Uncertainty in helium diffusion in zircon limits thermochronologic resolution: application to the Great Unconformity

Abstract. Thermochronology provides a unique perspective on the timing and magnitude of erosion during the generation of unconformities. Recently, thermochronology has been used to reinvigorate a long-standing debate about the origin of the Great Unconformity, a geological feature representing a period of missing time of almost a billion years from the stratigraphic record at the end of the Precambrian. Our ability to resolve thermal histories is fundamentally limited by how well we know parameters describing the temperature sensitivity of a thermochronometric system. The (U-Th)/He in zircon system is particularly well suited to examine the origin of the Great Unconformity because it is very sensitive to long durations of time at relatively low temperatures (< 200–250 °C). 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 accumulation and annealing. We show that the dispersion in predicted zircon (U-Th)/He ages for a given thermal history can be 100's of Ma for a specific amount of radiation damage highlighting 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.