Applications of the high-resolution scanning proton microprobe in the Earth sciences: An overview

Chemical Geology, 83 (1990) 27-37 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

27

Applications of the high-resolution scanning proton microprobe in the Earth sciences: An overview Donald G. Fraser Department of Earth Sciences, University of Oxford, Oxford, Oxon. OX1 3PR (Great Britain) (Accepted for publication January 30, 1990)

ABSTRACT Fraser, D.G., 1990. Applications of the high-resolution scanning proton microprobe in the Earth sciences: An overview. In: P.J. Potts, C. Dupuy and J.F.W. Bowles (Guest-Editors), Microanalytical Methods in Mineralogy and Geochemistry. Chem. Geol., 83: 27-37. The proton microprobe uses a focused beam of high-energy protons to generate X-rays in a sample that is to be analyzed. The major advantage of the proton microprobe as compared with the electron microprobe lies in its much lower X-ray background. This makes possible analyses at the 5-10-ppm concentration level with a resolution of 1/~m. The principles of operation of the proton microprobe are reviewed and the instrument compared with the ion probe and synchrotron Xray microprobe. Examples of its use in analyzing finely zoned dolomite and in investigating the late-stage concentration of elements on grain boundaries in garnet lherzolite xenoliths are reviewed. The proton and X-ray microprobes are likely to contribute increasing amounts of high-quality trace-element data in the near future.

1. Introduction

The development of the electron microprobe and its ability to perform full major-element analyses on the micrometer scale led to a revolution in chemical petrology. Studies of trace-element distribution within and between phases are of complementary importance. Whereas the bulk-phase petrology is determined by the major components present, minor and trace elements can offer unique additional information. By definition, their concentrations are low so that very large changes in absolute concentration are possible, making them potentially more sensitive monitors of change than major elements; there are often many trace components present in a sample so that multivariate information is enhanced; and at low concentrations, they may obey Henry's law, thus yielding very simple thermodynamic relationships. However, stud0009-2541/90/$03.50

ies of trace-element distribution have made much less progress than comparable work with major elements, because their concentrations usually lie well below the routine detection limits of the electron microprobe. Recently, the determination of trace-element concentrations on the/zm scale has become possible by means of three different instruments: (1) the ion microprobe, or secondary ion mass spectrometer (SIMS); (2) the synchrotron X-ray microprobe; and ( 3 ) the high-resolution proton microprobe. The X-ray and proton microprobes are both essentially non-destructive X-ray analytical instruments which function by generating X-rays in a sampie. They thus differ fundamentally in their operation from the ion microprobe which uses as a beam of ions to sputter material from a sample. The ejected material is then ionized and analyzed using a mass spectrometer. Although advances are being made in the calibra-

© 1990 Elsevier Science Publishers B.V.

28

D.G. FRASER

tion of the ion probe for chemical, as opposed to isotopic, analysis, the technique is still complicated by problems of molecular interference and surface chemistry so that reliable results can be difficult to achieve. In addition, although the best beam spot currently available is in the range 5-10/zm, trace-element analyses are usually performed with a beam of ~ 50/tm in diameter (e.g., Hickmott et al., 1987). The synchrotron X-ray microprobe and the proton microprobe both depend on the use of focused high-energy particle beams to generate secondary X-rays which are then used for analytical purposes. This is directly comparable with the electron microprobe which is now highly developed and can routinely produce analyses down to concentrations of ~ 1000 ppm. With more difficulty, concentrations at the level of a few 100 p p m can be determined and in favourable cases some elements have been analyzed from individual spots with concentrations in the range 50-100 ppm. However, the detection limit of the electron microprobe is ultimately limited by the relatively high non-characteristic X-ray background (bremsstrahlung) caused by deceleration of the incident electron beam as it interacts with the atoms in the target sample. The X-ray and proton microprobes minimize this problem in different ways by using beams of photons and protons, respectively. The X-ray microprobe uses a synchrotron as the source of an intense X-ray beam which is then collimated and focused using modern X-ray mirrors (e.g., Frantz et al., 1988 ). This new technique currently has a spatial resolution of ~ 5/tm and requires access to a synchrotron source. However, there will undoubtedly be major advances in this technology in the near future.

