Melt inclusions from Marianas arc lavas: implications for the composition and formation of island arc magmas

We have measured the major and trace element compositions of a suite of olivine-hosted melt inclusions in basaltic lavas from the islands of Agrigan and Guguan (one sample each) from the Mariana arc, part of the larger Izu–Bonin–Mariana system. The two lava samples examined show distinctly different chemical signatures that are considered to represent the dominance of sediment melt (Agrigan) or fluid (Guguan) components derived from the subducting slab and transferred to the mantle wedge, and thus provide an opportunity to examine melt inclusions from both fluid- and sediment melt-dominated arc melting systems. ogenized melt inclusions from both samples examined, as well as other lavas from the same islands, contain unusual subsilicic amphiboles as mineral inclusions within melt inclusions. Textural evidence, as well as the compositions of laboratory-melted inclusions, suggests that these amphiboles may have been present as an early-formed phenocryst phase and trapped alongside melt within inclusions. Major element compositions of homogenized melt inclusions vary substantially, and in part are attributable to ‘accumulative’ amphibole contributions. Inclusions that appear to contain little or no amphibole component, however, typically have higher MgO and FeOT and lower CaO and Al2O3 contents than their host and associated lavas. nclusions also display a large range of incompatible element abundances. Although the relative abundances of incompatible elements are generally consistent with the composition of lavas from Agrigan and Guguan, melt inclusion compositions are also substantially more variable than lava compositions from these islands. Melt inclusions from the two samples analyzed have Ba/La and [La/Sm]N ratios (chemical indicators of addition of slab-fluid and slab-derived melt components, respectively) similar in magnitude to the entire field of Mariana lava compositions. In general inclusions from Agrigan have higher La/Nb, Th/Nb, U/Nb, [La/Sm]N, [La/Yb]N, K2O/TiO2 and lower Cl/K2O, Ba/La, Ba/Nb, Ba/Th and Ce/Ce* than Guguan melt inclusions. These signatures are consistent with addition of slab-sediment-derived melt and slab-derived fluid to the melting systems beneath Agrigan and Guguan, respectively. We suggest that the large variations apparent in melt inclusion compositions largely represent differences in the flux of slab-derived components to the mantle wedge beneath individual arc melting systems, although small-scale variations in the depletion of the mantle wedge could also play a role. Our data also indicates that the composition of the slab-derived sediment melt supplied to the mantle wedge beneath Agrigan may be compositionally heterogeneous, although a larger data set is required to examine this in further detail. esence of melt inclusions of highly variable composition in relatively evolved olivine phenocrysts (Fo64–Fo83) show that, regardless of the ultimate cause of the incompatible element variations in melt inclusions, discrete melts of distinctly different compositions persist without substantial intermixing within the magmatic plumbing systems beneath Agrigan and Guguan (and presumably other arc volcanoes) during long intervals of magmatic differentiation. This suggests a model of small batches of magma migrating and differentiating beneath arc volcanoes, with mixing occurring only at a very late and probably shallow stage.