LAUSR

Abstract Dating of biotite using the 40 Ar/ 39 Ar method is used extensively to determine the timing of cooling and exhumation in metamorphic rocks. Interpretations of 40 Ar/ 39 Ar dates commonly assume that 40 Ar diffuses out of biotite through temperature-dependent volume diffusion, and therefore that the date represents the time at which biotite cooled through a closure temperature. Several processes, however, may perturb Ar systematics such that the 40 Ar/ 39 Ar date does not uniquely represent the timing of cooling through a closure temperature, including incomplete re-setting of Ar systematics, incorporation of excess Ar, crystal defects acting as Ar traps or fast-pathways, or fluid-present recrystallization/dissolution. We present a series of numerical diffusion model results that show the percentage of radiogenic Ar that should theoretically be retained in biotite with different grain radii residing for various periods over a range of P–T conditions, in a perfect open system that loses Ar via volume diffusion alone. A second set of models demonstrates the effects of different cooling rates on biotite 40 Ar/ 39 Ar dates and intra-grain Ar distributions in a perfect open system. The model results are useful for constraining cooling and exhumation histories from 40 Ar/ 39 Ar biotite data in a variety of metamorphic settings. They also provide baseline data for biotite 40 Ar retention, 40 Ar/ 39 Ar ages and intra-grain age distributions that would theoretically be produced from volume diffusion acting alone. Consequently, the models can help evaluate the plausibility of alternative scenarios that may have affected biotite 40 Ar/ 39 Ar dates, including extraneous Ar contamination or Ar loss via processes other than diffusion. In conjunction with well-constrained petrogenetic histories, numerical diffusion models are a powerful tool for interpreting 40 Ar/ 39 Ar biotite ages, especially when linked with intra-grain 40 Ar/ 39 Ar age profiles.