MAGMATIC WATER CONTENTS ACROSS AN ACTIVE SUBDUCTION ZONE

Water plays a critical role in magma generation at subduction zones as hydrous fluids are released into the mantle wedge from subducting lithosphere. The elevated pre-eruptive water contents of many subduction zone basaltic magmas is taken as evidence of their hydrous origins. Multifarious geochemical proxies indicate that the influence of hydrous fluids falls across active subduction zones, but relevant across-arc water data are minimal. Here we examine the variable pre-eruptive water contents of basaltic magmas across the northern part of the Central American subduction zone. As expected, the water concentrations measured in melt inclusions of olivine phenocrysts from volcanic front basalts are high, although variable (1-6 wt% H₂O). Melt inclusions in behind-the-front basalts, by contrast, have, on average, lower H₂O (1-2 wt%). The water contents of the behind-the-front basalts, although less than those characteristic of the front, are still considerably higher than the water contents that characterize basaltic magmas erupted at diverging plate margins and intra-plate settings. Hence, hydrous melting may remain important over 100 km behind the volcanic front.