Glass Inclusions in Mariana Arc Phenocrysts: A New Perspective on Magmatic Evolution in a Typical Intra-oceanic Arc1

Major element compositions of glass inclusions in olivine and plagioclase phenocrysts from representative Mariana arc 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 felsic melts previously unrecognized among Mariana arc lavas and indicate that felsic melts are important, if cryptic, components of this magmatic system. Primitive, mantle-derived melts have not been found. Glass inclusion data indicate that the arc magma systems sampled by erupting lavas are compositionally bimodal, with Fe-rich mafic and high-silica (66 to 76% SiO2) modes. These observations are most simply interpreted as being due to shallow, compositionally zoned magma chambers. The restriction of felsic glass inclusions to plagioclase phenocrysts indicates 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 that these minerals formed in different melts. These data indicate that Mariana arc magmas reside in strongly zoned magma chambers, and that coupled magma mixing and plagioclase accumulation are important for controlling the spectrum of lavas erupted in this typical arc.