Variation of illite/muscovite Ar-40/Ar-39 age spectra during progressive low-grade metamorphism: an example from the US Cordillera

Verdel, Charles, van der Pluijm, Ben A. and Niemi, Nathan (2012) Variation of illite/muscovite Ar-40/Ar-39 age spectra during progressive low-grade metamorphism: an example from the US Cordillera. Contributions to Mineralogy and Petrology, 164 3: 521-536.


Author Verdel, Charles
van der Pluijm, Ben A.
Niemi, Nathan
Title Variation of illite/muscovite Ar-40/Ar-39 age spectra during progressive low-grade metamorphism: an example from the US Cordillera
Formatted title Variation of illite/muscovite 40Ar/39Ar age spectra during progressive low-grade metamorphism: an example from the US Cordillera
Journal name Contributions to Mineralogy and Petrology   Check publisher's open access policy
ISSN 0010-7999
1432-0967
Publication date 2012-09
Sub-type Article (original research)
DOI 10.1007/s00410-012-0751-7
Volume 164
Issue 3
Start page 521
End page 536
Total pages 16
Place of publication Heidelberg, Germany
Publisher Springer
Collection year 2013
Language eng
Formatted abstract 40Ar/ 39Ar step-heating data were collected from micron to submicron grain-sizes of correlative illite- and muscovite-rich Cambrian pelitic rocks from the western United States that range in metamorphic grade from the shallow diagenetic zone (zeolite facies) to the epizone (greenschist facies). With increasing metamorphic grade, maximum ages from 40Ar/ 39Ar release spectra decrease, as do total gas ages and retention ages. Previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation/recrystallization of neo-formed illite. While recognizing the importance of these processes in evaluating our results, we suggest that the inverse correlation between apparent age and metamorphic grade is controlled, primarily, by thermally activated volume diffusion, analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments. Our results suggest that complete resetting of the illite/muscovite Ar thermochronometer occurs between the high anchizone and epizone, or at roughly 300 °C. This empirical result is in agreement with previous calculations based on muscovite diffusion parameters, which indicate that muscovite grains with radii of 0. 05-2 μm should have closure temperatures between 250 and 350 °C. At high anchizone conditions, we observe a reversal in the age/grain-size relationship (the finest grain-size produces the oldest apparent age), which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo-formed illite/muscovite crystallites typically surpass the size of detrital muscovite grains. It is also approximately the stage at which neo-formed illite/muscovite crystallites develop sufficient Ar retentivity to produce geologically meaningful 40Ar/ 39Ar ages. Results from our sampling transect of Cambrian strata establish a framework for interpreting illite/muscovite 40Ar/ 39Ar age spectra at different stages of low-grade metamorphism and also illuminate the transformation of illite to muscovite. At Frenchman Mtn., NV, where the Cambrian Bright Angel Formation is at zeolite facies conditions, illite/muscovite 40Ar/ 39Ar data suggest a detrital muscovite component with an apparent age ≥967 Ma. The correlative Carrara Fm. is at anchizone conditions in the Panamint and Resting Spring Ranges of eastern California, and in these locations, illite/muscovite 40Ar/ 39Ar data suggest an early Permian episode of subgreenschist facies metamorphism. The same type of data from equivalent strata at epizone conditions (greenschist facies) in the footwall of the Bullfrog/Fluorspar Canyon detachment in southern Nevada reveals a period of slow-to-moderate Late Cretaceous cooling.
Keyword Geochronology
Thermochronology
Illite/muscovite
Ar-40/Ar-39 dating
US Cordillera
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: School of Earth Sciences Publications
Official 2013 Collection
 
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