The role of celadonite on mantle metasomatism in subduction settings

Abstract Celadonite is the most abundant K-rich hydrated mineral formed during basalt alteration. In convergent margins, celadonite from the altered oceanic crust sinks into the mantle and contributes to the mantle wedge metasomatism, recorded by incompatible element enrichment in lavas and mantle xenoliths samples from these settings. However, experimental data on the role of celadonite in subduction zones as a function of P-T are still scarce. Therefore, celadonite stability under “hot” and “cold” subduction settings is investigated experimentally in this study. Results show an early silica loss, with quartz/coesite replacing celadonite from 400 to 600 °C and 2.5–7.7 GPa. At 700 °C and 2.5 GPa, celadonite is partially converted to phlogopite-annite, and at 800 °C and 4.0 GPa celadonite is absent. At temperatures above 800 °C, enstatite and phlogopite are the stable silicate phases from 2.5 to 7.7 GPa. Mossbauer spectroscopy and X-ray diffraction analyses show that the reaction of celadonite to phlogopite-annite is progressive and controlled by the Fe3+ to Fe2+ reduction in the octahedral site. Coupled to this, there is a transition of pyrite to pyrrhotite and the release of reducing fluids enriched in H2S, SiO2, OH− and K+ in the mantle wedge. These fluids have the potential to mobilize chalcophile elements (e.g. Cu, Zn) and act as mineralizing agents in shallower and more oxidizing crustal environments. Data in this study demonstrate that celadonite-phlogopite-annite series minerals are stable down to at least 230 km in the mantle, contributing to the sources of mantle metasomatism, arc-related magma genesis and possibly chalcophile mineralization.