Generation and Modification of the Mantle Wedge and Lithosphere beneath the West Bismarck Island Arc: Melting, Metasomatism and Thermal History of Peridotite Xenoliths from Ritter Island

Abstract

Peridotite xenoliths dredged from the seafloor NW of Ritter Island in the West Bismarck island arc offer a rare insight into the petrogenetic processes operating in the upper mantle wedge of an active oceanic subduction zone. Harzburgitic xenoliths and subordinate dunites and pyroxenites display significant textural and compositional variability between samples, which are interpreted as fragments of heterogeneous mantle that has experienced a complex petrogenetic history. Based on textures and in situ major and trace element analyses of olivine, orthopyroxene and clinopyroxene, five significant petrogenetic stages have been deduced. The first stage was a period of partial melting at temperatures > 1100 °C in a previously active arc (probably the now extinct Vitiaz-West Melanesian arc), indicated by the high Mg# and Cr#, low Al2O3 and very low concentrations of incompatible trace elements in residual phases and the absence of residual clinopyroxene. Modelling the concentrations of Y and Yb in orthopyroxene indicates depletion by high degrees (~30%) of wet fractional melting of a depleted mantle source. This was followed by metasomatism also related to this previous period of subduction, resulting in refertilization of the harzburgite residues with clinopyroxene and the formation of dunite and pyroxenite channels. Three populations of secondary clinopyroxene are identified based on their trace element compositions, with both light rare earth element (LREE)-depleted and (more unusually) sinusoidal REE patterns identified, indicating that a spectrum of fluid compositions was involved in the metasomatism. The third stage involved a period of cooling and chemical re-equilibration below the wet solidus, combined with decompression to shallow lithospheric mantle depths. Geothermometers with different closure temperatures reveal large temperature discrepancies, indicating that mantle cooling was both unusually extensive (down to ~600 °C), but also very slow (~20 °CMa-1). The fourth stage marked the cessation of cooling and formation of the modern mantle wedge as part of the West Bismarck island arc. Silicate melts percolated through networks of veins and reacted with the residual mantle, generating a variety of new disequilibrium textures, most notably orthopyroxene-clinopyroxene- glass reaction patches. This was accompanied by increases in spinel Cr# and olivine trace element concentrations, and higher mineral-mineral temperatures. These disequilibrium textures were preserved through the final stage of entrainment in the host basalt and rapid transport to the seafloor. The Ritter suite thus provides a remarkably detailed insight into the broad diversity of melt-fluid compositions and fluid-rock reaction processes in oceanic sub-arc mantle. Several of these features can be found both in other samples of sub-arc mantle and also in cratonic mantle, demonstrating the ubiquity of such processes beneath modern arcs and also the potential genetic relationship between subduction-zone processes and the formation of cratons. In this way, sub-arc xenoliths such as the Ritter suite, although at present under-sampled, can provide crucial insights for understanding the relationship between the mechanisms by which modern arc systems are generated and evolve and the nature of the upper mantle once subduction processes have ceased.