The strains of dating: Rb/Sr to the rescue?

Ductile shear zones accommodate vast amounts of strain in the crust and are a focus for fluids, magmas and elemental transfer. The rates and timescales of shear zone deformation, and the timing of associated fluid, magma flow or mineralisation events, are critical for constraining large-scale tectonic processes, the rheological behaviour of the lithosphere and the formation of economically

important resources through geological time [e.g. 1]. As micas deform and recrystallise during shearing, the geochronological information that they record may be modified or completely re-set. Following recent analytical advances, micas can now be dated in-situ by Rb-Sr LA-ICP-QQQ-MS analysis [e.g. 2], with trace element datasets gathered from the same ablation spot. Micas incorporate trace

elements that allow processes such as deformation, fluid infiltration and/or changes in pressure and temperature to be 'fingerprinted', and when linked to their geochronolgoical record, provide rich data about their history. However, there are still challenges to be overcome with the precision and accuracy

of in-situ Rb-Sr geochronology techniques such as finding and validating community agreed reference materials, determining crystallographic effects on data quality, and choice of analytical parameters and their influence on fractionation and matrix effects.