Rare-earth element geochemistry of Mariana fore-arc volcanics: Deep Sea Drilling Project Site 458 and Hole 459B.

Abstract

Volcanic basement rocks cored at Site 458 and Hole 459B in the Mariana fore-arc were analyzed for rare-earth elements. Samples recovered from the sites represent two petrologic and chemical groups: orthopyroxene-bearing, plagioclase-poor lavas (high-Mg bronzite andesites) - similar to 'boninite' - which occur in the upper part of Site 458, and plagioclase-clinopyroxene basalts of arc tholeiite type, which occur in the lower part of Site 458 and at Hole 459B. Least fractionated, high-Mg bronzite andesite samples have rare-earth element (REE) abundances within the range 2-6 X chondrites, and arc tholeiite samples have REE abundances in the range 4-20 X chondrites. The difference in relative REE distribution between the two volcanic groups is believed to reflect source characteristics. The data suggest that a shift or compositional modification of the volcanic source material for the Mariana fore-arc occurred with the initiation of the high-Mg bronzite andesite volcanism. This may have been related to an influx of H2O, which is important in the generation of high SiO2-high MgO liquids, into the sub-arc mantle.-Authors*