Published online by Cambridge University Press: 01 February 2018
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 represent 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.
Isotopic analysis of oceanic basalts can be used both to probe the structure of the mantle and to model its evolution over time. The approach taken here will be to examine the constraints on mantle structure from single isotopic systems (mainly Sr and Pb), then to examine the constraints on mantle evolution from multiple isotopic systems (Sr–Nd), (U–Th–Pb), (Sr–Nd–Pb). Evidence from other systems will be examined in later chapters.
Isotopic Tracing of Mantle Structure
Isotopic 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 isotope compositions of mid ocean ridge basalts (MORB) and OIB (Tatsumoto, 1966). Some of the key evidence for mantle structure comes from comparing isotope heterogeneity within and between these groups, and especially from case studies of ocean islands located on spreading ridges.
Contamination and Alteration
Before oceanic volcanics can be used to deduce mantle compositions, we must examine and quantify the amounts of alteration and contamination that could occur during magma transport and eruption on ocean islands or the ocean floor.