What are we dating, volume diffusion or recrystallisation? Isotopic modelling and trace-element analysis as tools to interpret the high-temperature U-Pb thermochronometer in apatite and rutile

The availability of high-temperature thermochronometers suitable for generation of continuous thermal histories at mid- to lower-crustal temperatures (i.e., ≥ 400 °C) is limited. Available thermochronometers include the recently developed apatite and rutile U-Pb thermochronometers ( ≤ 550 and 640 °C; Kooijman et al., 2010; Cochrane et al., 2014) and arguably the K-Ar system in white mica (sensitive to temperatures ≤ 500 °C. Recent work has focussed on micro-beam U-Pb analysis of apatite and rutile by sector-field and multi-collector LA-ICPMS to generate single-crystal U-Pb age profiles. Such profiles can be inverted to yield continuous thermal histories for high-temperature processes (e.g., Smye et al., 2018). However, both apatite and rutile can exhibit crystal growth and dissolution-reprecipitation reactions in the same temperature ranges at which measurable Pb diffusion occurs: neither behaves as a pure thermochronometer in all circumstances (e.g., Chambers and Kohn, 2012; Harlov et al., 2005). Thus, it is critical to develop protocols which unequivocally identify age profiles arising from volume diffusion.

Here, we present case studies from greenschist- to granulite-facies-grade metamorphic systems from the Eastern Alps and the Western Gneiss Region of Norway. We demonstrate the utility of trace-element analysis (Sr-Y-REE-Th-U) and isotopic forward modelling to discriminate age resetting arising from (re)crystallisation from diffusion. Both rutile and especially apatite routinely incorporate non-trivial amounts of common-Pb during crystallisation (as opposed to radiogenic Pb generated by in-situ radionuclide decay), rendering them discordant in U-Pb isotope space. This common-Pb must be corrected for during age calculation. However, common-Pb is isotopically distinct from radiogenic Pb but exhibits the same diffusion behaviour, so the predicted U-Pb isotopic distribution for a given crystal arising from a proposed thermal history can be estimated by isotopic forward modelling. Thus, common-Pb can be exploited to validate both the assumption of Pb-loss by volume diffusion, and the thermal history predicted by age profile inversion.

Chambers, J.A., & Kohn, M.J., Am. Mineral., 97, 543–555 (2012); Cochrane, R., et al., Geochim. Cosmochim. Acta, 127, 39–56, (2014); Harlov, D.E., et al., Contrib. Mineral. Petrol, 150, 268–286 (2005); Kooijman, E., et al., Earth Planet. Sci. Lett, 293, 321–330, (2010); Smye, A.J., et al., Chem. Geol., 494, 1–18 (2018).