Fluid metasomatism in the cold nose of the Mariana subduction zone

Fluid-mediated mass transfer in subduction zones is crucial for chemical cycling on Earth. Particularly little is, however, known about such processes at shallow subduction levels. We used thermodynamic models to reproduce the metamorphic history of ocean island basalt (OIB) clasts recovered from the Mariana forearc during IODP Expedition 366. The OIBs were subducted to ~30 km depth, metamorphosed/metasomatized, and subsequently recycled to the seafloor via mud volcanism (Fryer et al., 2020). The rocks exhibit K2O contents (median = 4.6 wt.%) and H2O (median = 5.3 wt.%) much higher than OIBs situated on the Pacific plate (Deng et al., 2021), suggesting that these have been added during subduction. This interpretation is in line with the presence of abundant phengite in the samples. Additionally, mass balance calculations point to the addition of SiO2, and high Cs, Rb, Th, and U concentrations imply an uptake whereas low Ba and Sr contents indicate the removal of trace elements. We show that the metasomatic change in composition and the formation of phengite can be explained by (i) the dehydration of altered oceanic crust releasing K2O-rich fluids and (ii) the subsequent reaction of such fluids with OIB. These processes are predicted to initiate at temperatures of <200°C and pressures of <5 kbar. Our study provides direct evidence for fluid–rock interactions and metasomatism in an active subduction zone. We demonstrate that mass transfer from subducted oceanic crust initiates at low pressure/temperature conditions. Subducted volcanics can hence undergo significant compositional changes even at shallow depths.