Using Geochemical Data

Evaluation, Presentation, Interpretation

Hugh R. Rollinson

• Editorial: Routledge
• Año: 2014
• ISBN:
  9781317898184
• Edición: 1
• Categoría: Physical Sciences
• Categoría: Geography

Using Geochemical Data brings together in one volume a wide range of ideas and methods currently used in geochemistry, providing a foundation of knowledge from which the reader can interpret, evaluate and present geochemical data.

Chapter 1

Geochemical data

1.1 Introduction

This book is about geochemical data and how they can be used to obtain information about geological processes. Conventionally geochemical data are subdivided into four main categories: the major elements, trace elements, radiogenic isotopes and stable isotopes (see Table 1.1). These four types of geochemical data each form the subject of a chapter of this book. Each chapter shows how the particular form of geochemical data can be used and how it provides clues to the origin of the suite of rocks in question. Different methods of data presentation are discussed and their relative merits evaluated.

Table 1.1 Whole-rock geochemistry of komatiite flows from the Belingwe greenstone belt, Zimbabwe (Data from Nisbet et al., 1987)

	ZV14	ZV85	ZV10
Major element oxides (wt%)
SiO2	48.91	45.26	45.26
TiO2	0.45	0.33	0.29
Al2O3	9.24	6.74	6.07
Fe2O3	2.62	2.13	1.68
FeO	8.90	8.66	8.70
MnO	0.18	0.17	0.17
MgO	15.32	22.98	26.21
CaO	9.01	6.94	6.41
Na2O	1.15	0.88	0.78
K2O	0.08	0.05	0.04
P2O5	0.03	0.02	0.02
S	0.04	0.05	0.05
H2O+	3.27	3.41	2.20
H2O−	0.72	0.57	0.28
CO2	0.46	0.84	1.04
Total	100.38	99.03	99.20
Selected trace elements (ppm)
Ni	470	1110	1460
Cr	2080	2770	2330
V	187	140	118
Y	10	6	6
Zr	21	16	14
Rb	3.38	1.24	1.38
Sr	53.3	32.6	31.2
Ba	32	12	10
Nd	2.62	1.84	2.31
Sm	0.96	0.68	0.85
Radiogenic isotope ratios
εNd	+2.4	+2.4	+2.5
^87Sr/^86Sr	0.7056	0.70511	0.70501
Stable isotope ratios (‰)
δ^18O	+7.3	+7.0	+6.8

Note: Major elements and Ni, Cr, V, Y, Ba determined by XRF; FeO determined by wet chemistry; H₂O and CO₂ determined by gravimetry; Rb, Sr, Sm, Nd determined by IDMS.

The major elements (Chapter 3) are the elements which predominate in any rock analysis. They are Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K and P, and their concentrations are expressed as a weight per cent (wt %) of the oxide (Table 1.1). Major element determinations are usually made only for cations and it is assumed that they are accompanied by an appropriate amount of oxygen. Thus the sum of the major element oxides will total to about 100 % and the analysis total may be used as a rough guide to its reliability. Iron may be determined as FeO and Fe₂O₃, but is sometimes expressed as ‘total Fe’ and given as either FeO_(tot) or Fe₂O_3(tot).

Trace elements (Chapter 4) are defined as those elements which are present at less than the 0.1 % level and their concentrations are expressed in parts per million (ppm) or more rarely in parts per billion (ppb; 1 billion = 10⁹) of the element (Table 1.1). Convention is not always followed however, and trace element concentrations exceeding the 0.1 % (1000 ppm) level are sometimes cited. The trace elements of importance in geochemistry are identified in Table 1.5 and shown in Figure 4.1.

Table 1.5 Elements readily analysed by XRF, INAA, IDMS, AAS, ICP and ICP-MS

Some elements behave as a major element in one group of rocks and as a trace element in another group of rocks. An example is the element K, which is a major constituent of rhyolites, making up more than 4 wt % of the rock and forming an essential structural part of minerals such as orthoclase and biotite. In some basalts, however, K concentrations are very low and there are no K-bearing phases. In this case K behaves as a trace element.

Volatiles such as H₂O, CO₂ and S are normally included in the major element analysis (Table 1.1). Water combined within the lattice of silicate minerals and released above 110 °C is described as H₂O+. Water present simply as dampness in the rock powder and driven off by heating below 110 °C is quoted as H₂O− and is not an important constituent of the rock. Sometimes the total volatile content of the rock is determined by ignition at 1000 °C and is expressed as ‘loss on ignition’ (Lechler and Desilets, 1987).

