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Quantitative REE analysis of silicates by SIMS: Conventional energy filtering vs. specimen isolation mode
Chemical Geology, 103 (1993) 45-54 Elsevier Science Publishers B.V., Amsterdam
45
[AL]
Quantitative REE analysis of silicates by SIMS: Conventional energy filtering vs. specimen isolation mode N.D. MacRae a, P. Bottazzi b, L. Ottolini b and R. Vannucci c aDepartment of Geology, The Universio, of Western Ontario, London, Ont. N6A 5B7, Canada bCNR-Centro di Studio per la Cristallochimica e la Cristallografia, via Bassi 4, 27100 Pavia, Ital3 CDipartimento di Scienze della Terra, Se:. Mineralogia, Petrografia e Geochimica, via Bassi 4. 27100 Pavia, Italy (Received May 22, 1991 ; revised and accepted July 27, 1992 )
ABSTRACT MacRae, N.D., Bottazzi, P., Ottolini, L. and Vannucci, R., 1993. Quantitative REE analysis of silicates by SIMS: Conventional energy filtering vs. specimen isolation mode. Chem. Geol., 103: 45-54. REE relative ion yields obtained by secondary ion mass spectrometry (SIMS) from amphiboles and clinopyroxene samples are reported for both the conventional energy filtering (CEF) and specimen isolation (SI) modes of operation. Both techniques are equally quantitative, with particular advantages for each. Data obtained by SI are virtually free of molecular ion interferences, but the primary beam diameter is large (70-100 ~tm ) and a slight loss of accuracy may occur due to critical beam placement within the unique geometry of the sample holder. Molecular ions of light REEs overlap peaks of heavy REEs in CEE operation, requiring a correction procedure involving determination of MO + / M + yields and solution of simultaneous equations. The primary beam diameter, however, is approximately one quarter that for SI--an essential requirement for the analysis of small and chemically zoned crystals. Ion yields from crystals do not differ from those for glass of identical composition in CEF. but in SI are approximately 15 per cent lower.
1. Introduction
For the geochemical study of silicates by insitu quantitative analysis of major and minor element analyses, the electron microprobe continues to be the instrument of choice. However, analysis is normally limited to concentrations above 500 ppm. For analysis of the rare earth elements (REEs), the electron microprobe has a usefulness limited to those few minerals (e.g., apatite, zircon, chevkinite), which strongly concentrate the REEs. Of the more recently developed microanalysis techniques, secondary ion mass spectrometry (SIMS) offers many advantages to geochemCorrespondence to." N.D. MacRae, Department of Geology, The University of Western Ontario, London, Ontario N6A 5B7, Canada.
0009-2541/93/$06.00
ists: polished rock/mineral sections of thin sections are the normal sample material, sensitivity is in the ppb range, virtually all elements are available, the diameter of the analyzed area may be as small as 10-15 ¢tm, and very small quantities of sample are consumed. SIMS employs a primary beam of focused ions to sputter microvolumes of sample (target consumption being in the order of ng); the generated secondary ions are sent through a mass spectrometer for analysis (Lovering, 1975). The full potential of the technique is being only slowly realized, however, primarily because of the generation of molecular ions by sputtering and the high variability of secondary ion yield with chemical composition of the sample. Some attempt has been made to predict ion yields on the basis of theoretical as well as semi-empirical models (e.g., Deline and Ev-
© 1993 Elsevier Science Publishers B.V. All rights reserved.
46
ans, 1978; Reed, 1983; Hinton, 1990), but at present quantification relies upon the use of proper reference standards. Matrix variation effects may be calibrated by the use of standards of similar major element composition, while molecular ion overlaps may be reduced by using high mass resolution methods (Reed, 1983, 1984) or, more commonly, by some form of kinetic energy filtering. The latter procedure, which consists of adjusting (or 'offsetting') the energy 'window' of the spectrometer, is based on the fact that molecular ions have a much narrower kinetic energy distribution than atomic ions and thus can be discriminated against (see Shimizu et al., 1978; Ray and Hart, 1982 ). In addition, a further benefit in using energy filtering would be the reduction of matrix effects (Crozaz and Zinner, 1986; Bottazzi et al., 1992). In this study, we compare the REE data obtained from a number of amphibole and clinopyroxene samples using the two most commonly described methods of energy filtering-the "conventional energy filtering" (CEF) technique described by Zinner and Crozaz (1986a), Shimizu et al. (1978) and others and the "specimen isolation" (SI) technique described by Metson et al. ( 1983, 1984), MacRae ( 1987 ) and others. We also evaluate the wisdom of using silicate glass calibration standards for the analysis of crystals.
2. Experimental The compositions of the amphibole and clinopyroxene samples used as "calibration sampies" are listed in Table 1 together with those of amphibole H8 (our control sample ). For all samples except KSS, major elements were determined by electron microprobe analysis (JEOL JXA-8600 Superprobe, using appropriate silicate standards; Dept. of Geology, UWO). REE contents were determined by inductively coupled plasma mass spectrometry (ICP-MS; by XRAL, Don Mills, Ontario ); for the three clinopyroxenes, analyses were also
QUANTITATIVE REE ANALYSIS OF SILICATES BY SIMS
done by INAA (ACTLABS, Ancaster, Ontario) and the results recorded are based upon averages of the two data sets. Major element concentrations of amphibole K S S were determined by electron microprobe analysis (F.A.Caporuscio, pers. commun., 1989) and its REE concentrations by inductively coupled plasma emission spectrography (ICP-AES; by CNR-CSCC, Pavia). Inhomogeneity of major elements was tested by choosing random points for electron microprobe analysis; the resulting variations--including both signal uncertainties as well as chemical heterogeneity in the samplemwere less than 10%. An estimate of sample inhomogeneity at trace levels was obtained by SIMS analyses using the CEF mode; resultant coefficients for the REEs (except for Eu and Gd which require deconvolution procedures; see later) are generally less than 12%. For all samples except J W 8 7 amphibole and JW83 amphibole, we have studied both the fused products of pure mineral separates and the crystals themselves. All except amphibole K S S were fused in platinum crucibles at 1450°C in a high-temperature furnace (see MacRae, 1987 ); K S S powder was fused using a Pt-Rh strip heater according to the method outlined by Nicholls (1974). The same mounted glass fragments were used for both operating modes of SIMS analysis. A CAMECA IMS 4fion microprobe, housed at CNR-CSCC, Pavia, Italy, was used in this study. For all analyses the 1 6 0 - - primary beam was mass-filtered, accelerated by 12.5 kV. For each of the six fused calibration samples and one control sample and the corresponding mineral grains, a minimum of six arbitrarily chosen spots was analyzed by both techniques. Intensities were measured over seven cycles at each location, but in order to get secondary ion signals under steady state sputtering conditions, data for only the last five cycles were accepted. No corrections have been made to the data for peak drifting with time; however, 3°Si--the reference mass and that with the narrowest peakmwas normally checked at every
47
N.D. MacRAEETAL. TABLE 1
Major and RE element sample compositions Amphiboles
Clinopyroxenes
JW87
JW83
SiO2 TiOz A1203 Cr203 FeO .1 MnO MgO CaO Na20 K20 F CI
48.76 *2 1.17 6.04 0.01 16.35 0.05 12.69 11.18 1.14 0.35 0.03 0.03
47.73 *z 1.61 6.26 0.01 16.83 0.04 12.13 11.09 1.27 0.54 0.09 0.02
Total
97.80
97.62
(ppm) La Ce Nd Sm Eu Gd Dy Er Yb
15.3 *4 61.6 71.8 23.9 2.48 29.6 34.3 22.4 22.4
22.4 28.0 9.8 2.00 11.4 13.8 8.8 9.0
KSS
H8
A26C
41.69 *2 0.98 9.88 0.11 21.09 0.65 8.58 11.27 2.23 1.41 0.36
51.88 *z 0.06 1.14 0.00 18.20 0.83 6.62 20.10 0.91 0.00 -
98.25
99.74
A9
A1977
(wt.%)
5.7 *4
40.27 *3 5.36 13.76 0.01 17.02 0.00 9.87 10.36 2.91 1.17 100.72
21 *~ 67 56 15 3.45 9.9 8.93 5.28 4.7
3.5 *4 14.8 18.6 7.2 1.28 11,8 18.3 18.2 25.1
1.6 *4 6.0 8.0 4.4 0.32 3.9 2.7 1.6 3.8
51.09 *2 0.03 0.63 0.01 18.67 1.48 5.57 21.78 0.88 0.00
100.14
1.2 *4 4.6 5.3 2.7 0.23 2.2 1.2 0.7 2.5
50.80 .1 0.05 0.56 0.00 18.53 1.57 5.30 21.68 0.70 0.00
99.19
1.2 *4 4.4 4.6 2.0 0.20 1.4 0.9 0.5 1.8
*~Total Fe as FeO. *2EMP data from JEOL JXA-8600 Superprobe; analyst R.L. Barnett (U.W.O.). *3EMP data from F.A. Caporuscio (pers. commun., 1989). *4Analyses by ICP-MS (XRAL, Don Mills, Ontario) and INAA (ACTLABS, Ancaster, Ontario ). *SAnalysis by ICP-AES, CNR-CSCC, Pavia, Italy.
