Are arc-type rocks the products of magma crystallisation? Observations from a simple oceanic arc volcano: Raoul Island, Kermadec Arc, SW Pacific

Raoul Island is the emergent summit of a large intra-oceanic strato-volcano in what is globally one of the simplest of subduction settings. In this simple setting erupted magmas span the compositional range from basalt to dacite but none have the high Mg-numbers and high Ni and Cr expected of primitive mantle-derived melts. The lavas range from aphyric to highly porphyritic and are characterised by phenocryst assemblages dominated by plagioclase accompanied by clinopyroxene, olivine, orthopyroxene and spinel. Phenocryst core compositions and zoning profiles are remarkably uniform irrespective of total phenocryst content or geochemical composition, indicating a decoupling of melt and crystal components in the system. A consistent model for the Raoul magmatic system is that primitive high-Mg magma generated in a melt column within the underlying mantle wedge is transformed into a series of derivative low-Mg magmas by fractional crystallisation within the lower crust. Low-Mg magma accumulates variable quantities of crystal cargo as it ascends toward the surface through a crystal mush zone. These processes are essentially those that characterise continental subduction-related magmatic systems but differ only in the absence of an evolved crustal component.