11 Oct 2022
11 Oct 2022
Status: this preprint is currently under review for the journal GChron.

Origin of Great Unconformity Obscured by Thermochronometric Uncertainty

Matthew Fox1, Adam G. G. Smith1, Pieter Vermeesch1, Kerry Gallagher2, and Andrew Carter3 Matthew Fox et al.
  • 1London Geochronology Centre, Department of Earth Sciences, University College London, Gower St., London, WC16BT, UK
  • 2Géosciences Rennes/OSUR, University of Rennes, Rennes, France
  • 3London Geochronology Centre, Department of Earth Sciences, Birkbeck College, Gower St., London, WC16BT, UK

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.

Matthew Fox et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gchron-2022-23', Alexis Ault, 14 Nov 2022
  • CC1: 'Comment on gchron-2022-23', Kalin McDannell, 16 Nov 2022
  • RC2: 'Comment on gchron-2022-23', Kip Hodges, 21 Nov 2022
  • RC3: 'Comment on gchron-2022-23', Rebecca Flowers, 22 Nov 2022
  • RC4: 'Systematic uncertainty and thermochronology of the Great Unconformity? A review of Fox et al. 2022, gchron-2022-23', Brenhin Keller, 22 Nov 2022
  • RC5: 'Comment on gchron-2022-23', William Guenthner, 28 Nov 2022

Matthew Fox et al.

Matthew Fox et al.


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Short summary
The Great Unconformity represents an enormous amount of time lost from the sedimentary record. Its origin is debated, in part, due to different approaches used to interpret zircon (U–Th)/He ages. This thermochronometric system is ideal for this problem because the temperature sensitivity varies according to radiation damage. Here we explore the uncertainty associated with the radiation damage model and show how this limits our ability to resolve the origin of the Great Unconformity.