FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1
Quantitative analysis of rock-forming minerals and volcanic
glasses by electron probe microanalyzer
Hiroshi Shukuno
Research Program for Geochemical Evolution, Institute for Frontier Research on Earth Evolution (IFREE)
2mm and an outer diameter of 3mm. Their standards are composed of natural minerals and synthetic compounds, and a set
of standards can be selected for a given purpose by changing
the combination used from those available. The JEOL standards have high quality and flexibility. At JAMSTEC, fiftytwo JEOL standards and one standard prepared at JAMSTEC
are available, encompassing forty-three elements (B, C, F, Na,
Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn,
Ga, Ge, Sr, Y, Zr, Nb, Ag, Cd, Sn, Sb, Te, Ba, Hf, W, Pt, Hg,
Pb, Bi, La, Ce, Pr and Nd). A list of these standards and their
chemical composition is given in Table 1. Carbon-coating was
simultaneously applied to all the standard sets with a thickness
of 20nm. Therefore, any systematic errors arising from differences in the thickness of the carbon-coating are minimized.
Introduction
An analysis of a small area on the surface of a solid specimen can be carried out with an electron probe microanalyzer
(EPMA). An electron beam is focused on to the surface of the
specimen and interacts with it, resulting in emission of
backscattered primary electrons, low-energy photoelectrons,
Auger electrons and characteristic X-rays. An X-ray detector,
a wavelength- dispersive spectrometer (WDS) and/or an energy-dispersive spectrometer (EDS), records the X-ray spectrum
emitted from the specimens in the EPMA. EPMA technology
has been developed since 1960s, and EPMA chemical analyses for common rock-forming minerals have been technically
achieved by petrologists and mineralogists. The methods of
analysis by EPMA have been described by many researchers
(e.g., Soejima, 1987). An EPMA can perform analyses of
micron-scale areas, can qualitatively analyze elements from
5Be to 92U, and quantitatively detect amounts greater than
0.001wt.%.
In JAMSTEC, two EPMAs have been installed for use by
many researchers. One is a JEOL SUPERPROBE JXA-8900
equipped with five WDS-type detectors and an EDS-type
detector. The other is a JEOL SUPERPROBE JXA-8800
equipped with four WDS-type detectors. Chemical analyses of
rock-forming minerals and volcanic glasses have been carried
out with the EPMAs in IFREE, in order to resolve the genesis
and evolution of magmas from subduction zones and mantle
plumes. In this paper, quantitative analyses for major and
some minor components of rock-forming minerals and volcanic glasses using the EPMAs will be described.
Analytical procedures
The JXA-8900 and JXA-8800 microanalyzers at JAMSTEC are equipped with five and four WDS detectors, respectively. Each WDS detector contains two analyzing crystals
(spectroscopic crystals), which have different crystal lattice
planes for Bragg diffraction. These crystals can be adequately
changed through the computer system, according to the elements analyzed. The EPMAs at JAMSTEC have LDE1,
LDE2, TAP, PET and LiF analyzing crystals and cover a wide
variation of elements for analyses (Table 2). The EPMAs have
two types of X-ray detectors. The first type is a gas-flow
counter, and the second is a gas-filled counter. The analyzing
crystals with relatively wide distances between crystal lattice
planes are combined with gas-flow type detectors, e.g., LDE1,
LDE2, TAP and PET crystals. The gas-filled type detectors
are combined with PET and LiF crystals. A list of the analyzing crystals and the X-ray detectors is given in Table 2.
