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| Titel |
Re-evaluating the closure temperature concept in metamorphic rocks: when does 40Ar/39Ar dating constrain exhumation? |
| VerfasserIn |
Clare J. Warren, Felix Hanke, Simon P. Kelley |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052300
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| Zusammenfassung |
| The determination of metamorphic cooling/exhumation rates using the apparent
40Ar/39Ar age of muscovite, important for underpinning tectonic models, is based
on several simplifying assumptions including the concept of the Dodson closure
temperature. These assumptions are shown to be invalid in a significant number of
metamorphic scenarios where Ar diffusion is hindered, for example under certain
pressure-temperature (PT) conditions, or when removal of Ar from the rock volume is
inefficient due to low fluid contents and low permeability. Using numerical diffusion models
which include a recently reported significant pressure dependence of Ar diffusion in
muscovite, we systematically interrogate all the assumptions associated with 40Ar/39Ar
dating. A simple graphic method for evaluating the PT regions in which 40Ar/39Ar
dates could meaningfully constrain the timing of exhumation in an open system
is presented. The link between apparent 40Ar/39Ar age and traditional “Dodson
closure temperature” is found to be valid only when muscovite crystallizes under, or
subsequently reaches, high temperature and relatively low pressure conditions in an
open system. We show that open system behavior may be most reliably verified by
comparing theoretical and measured 40Ar/39Ar age profiles across muscovite grains. We
suggest that the most robust method for determining whether metamorphic muscovite
40Ar/39Ar ages relate to the timing of exhumation involves: (1) the determination of
the pressure, temperature and time (PTt) conditions of muscovite crystallization,
(2) the collection of high-precision and high spatial resolution 40Ar/39Ar profiles
across muscovite and (3) the comparison between analytical data and numerical
diffusion models. The results and discussion presented here for muscovite are equally
applicable to other metamorphic minerals commonly dated using the 40Ar/39Ar system. |
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