Multiple dates in millimetres; diffusion as an explanation for Rb-Sr age discrepancies in biotite

In situ analysis of the Rb–Sr isotopic composition of biotite via triple quadropole LA–ICPMS is an increasingly popular method for constraining the time through the Sr closure temperature in rocks. Although interpreting the radiogenic product can be complicated by various factors that can affect diffusion of Rb and Sr, the role of the different minerals that may be in contact with biotite in regard to local diffusion gradients is poorly understood. In this study, we show the importance of analysing Rb–Sr isotopic data in the context of detailed petrographic observations, which reveals that the ratios obtained are affected by various diffusion pathways between like material and minerals that preferentially incorporate Sr. The studied samples are metapelites from the Fraser Zone (Western Australia) that have peak metamorphic conditions of about 850 °C and 9 kbar and a history of cryptic biotite Ar-Ar ages of ~1205 Ma, which on face value could imply exhumation rates that are some of the fastest recorded in Earth’s history. However, new biotite data from in-situ Rb-Sr analysis highlights differences in Sr retentivity. While calculated isochrons may at first yield large errors, sorting based on the location of the grains in terms of surrounding minerals yields a possible solution for varying Sr values skewing the ages in the sample. This results in an average age of 1205 Ma for biotite and sillimanite surrounded grains and 1107 Ma, from biotite and sillimanite surrounded grains and quartz and K-feldspar surrounded grains. This shows that the diffusive properties of Sr between biotite and the surrounding minerals creating variable re-equilibration between the different domains surrounding biotite. The complexities of Sr diffusion within between the various phases are still unknown, but the apparent effect between the surrounding material on the biotite and the measured initial Sr values does play a key factor in the final calculated ages and the interpretations they represent.