The conditions for analysis, which are composed of an
accelerating voltage, probe current, probe diameter, count time,
peak position and background position, are carefully chosen
depending on target material and elements analyzed. Sample
resistibility to the electron beam under analyzing conditions
was verified empirically using the standards and the target
materials. The conditions for analysing alkali elements, such as
Na and K, were checked with particular caution, to prevent loss
of X-ray intensity. Lower and upper background positions for
the analyzed elements were determined by doing a peak scan
near the peak for the element, both in the standards and in minerals similar in composition to the material analyzed. The peak
count time for each element was selected to obtain sufficient
X-ray intensities for the analysis. The probe diameter must be
selected, taking specimen damage, loss of X-ray intensity, and
defocus of the spectrometer into consideration. It was checked
that the probe diameter selected in the analyzing condition
minimized specimen damage. Consequently, a phenocryst-
Selection of standards
EPMA analyses are relative analyses. Therefore, the selection of standard materials is very important. The requirements
for standards for EPMA analysis are as follows: (1) accurately
determined chemical composition, (2) micro-scale homogeneity and (3) stability under electron exposure during measurement. Many kinds of materials are prepared as standard materials, for example, pure metals, alloys, oxides and boron minerals. It is preferred that standards with pure chemical compositions or standards similar in composition to the material analyzed are selected for EPMA analyses, to minimize systematic
errors from matrix corrections (Soejima, 1987; Scott et al.,
1995). It is difficult to collect a large number of high quality
standard materials for EPMA analyses, and to prepare them
for each instrument. At present, several companies produce
high quality standards available for electron microprobe analysis (e.g., Astimex Scientific Ltd. and JEOL DATUM Ltd.,
etc.). For example, the standards prepared by JEOL are
mounted in pipes with a length of 10mm, an inner diameter of
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FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1
sized grain was analyzed with a probe diameter of 5µm, and a
focused beam was used for a micro-grain, such as inclusions, at
conditions of 15-20kV and 12-25nA. Volcanic glass sensitive
to an electron beam, was analyzed with a probe diameter of
10µm at 15kV and 10nA. A matrix correction for all analyses
was processed by the ZAF correction prepared by JEOL. The
combinations of analyzing crystals and count times for some
examples are shown in Table 2. The standard materials used
are also summarized in Table 2. A summary of the conditions
used for some analyses is as follows.
Results and discussion
Representative analyses are shown in Tables 3-5. Averages
and standard deviations of more than ten analyses are also listed in Tables 3-5. Most of the analytical results satisfy the stoichiometry and the total amount of each mineral analyzed. The
low standard deviations indicate that the analyzed minerals
were homogeneous. The results between JXA-8900 and JXA8800 are almost identical (Tables 3-5), although there are
some exceptions. The results of SiO2 and Al2O3 contents in
enstatite indicate slightly different values between JXA-8900
and JXA-8800 (Table 3). This difference is probably due to
heterogeneity within each enstatite grain analyzed. K2O content of K-bearing minerals such as K-feldspar and adularia, is
also slightly different between JXA-8900 and JXA-8800
(Table 3). This difference is caused by the standard materials
selected for K analysis. K-feldspar (K 2O=5.62wt.%) and
potassium titanium phosphate (K2O=23.80wt.%), which are
prepared by JEOL, are used in JXA-8900 and JXA-8800,
respectively. The K2O content of the K-feldspar is too low as a
standard for K-analysis. In addition, the true value may be
slightly lower than the recommended value reported by JEOL.
The analytical results of olivine by 20kV-25nA are listed in
Table 4 with the recommended compositions. The analytical
results, including minor components such as MnO, CaO and
NiO, have good agreement with the recommended composition. The results between JXA-8900 and JXA-8800 are almost
equivalent (Table 4). Table 5 shows analytical results of the
submarine volcanic glass. The compositions determined by
XRF in JAMSTEC are also shown in Table 5. There is good
agreement in each element, although the SiO2 content and the
total amount by EPMA are slightly higher and lower than
those by XRF, respectively. Table 5 shows analytical results
of three individual grains from the volcanic glass. These
results are almost equivalent, indicating that the analyzed glass
is almost homogeneous and a good reference sample.
Most of the analytical results in this study are quantitatively
and stoichiometricaly satisfactory for our purpose.