uses a highly focused beam of protons to excite the sample. As in the case of electrons, the incident proton beam rapidly dissipates its energy on entering a sample by means of inelastic collisions with the electrons of the sample atoms. Since a proton is 1836 times as massive as the electrons in the target atoms, the incident protons lose only a small fraction of their energy per collision with an electron. This gives the proton microprobe two major advantages when compared with the electron microprobe: ( 1 ) higher spatial resolution because the beam is scattered less within the sample; and (2) a lower X-ray background (bremsstrahlung) caused by deceleration of the incident beam. The low X-ray background makes it possible to use the proton microprobe to measure traceelement concentrations down to levels of ~ 1 p p m on a 1-/zm beam spot, depending on the particular element and matrix. This is far below the best levels attainable using the electron microprobe and approaches the best detection limits currently available using the ion probe and synchrotron X-ray microprobes, but with a higher spatial resolution.

2. The proton microprobe technique

where a is the acceleration; f t h e force exerted on the decelerating particle; and m, its mass. Since the mass of the proton is 1836 times greater than that of an electron, and it experiences the same repulsive force, the primary proton bremsstrahlung should be a factor of 1/

The high-resolution scanning proton microprobe (SPM) is an analytical microbeam instrument which functions like an electron microprobe, but, instead of an electron beam, it

2.1. X-ray background

When a charged particle decelerates, it radiates its excess energy as braking radiation (bremsstrahlung). In the electron microprobe, the primary electron beam decelerates very rapidly when it meets the electrons of the target atoms and the intensity of primary electron bremsstrahlung is high, thus limiting the lower detection limit of characteristic X-rays from elements in the sample. The intensity of projectile bremsstrahlung can be approximated by: I oc a2 oc ( f / m ) 2

(1)

29

HIGH-RESOLUTION SCANNINGPROTON MICROPROBEIN THE EARTH SCIENCES

(1836) 2, or ,-, 1 0 6 lower than that in the electron microprobe. In practice, this factor is lowered because the proton undergoes more collisions than the electron during its passage into the target. A plot of proton bremsstrahlung as a function of X-ray energy is shown in Fig. 1. Unfortunately, however, the detection limits of the proton microprobe are not 106 times lower than those of the electron microprobe. In order to generate characteristic X-rays, an inner electron must be ejected from an atom within the sample by interaction with the incident beam. These secondary electrons, which are generated within the sample, also slow down as they interact with atoms in the target thus generating secondary electron bremsstrahlung. A plot of secondary electron bremsstrahlung as a function of X-rays energy is also shown in Fig. 1, and it can be clearly seen that for energies up to ,,, 11 keV (about Se-K, ), the background is dominated by the secondary electron bremsstrahlung. A comparison of the X-ray spectra 10

of a dolomite sample excited both by the electron microprobe and by the proton microprobe is given in Fig. 2 which indicates the substantial improvement in peak/background ratio that can be achieved using the proton microprobe. 2.2. Thin targets The ionization cross-section of an element decreases with decreasing energy of the exciting particle. Thus proton-induced X-ray emission or PIXE is simplest in either of two extreme cases. In the first, the target is kept very thin so that the energy of the proton beam does not decrease significantly during its passage through the specimen. In the second, the sample is kept thick enough that the proton beam is stopped completely within the specimen. For the investigation of geological samples using a 3.5-MeV beam, this thickness is < 100 #m, depending on the composition of the sample. The "stopping power" of a sample varies with the density and can be simplified as: S(E) = 1 /p .d E /d X

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30

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HIGH-RESOLUTIONSCANNING PROTON MICROPROBE IN THE EARTH SCIENCES

tion limits of 0.1-1 ppm can be achieved for many elements supported on a low-atomicnumber, e.g. carbon, film. However, such thin samples are difficult to prepare for geological materials of thin section size and the usual approach is to use the other relatively simple case in which the beam is completely stopped within the target.

