Structural and chemical resetting processes in white mica and their effect on K-Ar data during low temperature metamorphism

Akker, Ismay Vénice; Berger, Alfons; Zwingmann, Horst; Todd, Andrew; Schrank, Christoph E.; Jones, Michael W.M.; Kewish, Cameron M.; Schmid, Timothy C.; Herwegh, Marco (2021). Structural and chemical resetting processes in white mica and their effect on K-Ar data during low temperature metamorphism. Tectonophysics, 800, p. 228708. Elsevier 10.1016/j.tecto.2020.228708

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White mica has been widely used to date microstructures and tectonic events in faults, shear-zones and folds because of its suitability for radiogenic dating. However, complex (i) microstructural evolution, (ii) individual chemical evolution of the K-bearing phases, (iii) mixing of ‘detrital grains’ with newly formed and/or recrystallized or chemically reset grains as well as (iv) volume diffusion may result in apparent K-Ar ages. Here, specimens from a prograde sediment sequence of the exhumed fossil European Alpine accretionary wedge were used to investigate resetting processes of white mica by the type and intensity of deformation as well as peak metamorphic conditions. We combine the K-Ar system with mass and mineral quantities from grain size fractions to calculate the amount of recrystallized white mica in each grain size fraction along the metamorphic gradient. Increasing recrystallization with increasing metamorphic grade is related to thermally activated pressure solution and dissolution-precipitation creep, as seen by the formation of a spaced cleavage of recrystallized phyllosilicates documented through Synchrotron X-ray Fluorescence Microscopy and Scanning Electron Microscope imaging techniques. Increasing recrystallization by dissolution-precipitation processes induces chemical resetting of the isotopic system, resulting in a prograde decrease of apparent K-Ar ages. We demonstrate that Ar volume diffusion does not play a significant role for the low-temperature samples, promoting recrystallization as the important physico-chemical process for age resetting. However, white mica chemistry reveals that no simple relation between isotopic resetting and grain size exists along the prograde path. Reliable age information can therefore only be obtained in the case of (nearly) complete resetting, which accounts only for the smallest grain size fraction at the highest metamorphic temperature. These findings could shed new light on accurate dating of mica-rich fault rocks, where the time constraints depend not only on the temperature, but also on the amount and type of deformation.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences
08 Faculty of Science > Institute of Geological Sciences > Tectonics

UniBE Contributor:

Akker, Ismay Vénice; Berger, Alfons; Schmid, Timothy Chris and Herwegh, Marco

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

0040-1951

Publisher:

Elsevier

Funders:

Organisations 162340 not found.

Language:

English

Submitter:

Ismay Vénice Akker

Date Deposited:

23 Feb 2021 14:49

Last Modified:

11 Mar 2021 17:12

Publisher DOI:

10.1016/j.tecto.2020.228708

Uncontrolled Keywords:

Microstructure Phyllosilicates K-Ar Recrystallization Metamorphism

BORIS DOI:

10.48350/152530

URI:

https://boris.unibe.ch/id/eprint/152530

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