location while the REE peaks were checked less frequently. Although no corrections were made for background noise, background intensity was measured at mass 174.5 for each cycle, and was routinely less than 1 count per run (i.e. over 25 s acquisition time). 2.1. CEF In the CEF mode of operation the primary
160-- beam was focused to produce a sputter crater at the sample surface approximately 20 #m in diameter. Samples were coated with a layer of gold approximately 400/~ thick. Posi-
tive secondary ions were accelerated through a nominal 4.5 kV sample voltage offset by - 8 0 V (Bottazzi et al., 1990). More precisely, we closed the energy window almost completely, tuned the mass spectrometer to the maximum signal, opened the energy slit symmetrically to a total width corresponding to 50 eV and offset the sample voltage by 80 V. Using the 150 #m imaged field and wide open entrance and exit slits, we collected data for the following peaks:
30, 138, 139, 140, 146, 149, 151,153, 156, 158, 161,162, 163, 167 and 174. Counting times of 20 s per cycle were used for all the REE peaks and 2 s for 30 (Si).
48
QUANTITATIVEREE ANALYSISOF SILICATESBY SIMS
The voltage offset was sufficient to filter interferences from masses 138Ba, 139La, 14°Ce, 146Nd and 149Sm. EU was resolved from BaO interferences by solving simultaneous equations for data from masses 151 and 153. To resolve Gd and Dy from CeO and NdO, respectively, a least squares procedure (consisting of a simplification of the method described by Zinner and Crozaz, 1986a) was adopted for the signals from masses 156, 158, 161 and 162. This included assuming a fixed oxide-to-element ratio pattern (determined previously from accurately known standards) and the determination of an overall multiplication factor that compensates for the effects of possible slight changes in experimental conditions. Dy was also determined from the signal at mass 163, which suffers only from a negligible interference of SmO; the results of both Dy determinations agree. Ion intensities were corrected for isotopic abundances and normalized to Si to remove the effects of instrumental variations. REE and Si intensities from the standard samples were used in combination with the known REE and Si atomic concentrations (recalculated from the analyses of Table 1 ) to compute relative ion yields according to the formula: (ion-yield) = (IREE/Isi)/( [ R E E ] / [ S i ] ) (1) where [REE ] and [Si] are atomic concentrations and IREE and Is, are corrected ion intensities. Relative ion yields are reported in Table 2.
2.2. SI The major difference between the CEF and SI methods operation is that in the SI mode the sample is sufficiently insulated from all conductive materials that it will charge freely under the primary beam. The samples not only are uncoated but also are isolated from the metal sample holder by Teflon rings and discs. The top is covered by a stainless steel mask,
backed with a thin Teflon disc, both drilled with fifteen evenly spaced 3-mm diameter holes, exposing the sample surface (MacRae, 1987). Potential contour mapping by Lau et al. (1985) within the instrument's specimen chamber showed that the geometry of a charged aperture (the steel mask) above an insulated specimen produces a stable voltage some 450600 V below the secondary accelerating voltage (4.5 kV) and causes trapping of low kinetic energy ions, thus providing the extreme energy filtering of secondary ions characteristic of SI analysis. Thus, only high-energy ions can enter the magnetic sector and be analyzed. The 3-mm diameter aperture was chosen to optimize simply charged ion intensity while maintaining m a x i m u m molecular ion suppression (Metson et al., 1983; Mclntyre et al., 1985 ). While the spectra processed through the mass spectrometer are virtually free of molecular ion interferences, intensities are low unless large primary currents are used; for this study we used a primary current of approximately 750 nA, resulting in sputter craters in the samples approximately 100 ¢zm in diameter. In order to obtain m a x i m u m intensity, the energy slits and all apertures of the secondary column were fully opened. We collected data for the following peaks: 3°Si ' 138Ba ' 139La ' 140Ce ' 146Nd ' J52Sm ' 153Eu ' 158Gd, 163Dy, 166Er and 174yb. Counting time f o r 3°Si was 2 s, for 138Ba 10 S, and 15 s for all the REEs. As for CEF, we collected data for seven cycles, but accepted only the last five. Intensity ratios and analytical precision for the SI mode of operation appear with the corresponding CEF data in Table 2.
3. Results and discussion
Published studies of secondary ion yields are still scarce, particularly those that document data for a variety of samples. Table 2 lists the REE ion yields, normalized to Si ÷, from our
49
N.D. MacRAE ET AL. TABLE 2 REE ion yields and CEF/SI ion yield ratios
Fused amphiboles JVV87
JH'83
Reference values
Fused clinopyroxenes KSS
avg
s.d.
A26C
.49
.41977
avg
s.d.
N B S 6 1 0 *l
D / W g1.2
(%)
(%) Ion yields: REE+/Si+ / [ (REE~t) / (Sia,) ] CEF: La Ce Nd Sm Eu Gd Dy Er Yb
2.42 2.35 2.57 2.83 2.65 2.67 2.66 2.41 2.67
2.35 2.37 2.49 2.61 2.32 2.66 2.58 2.46 2.67
2.52 2.41 2.55 2.58 3.22 3.08 3.43 3.07 3.12
2,43 2.38 2.54 2.67 2.73 2.80 2.89 2.65 2.82
2.9 1.0 1.3 4.2 13.6 7.0 13.3 11.3 7.5
3.14 2.91 3.38 3.76 2.83 3.38 3.66 3.06 3.82
3.14 2.80 3.49 3.70 2.75 3.40 3.53 3.04 3.59
3.26 2.91 3.46 3.73 2.56 3.46 3.34 3.11 3,77
3.18 2.87 3.44 3.73 2.71 3.41 3.51 3.07 3.73
1.9 1.7 1.5 0,5 4,1 0,9 3.7 1.0 2.7
SI: La Ce Nd Sm Eu Gd Dy Er Yb
1.65 1.61 1.76 1.99 1.92 2.11 1.97 1.87 1.74
1.36 1.35 1.44 1.59 1.46 1.78 1.63 1.54 1.45
1.36 1.30 1.37 1.40 1.88 2.21 1.90 1.65 1.43
1.46 1.42 1.52 1.66 1.75 2.03 1.83 1.69 1.54
9.4 9.6 11.1 14.8 11.9 9.0 8.0 8.1 9.2
2.08 1,94 2,23 2.60 1.93 2.63 2.38 2.16 2.28
1.81 1.77 2.15 2.37 1.85 2.38 2.20 1.92 2.00
1.78 1.71 1.99 2.17 1.60 2.50 1.94 1.83 2.03
1.89 1.81 2.12 2.38 1.79 2.50 2.17 1.97 2.10
6.9 5.5 4.7 7.6 7.8 4.0 8.3 7.1 6.2
La Ce Nd Sm Eu Gd Dy Er Yb
Ion yield ratios: [ (REE+/Si + )CEF/(REE+/Si + )s~ 1.46 1.72 1.86 1.68 10.1 1.51 1.46 1.75 1.86 1.69 10.1 1.50 1.45 1.73 1.85 1.68 10.1 1.52 1.43 1.64 1.84 1.64 10.4 1.45 1.38 1.59 1.71 1.56 9.0 1.47 1.29 1.57 1.43 1.43 7.7 1.29 1.35 1.58 1.81 1.58 12.0 1.54 1.29 1.60 1.87 1.59 15.1 1.42 1.53 1.84 2.18 1.85 14.6 1.68
1.73 1.58 1.62 1.56 1.49 1.43 1.60 1.58 1.80
1.83 1.70 1.74 1.72 1.56 1.38 1.72 1.70 1.86
1.69 1.59 1.63 1.58 1.51 1.37 1.62 1.57 1.78
7.9 5.2 5.5 7.0 2.6 4.2 4.6 7.3 4.2
2.27 2.2l 2.59 2.78 2.92 2.75 2.50 2.45 2.47
Total av. 1.69 1.64 1.65 1.61 1.53 1.40 1.60 1.58 1.82
2.51 2.19 2.89 3.30 3.57 2.96 2.61 2.39 2.72
s.d. (%) 8.9 8.6 8.3 9.0 6.8 6.9 9.0 11.8 10.9
*~Hinton ( 1990, table 1; National Bureau of Standards silicate glass NBS610. *2Zinner and Crozaz ( 1986b, table 1 ); Drake and Weill REE silicate glass.
