Oceanic islands and the mantle: historical perspectives

Abstract

Coral reefs, the controversy over atolls and the implications for subsidence provided an early focus of geological interest in oceanic islands. At the same time, studies on the volcanic rocks suggested a crystal fractionation link between basalt and more differentiated lavas. Apart from the diversion of the “Daly Gap” there has been remarkably little dissent from this view which continues to be reinforced by increasingly sophisticated petrogenetic modelling. Compositional variations in oceanic volcanic suites gradually became apparent during the first half of the twentieth century and differences were usually ascribed to contamination by patches of siliceous continental crust. The layered structure of the Earth was established by the late 1930's although there was no consensus on the composition of the layers. In the late 1950's and early 1960's petrologists turned to the mantle as a means of explaining variations in basalt chemistry and their investigations were supported by high-pressure experimental studies. Particular importance was attached to oceanic islands, especially Hawaii, as repositories of basalt uncontaminated by continental crust. Recognition of the compositional distinction between ocean floor and ocean island basalts led to the emergence of a new generation of Earth models. Ocean-island or hot-spot volcanism was now viewed as the surface manifestation of mantle plumes. Trace-element and isotopic compositions of basalts permitted the identification of different mantle sources, the apparent number of which multiplied rapidly. The emphasis was now on mantle composition, dynamics and evolution, with ocean island basalts being used as tracers of source heterogeneities, and at the same time providing an insight into mantle processes and their timing. Isotopic evidence for crustal recycling added a new dimension to the problem and oceanic islands are now seen as an integral part of a dynamic crustmantle system.