Origin of Great Unconformity Obscured by Thermochronometric Uncertainty

Abstract. Thermochronology provides a unique perspective on the magnitude of rock that is eroded during, and the timing of, unconformities in the rock record. Recently, thermochronology has been used to stoke a long-standing debate about the origin of the Great Unconformity, a global erosional event that represents a time period of almost a billion years at the end of the Precambrian. The (U–Th)/He in zircon system is particularly well suited to provide this perspective because it is very sensitive to long durations of time at relatively low temperatures (< 200–250 °C). However, the diffusion kinetics of 4He in zircon change dramatically as a result of radiation damage to the crystal lattice. Therefore, our ability to resolve thermal histories is fundamentally limited by how well we know parameters controlling helium diffusion and their uncertainties. Currently, there is no estimate of how these uncertainties impact the inferred thermal histories. Here we determine uncertainties in the Zircon Radiation Damage and Annealing Model (ZRDAAM, Guenthner et al. 2013) that describes changes in 4He diffusion kinetics as a function of radiation damage. We show that the dispersion in predicted zircon (U-Th)/He ages for a given thermal history can be 100s Ma for a specific amount of radiation damage and that thermal histories are less well resolved than previously appreciated. Additional diffusion experiments and calibration with natural laboratories would provide better constraints on diffusion kinetic parameters.