six fused amphibole and clinopyroxene samples obtained by both CEF and SI modes of instrument operation. Listed for comparison are literature values for the synthetic silicate glass NBS 610 (Hinton, 1990) and those from the synthetic Drake and Weill REE glass standards (Zinner and Crozaz, 1986b), both obtained by quite similar CEF configurations. The CEF ion yield data are all similar, but variations are evident. In fact, we note an inverse correlation of
ion yield to REE content, with yields from amphibole glasses generally lower than from clinopyroxene glasses. While we are unable to account fully for it, part of that correlation could be due to matrix differences. Although not as strong, the same correlation is evident for the SI data. Table 2 also contains the ratios of CEF to SI intensities, which vary by + 0.2 about 1.6. Under the adopted experimental conditions,
50
QUANTITATIVE REE ANALYSISOF SILICATES BY SIMS
REE intensities were practically the same for both CEF and SI modes, whereas the 3°Si signal was about 30% higher in SI than in CEF. The significant difference in the two methods can be ascribed mainly to the different energy ranges being monitored in the two experimental conditions. Using CEF operating conditions, a wideopen energy slit, and a Bertan High Voltage power supply, we measured the secondary ion intensity ratios 139La/3°Si and ~4°Ce/3°Si from a silicate glass (REE3; Drake and Weill, 1972 ) through 50 V increments of offset from - 100 to - 6 0 0 V, thus covering the range of offset experienced for SI operation (Fig. 1 ). The variations were systematic and reproducible (std. dev. < 5 % ) from different sample points over 6 h of data collection. From Table 2, we note that the variation in ion yields among REE is negligible with respect to the uncertainty of measurements, indicating that the application of a large voltage offset in SI does not introduce a discrimination among REE signals with respect to CEF (i.e. the energy distributions of REE are very similar in the range from moderate to very high energies). This fact was already documented by Muir et al. ( 1987b ) for analyses of the glass standard NBS 610.
5 o
139•30 3-
1O0
200
300 400 OFFSET (VOLTS)
500
600
Fig. 1. Secondary ion intensity ratios relative to progressive offsets of the sample voltage from - 100 V to - 600 V (4.4 kV to 3.9 kV).
Another difference between CEF and SI experimental conditions concerns the primary beam intensity; its increase from CEF to SI, however, is roughly matched by a proportional increase in sputter area, indicating that the beams have practically the same densities in CEF and SI modes. We then expect in principle the mechanism of sputtering to be similar and not to contribute to significant changes in ion yields. Measurements of sputtered craters were carried out on one of the fused amphiboles (JW87) using a DEKTAC II profilometer. A typical SI crater was about 5.4 #m deep, representing sputtering of approximately 1.1 × 10 - l l c m 3 / s (a little less than 3.5× 10 -z ng/s of sample). A corresponding CEF crater was typically 2.5/zm deep, representing sputtering of 2 E l 0 -13 c m 3 / s and less than 6.3× 10 -4 ng/s of sample. Notwithstanding the use of the O - primary beam and gold coating of the samples in CEF mode to reduce charge pile-up on the surface (Reed, 1989), it is known that charge accumulation occurs on the sputtered area, which mainly results in a shift of the energy distribution of secondary ions. In order to check its possible influence on relative ion yields in crystalline and glassy matrices, we monitored the position of the low-energy edge at 10% of the maximum intensity (Zinner and Crozaz, 1986a) of the energy distribution for mass 32 (O2 + ) as well as mass 16 (O + ), on crystalline and glassy kaersutite KSS. We noted that charging, after a sudden increase by 5-10 V during the first 5 min, follows a nearly linear trend (approx. 1 V/10 min) in both structures, with a constant difference between the two of less than 2 V; this process is typically reproducible within 1-2 V over a one-day span under the same operating conditions. A shift by 10-15 V is thus added to that instrumentally obtained by offsetting the sample accelerating voltage ( + 4.5 kV ). REE ion yields relative to Si were then determined over an offset voltage range from - 1 0 0 V to - 6 0 V and compared to those obtained at - 80 V; the re-
N.D. MacRAEET AL.
sults were reproducible to within 10% over the span of one day with the exception of the Yb data, which were higher. These results are consistent with the 13% variation reported by Crozaz and Zinner ( 1985 ) who measured REE intensity ratios relative to Ca + in phosphate grains for different levels of energy filtering over the 60 to 100 V offset range. Muir et al. (1987a) examined charge stabilization for SI and CEF modes of SIMS operation, using glass, ceramic and crystal of identical composition (titanite). They found that the results using SI showed very similar charging for all samples and that the reproducibility of the data was as good, if not better, than that from the CEF mode of operation. The degree of sample charging during SI operation was measured and discussed by Lau et al. ( 1985 ). In our experiments, precision of CEF determinations was commonly higher than for the same sample done by SI, particularly for amphibole samples. Several factors might be considered to influence the difference in standard deviations between data collected for the two techniques. Firstly, the counting time for secondary ions at each REE mass was 1/3 less for SI than for CEF; the shorter time should, however, have been compensated for by the larger sample volume sputtered for SI; this implies that any possible small-scale inhomogeneities would be averaged during analysis. Further, considering that relative REE ion yields are generally dependent on secondary ion energy, this dependence could explain the larger errors encountered during SI analysis; since the amount of charging during SI is slightly variable, characteristic of the matrix under investigation, the corresponding variations in the selected secondary ion energy could lead to variations in relative ion yields. Finally, we must consider a unique aspect of the SI sample holder geometry. As noted by Lau et al. ( 1985 ), the potential contours above an insulating specimen are not regular across the dimensions of the aperture, varying with distance from the geometric centre. The variation
51
is readily demonstrated, and despite an effort to maintain optimal beam positioning, the noted standard deviations are strongly influenced by the positioning of the sputtering site relative to the aperture. We have analyzed both crystalline and fused material of amphiboles KSS and H8 (only CEF) and of the three clinopyroxenes (both CEF and SI) (Table 3). The crystal/glass ion yield ratios are approximately unity for the CEF data, but approximately 0.85 for SI determinations. Ray and Hart ( 1982 ), using CEF, noted lower ion yields for glass than for crystals of the same compositions. Since then, however, Muir et al. ( 1987a, using CEF and SI on glass, ceramic and crystal of identical titanite composition), MacRae (1987, using SI) and Hinton (1990, using CEF) report virtually identical ion yields for materials of equivalent composition, regardTABLE3 Crystal to glass ion yield ratios Amphiboles
Clinopyroxenes
KSS*
A26
Avg.
St. dev.
(%) CEF: La 0.90 Ce 0.91 Nd 0.98 Sm 1.00 Eu 1.05 Gd 1.06 Dy 1.11 Er 1.04 Yb 1.11 SI: La Ce Nd Sm Eu Gd Dy Er Yb
H8
0.83 0.63 0.96 1.03 1.03 1.18 1.02 1 . 2 7 1.00 0.92 1.06 1.04 1.08 1.10 1.06 0.98 1.03 1.16
0.73 0.80 0.84 0.85 0.83 0.89 0.80 0.67 0.73
,49
A1977
0.73 0.93 0.99 1.00 1.05 0.69 1.02 0.94 1.01
0.78 0.95 0.97 1.02 0.98 0.98 1.01 1.00 0.95
0.77 0.96 1.03 1.06 1.00 0.97 1.06 1.00 1.05
11.8 4.3 7.6 9.8 4.9 14.6 3.9 4.3 7.1
0.60 0.79 0.82 0.81 0.70 0.88 0.80 0.87 0.87
1.41 1.03 0.80 0.83 0.80 0.82 0.89 1.00 0.85
0.91 0.87 0.82 0.83 0.78 0.86 0.83 0.85 0.87
38,9 12,7 2.0 2.0 7.2 3.6 5.1 16.1 7.6
*Ratios from Ottolini et al. ( 1991 ).
