High pressure phase relations of subducted volcaniclastic sediments from the west pacific and their implications for the geochemistry of Mariana arc magmas

The phase relations of natural volcaniclastic sediments from the west Pacific Ocean were investigated experimentally at conditions of 3-6 GPa and 800-900 degrees C with 10 wt.% added H2O (in addition to similar to 10 wt.% structurally-bound H2O) to induce hydrous melting. Volcaniclastic sediments are shown to produce a sub-solidus assemblage of garnet, clinopyroxene, biotite, quartz/coesite and the accessory phases rutile +/- Fe-Ti oxide +/- apatite +/- monazite +/- zircon. Hydrous melt appears at temperatures exceeding 800-850 degrees C, irrespective of pressure. The melt-producing reaction consumes clinopyroxene, biotite and quartz/coesite and produces orthopyroxene. These phase relations differ from those of pelagic clays and K-bearing mid ocean ridge basalts (e.g. altered oceanic crust) that contain phengite, rather than biotite, as a sub-solidus phase. Despite their relatively high melt productivity, the wet solidus for volcaniclastic sediments is found to be higher (825-850 degrees C) than other marine sediments (700-750 degrees C) at 3 GPa. This trend is reversed at high-pressure conditions (6 GPa) where the biotite melting reaction occurs at lower temperatures (800-850 degrees C) than the phengite melting reaction (900-1000 degrees C).

Trace element data was obtained from the 3 GPa run products, showing that partial melts are depleted in heavy rare earth elements (REE) and high field strength elements (HESE), due to the presence of residual garnet and rutile, and are enriched in large ion lithophile elements (LILE), except for Sr and Ba. This is in contrast to previous experimental studies on pelagic sediments at sub-arc depths, where Sr and Ba are among the most enriched trace elements in glasses. This behavior can be partly attributed to the presence of residual apatite, which also host some light REE in our supra-solidus residues. Our new experimental results account for a wide range of trace element and U-series geochemical features of the sedimentary component of the Mariana arc magmas, including imparting a substantial Nb anomaly to melts from an anomaly-free protolith. (C) 2013 Elsevier B.V. All rights reserved.