Variable Sr Diffusion and Implications for Rb–Sr Biotite Geochronology

ABSTRACT The rubidium–strontium (Rb–Sr) system has a long pedigree as a geochronological tool that relies on the application of an isochron approach. In particular, biotite contains strongly radiogenic Rb–Sr isotopes, and thus can yield meaningful crystallisation or cooling ages via this approach. However, the response of the Rb–Sr system in biotite to multiple tectonothermal events, and interpretation of the resulting apparent ages, requires a nuanced understanding of the degree to which isotopic equilibration has been attained. Here, we investigate biotite in metapelitic rocks from the Fraser Zone, Albany–Fraser Orogen, Western Australia, a fault‐bound block of Mesoproterozoic crust. Using a combination of phase equilibrium modelling, in situ laser ablation Rb–Sr analyses of biotite and U–Pb monazite dating, we show that these rocks reached peak metamorphic conditions of 830°C–875°C and 7.8–11.3 kbar at 1280 ± 5 Ma, during late Stage I of the Albany Fraser Orogeny. Rb–Sr biotite model ages, ages calculated from single analyses and a set initial 87 Sr/ 86 Sr i , reveal a direct relationship to their petrographic setting; grains associated with quartz–K‐feldspar segregations have lower 87 Sr content and younger model ages than grains within biotite‐ and sillimanite‐rich bands. The model ages imply that biotite closed to Sr isotope exchange at different times depending on petrological context. Ages of c. 1200 Ma associated with biotite–sillimanite matrix bands likely record cooling through 300°C–400°C during Stage II of the Albany Fraser Orogeny, whereas younger, c. 1100 Ma Rb–Sr biotite ages are associated with leucosome segregations and were influenced by later Sr loss. The presence of variable apparent ages from the same fabric within single samples implies differential Sr equilibration controlled by those minerals in contact with biotite.