52
less of structural state. Certainly, any determinations of crystals are expected to show more compositional variation--as was the case in this study--simply because of natural variations due to chemical zoning and minute inclusions. Clearly, fused samples would make the best calibration material, at least for CEF operation. Our samples, analyzed by CEF in both matrix structures, did not show any systematic differences. Only La showed crystal-to-glass intensity ratios lower than 1 in amphiboles (average 0.87) and augites (average 0.71 ), the minimum value being 0.63 in A26. All other REEs yielded crystal-to-glass intensity ratios which are comparable and close to 1. As for the La contents, we suspect that some laboratory contamination during fusion could be responsible for higher La in the glasses. A second possibility is the presence of a light REE-enriched phase, such as apatite, which escaped detection during SEM investigation of the grains fused. Crystal-to-glass analyses (of the three clinopyroxenes) in SI mode again showed a problem with La, but all ratios were nevertheless less than 1 (average over REEs is 0.84 exclusive of La, as opposed to 1.02 for CEF). Major element variations within the amphibole group appear not to significantly affect REE ion yields (because the clinopyroxene samples are so similar in chemical composition, we are unable to draw conclusions in their case). This agrees with studies by Ray and Hart (1982) and Ottolini et al. (1992) who reported that for CEF determinations of such minerals, the Si-normalized ion yields show no consistent variations with major element chemistry. The CEF relative ion yields are quite similar for different REEs, with values from amphibole glasses generally lower than from clinopyroxene glasses. In amphiboles they range from 2.38 (Ce) to 2.89 (Dy); the variation for clinopyroxenes, with the exception of Eu, is 2.87 (Ce) to 3.77 (Yb). As far as Eu is con-
QUANTITATIVE REE ANALYSISOF SILICATES BY SIMS
cerned, we must point out that the scatter of bulk determinations is rather large, on the order of 20-25% in the three augites, whereas the range of variations of most REE experimental data, with respect to the assumed reference values (see Table 1 ), was very limited (typically about 10%). Such variations in Eu data are attributable to ( 1 ) primary inhomogeneity of the low primary Eu content in the subsampies used for analyses, and (2) different uncertainties of experimental data obtained by ICPMS and INAA methods. As a result, no definite value for Eu ion yield in clinopyroxene can be assessed at this point. The average value of 2.71 reported in Table 2 must be considered a rough estimate. Furthermore, if we calculate the ratios clinopyroxene/amphibole REE ion yields, we can see that they are almost identical for both CEF and SI techniques, including the increase from 1.16 (CEF) and 1.17 (SI) for Er to 1.32 (CEF) and 1.36 (SI) for Yb. On this basis, the differences observed in relative REE ion yields between amphiboles and clinopyroxenes may be reasonably ascribed to matrix effects deriving from differences in bulk chemistry of the two silicate families. The (chemical) matrix effects observed between fused amphiboles and clinopyroxenes are the same in both techniques, i.e. for REE ions sputtered with moderate and with high energies. It is also noted that the application of higher voltage offsets, as implied in SI, does not reduce matrix effects below those of CEF analyses. Therefore, in order to obtain the most accurate quantitative data, it is essential to reduce as much as possible the matrix differences between standards and samples. The REE ion yields obtained from the amphibole calibration samples were used to calculate the REE concentrations in the control sample H8 by both CEF and SI methods of operation. SIMS results are compared with ICPMS results in Fig. 2. Variations among the different data sources are small with the exception of Sm results (approx. 25%).
53
N.D. MacRAE ET A L
100-
jj. 0~
/z o
L)
10 Ii" ~ i / CEF ×
03
H8 Glass
SI
!
ICP-MS
La de Nd Sm Eu ad By Er V~ Fig. 2. Chondrite-normalized analyses of test sample H8, using the amphibole average ion yields of CEF and SI techniques, as compared with ICP-MS data.
4. Conclusions The data presented here show that the CEF and SI techniques of SIMS operation are equally quantitative. The (chemical) matrix effects observed between fused amphiboles and clinopyroxenes are the same in both techniques, i.e. for REE ions sputtered with moderate and with high energies. The effects of major element variation on REE yields are generally very limited; nevertheless, this study shows them to be sufficient that, for high accuracy, we cannot recommend the use of common quantification factors for both groups. Comparison of ion yields from crystals and glass of the same material shows that for CEF there are no significant differences. Calibration curves may thus be constructed using homogeneous glass standards and used for analysis of crystals. Yields from crystals using SI, however, appear to be approximately 15% lower than from glass. The major difference between the two techniques is that with SI molecular interferences are much more suppressed over the whole REE spectrum, while with CEF interferences of light REE oxides on the heavy REE peaks must be dealt with. Until we learn more about the factors which control ion yields of elements and compounds from a given matrix, interference handling in CEF relies upon either peak strip-
ping procedures--whose drawback is errors magnification--or upon the knowledge of M O + / M ÷ ratios (for very specific operating conditions) obtained from well-characterized standard samples. Two disadvantages of SI are that ( 1 ) in order to get a sensitivity equivalent to CEF, a large diameter primary beam (70-100 pm) must be used, and (2) primarily due to the critical placement of the beam in electric fields with strong gradients, some slight loss of precision is common. Precisions and accuracies in the order of 10% can generally be achieved with both techniques. This investigation shows the validity of the two energy filtering approaches for REE quantification of silicate minerals.
Acknowledgements We wish to acknowledge receipt of Italian funds through a NATO-CNR Senior Guest Fellowship (n. 217.23, 6/22/1989) and Canadian funds through NSERC (OGP0005539). Our thanks to J.B. Whalen for the JWamphiboles, to F.A. Caporuscio for the K S S amphibole and its analysis, and to M. Palenzona for his technical assistance and sample preparation. I.J. Muir kindly reviewed an early version of the manuscript and gave great assistance to the first author to sort out the revision problems. We gratefully acknowledge the constructive review and numerous suggestions for manuscript improvement by E. Zinner.
References Bottazzi, P., Ottolini, L. and Vannucci, R., 1990. Quantitative SIMS analysis of rare earth elements in maficultramafic rock samples. In: A. Benninhoven, C.A. Evans, K.D. McKeegan, H.A. Storms and H.W. Werner (Editors), Secondary Ion Mass Spectrometry, SIMS VII. Wiley, Chichester, pp. 413-416. Bottazzi, P., Ottolini, L. and Vannucci, R., 1992. SIMS analyses of rare earth elements in natural minerals and glasses: an investigation of structural matrix effects on ion yields. Scanning Electron Microsc., 14: 160-168. Crozaz, G. and Zinner, E., 1986. Quantitative ion micro-
54 probe analysis of the rare earth elements in minerals. Scanning Electron Microsc., 1986/II: 369-376. Crozaz, G. and Zinner, E., 1985. Ion probe determinations of the rare earth concentrations of individual meteoric phosphate grains. Earth Planet. Sci. Lett., 73: 41-52. Deline, V.R. and Evans, C.A., 1978. A unified explanation for secondary ion yields. Appl. Phys. Lett., 33: 578-580. Drake, M.J. and Weill, D.F., 1972. New rare earth element standards for electron microprobe analysis. Chem. Geol., 10: 179-181. Hinton, R.W., 1990. Ion microprobe trace-element analysis of silicates: Measurement of multi-element glasses. Chem. Geol., 83:11-25. kau, W.M., Mclntyre, N.S., Metson, J.B., Cochrane, D. and Brown, J.D., 1985. Stabilization of charge on electrically insulating surfaces--Experimental and theoretical studies of the specimen isolation method. Surface Interface Anal., 7:275-281. Lovering, J.F., 1975. Application of SIMS microanalysis techniques to element and isotopic studies in geochemistry and cosmochemistry. Natl. Bur. Stand., Spec. Publ., 427: 135-178. MacRae, N.D., 1987. Quantitative analysis of REEs by SIMS. Am. Mineral., 72: 1263-1268. Mclntyre. N.S., Fichter, D., Metson, J.B., Robinson, W.H. and Chauvin, W., 1985. Suppression of molecular ions in SIMS spectra of metals and semiconductors. Surface Interface Anal., 7: 69-73. Metson, J.B., Bancroft, G.M., McIntyre, N.S. and Chauvin, W.J., 1983. Suppression of molecular ion in the secondary ion mass spectra of minerals. Surface Interface Anal., 5: 181-185. Metson, J.B., Bancroft, G.M., Nesbitt, H.W. and Jonasson, R.G., 1984. Analysis for rare earth elements in accessory minerals by specimen isolated secondary ion mass spectrometry. Nature (London), 307: 347-349. Muir, I.J., Bancroft, G.M. and Metson, J.B., 1987a. A
QUANTITATIVEREE ANALYSIS OF SILICATES BY SIMS
comparison of conventional and specimen isolation filtering techniques for the SIMS analyses of geologic materials. Int. J. Mass Spectrom. Ion Proc., 75: 159170. Muir, I.J., Bancroft, G.M., MacRae, N.D. and Metson, J.B., 1987b. Quantitative analyses of rare-earth elements in minerals by secondary ion mass spectrometry. Chem. Geol., 64: 269-278. Nicholls, I.A., 1974. A direct fusion method of preparing silicate rock glasses for energy-dispersive electron microprobe analysis. Chem. Geol., 14:151-157. Ottolini, L., Bottazzi, P. and Vannucci~ R., 1992. Quantitative SIMS analyses of REEs in silicate minerals: Choice of the standard for natural and glassy matrix. Proc. Geoanalysis 90, Geol. Surv. Can. (in press). Ray, G. and Hart, S.R., 1982. Quantitative analysis of silicates by ion microprobe. Int. J. Mass Spectrom. Ion Phys., 44: 231-255. Reed, S.J.B., 1983. Secondary-ion yields of rare earths. Int. J. Mass Spectrom. Ion Phys., 54:31-40. Reed, S.J.B., 1984. Secondary ion mass spectrometry-ion probe analysis for rare earths. Scanning Electron Microsc., II: 529-535. Reed, S.J.B., 1989. Ion microprobe analysis--Review of geological application. Mineral. Mag., 53: 3-24. Shimizu, N., Semet, M.P. and All6gre, C.J., 1978. Geochemical applications of quantitative ion microprobe analysis. Geochim. Cosmochim. Acta, 42:1321-1334. Zinner, E. and Crozaz, G., 1986a. A method for quantitative measurement of rare earth elements in the ion microprobe. Int. J. Mass Spectrom. Ion Phys., 69:1738.
