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TitleNumerical models of P-T, time and grain-size controls on Ar diffusion in biotite: an aide to interpreting 40Ar/39Ar ages
AuthorSkipton, D R; Warren, C J; Hanke, F
SourceChemical Geology vol. 496, 2018 p. 14-24, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20180099
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf (Adobe® Reader®); html; xlsx (Microsoft® Excel®)
Subjectsgeochronology; mathematical and computational geology; radiometric dating; argon argon dating; biotite; pressure-temperature conditions; grain size analyses; traps; morphology, crystal; crystallization; recrystallization; fluid dynamics; models; tectonic setting; metamorphism; orogenies; thermal history; Methodology
Illustrationsphase diagrams; graphs
ProgramGEM2: Geo-mapping for Energy and Minerals Mary River North Baffin
Released2018 06 15
AbstractDating of biotite using the 40Ar/39Ar method is used extensively to determine the timing of cooling and exhumation in metamorphic rocks. Interpretations of 40Ar/39Ar dates commonly assume that 40Ar 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 40Ar/39Ar 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 40Ar/39Ar dates and intra-grain Ar distributions in a perfect open system. The model results are useful for constraining cooling and exhumation histories from 40Ar/39Ar biotite data in a variety of metamorphic settings. They also provide baseline data for biotite 40Ar retention, 40Ar/39Ar 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 40Ar/39Ar 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 40Ar/39Ar biotite ages, especially when linked with intra-grain 40Ar/39Ar age profiles.
Summary(Plain Language Summary, not published)
Isotopic analysis of the mineral biotite is often used to determine the age of low-temperature (~300°C) events that affect rocks, including the timing of uplift and cooling of the rocks from deep in the Earth's crust. An age can be calculated based on measurements of the isotopic ratio of argon (40Ar/39Ar) in biotite, based on the known temperature-controlled diffusion behaviour of argon in biotite. In this study, we present a series of numerical models of diffusion of argon (Ar) in biotite that show the percentage of Ar that should theoretically be retained in biotite grains of different sizes, residing in the Earth's crust for various periods over a range of pressure-temperature conditions. The model results are useful for constraining low-temperature geological histories from 40Ar/39Ar biotite data in a variety of geological settings.

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