The crustal element cycle allows continuous redistribution of elements through different crustal reservoirs. Although retrogression after peak metamorphism is commonly known to affect both major and trace elements, particularly in the presence of fluids, much is still unknown about the trace element mass transfer associated with specific retrograde reactions. This study investigates the extent to which the retrograde breakdown of biotite and cordierite – key hosts of Li, Be, Cs, Sn, Ta and W in metapelites and granites – can mobilise their hosted critical elements. We combined petrography, in-situ mineral major and trace element analysis, bulk-rock major element analysis, geothermometry, phase equilibria modelling and mass transfer estimates to investigate the breakdown of biotite and cordierite to chlorite and pinite, respectively. The biotite to chlorite reaction (350–500 °C) requires fluid influx to produce the observed volumes of chlorite, muscovite, ilmenite, and rutile. This reaction results in substantial mobilisation of Cs and moderate losses of Li and Sn, while W, Nb, and Ta remain relatively immobile. Cordierite breaks down below ~ 200 °C into fine-grained aggregates of kaolinite and smectite clays (known as pinite), with element mobilisation varying with the degree of replacement, fluid composition and clay mineralogy. Li, Be, and Cs show contrasting behaviours, with mobilisation enhanced in Fe- and Mg-poor clays under open-system conditions. Our findings highlight that the breakdown of biotite and cordierite releases critical elements stored in these phases, which could potentially generate enriched fluids.
Costa et al. (Fri,) studied this question.