Zinner, E. and Crozaz, G., 1986b. Ion probe determination of the abundances of all the rare earth elements in single mineral grains. In: A. Benninghoven, R.J. Colton, D.S. Simons and H.W. Werner (Editors), Secondary Ion Mass Spectrometry, SIMS V. Springer, Heidelberg, pp. 444-446.
45
[AL]
Quantitative REE analysis of silicates by SIMS: Conventional energy filtering vs. specimen isolation mode N.D. MacRae a, P. Bottazzi b, L. Ottolini b and R. Vannucci c aDepartment of Geology, The Universio, of Western Ontario, London, Ont. N6A 5B7, Canada bCNR-Centro di Studio per la Cristallochimica e la Cristallografia, via Bassi 4, 27100 Pavia, Ital3 CDipartimento di Scienze della Terra, Se:. Mineralogia, Petrografia e Geochimica, via Bassi 4. 27100 Pavia, Italy (Received May 22, 1991 ; revised and accepted July 27, 1992 )
ABSTRACT MacRae, N.D., Bottazzi, P., Ottolini, L. and Vannucci, R., 1993. Quantitative REE analysis of silicates by SIMS: Conventional energy filtering vs. specimen isolation mode. Chem. Geol., 103: 45-54. REE relative ion yields obtained by secondary ion mass spectrometry (SIMS) from amphiboles and clinopyroxene samples are reported for both the conventional energy filtering (CEF) and specimen isolation (SI) modes of operation. Both techniques are equally quantitative, with particular advantages for each. Data obtained by SI are virtually free of molecular ion interferences, but the primary beam diameter is large (70-100 ~tm ) and a slight loss of accuracy may occur due to critical beam placement within the unique geometry of the sample holder. Molecular ions of light REEs overlap peaks of heavy REEs in CEE operation, requiring a correction procedure involving determination of MO + / M + yields and solution of simultaneous equations. The primary beam diameter, however, is approximately one quarter that for SI--an essential requirement for the analysis of small and chemically zoned crystals. Ion yields from crystals do not differ from those for glass of identical composition in CEF. but in SI are approximately 15 per cent lower.
1. Introduction
For the geochemical study of silicates by insitu quantitative analysis of major and minor element analyses, the electron microprobe continues to be the instrument of choice. However, analysis is normally limited to concentrations above 500 ppm. For analysis of the rare earth elements (REEs), the electron microprobe has a usefulness limited to those few minerals (e.g., apatite, zircon, chevkinite), which strongly concentrate the REEs. Of the more recently developed microanalysis techniques, secondary ion mass spectrometry (SIMS) offers many advantages to geochemCorrespondence to." N.D. MacRae, Department of Geology, The University of Western Ontario, London, Ontario N6A 5B7, Canada.
0009-2541/93/$06.00
ists: polished rock/mineral sections of thin sections are the normal sample material, sensitivity is in the ppb range, virtually all elements are available, the diameter of the analyzed area may be as small as 10-15 ¢tm, and very small quantities of sample are consumed. SIMS employs a primary beam of focused ions to sputter microvolumes of sample (target consumption being in the order of ng); the generated secondary ions are sent through a mass spectrometer for analysis (Lovering, 1975). The full potential of the technique is being only slowly realized, however, primarily because of the generation of molecular ions by sputtering and the high variability of secondary ion yield with chemical composition of the sample. Some attempt has been made to predict ion yields on the basis of theoretical as well as semi-empirical models (e.g., Deline and Ev-
© 1993 Elsevier Science Publishers B.V. All rights reserved.
46
ans, 1978; Reed, 1983; Hinton, 1990), but at present quantification relies upon the use of proper reference standards. Matrix variation effects may be calibrated by the use of standards of similar major element composition, while molecular ion overlaps may be reduced by using high mass resolution methods (Reed, 1983, 1984) or, more commonly, by some form of kinetic energy filtering. The latter procedure, which consists of adjusting (or 'offsetting') the energy 'window' of the spectrometer, is based on the fact that molecular ions have a much narrower kinetic energy distribution than atomic ions and thus can be discriminated against (see Shimizu et al., 1978; Ray and Hart, 1982 ). In addition, a further benefit in using energy filtering would be the reduction of matrix effects (Crozaz and Zinner, 1986; Bottazzi et al., 1992). In this study, we compare the REE data obtained from a number of amphibole and clinopyroxene samples using the two most commonly described methods of energy filtering-the "conventional energy filtering" (CEF) technique described by Zinner and Crozaz (1986a), Shimizu et al. (1978) and others and the "specimen isolation" (SI) technique described by Metson et al. ( 1983, 1984), MacRae ( 1987 ) and others. We also evaluate the wisdom of using silicate glass calibration standards for the analysis of crystals.