Common rock-forming minerals
The conditions for analysis of common rock-forming minerals are an accelerating voltage of 15kV, a beam current of
15nA on the Faraday cup, a peak count time of 20-30sec, and
a probe diameter of 5µm for phenocryst-sized grains. A
focused beam is used for analysis of small grains, such as
inclusions. When using a focused beam, a beam current of
12nA and a peak count time of 10-20sec are used. 11 major
elements (Si, Ti, Al, Cr, Fe, Mn, Mg, Ca, Na, K and Ni) are
analyzed.
Minor components for olivine
Olivine is composed of cations of Mg, Fe and Si, and minor
Mn, Ca and Ni. Minor components in mafic minerals give us
information about magma generation. Consequently, they
must be analyzed with high precision. A sufficient X-ray
intensity for this analysis is not obtained with the conditions
shown above. An incident electron energy at least 2 to 3 times
of the critical excitation energy is required in order to obtain
precise analyses. Although a high accelerating voltage can
obtain a relatively high X-ray intensity, the following problems occur: (1) penetration of the beam into the specimen, (2)
necessity of a large absorption correction, and (3) loss of spatial resolution. Suitable conditions for the analysis of minor
components of olivine were obtained in this study: an accelerating voltage of 20kV, a beam current of 25nA and a probe
diameter of 5µm. The peak count times are 10-30sec for major
components (Si, Mg and Fe) and 100-200sec for minor components (Ca, Mn and Ni). A combination of high sensitive
analyzing crystal (LiFH) and detector was used in the analyses
for Mn and Ni. This combination can obtain about 3 times the
X-ray intensity relative to the regular combination. Therefore,
the total analyzing time was relatively decreased.
Acknowledgements. I wish to thank N. Irino and K. Tani for XRF
analysis of the volcanic glass. I am also greatly indebted to Y. Tamura, T. Tsujimori and T. Morishita for their suggestion. K. Uematsu
assisted with operation of JXA-8900.
References
Scott, V. D., G. Love, and S. J. B. Reed, Quantitative electron-probe
microanalysis, 311pp., Ellis Horwood Limited, 1995.
Soejima, H., Electron probe microanalysis, 597pp., The Nikkan
Kogyo Shinbun, LTD., 1987.
Volcanic glass
Glass is very sensitive to an electron beam. The analytical
conditions must be carefully chosen. Fig. 1 shows the variations of X-ray intensity for Na-Kα with count time under some
conditions. It indicates that a focused beam decreases the X-ray
intensity of Na-Kα with time. The preferred analytical conditions are: 15kV, 10nA, and a probe diameter of 10µm. The
peak count time is 10-30sec. Ten major elements (Si, Ti, Al,
Fe, Mn, Mg, Ca, Na, K and P) can be analyzed. The total analyzing time for a point is less than 4 minutes. It was checked
that the submarine volcanic glass as a reference sample could
survive damage from electron exposure during analysis.
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Figure 1. Variations of X-ray intensity of Na-Kα to count time in the volcanic glass. (a) 15kV, 10nA and focused beam. (b) 15kV, 20nA and focused
beam. (c) 15kV, 10nA and broad beam (10µm). The intensity of Na-Kα decreases with count time in the conditions using a focus beam (a and b).
However, the intensity of Na-Kα is almost constant with count time in the condition using a broad beam (10µm).
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Table 1. Lists of standard materials and their compositions
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Table 2. Lists of analyzing crystals and analytical conditions. (a) Combinations of analyzing crystals and X-ray detectors. (b) Combinations of analyzing crystals and analyzing time. (c) Combinations of standard materials for each target material.
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Table 3. Analyses of silicate and oxide minerals in 15kV-15nA. Representative analyses, average and standard deviations (>15) are listed.
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Table 3. (Continued)
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Table 4. Analyses of olivine in 20kV-25nA. Recommended compositions, represent
analyses, average and standard deviations (>20) are listed.
Table 5. Analytical results of submarine volcanic glass by broad beam (10µm) in 15kV10nA. The compositions determined by XRF are also listed. The results of three individual grains are almost equivalent. It suggests that the analyzed glass is homogeneous.
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