GLASS INCLUSIONS IN MARIANA ARC PHENOCRYSTS - A NEW PERSPECTIVE ON MAGMATIC EVOLUTION IN A TYPICAL INTRAOCEANIC ARC

Major element compositions of glass inclusions in olivine and plagiocl ase phenocrysts from representative Mariana are lavas show that mafic Mariana arc liquids are tholeiitic and not high-alumina, implying that the high-alumina characteristic of these lavas reflects accumulation of plagioclase. Glass inclusions also show the common occurrence of fe lsic melts previously unrecognized among Mariana are lavas and indicat e that felsic melts are important, if cryptic, components of this magm atic system. Primitive, mantle-derived melts have not been found. Glas s inclusion data indicate that the are magma systems sampled by erupti ng lavas are compositionally bimodal, with Fe-rich mafic and high-sili ca (66 to 76% SiO2) modes. These observations are most simply interpre ted as being due to shallow, compositionally zoned magma chambers. The restriction of felsic glass inclusions to plagioclase phenocrysts ind icates that felsic melts reside in the upper part of the magma chamber , underlain by Fe rich mafic melts. Plagioclase phenocrysts accumulate between mafic and felsic zones. Glass inclusions from plagioclase and olivine in the same sample are compositionally distinct, indicating t hat these minerals formed in different melts. These data indicate that Mariana are magmas reside in strongly zoned magma chambers, and that coupled magma mixing and plagioclase accumulation are important for co ntrolling the spectrum of lavas erupted in this typical arc.