2.3. Analysis of geological ("thick") targets As shown above, a geological sample with a mean atomic number in the range Si to Fe, prepared as a polished thin section 100/zm thick, will be thick enough to completely stop protons with energies of up to 3.5 MeV. This technique, in which the proton is completely stopped by the target is sometimes referred to as thick-target PIXE, or TTPIXE. The calibration of TTPIXE analyses is usually made either by comparison with standards of similar composition which have been analyzed by other techniques (e.g., Fraser et al., 1984), or by means of sophisticated peak-stripping and modelling software which may allow standardless analysis (e.g., Ryan et al., 1988). The latter standardless approach may also make use of prior knowledge of the major-element composition of the sample obtained, for example, using the electron microprobe. This minimizes any systematic errors which may occur such as errors in the measurement of total beam charge deposited. Using this method, analyses of traceelement concentrations down to 5 ppm n rock samples have recently been reported (Griffin et al., 1988, 1989) and their results agree well with the published values of the standards analyzed by them. A full description of this technique is given in Ryan et al. ( 1988 ).

31

tectors. A good crystal spectrometer can give energy resolution of 10 eV, depending on the X-ray energy, which is far better than the 120140 eV typical of a good Si (Li) detector. However, this resolution is achieved at the cost of a greatly reduced solid angle of detection and therefore intensity. In addition, only one element per spectrometer can be analyzed simultaneously. Since the main advantage of the proton microprobe lies in the analysis of traceelement concentrations, beam current and counting time are critical and so the much higher angular efficiency of a large Si (Li) detector is usually preferred. A further problem encountered in the determination of trace-element concentrations in geological samples lies in the high X-ray yields produced by the major elements in the sample. High X-ray intensities lead to increased dead time in the detector and an enhanced background from which it is difficult to resolve the trace-element peaks. For these reasons, X-ray filters are usually interposed between the sample and the detector in order to reduce the intensitY of low-energy X-rays. A wide variety of filters has been used including plastic of different thicknesses, metal foils, compound matedais, films containing a critical absorber and "magic" or "funny" filters containing one or more holes which may be used to manicure the X-ray background. All of these filters must be calibrated experimentally, but are almost essential for the analysis of trace elements in geological materials. Further details of the proton microprobe technique are given in recent reviews elsewhere (Watt and Grime, 1987; Johansson and Campbell, 1988 ).

3. Geological applications 2.5. Detectors and filters Although wavelength-dispersive (crystal) spectrometers have been widely used on the electron microprobe, most PIXE work has been carded out using energy-dispersive Si (Li) de-

3.1. Trace-element zoning in a secondary dolomite Although there have been numerous analyses of geological materials using the PIXE

32

technique, very few have used the high resolution which is now available using focused beams. A good example of the resolution now possible is given in a recent paper (Fraser et al., 1989) in which a dolomite crystal, which showed well-developed cathodoluminescent zoning, was shown to have/~m scale zoning of Fe, Mn and Zn. The Mn zones coincided with the cathodoluminescence. A two-dimensional map of the concentrations of these elements is shown in Fig. 3 and line profiles obtained by

D.G.

FRASER

scanning across the zoning pattern are shown in Fig. 4. X-ray spectra for this sample have been shown in Fig. 2. The complex zoning shown indicates that the chemical potentials of the Fe, Mn and Zn components varied abruptly and independently in the pore-water solution. Comparison with the concentration profile of Sr suggests that these abrupt variations were probably caused by changing redox conditions. Peak-stripping and back-ground correction for these data were carried out us:

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tribution in mantle xenoliths have been made by Fraser et al. (1984) and by Griffin (1988, 1989). In the first study, Fraser et al. used the highresolution and scanning capabilities o f the Oxford proton microprobe to study the distribu-