Isotopes are subdivided into radiogenic and stable isotopes. Radiogenic isotopes (Chapter 6) include those isotopes which decay spontaneously due to their natural radioactivity and those which are the final daughter products of such a decay scheme. They include the parent-daughter element pairs Rb–Sr, Sm–Nd, U–Pb, Th–Pb and K–Ar. They are expressed as ratios either in absolute terms ⁸⁷Sr/⁸⁶Sr (e.g.) or relative to a standard (the ε-notation) (Table 1.1).

Stable isotope studies in geology (Chapter 7) concentrate on the naturally occurring isotopes of light elements such as H, O, C and S which may be fractionated on the basis of mass differences between the isotopes of the element. For example, the isotope ¹⁸O is 12.5 % heavier than the isotope ¹⁶O and the two are fractionated during the evaporation of water. Stable isotopes contribute significantly to an understanding of fluid and volatile species in geology. They are expressed as ratios relative to a standard using the δ-notation (Table 1.1)

The major part of this book discusses the four main types of geochemical data outlined above and shows how they can be used to identify geochemical processes. In addition, Chapter 5 has been included to show the way in which trace and major element chemistry is used to determine the tectonic setting of some igneous and sedimentary rocks. Chapter 2 discusses some of the particular statistical problems which arise when analysing geochemical data-sets, and some recommendations are made about permissible and impermissible methods of data presentation.

In this introductory chapter we consider three topics: (1) the geochemical processes which are likely to be encountered in nature and their geochemical signatures; (2) the interaction between geological fieldwork and the interpretation of geochemical data; and (3) the different analytical methods currently in use in modern geochemistry.

1.2 Geological processes and their geochemical signatures

A major purpose of this text is to show how geochemical data can be used to identify geological processes. In this section the main geochemical signatures of igneous, sedimentary and metamorphic processes are briefly summarized and presented in graphical (Figures 1.1 to 1.3) and in tabular form (Tables 1.2 to 1.4). This brief survey is augmented by fuller discussions elsewhere in the text. Each of the Tables 1.2 to 1.4 lists the geological processes which may have a geochemical signature and identifies the sections in the book where the particular process is described and characterized using major or trace elements, and radiogenic or stable isotopes.

Table 1.2 The geochemical signatures of igneous processes identified by section in subsequent chapters

Table 1.3 The geochemical signatures of sedimentary processes, identified by section in subsequent chapters

Table 1.4 The geochemical signatures of metamorphic processes, identified by section in subsequent chapters

Figure 1.1 Flow diagram showing the principal processes which control the chemical composition of igneous rocks.

Figure 1.2 Flow diagram showing the principal processes which control the chemical composition of sedimentary rocks.

Figure 1.3 Flow diagram showing the principal processes which control the chemical composition of metamorphic rocks.

1.2.1 Processes which control the chemical composition of igneous rocks

The chemical composition and mineralogy of the source region exerts a fundamental control over the chemistry of magmatic rocks. The major and trace element composition of a melt is determined by the type of melting process and the degree of partial melting, although the composition of the melt can be substantially modified en route to the surface (Figure 1.1). The source region is best characterized by its radiogenic isotope composition because isotope ratios are not modified during partial melting and magma chamber processes. The composition of the source itself is a function of mixing processes in the source region. This is particularly pertinent to studies of the mantle, and in the last decade important advances have been made in understanding mantle dynamics through the isotopic study of mantle-derived oceanic basalts (see Section 6.3.6).

Most magmatic rocks are filtered through a magma chamber prior to their emplacement at or near the surface. Magma chamber processes frequently modify the chemical composition of the primary magma, produced by partial melting of the source, through fractional crystallization, magma mixing, contamination or a dynamic mixture of several of these processes. Resolving the chemical effects of these different processes requires the full range of geochemical tools — major and trace element studies coupled with the measurement of both radiogenic and stable isotope compositions. Excellent and detailed discussions of magma chamber processes are given by Hall (1987 — Chapter 7) and Wilson (1989 — Chapter 4).

Following emplacement or eruption, igneous rocks may be chemically modified, either by ...