Chapter 1
Analytical Geochemistry

1.1 Overview of Analytical Geochemistry

In this chapter, we reconsider why we determine the concentration of elements or isotopic ratios in silicate materials. In this book, the application of analytical chemistry techniques to earth sciences is named as "analytical geochemistry."

The purposes of the analytical geochemistry are to reveal the distribution of elements and to unravel origin and evolution of the solar system including the Earth, the Moon, other planets, asteroids, dwarf planets, and comets from atomic scale to solar system scale, namely, from nanometer to tetrameter scales (the SI prefixes are summarized in Table 1.1).

Table 1.1 SI prefixes

Empirical expressions are sometimes used, but they are not recommended.

It is easy to say but difficult to accomplish. In order to reach the result, we need a strategy. It took more than a century to establish five strategies in analytical geochemistry. To execute these strategies, we had to wait for developments of analytical methods. In other words, the evolution of the analytical methods was directly related to the evolution in analytical geochemistry and earth sciences. The evolution includes the new strategies to determine as many elements as possible, to measure isotopic ratios as precisely as possible, and to analyze as small an amount of the samples as possible. For example, TIMS (thermal ionization mass spectrometry) is one of the analytical methods that seemed to satisfy the requirements of analytical geochemistry. In this chapter, the strategies in analytical geochemistry are briefly reviewed.

We have five strategies in analytical geochemistry: (i) major element geochemistry; (ii) trace element geochemistry; (iii) determination of mass fractionation; (iv) age dating; and (v) radiogenic isotopes for geochemical tracers.

1.1.1 Major Element Geochemistry

When geochemistry started, the strategy was only to determine the bulk major elements using classic wet chemistry. Then the classification of rocks was the first thing we would do. For the classification of igneous rocks, the use of a TAS (total alkali versus silica) diagram was one of the most common way (see Figure 1.1). This plots SiO2 versus Na2O + K2O abundances. Classifying the rocks is simple but important, but precise determination of the major elements is required.

Figure 1.1 Classification of igneous rocks by SiO2 and Na2O + K2O abundances. The classification is after Le Maitre [8]. The discrimination line between alkaline and subalkaline is after [9].

In order to replace the time-consuming and complex classic wet chemistry, X-ray fluorescence spectroscopy (XRF, see Section 1.1.1.1) was invented, which has been widely used since then. Furthermore, in order to observe and determine the major elements in spot areas, secondary electron microprobe with the energy dispersive spectrometry (SEM-EDS; see Figure 1.2) and electron probe micro analysis (EPMA; see Figure 1.3) were developed. The geochemists today first observe, describe, and analyze using these techniques to retrieve as much as possible the phase and information on the major elements from samples.

Figure 1.2 Schematic diagram of the scanning electron microscope (SEM). The electron beam is produced at the top of the column, and the shape of the electron beam is reformed using electron lenses and apertures. Finally, the beam is projected onto the sample. The electron beam is scanned over a small area, and the secondary electrons, scattered electrons, and the characteristic X-rays are detected by electron detectors and an X-ray detector, respectively. A semiconductor detector is used for X-ray detection. This configuration is called the energy dispersive spectrometry (EDS). The merits of SEM-EDS are (i) the spectra for all elements are recorded in one scan, so measurement time is very short; (ii) secondary and backscattering electron images are better than that of an electron probe micro analyzer (EPMA) (see Figure 1.3), because the detector positions are designed to collect these electrons; and (iii) the price of SEM-EDS is on-third to one-fourth of that of EPMA. In case that the electron beam does not cover the whole sample, an x-y stage is equipped to move the whole sample.

Figure 1.3 Schematic diagram of electron probe micro analyzer (EPMA). The electron gun, lenses, apertures are almost the same as those of SEM. The difference is in the collection and determination of characteristic X-rays. Characteristic X-rays emitted from the sample are diffracted by the crystals and collected by X-ray detectors. The position and material of the diffracting crystals are changed according to the wavelengths of characteristic X-rays of the target elements. This is called the wave length dispersive spectrometry (WDS). Five sets of diffracting crystals are maximally placed, and 10 elements can be determined in two scans. The merits of WDS are (i) the resolution of the characteristic X-ray is higher; therefore (ii) the background is lower; and (iii) X-ray diffraction is independent of the detector. Therefore, the diffracting crystal and the detector are chosen separately. Thus (iv) detection and diffraction correction are independent of the X-ray wavelength. Spot analysis of EPMA is simple. The polished and carbon-coated sample is set on the sample stage. The sample is bombarded by the electron beam, and the characteristic X-rays are measured. If more than five elements are to be measured, the diffraction crystals and their positions are changed, and the characteristic X-rays are measured again. The concentration is calculated by a so-called ZAF correction method using standard materials. Z, A, and F mean influences from the atomic number, X-ray absorption, and secondary fluorescence, respectively. The precision of elemental analysis is highly dependent on the standard materials. The stage is moved along the x and y directions on the x-y stage. The scanning (mapping) analysis is a feature of SEM-EDS and EPMA. The measurement time of each point is the X-ray integration time. Scanning measurement by WDS takes a very long time, and therefore only the spot analysis should be done by WDS. The size of the X-ray is 2.5 µm in diameter, and therefore a scanning stage of <2.5 µm is meaningless.

1.1.1.1 X-ray Fluorescence Spectrometer

Details of the XRF spectrometer are shown in Figure 1.4. XRF is mainly applied for the analysis of solid samples. Generally, about 100 mg sample is diluted with 10 times a flux, which is composed of a mixture of pure LiBO2 and Li2B4O7, and melted into a glass bead in a Pt crucible. As the mass number increases, the absorption of X-rays also increases, and therefore lithium borate is ideal material to make the glass bead for measurement of the emitted X-rays.

Figure 1.4 Schematic diagrams of (a) an X-ray fluorescence spectrometer and (b) a gas proportional counter. X-rays produced by the X-ray tube are made to fall on a sample bead. A Rh anode X-ray tube is often used. Secondary characteristic X-rays are emitted from the sample. X-rays are collimated onto the diffracting crystal by the primary collimator. From the entrance of the primary collimator to the detector, the chamber is kept in vacuum. Then the beams enter the diffracting crystal. There are various crystals suitable for the energy of X-rays, such as LiF, pentaerythritol crystal (PET), thallium acid phthalate crystal (TAP), and so on, and crystals can be exchanged easily without breaking the vacuum. The diffracted characteristic X-rays are measured by (b) a gas flow proportional counter, which counts the pulses. The counter is a metal box filled with a mixture of Ar and CH4. There are two windows made of PP (polypropylene) film for X-ray input and output. There is a wire in the center to which a high voltage is supplied and the preamplifier is connected. When X-rays fall on the counter, they ionize the gas and electrons are generated and amplified. Thus an electric pulse is counted. In a modern XRF spectrometer, wavelengths of characteristic X-rays of each element are automatically chosen, characteristics of the counter are automatically compensated, and deconvolution of peak overlaps is automatically performed.

To minimize loss of the secondary X-rays, the chamber is kept under vacuum. Air is made of N2 and O2, and their mass numbers are higher than those of Li or B....