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Micro-PIXE and quantitative cathodoluminescence spectroscopy: Combined high resolution trace element analyses in minerals
Nuclear Instruments and Methods in Physics Research B 150 (1999) 470±477
Micro-PIXE and quantitative cathodoluminescence spectroscopy: Combined high resolution trace element analyses in minerals D. Habermann
a,c,*
, J. Meijer b, R.D. Neuser a, D.K. Richter a, C. Rolfs b, A. Stephan
b
a
Institute of Geology, Ruhr-University Bochum, D-44780 Bochum, Germany Institute of Physics, Ruhr-University Bochum, D-44780 Bochum, Germany Institute of Experimental Physics, TU-Bergakademie Freiberg, D-09596 Freiberg, Germany b
c
Abstract We combined high resolution Cathodoluminescence (CL)-spectroscopy and micro-PIXE to study the correlation of the activator concentration and the CL-intensity. Based on these results the Quantitative High Resolution Spectral analysis of Cathodoluminescence (QHRS-CL) is developed. Micro-PIXE and the new method (QHRS-CL) have been used to investigate trace elements in minerals. Using micro-PIXE and related methods the crystal lattice site and charge state of the analysed elements cannot be determined. This can be analysed exactly by using QHRS-CL. So the combination of micro-PIXE and QHRS-CL is a powerful tool for analysing trace element concentration above 100 ppb, the charge state and the lattice site of these elements in crystal structures. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Since cathodoluminecence (CL) is used to detect trace element distribution in minerals a possible quantitative analysis of activator elements by their CL-intensity was the incentive to numerous investigations. The CL of minerals is predominantly controlled by the activator ions Mn2 , Fe3 , Cr3 and rare earth elements (REE2=3 ) and the quencher ions like Fe2 . There is a general agreement in literature about the dominant role of the Mn2 -ion as the most important activator element and Fe2 as the most ecient quencher el-
* Corresponding author. E-mail: [email protected]
ement in minerals (e.g. Refs. [1±6]). The correlation of CL-intensity and activator concentration and the low limit of activator concentrations, above which CL occurs, is often controversially discussed [7,5]. There is a widespread assumption in geosciences, that a concentration level of 10±30 ppm is needed before Mn2 activation in calcite is present (e.g. Refs. [8, 9]). As these authors based their de®nition on visible determination the CL-intensity, this interpretation is not objective. However, the CL is very sensitive to low variations in the activator and quencher elements concentrations of minerals. In this paper spectroscopic investigation of CL combined with micro-PIXE will be used to analyse the correlation between CL-intensity and activator element concentration. The powerful combination
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 9 2 6 - 4
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 470±477
of quantitative CL-spectroscopy and micro-PIXE for trace element analyses in minerals will be demonstrated. 2. Method The samples are calcite, dolomite and feldspar minerals from sedimentary, hydrothermal or magnetic source. Polished thin sections of the samples are measured by micro-PIXE and Quantitative High Resolution Spectroscopy of CLemission (QHRS-CL). The QHRS-CL-device developed at the RuhrUniversity Bochum based on the combination of 1. the ``hot cathode'' CL-microscope (type HC1LM) [10], 2. an EG&G digital triple grating spectrograph with a LN2 -cooled CCD-camera, 3. a noncommercial software for quantitative CLspectral analyses. For the ®rst time the QHRS-CL allows to apply CL for quantitative trace element analyses in minerals. This method is calibrated e.g. for calcite by using an island spar analysed by micro-PIXE and CL-spectroscopy (Table 1) [11]. CL-Spectra were accumulated within an exposure time of 5±120 s, depending on the CLintensity. For high lateral resolution the analysed area was focused to a spot of 30 lm in diameter. To prevent charging during the CL analyses thick gold the samples are covered by a few A coating. To avoid variations in the coating thickness and on that attributed variations in CL-intensity the coating was done at standard conditions. The CL-intensity is also controlled by other physical processes ± e.g. thermal quenching, generating of lattice defects ± depending on the physical properties of the mineral and the analyses conditions. For each mineral group we de®ned standard analyses conditions in experiments. More details are described in Ref. [11]. At present, the calculated detection limit of QHRS-CL is in the range of 0.1 ppm. To keep the calculated detection limit of micro-PIXE, QHRSCL and moreover the quantitative ionoluminescence [12] comparable this is calculated on the
471
same way basing on the LOD calculation of the GUPIX [13] software package. At a discriminated CL-peak with a con®dence level of 99.86% the smallest p acceptable CL-peak area in a spectrum is 3 Nb , where Nb is the background at the peak position. The micro-PIXE analyses were done at the Dynamitron Tandem Laboratorium (DTL) at the Ruhr-University Bochum (method in Ref. [14]). At an average spot-size of 10 lm and a proton energy of 3 MeV, the detection limit for Mn is 10 ppm and for most REE >n*10 ppm. The spectra were analysed by the GUPIX [13] software package. The lattice site of Mn2 in calcite, dolomite and feldspar samples are analysed by Electron Spin Resonance (ESR) with a Bruker ESP 300e (XBand) at the Institute of Experimental Physics at the University of Mining and Technology Freiberg. 3. Experiment results and discussion 3.1. Correlation between CL-spectroscopy and electron spin resonance spectroscopy The wavelength of Mn2 -activated CL is controlled by symmetry and strength of the crystal ®eld. Small variations either in symmetry and/or the atomic distance ± between the central metal ion and the surrounding oxygen-ligands ± caused a shift in wavelength of Mn2 -activated CL. For example, in the dolomite structure the Mn2 -ion can be incorporated in Ca- as well as Mg-position (e.g. Ref. [15]). From there, the Mn2 -peaks emit at 575 nm (Mn2 in Ca-position) and 660 nm (Mn2 in Mg-position) (e.g. Refs. [16,11]) (Fig. 1(a)). The CL-data of Mn-activated dolomite (Fig. 1(a)) and feldspar (Fig. 1(b)) are comparable with the data of ESR spectroscopy, where Mn2 incorporation in the dierent lattice positions can be documented. Only powder sample and orientated single crystals can be analysed by ESR. So, the advantage of CL-spectroscopy compared to ESR is, that Mn2 -partition in the crystal lattice can be analysed in thin sections with a point size of 30 lm.
