The water and trace element contents of melt inclusions across an active subduction zone

Water concentrations of olivine-hosted melt inclusions show no consistent variation across the northern part of the Central American subduction zone in southeastern Guatemala. Magmatic water contents remain moderately high (~2 wt%) throughout the back-arc region. Melt inclusions from some of these back-arc basalts also have notably high CO₂ contents (>900 ppm CO₂). The B and B/Ce ratios of melt inclusions systematically decline across the arc, the first parameters to exhibit systematic changes across southeastern Guatemala. It appears, therefore, that dehydration-driven, flux-melting persists across the arc, although decompression melting is of approximately equal importance in the back-arc region. Dehydration of the slab/wedge region is regarded as semi-continuous down-dip, to depths of at least 175–200 km. Moderate water contents are maintained by stepwise dehydration reactions, while truly incompatible fluid mobile elements are progressively stripped from the Cocos plate. The notably high CO₂ contents of some back-arc basalts may indicate increasing devolatilization of subducted carbonate sediments with slab depth. The moderate H₂O contents of back-arc basaltic magmas has likely contributed to their early fractionation of clinopyroxene around the Moho.Editorial responsibility: T.L. Grove