A Tale of Two Arcs: Petrogenesis of Ultramafic Xenoliths Sampling the Upper Mantle Wedge Beneath the West Bismarck Island Arc

Abstract

Peridotite xenoliths transported to the surface in basaltic magma from the upper mantle wedge beneath the West Bismarck island arc, Papua New Guinea, present a rare opportunity to assess the nature of the mantle wedge in an active intra-oceanic island arc. This thesis reports comprehensive new geochemical and isotopic data for harzburgites, pyroxenites and dunites, from the island of Ritter, in order to understand how partial melting and hydrous metasomatism generate chemically, isotopically and physically distinct mantle.

The highly depleted major and moderately incompatible trace element composition of residual phases and the radiogenic strontium isotope composition of texturally well-equilibrated harzburgites are best explained by hydrous partial melting and metasomatism associated with a previous period of subduction. Harzburgites that record textural evidence for melt-rock reaction, meanwhile, have elevated equilibration temperatures, oxidised spinel compositions, elevated olivine water contents and strontium isotope compositions identical to regionally erupted basalts. These features reflect interaction between ambient mantle and primary hydrous, oxidised basaltic melts in the upper mantle wedge. Modelling of trace element diffusion profiles in olivine constrains this event to approximately one year before exhumation.

The low water contents of both coarse-grained olivine and orthopyroxene are consistent with equilibrium in chemically depleted upper mantle. The absence of hydrated silicon vacancies in olivine despite overall increases in water content during melt-rock reaction indicates that the mantle wedge may not change significantly in mechanical strength during hydrous melting and metasomatism. Chemical and radiogenic signatures of subduction are thus more likely to survive convective homogenisation.

The whole-rock budget of highly siderophile elements (HSE) is contained within heterogeneously distributed trace sulphides and inferred alloy phases, and is controlled by both partial melting and metasomatism. An absence of any correlation of Sr isotopes with either HSE or Os isotopes indicates these elements may be immobile in slab-derived fluids. Elevated concentrations of Pt and Pd in pyroxenite are mirrored by depletions in dunite, demonstrating that melt-rock reaction is instead responsible for enrichments in these elements in arc mantle. A correlation between whole-rock 187Os/188Os and phosphorous in olivine offers clues to ancient processes unrelated to active subduction, not recorded by any other chemical or isotopic system.