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6 - Isotope geochemistry of oceanic volcanics

Alan P. Dickin
Affiliation:
McMaster University, Ontario
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Summary

Some of the most important questions in geology concern the processes which operate in the Earth's mantle. Mantle convection is clearly the driving force behind plate tectonics (e.g. Turcotte and Oxburgh, 1967), but the details of its operation are still unclear. The depth of mantle convection cells, the fate of subducted lithosphere and the source of upwelling mantle plumes are all questions that remain poorly understood. Isotope geochemistry may help to answer these questions by revealing the progress of mantle differentiation into different reservoirs and the extent to which these reservoirs are re-mixed by convective stirring.

The inaccessibility of the mantle presents a severe problem for geochemical sampling. However, mantle-derived basic magmas provide a prime source of evidence about the chemical structure of the mantle. Isotopic tracers are a particularly powerful tool for such studies, because, unlike elemental concentrations, isotope ratios are not affected by crystal fractionation. However, isotope ratios are susceptible to contamination in the continental lithosphere. Therefore the simplest approach to studying mantle chemistry through basic magmas is to analyse oceanic volcanics, which are expected to have suffered minimal contamination in the thin oceanic lithosphere.

Isotope analysis of ocean-island basalts (OIB) was first used to demonstrate the existence of mantle heterogeneity (Faure and Hurley, 1963; Gast et al., 1964). Subsequently, variations were found between the isotopic compositions of mid-ocean-ridge basalts (MORB) and OIB (Tatsumoto, 1966). Isotopic analysis of oceanic basalts can be used both to probe the structure of the mantle and to model its evolution over time.

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Publisher: Cambridge University Press
Print publication year: 2005

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