BAYESIAN AGE-SEQUENCE MODELING REFINES PETROCHRONOLOGIC DATES AND RATES

Constraining the absolute time and duration of geologic processes is one of the great challenges and goals in Earth sciences. Increasingly, the integration of geochronologic constraints with petrologic information is being applied to understand the timescales of metamorphic, igneous, tectonic, and fluid-related processes. These developments have been largely facilitated by analytical advances that permit the analysis of smaller analytical volumes and the collection of complimentary trace element/isotopic data. However, an enduring challenge is resolving the ages and timescales of relatively short-lived processes and events within the uncertainty of these in-situ analytical techniques (~1-2%). Many geochronometers, including monazite, xenotime, zircon, titanite, and garnet, preserve relative age constraints in the form of compositional zoning and/or petrologic context. This information can be leveraged in a Bayesian framework (age-sequence modeling) which combines age constraints (likelihoods) with prior information to generate a probabilistic posterior chronology. Calculations carried out in Isoplot 4.0 provide enhanced precision on geochronologic dates and rates, as highlighted by several applications. Bayesian modeling of complex, concentrically zoned monazite from the Amherst block in the northern Appalachian orogen significantly reduced uncertainties (up to 40-70%). Pairing monazite compositions with coexisting xenotime yielded a detailed temperature-time history, resolving heating and cooling associated with the Acadian (~405 Ma) and Neoacadian (~380 Ma) orogenies. Application to zoned monazite from an ultra-high temperature granulite sample from the southern Trans-Hudson orogen yielded durations of 0.5⁺⁹/_-0.4 Ma and 20⁺⁵/_-8 Ma for biotite dehydration melting and suprasolidus conditions, respectively. The relatively short intervals of heating and peak conditions are consistent with UHT metamorphism in a back-arc tectonic setting. Finally, we pair age-sequence modeling with Bayesian step-change analysis to constrain the duration of garnet stability in metamorphic rocks from the New England Appalachians. We propose that age-sequence modeling represents a powerful new tool that can be widely applied to refine the absolute time and duration of geologic processes.