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The central thrust of this book is that geochemical data can be used to identify and interpret geological processes in igneous, metamorphic and sedimentary rocks. This chapter categorises geochemical data into major element oxides, trace elements, radiogenic isotopes and stable isotopes. The text discusses the main processes which control the chemical composition of planetary bodies, which operate in igneous and metamorphic rocks and at the Earth’s surface and describes the main analytical methods currently in use. These include the methods of X-ray fluorescence, mass spectrometry and inductively coupled plasma mass spectrometry for both whole-rock analysis and in situ micro-analysis. Sampling protocols are briefly described and the choice of a suitable analytical method is discussed. Potential sources of error in geochemical analysis are identified and discussed.
The behaviour of trace elements in geochemistry may be understood in terms of the chemical properties of their ions and the way in which they are partitioned between solid (mineral) and melt phases during the melting and solidification of igneous rocks and their sources. However, partition coefficients vary according to a large number of physical parameters such as temperature, pressure, melt composition and oxygen fugacity, and these effects can be modelled using the results of experimental petrology and thermodynamic analysis. Trace element data for both igneous and sedimentary rocks are plotted according to their geochemical properties and may be displayed on bivariate diagrams, rare earth element (REE) plots, multi-element plots normalised to the composition of the Earth’s primitive mantle or as highly siderophile elements (HSE). The physical understanding of trace element distributions means that they can be modelled during partial melting, fractional crystallisation and assimilation and comparisons made with the measured compositions of natural rock samples.
Geochemists use radiogenic isotopes in geochronology and in petrogenetic studies. In geochronology isotopes from the K–Ar, Rb–Sr, Sm–Nd, Lu–Hf, U–Th–Pb, and Re–Os systems are used in isochron calculations and in the calculation of model ages. The concepts of closure temperature and the meaning of a geological ‘age’ are discussed. In petrogenesis radiogenic isotopes are used to identify different crust and mantle reservoirs as sources of magmatic rocks and to chart the isotopic evolution of the crust and mantle over time. The epsilon and gamma notations for normalising isotopic compositions relative to that of the chondritic uniform reservoir (CHUR) are described. The recognition of different mantle reservoirs and their evolution over time provides the basis for understanding large-scale processes in the mantle which feed into geodynamic models for the crust and mantle.
The process of discriminant analysis has been applied to major and trace elements in igneous and sedimentary rocks to seek to identify the original tectonic setting in which the rocks formed. A ‘training set’ of data from known environments is used to construct a discrimination diagram which is then used with data from unknown sources. Normally, the discrimination diagrams are based upon immobile trace element data and they have been applied predominantly to mafic igneous rocks, although there are also applications to felsic rocks and sediments. In the past, diagrams of this type have been used indiscriminately and here a robust approach is advocated for statistical analysis. Some of the diagrams presented are based upon elemental data, while others are based upon calculated discriminant functions and require some specific pre-calculation. Diagrams of this type are paradoxically accurate and at the same time geochemically opaque.
Major element data in geochemistry are used in rock classification, in plotting variation diagrams and in plotting diagrams in which the rock chemistry is shown together with the results of experimentally determined data. The main approaches to major element rock classification in igneous and sedimentary rocks include the use of oxide–oxide plots, the calculated normative mineralogy and the rock composition recast as cations. Binary and ternary variation diagrams are widely used to display geochemical data, and a major part of the interpretation of geochemical data is the identification and the replication, through modelling, of trends on these diagrams. The wealth of experimental data on igneous rocks means that natural rock compositions may be compared with experimental data gathered at particular pressures and temperatures to understand melting and fractionation processes in the Earth’s mafic and felsic crusts and mantle.
The geochemical applications of the traditional stable isotopes H, O, C, S and N and the non-traditional stable isotopes Li, Mg, Si, Cr and Fe are presented in this chapter. The standard isotopic notation, reference standards and isotopic distributions in some planetary reservoirs and the major Earth reservoirs are given for each isotope system. In each case the processes of equilibrium and kinetic fractionation are discussed and applied to high- and low-temperature (Earth surface and biological) systems and the fractionation factors listed. The utility of each isotope system as a tracer in geological and biological processes is discussed with applications to planetary differentiation, high-temperature igneous processes and low-temperature processes at the Earth’s surface in the terrestrial and aqueous environment and in living organisms. Stable isotopes are shown to be an important tracer for the evolution of life on Earth and the development of the Earth’s atmosphere.
The statistical analysis of geochemical data employs the main statistical techniques of averaging, probability distributions, correlation, regression, multivariate analysis and discriminant analysis. A particular problem with major element geochemical data is that it is constrained; that is, the compositions sum to 100% and the data are ‘closed’. A related problem arises when ternary plots are used to display geochemical data. Techniques are described to accommodate the problems associated with compositional data which include log-ratio conversions and the biplot diagram. Further statistical problems arise in the area of ratio correlation as advocated in Pearce element ratio diagrams, which is not recommended. Applications to trace elements and radiogenic isotope correlations are discussed. The details of discriminant analysis are outlined as a prelude to a more detailed discussion of tectonic discrimination diagrams considered in Chapter 5.
This textbook is a complete rewrite, and expansion of Hugh Rollinson's highly successful 1993 book Using Geochemical Data: Evaluation, Presentation, Interpretation. Rollinson and Pease's new book covers the explosion in geochemical thinking over the past three decades, as new instruments and techniques have come online. It provides a comprehensive overview of how modern geochemical data are used in the understanding of geological and petrological processes. It covers major element, trace element, and radiogenic and stable isotope geochemistry. It explains the potential of many geochemical techniques, provides examples of their application, and emphasizes how to interpret the resulting data. Additional topics covered include the critical statistical analysis of geochemical data, current geochemical techniques, effective display of geochemical data, and the application of data in problem solving and identifying petrogenetic processes within a geological context. It will be invaluable for all graduate students, researchers, and professionals using geochemical techniques.