2. Experimental The compositions of the amphibole and clinopyroxene samples used as "calibration sampies" are listed in Table 1 together with those of amphibole H8 (our control sample ). For all samples except KSS, major elements were determined by electron microprobe analysis (JEOL JXA-8600 Superprobe, using appropriate silicate standards; Dept. of Geology, UWO). REE contents were determined by inductively coupled plasma mass spectrometry (ICP-MS; by XRAL, Don Mills, Ontario ); for the three clinopyroxenes, analyses were also
QUANTITATIVE REE ANALYSIS OF SILICATES BY SIMS
done by INAA (ACTLABS, Ancaster, Ontario) and the results recorded are based upon averages of the two data sets. Major element concentrations of amphibole K S S were determined by electron microprobe analysis (F.A.Caporuscio, pers. commun., 1989) and its REE concentrations by inductively coupled plasma emission spectrography (ICP-AES; by CNR-CSCC, Pavia). Inhomogeneity of major elements was tested by choosing random points for electron microprobe analysis; the resulting variations--including both signal uncertainties as well as chemical heterogeneity in the samplemwere less than 10%. An estimate of sample inhomogeneity at trace levels was obtained by SIMS analyses using the CEF mode; resultant coefficients for the REEs (except for Eu and Gd which require deconvolution procedures; see later) are generally less than 12%. For all samples except J W 8 7 amphibole and JW83 amphibole, we have studied both the fused products of pure mineral separates and the crystals themselves. All except amphibole K S S were fused in platinum crucibles at 1450°C in a high-temperature furnace (see MacRae, 1987 ); K S S powder was fused using a Pt-Rh strip heater according to the method outlined by Nicholls (1974). The same mounted glass fragments were used for both operating modes of SIMS analysis. A CAMECA IMS 4fion microprobe, housed at CNR-CSCC, Pavia, Italy, was used in this study. For all analyses the 1 6 0 - - primary beam was mass-filtered, accelerated by 12.5 kV. For each of the six fused calibration samples and one control sample and the corresponding mineral grains, a minimum of six arbitrarily chosen spots was analyzed by both techniques. Intensities were measured over seven cycles at each location, but in order to get secondary ion signals under steady state sputtering conditions, data for only the last five cycles were accepted. No corrections have been made to the data for peak drifting with time; however, 3°Si--the reference mass and that with the narrowest peakmwas normally checked at every
47
N.D. MacRAEETAL. TABLE 1
Major and RE element sample compositions Amphiboles
Clinopyroxenes
JW87
JW83
SiO2 TiOz A1203 Cr203 FeO .1 MnO MgO CaO Na20 K20 F CI
48.76 *2 1.17 6.04 0.01 16.35 0.05 12.69 11.18 1.14 0.35 0.03 0.03
47.73 *z 1.61 6.26 0.01 16.83 0.04 12.13 11.09 1.27 0.54 0.09 0.02
Total
97.80
97.62
(ppm) La Ce Nd Sm Eu Gd Dy Er Yb
15.3 *4 61.6 71.8 23.9 2.48 29.6 34.3 22.4 22.4
22.4 28.0 9.8 2.00 11.4 13.8 8.8 9.0
KSS
H8
A26C
41.69 *2 0.98 9.88 0.11 21.09 0.65 8.58 11.27 2.23 1.41 0.36
51.88 *z 0.06 1.14 0.00 18.20 0.83 6.62 20.10 0.91 0.00 -
98.25
99.74
A9
A1977
(wt.%)
5.7 *4
40.27 *3 5.36 13.76 0.01 17.02 0.00 9.87 10.36 2.91 1.17 100.72
21 *~ 67 56 15 3.45 9.9 8.93 5.28 4.7
3.5 *4 14.8 18.6 7.2 1.28 11,8 18.3 18.2 25.1
1.6 *4 6.0 8.0 4.4 0.32 3.9 2.7 1.6 3.8
51.09 *2 0.03 0.63 0.01 18.67 1.48 5.57 21.78 0.88 0.00
100.14
1.2 *4 4.6 5.3 2.7 0.23 2.2 1.2 0.7 2.5
50.80 .1 0.05 0.56 0.00 18.53 1.57 5.30 21.68 0.70 0.00
99.19
1.2 *4 4.4 4.6 2.0 0.20 1.4 0.9 0.5 1.8
*~Total Fe as FeO. *2EMP data from JEOL JXA-8600 Superprobe; analyst R.L. Barnett (U.W.O.). *3EMP data from F.A. Caporuscio (pers. commun., 1989). *4Analyses by ICP-MS (XRAL, Don Mills, Ontario) and INAA (ACTLABS, Ancaster, Ontario ). *SAnalysis by ICP-AES, CNR-CSCC, Pavia, Italy.
location while the REE peaks were checked less frequently. Although no corrections were made for background noise, background intensity was measured at mass 174.5 for each cycle, and was routinely less than 1 count per run (i.e. over 25 s acquisition time). 2.1. CEF In the CEF mode of operation the primary
160-- beam was focused to produce a sputter crater at the sample surface approximately 20 #m in diameter. Samples were coated with a layer of gold approximately 400/~ thick. Posi-
tive secondary ions were accelerated through a nominal 4.5 kV sample voltage offset by - 8 0 V (Bottazzi et al., 1990). More precisely, we closed the energy window almost completely, tuned the mass spectrometer to the maximum signal, opened the energy slit symmetrically to a total width corresponding to 50 eV and offset the sample voltage by 80 V. Using the 150 #m imaged field and wide open entrance and exit slits, we collected data for the following peaks:
30, 138, 139, 140, 146, 149, 151,153, 156, 158, 161,162, 163, 167 and 174. Counting times of 20 s per cycle were used for all the REE peaks and 2 s for 30 (Si).
48
QUANTITATIVEREE ANALYSISOF SILICATESBY SIMS
The voltage offset was sufficient to filter interferences from masses 138Ba, 139La, 14°Ce, 146Nd and 149Sm. EU was resolved from BaO interferences by solving simultaneous equations for data from masses 151 and 153. To resolve Gd and Dy from CeO and NdO, respectively, a least squares procedure (consisting of a simplification of the method described by Zinner and Crozaz, 1986a) was adopted for the signals from masses 156, 158, 161 and 162. This included assuming a fixed oxide-to-element ratio pattern (determined previously from accurately known standards) and the determination of an overall multiplication factor that compensates for the effects of possible slight changes in experimental conditions. Dy was also determined from the signal at mass 163, which suffers only from a negligible interference of SmO; the results of both Dy determinations agree. Ion intensities were corrected for isotopic abundances and normalized to Si to remove the effects of instrumental variations. REE and Si intensities from the standard samples were used in combination with the known REE and Si atomic concentrations (recalculated from the analyses of Table 1 ) to compute relative ion yields according to the formula: (ion-yield) = (IREE/Isi)/( [ R E E ] / [ S i ] ) (1) where [REE ] and [Si] are atomic concentrations and IREE and Is, are corrected ion intensities. Relative ion yields are reported in Table 2.
2.2. SI The major difference between the CEF and SI methods operation is that in the SI mode the sample is sufficiently insulated from all conductive materials that it will charge freely under the primary beam. The samples not only are uncoated but also are isolated from the metal sample holder by Teflon rings and discs. The top is covered by a stainless steel mask,
backed with a thin Teflon disc, both drilled with fifteen evenly spaced 3-mm diameter holes, exposing the sample surface (MacRae, 1987). Potential contour mapping by Lau et al. (1985) within the instrument's specimen chamber showed that the geometry of a charged aperture (the steel mask) above an insulated specimen produces a stable voltage some 450600 V below the secondary accelerating voltage (4.5 kV) and causes trapping of low kinetic energy ions, thus providing the extreme energy filtering of secondary ions characteristic of SI analysis. Thus, only high-energy ions can enter the magnetic sector and be analyzed. The 3-mm diameter aperture was chosen to optimize simply charged ion intensity while maintaining m a x i m u m molecular ion suppression (Metson et al., 1983; Mclntyre et al., 1985 ). While the spectra processed through the mass spectrometer are virtually free of molecular ion interferences, intensities are low unless large primary currents are used; for this study we used a primary current of approximately 750 nA, resulting in sputter craters in the samples approximately 100 ¢zm in diameter. In order to obtain m a x i m u m intensity, the energy slits and all apertures of the secondary column were fully opened. We collected data for the following peaks: 3°Si ' 138Ba ' 139La ' 140Ce ' 146Nd ' J52Sm ' 153Eu ' 158Gd, 163Dy, 166Er and 174yb. Counting time f o r 3°Si was 2 s, for 138Ba 10 S, and 15 s for all the REEs. As for CEF, we collected data for seven cycles, but accepted only the last five. Intensity ratios and analytical precision for the SI mode of operation appear with the corresponding CEF data in Table 2.
3. Results and discussion
Published studies of secondary ion yields are still scarce, particularly those that document data for a variety of samples. Table 2 lists the REE ion yields, normalized to Si ÷, from our
49
N.D. MacRAE ET AL. TABLE 2 REE ion yields and CEF/SI ion yield ratios
Fused amphiboles JVV87
JH'83
Reference values
Fused clinopyroxenes KSS
avg
s.d.
A26C
.49
.41977
avg
s.d.
