Scale

Abstract:

Recent advances in zircon U-Pb geochronology can yield radioisotopic ²⁰⁶Pb/²³⁸U dates with nominal precisions that approach the thousand year level for young samples, most often achieved by pooling several analyses. These advances are prompting a reevaluation of a number of aspects of the geology and the radioisotopic systematics of zircon that are only relevant at the scale of thousands of years, and must be addressed in order to extract a meaningful age that reflects a geological event. For example, in young volcanic rocks, individual crystals may take more than 1 ka to crystallize and growth events may take place over 10s or 100s of ka prior to an eruption. In order to best leverage this chronological information for stratigraphic purposes, e.g., where eruption is the key, we require tools to predict the timescales of zircon crystallization relative to eruption and to quantitatively extract this from measured dates. In this context it is also important to study the petrographic and petrologic relations of zircon contained in volcanic rocks. Recent studies suggest that, in some cases, the majority of zircon crystals are armored and thus cease to crystallize prior to an eruptive event. In addition to geological complexities, the systematics of short lived radionuclides with respect to initial disequilibrium and open system, including ²³⁰Th, ²²⁶Ra, ²²²Rn and ²³⁴U, are not fully understood, and may contribute to uncertainty in ²⁰⁶Pb/²³⁸U dates at timescales of 1 ka or greater. Potential additional effects from a subtle miscalibration of l²³⁸U have no bearing when ²⁰⁶Pb/²³⁸U ages are compared internally but must be considered for comparison with ages derived from other isotopic systems (most prominently ⁴⁰Ar/³⁹Ar). Efforts are currently underway to address some of these issues whereas others have been neglected. The combined effects of these limiting factors have yet to be fully understood, but must be quantified before analytical precision and accuracy of a reported weighted mean ²⁰⁶Pb/²³⁸U can be adequately translated to the age of a geological event such as a volcanic eruption.