WHITE MICA LATTICE DEFECTS GOVERN MULTIPATH DIFFUSION OF SR AND 40AR

In-situ Rb/Sr and ⁴⁰Ar/³⁹Ar geochronology was conducted on white mica with optically distinct grain sizes and deformation intensities from four paragneisses of the Seve Nappe Complex, Scandinavian Caledonides. Mica textures range from undeformed inclusions in plagioclase to weakly deformed porphyroblasts surrounded by neoblasts, isolated white mica fish, and porphyroblasts with localized grain-scale kinking. Rb/Sr and ⁴⁰Ar/³⁹Ar dates are c. 445-440 Ma and c. 430-425 Ma, respectively, representing cooling events of the Seve Nappe Complex through upper amphibolite then lower amphibolite/greenschist facies temperatures. Deformation of the rocks is interpreted to have occurred before, or during, the first of the two events. Thus, the dates record closure of Sr then ⁴⁰Ar between the white mica and corresponding bulk rock volumes during cooling of the rocks. However, we do not observe a correlation between grain size and date, as predicted by volume diffusion theory. To examine the potential diffusion pathways for Sr and ⁴⁰Ar, select porphyroblasts/neoblasts, white mica fish, and kinked porphyroblasts were studied in detail using back-scattered electron diffraction and electron channeling contrast imaging. The examined white mica all contain grain- to µm-scale, undulose misorientation domains, which is evidence for crystal plasticity. Grain-scale domains have a maximum misorientation of ~48° relative to the grain average. The µm-scale undulations are locally delimited by both cleavage-normal low-angle grain boundaries (2–10°) and cleavage-parallel fractures. The low-angle grain boundaries are interpreted as µm-kink bands boundaries and cleavage fractures are interpreted as manifestations of stacking faults that formed in response to dislocation glide. The µm-kink band rotation axes are variable and depend on the orientation of the lattice relative to the principal stress axes. The results demonstrate the sensitivity of white mica to develop crystal lattice defects, signalling that Rb/Sr and ⁴⁰Ar/³⁹Ar dates from metamorphic white mica may best be explained by multipath diffusion rather than volume diffusion.

Research funded by NCN grant no. UMO-2021/40/C/ST10/00264 (C.J. Barnes), and “Juan de la Cierva” Fellowship JFJC2021-047505-I by MCIN/AEI/10.13039/501100011033 and CSIC (M. Bukała)