Crystallisation sequence of a REE-rich carbonate melt: an experimental approach

Carbonatites host the main REE deposits in the world, with bastnaesite being the main REE-bearing mineral of interest. However, the nature of the enrichment process, magmatic vs hydrothermal, is still debated. This study aims to experimentally determine the behaviour of REE elements during carbonatite crystallisation, and if bastnaesite can be directly crystallised from a carbonate melt. Crystallisation experiments have been done from 900 to 600°C at 1 kbar on a REE-rich calciocarbonatitic composition. Calcite (Ca,REE)CO3 is the dominant magmatic mineral, so the residual melt evolves toward natrocarbonatitic compositions as crystallisation proceeds. A small amount of britholite (REE,Ca)5((Si,P)O4)3(OH,F) is observed at high temperatures and is replaced by phlogopite KMg3(AlSi3O10)(OH)2 and apatite (Ca,REE)5(PO4)3(F,OH) at T < 650°C. A small amount of pyrochlore (Ca,Na,REE)2Nb2O6(OH,F) is observed at T < 700°C. No bastnaesite has been found in any crystallisation experiment. We thus performed a bastnaesite saturation experiment at 600°C. The melt saturated with bastnaesite however contains 20 wt% of REE: such high value implies that magmatic saturation of bastnaesite is unlikely to happen in nature. F, Cl and water decrease the temperature of calcite saturation, allowing the system to crystallise at lower temperatures. REE are slightly incompatible with calcite, especially at low temperatures. The residual carbonate melt is thus enriched as crystallisation proceeds. Finally, we collected textural and chemical evidence suggesting the presence of a Na,Cl,REE-rich fluid at high temperatures and under hydrous conditions. Such Na,Cl,REE-rich fluids may play a critical role in the remobilisation of REE and the bastnaesite crystallisation during subsolidus reactions.