N B S 6 1 0 *l
D / W g1.2
(%)
(%) Ion yields: REE+/Si+ / [ (REE~t) / (Sia,) ] CEF: La Ce Nd Sm Eu Gd Dy Er Yb
2.42 2.35 2.57 2.83 2.65 2.67 2.66 2.41 2.67
2.35 2.37 2.49 2.61 2.32 2.66 2.58 2.46 2.67
2.52 2.41 2.55 2.58 3.22 3.08 3.43 3.07 3.12
2,43 2.38 2.54 2.67 2.73 2.80 2.89 2.65 2.82
2.9 1.0 1.3 4.2 13.6 7.0 13.3 11.3 7.5
3.14 2.91 3.38 3.76 2.83 3.38 3.66 3.06 3.82
3.14 2.80 3.49 3.70 2.75 3.40 3.53 3.04 3.59
3.26 2.91 3.46 3.73 2.56 3.46 3.34 3.11 3,77
3.18 2.87 3.44 3.73 2.71 3.41 3.51 3.07 3.73
1.9 1.7 1.5 0,5 4,1 0,9 3.7 1.0 2.7
SI: La Ce Nd Sm Eu Gd Dy Er Yb
1.65 1.61 1.76 1.99 1.92 2.11 1.97 1.87 1.74
1.36 1.35 1.44 1.59 1.46 1.78 1.63 1.54 1.45
1.36 1.30 1.37 1.40 1.88 2.21 1.90 1.65 1.43
1.46 1.42 1.52 1.66 1.75 2.03 1.83 1.69 1.54
9.4 9.6 11.1 14.8 11.9 9.0 8.0 8.1 9.2
2.08 1,94 2,23 2.60 1.93 2.63 2.38 2.16 2.28
1.81 1.77 2.15 2.37 1.85 2.38 2.20 1.92 2.00
1.78 1.71 1.99 2.17 1.60 2.50 1.94 1.83 2.03
1.89 1.81 2.12 2.38 1.79 2.50 2.17 1.97 2.10
6.9 5.5 4.7 7.6 7.8 4.0 8.3 7.1 6.2
La Ce Nd Sm Eu Gd Dy Er Yb
Ion yield ratios: [ (REE+/Si + )CEF/(REE+/Si + )s~ 1.46 1.72 1.86 1.68 10.1 1.51 1.46 1.75 1.86 1.69 10.1 1.50 1.45 1.73 1.85 1.68 10.1 1.52 1.43 1.64 1.84 1.64 10.4 1.45 1.38 1.59 1.71 1.56 9.0 1.47 1.29 1.57 1.43 1.43 7.7 1.29 1.35 1.58 1.81 1.58 12.0 1.54 1.29 1.60 1.87 1.59 15.1 1.42 1.53 1.84 2.18 1.85 14.6 1.68
1.73 1.58 1.62 1.56 1.49 1.43 1.60 1.58 1.80
1.83 1.70 1.74 1.72 1.56 1.38 1.72 1.70 1.86
1.69 1.59 1.63 1.58 1.51 1.37 1.62 1.57 1.78
7.9 5.2 5.5 7.0 2.6 4.2 4.6 7.3 4.2
2.27 2.2l 2.59 2.78 2.92 2.75 2.50 2.45 2.47
Total av. 1.69 1.64 1.65 1.61 1.53 1.40 1.60 1.58 1.82
2.51 2.19 2.89 3.30 3.57 2.96 2.61 2.39 2.72
s.d. (%) 8.9 8.6 8.3 9.0 6.8 6.9 9.0 11.8 10.9
*~Hinton ( 1990, table 1; National Bureau of Standards silicate glass NBS610. *2Zinner and Crozaz ( 1986b, table 1 ); Drake and Weill REE silicate glass.
six fused amphibole and clinopyroxene samples obtained by both CEF and SI modes of instrument operation. Listed for comparison are literature values for the synthetic silicate glass NBS 610 (Hinton, 1990) and those from the synthetic Drake and Weill REE glass standards (Zinner and Crozaz, 1986b), both obtained by quite similar CEF configurations. The CEF ion yield data are all similar, but variations are evident. In fact, we note an inverse correlation of
ion yield to REE content, with yields from amphibole glasses generally lower than from clinopyroxene glasses. While we are unable to account fully for it, part of that correlation could be due to matrix differences. Although not as strong, the same correlation is evident for the SI data. Table 2 also contains the ratios of CEF to SI intensities, which vary by + 0.2 about 1.6. Under the adopted experimental conditions,
50
QUANTITATIVE REE ANALYSISOF SILICATES BY SIMS
REE intensities were practically the same for both CEF and SI modes, whereas the 3°Si signal was about 30% higher in SI than in CEF. The significant difference in the two methods can be ascribed mainly to the different energy ranges being monitored in the two experimental conditions. Using CEF operating conditions, a wideopen energy slit, and a Bertan High Voltage power supply, we measured the secondary ion intensity ratios 139La/3°Si and ~4°Ce/3°Si from a silicate glass (REE3; Drake and Weill, 1972 ) through 50 V increments of offset from - 100 to - 6 0 0 V, thus covering the range of offset experienced for SI operation (Fig. 1 ). The variations were systematic and reproducible (std. dev. < 5 % ) from different sample points over 6 h of data collection. From Table 2, we note that the variation in ion yields among REE is negligible with respect to the uncertainty of measurements, indicating that the application of a large voltage offset in SI does not introduce a discrimination among REE signals with respect to CEF (i.e. the energy distributions of REE are very similar in the range from moderate to very high energies). This fact was already documented by Muir et al. ( 1987b ) for analyses of the glass standard NBS 610.
5 o
139•30 3-
1O0
200
300 400 OFFSET (VOLTS)
500
600
Fig. 1. Secondary ion intensity ratios relative to progressive offsets of the sample voltage from - 100 V to - 600 V (4.4 kV to 3.9 kV).
Another difference between CEF and SI experimental conditions concerns the primary beam intensity; its increase from CEF to SI, however, is roughly matched by a proportional increase in sputter area, indicating that the beams have practically the same densities in CEF and SI modes. We then expect in principle the mechanism of sputtering to be similar and not to contribute to significant changes in ion yields. Measurements of sputtered craters were carried out on one of the fused amphiboles (JW87) using a DEKTAC II profilometer. A typical SI crater was about 5.4 #m deep, representing sputtering of approximately 1.1 × 10 - l l c m 3 / s (a little less than 3.5× 10 -z ng/s of sample). A corresponding CEF crater was typically 2.5/zm deep, representing sputtering of 2 E l 0 -13 c m 3 / s and less than 6.3× 10 -4 ng/s of sample. Notwithstanding the use of the O - primary beam and gold coating of the samples in CEF mode to reduce charge pile-up on the surface (Reed, 1989), it is known that charge accumulation occurs on the sputtered area, which mainly results in a shift of the energy distribution of secondary ions. In order to check its possible influence on relative ion yields in crystalline and glassy matrices, we monitored the position of the low-energy edge at 10% of the maximum intensity (Zinner and Crozaz, 1986a) of the energy distribution for mass 32 (O2 + ) as well as mass 16 (O + ), on crystalline and glassy kaersutite KSS. We noted that charging, after a sudden increase by 5-10 V during the first 5 min, follows a nearly linear trend (approx. 1 V/10 min) in both structures, with a constant difference between the two of less than 2 V; this process is typically reproducible within 1-2 V over a one-day span under the same operating conditions. A shift by 10-15 V is thus added to that instrumentally obtained by offsetting the sample accelerating voltage ( + 4.5 kV ). REE ion yields relative to Si were then determined over an offset voltage range from - 1 0 0 V to - 6 0 V and compared to those obtained at - 80 V; the re-
N.D. MacRAEET AL.
sults were reproducible to within 10% over the span of one day with the exception of the Yb data, which were higher. These results are consistent with the 13% variation reported by Crozaz and Zinner ( 1985 ) who measured REE intensity ratios relative to Ca + in phosphate grains for different levels of energy filtering over the 60 to 100 V offset range. Muir et al. (1987a) examined charge stabilization for SI and CEF modes of SIMS operation, using glass, ceramic and crystal of identical composition (titanite). They found that the results using SI showed very similar charging for all samples and that the reproducibility of the data was as good, if not better, than that from the CEF mode of operation. The degree of sample charging during SI operation was measured and discussed by Lau et al. ( 1985 ). In our experiments, precision of CEF determinations was commonly higher than for the same sample done by SI, particularly for amphibole samples. Several factors might be considered to influence the difference in standard deviations between data collected for the two techniques. Firstly, the counting time for secondary ions at each REE mass was 1/3 less for SI than for CEF; the shorter time should, however, have been compensated for by the larger sample volume sputtered for SI; this implies that any possible small-scale inhomogeneities would be averaged during analysis. Further, considering that relative REE ion yields are generally dependent on secondary ion energy, this dependence could explain the larger errors encountered during SI analysis; since the amount of charging during SI is slightly variable, characteristic of the matrix under investigation, the corresponding variations in the selected secondary ion energy could lead to variations in relative ion yields. Finally, we must consider a unique aspect of the SI sample holder geometry. As noted by Lau et al. ( 1985 ), the potential contours above an insulating specimen are not regular across the dimensions of the aperture, varying with distance from the geometric centre. The variation
51
is readily demonstrated, and despite an effort to maintain optimal beam positioning, the noted standard deviations are strongly influenced by the positioning of the sputtering site relative to the aperture. We have analyzed both crystalline and fused material of amphiboles KSS and H8 (only CEF) and of the three clinopyroxenes (both CEF and SI) (Table 3). The crystal/glass ion yield ratios are approximately unity for the CEF data, but approximately 0.85 for SI determinations. Ray and Hart ( 1982 ), using CEF, noted lower ion yields for glass than for crystals of the same compositions. Since then, however, Muir et al. ( 1987a, using CEF and SI on glass, ceramic and crystal of identical titanite composition), MacRae (1987, using SI) and Hinton (1990, using CEF) report virtually identical ion yields for materials of equivalent composition, regardTABLE3 Crystal to glass ion yield ratios Amphiboles
Clinopyroxenes
KSS*
A26
Avg.
