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Using white mica 40 Ar/ 39 Ar data as a tracer for fluid flow and permeability under high‐ P conditions: Tauern Window, Eastern Alps
Author(s) -
WARREN C. J.,
SMYE A. J.,
KELLEY S. P.,
SHERLOCK S. C.
Publication year - 2012
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.2011.00956.x
Subject(s) - geology , greenschist , mylonite , muscovite , lithology , metamorphic rock , geochemistry , mineralogy , metamorphic facies , eclogite , mica , facies , quartz , shear zone , geomorphology , subduction , tectonics , structural basin , seismology , paleontology
New single‐grain‐fusion muscovite and paragonite 40 Ar/ 39 Ar data from eclogite and blueschist units exposed in the Tauern Window, Eastern Alps yield a range of apparent ages from 90 to 23 Ma. These apparent ages are generally older than expected for 40 Ar/ 39 Ar cooling ages, given constraints from other geochronological systems such as Rb–Sr and U–Pb. Numerical Ar‐in‐muscovite diffusion models for Tauern Window nappe P–T paths in an open system suggest that 40 Ar/ 39 Ar ages should lie between 29 and 24 Ma, and that they should constrain cooling and decompression following the post‐high pressure Barrovian overprint. The measured ranges of apparent 40 Ar/ 39 Ar dates suggest that the assumption of open system behaviour is not valid for this region. The local and/or regional generation of fluid during exhumation promoted pervasive recrystallization of high pressure lithologies throughout the Tauern Window to greenschist and amphibolite facies assemblages. The old apparent 40 Ar/ 39 Ar white mica dates in all lithologies are therefore interpreted as being due to inefficient removal of grain boundary Ar by the grain boundary fluids during the Barrovian overprint, due to high Ar concentrations or limited connectivity or both. This caused spatially (mm‐scale) and temporally variable fluxes of Ar out of, and probably into, white mica in both metasedimentary and metabasic lithologies.