| Title | Numerical models of P-T, time and grain-size controls on Ar diffusion in biotite: an aide to interpreting 40Ar/39Ar ages |
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| Author | Skipton, D R; Warren, C J; Hanke, F |
| Source | Chemical Geology vol. 496, 2018 p. 14-24, https://doi.org/10.1016/j.chemgeo.2018.06.005  Open Access |
| Year | 2018 |
| Alt Series | Natural Resources Canada, Contribution Series 20180099 |
| Publisher | Elsevier BV |
| Document | serial |
| Lang. | English |
| Media | paper; on-line; digital |
| File format | pdf (Adobe® Reader®); html; xlsx (Microsoft® Excel®) |
| Subjects | geochronology; 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 |
| Illustrations | phase diagrams; graphs |
| Program | GEM2: Geo-mapping for Energy and Minerals Mary River North Baffin |
| Released | 2018 06 15 |
| Abstract | Dating 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. |
| GEOSCAN ID | 308325 |
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