St. dev.
(%) CEF: La 0.90 Ce 0.91 Nd 0.98 Sm 1.00 Eu 1.05 Gd 1.06 Dy 1.11 Er 1.04 Yb 1.11 SI: La Ce Nd Sm Eu Gd Dy Er Yb
H8
0.83 0.63 0.96 1.03 1.03 1.18 1.02 1 . 2 7 1.00 0.92 1.06 1.04 1.08 1.10 1.06 0.98 1.03 1.16
0.73 0.80 0.84 0.85 0.83 0.89 0.80 0.67 0.73
,49
A1977
0.73 0.93 0.99 1.00 1.05 0.69 1.02 0.94 1.01
0.78 0.95 0.97 1.02 0.98 0.98 1.01 1.00 0.95
0.77 0.96 1.03 1.06 1.00 0.97 1.06 1.00 1.05
11.8 4.3 7.6 9.8 4.9 14.6 3.9 4.3 7.1
0.60 0.79 0.82 0.81 0.70 0.88 0.80 0.87 0.87
1.41 1.03 0.80 0.83 0.80 0.82 0.89 1.00 0.85
0.91 0.87 0.82 0.83 0.78 0.86 0.83 0.85 0.87
38,9 12,7 2.0 2.0 7.2 3.6 5.1 16.1 7.6
*Ratios from Ottolini et al. ( 1991 ).
52
less of structural state. Certainly, any determinations of crystals are expected to show more compositional variation--as was the case in this study--simply because of natural variations due to chemical zoning and minute inclusions. Clearly, fused samples would make the best calibration material, at least for CEF operation. Our samples, analyzed by CEF in both matrix structures, did not show any systematic differences. Only La showed crystal-to-glass intensity ratios lower than 1 in amphiboles (average 0.87) and augites (average 0.71 ), the minimum value being 0.63 in A26. All other REEs yielded crystal-to-glass intensity ratios which are comparable and close to 1. As for the La contents, we suspect that some laboratory contamination during fusion could be responsible for higher La in the glasses. A second possibility is the presence of a light REE-enriched phase, such as apatite, which escaped detection during SEM investigation of the grains fused. Crystal-to-glass analyses (of the three clinopyroxenes) in SI mode again showed a problem with La, but all ratios were nevertheless less than 1 (average over REEs is 0.84 exclusive of La, as opposed to 1.02 for CEF). Major element variations within the amphibole group appear not to significantly affect REE ion yields (because the clinopyroxene samples are so similar in chemical composition, we are unable to draw conclusions in their case). This agrees with studies by Ray and Hart (1982) and Ottolini et al. (1992) who reported that for CEF determinations of such minerals, the Si-normalized ion yields show no consistent variations with major element chemistry. The CEF relative ion yields are quite similar for different REEs, with values from amphibole glasses generally lower than from clinopyroxene glasses. In amphiboles they range from 2.38 (Ce) to 2.89 (Dy); the variation for clinopyroxenes, with the exception of Eu, is 2.87 (Ce) to 3.77 (Yb). As far as Eu is con-
QUANTITATIVE REE ANALYSISOF SILICATES BY SIMS
cerned, we must point out that the scatter of bulk determinations is rather large, on the order of 20-25% in the three augites, whereas the range of variations of most REE experimental data, with respect to the assumed reference values (see Table 1 ), was very limited (typically about 10%). Such variations in Eu data are attributable to ( 1 ) primary inhomogeneity of the low primary Eu content in the subsampies used for analyses, and (2) different uncertainties of experimental data obtained by ICPMS and INAA methods. As a result, no definite value for Eu ion yield in clinopyroxene can be assessed at this point. The average value of 2.71 reported in Table 2 must be considered a rough estimate. Furthermore, if we calculate the ratios clinopyroxene/amphibole REE ion yields, we can see that they are almost identical for both CEF and SI techniques, including the increase from 1.16 (CEF) and 1.17 (SI) for Er to 1.32 (CEF) and 1.36 (SI) for Yb. On this basis, the differences observed in relative REE ion yields between amphiboles and clinopyroxenes may be reasonably ascribed to matrix effects deriving from differences in bulk chemistry of the two silicate families. The (chemical) matrix effects observed between fused amphiboles and clinopyroxenes are the same in both techniques, i.e. for REE ions sputtered with moderate and with high energies. It is also noted that the application of higher voltage offsets, as implied in SI, does not reduce matrix effects below those of CEF analyses. Therefore, in order to obtain the most accurate quantitative data, it is essential to reduce as much as possible the matrix differences between standards and samples. The REE ion yields obtained from the amphibole calibration samples were used to calculate the REE concentrations in the control sample H8 by both CEF and SI methods of operation. SIMS results are compared with ICPMS results in Fig. 2. Variations among the different data sources are small with the exception of Sm results (approx. 25%).
53
N.D. MacRAE ET A L
100-
jj. 0~
/z o
L)
10 Ii" ~ i / CEF ×
03
H8 Glass
SI
!
ICP-MS
La de Nd Sm Eu ad By Er V~ Fig. 2. Chondrite-normalized analyses of test sample H8, using the amphibole average ion yields of CEF and SI techniques, as compared with ICP-MS data.
4. Conclusions The data presented here show that the CEF and SI techniques of SIMS operation are equally quantitative. The (chemical) matrix effects observed between fused amphiboles and clinopyroxenes are the same in both techniques, i.e. for REE ions sputtered with moderate and with high energies. The effects of major element variation on REE yields are generally very limited; nevertheless, this study shows them to be sufficient that, for high accuracy, we cannot recommend the use of common quantification factors for both groups. Comparison of ion yields from crystals and glass of the same material shows that for CEF there are no significant differences. Calibration curves may thus be constructed using homogeneous glass standards and used for analysis of crystals. Yields from crystals using SI, however, appear to be approximately 15% lower than from glass. The major difference between the two techniques is that with SI molecular interferences are much more suppressed over the whole REE spectrum, while with CEF interferences of light REE oxides on the heavy REE peaks must be dealt with. Until we learn more about the factors which control ion yields of elements and compounds from a given matrix, interference handling in CEF relies upon either peak strip-
ping procedures--whose drawback is errors magnification--or upon the knowledge of M O + / M ÷ ratios (for very specific operating conditions) obtained from well-characterized standard samples. Two disadvantages of SI are that ( 1 ) in order to get a sensitivity equivalent to CEF, a large diameter primary beam (70-100 pm) must be used, and (2) primarily due to the critical placement of the beam in electric fields with strong gradients, some slight loss of precision is common. Precisions and accuracies in the order of 10% can generally be achieved with both techniques. This investigation shows the validity of the two energy filtering approaches for REE quantification of silicate minerals.
Acknowledgements We wish to acknowledge receipt of Italian funds through a NATO-CNR Senior Guest Fellowship (n. 217.23, 6/22/1989) and Canadian funds through NSERC (OGP0005539). Our thanks to J.B. Whalen for the JWamphiboles, to F.A. Caporuscio for the K S S amphibole and its analysis, and to M. Palenzona for his technical assistance and sample preparation. I.J. Muir kindly reviewed an early version of the manuscript and gave great assistance to the first author to sort out the revision problems. We gratefully acknowledge the constructive review and numerous suggestions for manuscript improvement by E. Zinner.
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QUANTITATIVEREE ANALYSIS OF SILICATES BY SIMS
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