Nonmonotonic thermal histories and contrasting kinetics of multiple thermochronometers

Abstract

Thermochronologic data are typically interpreted as point- or path-wise constraints on monotonic cooling histories. This is at least partly because allowing for the possibility of nonmonotonic thermal histories both precludes straightfoward use of closure temperature ( T c ) concepts and introduces ambiguities in modeling continuous time–temperature paths from multiple thermochronometers or closure profile, multi-domain, or fission-track length data. However, the monotonic cooling assumption severely limits the ability to elucidate reheating episodes with potentially important geologic significance. Here I show that in some cases multiple thermochronometers with contrasting kinetic properties can be used to both diagnose reheating events and constrain their duration and temperature. Thermochronometric systems with varying activation energies display “kinetic crossovers,” whereby relative diffusivities are reversed at certain temperatures. For reheating events of certain durations and temperatures, this results in “inverted” ages, whereby systems with higher nominal T c s have younger ages than systems with lower T c s. However, even if reheating does not cause age inversion, in cases where two systems are partially reset and constraints on the timing of a reheating event are available, the relative fractional resetting extents can be inverted to estimate a square-pulse equivalent duration and temperature of the reheating event. Here I outline this approach for diagnosing nonmonotonic thermal histories and for deducing features of reheating events from thermochronometric data, and review several examples from previously published data that illustrate its use and potential in a range of applications.