Shallow-Level Processes in Ocean-island Magmatism: Editorial

Abstract

surface. Some of the most detailed and comprehensive Magmatism at ocean islands typically has been associated recent studies of individual ocean-island magma systems with deep-rooted mantle plumes. Rarely, these are cosuggest, however, that many of the important criteria incident with oceanic spreading centres (e.g. Iceland), that we use to infer mantle sources may be compromised but more commonly they form intra-plate hotspots (e.g. by interaction between magmas and the oceanic and Hawaii, Canary Islands). If magmatism at ocean islands island crust through which they ascend. The occurrence is indeed intimately linked with mantle plumes and of contamination at ocean islands has been deemed less these plumes are derived from great depths (670 km likely than in continental settings, where magmas pass discontinuity or core–mantle boundary), effectively conthrough thicker, lower density crust. This contention is stituting the major mode of deep upwelling in the global apparently supported by the numerous case studies of mantle convection system, then their surface manicontamination of continental magmas. The difference in festation should provide some of our best constraints on the observable chemical consequences of contamination the composition and nature of the deep mantle. In in the two environments must be appreciated in the contrast, the dominant volume of present-day volcanic context of the compositional distinctions of the crust. activity, which is concentrated along plate boundaries, The compositional contrast between continental crust provides little definitive information on the deep mantle. and mantle-derived magma is commonly large, and Over the past 20 years considerable progress in our therefore the effects of contamination, in terms of standunderstanding of the composition of the upper mantle ard indicators such as isotope ratios, are readily aphas been inextricably linked to advances in analytical preciated. In contrast, basaltic magmas erupted on ocean geochemistry, particularly the use of isotope and inislands pass through dense basaltic crust, and any comcompatible trace element ratios in oceanic basalts as positional differences between magma and contaminant tracers of their mantle sources. Early studies have now may be subtle. It is important to point out, however, that been followed up by a number of very detailed volthe potentially subtle effects of contamination do not canological–petrological–geochemical studies of inimply that such effects are irrelevant in the grand picture dividual oceanic islands. Although Hawaii has of mantle geodynamics, which itself is dependent on traditionally served as our de facto ‘model’ of ocean-island compositional subtlety. magmatism by virtue of the sheer volume of geochemical Given the large volume of high-quality data, and the and geophysical data accumulated there, we now realize large number of studies of individual ocean islands that that a spectrum of volcano types and compositions exists, now exist, the time has come to ask what effects shallowand these have been used to map mantle domains, to infer level processes at ocean islands may have in modifying processes of partial melting, and to calculate recycling magma compositions. This question was addressed at budgets between crust and mantle. a Chapman Conference sponsored by the American The use of ocean-island basalt (OIB) geochemistry Geophysical Union from 9 to 16 November 1996 in to define mantle sources and partial melting processes Tenerife, Canary Islands. The conference was attended depends implicitly on the assumption that the primary by 45 scientists from seven nations, representing interests magmas are not significantly modified, other than by closed system fractional crystallization, en route to the varying from isotope geochemistry to geophysics. This