3.2. S t u d i e s o f m a n t l e m a t e r i a l s

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tion of Sr in garnet lherzolite xenoliths. No software was available at that time for standardless analysis and so the Sr X-ray yields were calibrated with reference to clinopyroxene samples in which Sr had been previously determined by isotope dilution mass spectrometry. The results showed clearly that in a

garnet lherzolite xenolith from which radiogenic S7Sr could be readily acid leached, the Sr was located predominantly along grain boundaries and cracks in the specimen. This was interpreted as showing that the Sr had been introduced at a late stage, after the primary texture of the rock had formed (Fig. 5). In

36

D.G. FRASER

contrast a different xenolith, from which Sr could not easily be acid leached, showed no late-stage enrichment of Sr in the sample as is shown in Fig. 6. More recently, Griffin et al. have followed up this work by using the proton microprobe to examine trace-element distributions in garnets from sheared mantle xenoliths (Griffin et al., 1989) and in silicate inclusions in diamonds (Griffin et al., 1988). The trace-element data presented in these studies showed marked trace-element zoning in garnets from high-temperature garnet lherzolite xenoliths and that, as in the case of Sr noted above, the concentrations of several elements including Ti, Zr Y and Cr were increased in late-stage processes. These data indicate that considerable care should be taken in using the chemical compositions of garnet xenoliths to infer compositions of primary mantle material. Other applications of the proton microprobe include spot analyses of lunar oxide minerals (Blank et al., 1984), studies of U / P b distribution in zircons (Clark et al., 1979; Lucas et al., 1981 ) and of trace-metal concentrations in sulphide minerals (e.g., Cabri et al., 1985). Recently, preliminary reports (Andersson et al., 1987; Sie et al., 1987) have also been given of attempts to use the proton microprobe to determine trace-element concentrations in fluid inclusions. This work is at an early stage, but the capability of the proton microprobe to yield information on the variation of concentration with depth makes these studies very promising.

reduce the detected X-ray intensities from major elements in the sample. In some cases, for example measurements of lanthanide concentrations using their L-lines, the extra resolution of a crystal spectrometer could be attractive to allow separation from the K-lines of major elements present, but this would be at the expense of longer counting times. The proton microprobe has advantages in terms of resolution and ease of beam scanning over the synchrotron X-ray microprobe, but for those with access to a synchrotron, the latter instrument should have shorter counting times and is likely to undergo rapid development in the near future. It is probably no exaggeration to predict that the routine availability of traceelement analyses on t h e / t m scale will have as much impact on geochemistry and petrology as did the introduction of the electron microprobe 25 years ago.

4. Conclusions

References

The high resolution proton microprobe is now capable of providing trace-element analyses at levels down to a few p p m from specimen areas of 1 # m in diameter or even below. The difficulties in performing quantitative analyses on thick geological samples now lie primarily in the selection of suitable filters to

Anderson,H.H. and Ziegler,J.F., 1977.The Stoppingand Rangesof Ions in Matter, Vol. 2. Pergamon,New York, N.Y. Andersson, A.J., Clark, A.H., Ma, X.P., Palmer, G.R., MacArthur, J.D., Bodnar, R.J. and Roedder, E., 1987. Geol. Assoc.Can.-Mineral. Assoc. Can., Annu. Meet., Saskatoon, Sask. Blank, H., E1 Goresy, A., Janicke, J., Nobiling, R. and

Acknowledgements This work is funded by the NERC under grant G R 3 / 7 3 4 2 which the author gratefully acknowledges. It is also a pleasure to acknowledge discussions with David Feltham and Mark Whiteman and the assistance of the Oxford proton microprobe group whose expertise was essential for the performance of some of the work described here. I also thank Dr. N. Charnley for his assistance in obtaining the electron microprobe X-ray spectrum shown in Fig. 2.

HIGH-RESOLUTION SCANNING PROTON MICROPROBE IN THE EARTH SCIENCES

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