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D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 470±477
Table 1 PIXE-analyses data of an (a) Island spar crystal (sample: I-spar) and (b) feldspar crystal (sample: spruce mine S-M) for calculating the Mn-calibration value of the QHRS-CL method. In both cases the quencher (e.g. Fe, Ni) and sensitiser elements (e.g. Yb) are in low concentration respectively not detected by micro-PIXE (average detection limit 10 ppm). Co-activators like most REE are only in the feldspar sample analysed by micro-PIXE and QHRS-CL. (c) The data of the synthetic Sm3 -doped calcite show that only Sm is incorporated in sucient quantities. Therefore quenching and sensitising by other trace elements can be excluded Part a
I-SPAT-1
I-SPAT-2
I-SPAT-3
I-SPAT-4
I-SPAT-5
I-SPAT-6
I-SPAT-7
25 26 28 38 39 56
159 241 n.d. 660 n.d. n.d.
159 203 n.d. 658 n.d. n.d.
160 177 n.d. 614 n.d. n.d.
198 248 n.d. 458 n.d. n.d.
173 128 n.d. 352 n.d. n.d.
134 46 n.d. 377 n.d. n.d.
118 42 n.d. 299 n.d. n.d.
Part b
S-M1
S-M2
S-M3
S-M4
S-M5
S-M6
S-M7
S-M8
25 26 28 38 39 56 57 58 59 60 61 62 63 59 65 66 67 68 69 70 71
Mn Fe Ni Sr Y Ba La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
1.3 99.1 n.d. ? 13.2 ? 7.8 n.a. 61.8? 122.4 n.d. 39.8 ? 12.9 n.d. n.d. n.d. n.d. n.d. ? 5.9 ? 4.1 n.d. ? 8.8 ? 2.2
7.3 93.9 n.d. 20.9 ? 9.5 n.a. ? 87.9 131.9 ? 49.1 ? 38.1 ? 9.3 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
27.2 90.8 n.d. 16.1 7.4 n.a. ? 59.8 93.9 ? 32.2 ? 36.3 ? 6.1 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.a. ? 2.8
? 4.8 108.9 n.d. 19.6 7.4 n.a. ? 82.0 133.8 ? 43.6 ? 31.3 ? 7.7 ? 18.3 ? 13.8 ? 19.0 n.d. n.d. n.d. ? 4.5 n.d. ? 3.8 n.d.
15.9 120.2 n.d. 12.7 7.1 n.a. 95.7 140.0 ? 40.0 ? 33.5 ? 9.4 ? 8.8 ? 5.9 n.d. n.d. n.d. n.d. ? 3.4 n.d. ? 2.0. n.d.
? 6.2 101.3 n.d. ? 9.2 ? 11.8 n.a. ? 159.6 163.7 ? 37.6 ? 79.3 ? 61,7 ? 58.3 ? 24.7 ? 11.7 ? 22.4 n.d. ? 22.9 ? 7.4 n.d. n.d. n.d.
18.3 123.9 n.d. ? 16.6 ? 8.4 n.a. ? 126.4 168.1 ? 70.3 ? 63.9 n.d. ? 23.3 ? 14.0 ? 25.6 ? 10.0 ? 11.7 ? 5.5 n.d. n.d. n.d. n.d.
? 6.3 117.8 n.d. ? 23.3 n.d. n.a. ? 111.1 ? 141.4 ? 70.4 ? 79.1 ? 97.5 ? 115.8 ? 29.1 ? 48.3 n.d. ? 99.6 ? 40.3 ? 50.0 ? 22.3 ? 43.8 n.d.
Part c
Sm-p1
Sm-p2
Sm-p3
Sm-p4
Sm-p5
Sm-p6
Sm-p7
Sm-p8
25 26 28 38 39 56 57 58 59 60 61 62
n.d. n.d. n.d. ? 6.4 n.d. n.d. n.a. n.a. n.d. n.d. n.d. 79.6
n.d. ? 18.1 ? 9.0 ? 3.2 n.d. n.d. n.a. n.a. n.d. n.d. n.d. 523.0
? 19.2 ? 14.6 n.d. ? 9.7 n.d. n.d. n.a. n.a. n.d. n.d. n.d. 202.4
? 75.8 ? 8.1 n.d. ? 16.4 n.d. n.d. n.a. n.a. n.d. n.d. n.d. 60.4
n.d. ? 19.9 n.d. ? 3.0 n.d. ? 203.0 n.a. n.a. n.d. n.d. n.d. 132.5
? 5.1 ? 7.0 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 83.4
n.d. ? 9.7 n.d. n.d. n.d. n.d. ? 78.8 ? 58.8 n.d. n.d. n.d. 167.2
? 5.9 ? 8.9 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. ? 14.3 104.9
Mn Fe Ni Sr Y Ba
Mn Fe Ni Sr Y Ba La Ce Pr Nd Pm Sm
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 470±477
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Table 1 (Continued) Part c
Sm-p1
Sm-p2
Sm-p3
Sm-p4
Sm-p5
Sm-p6
Sm-p7
Sm-p8
63 59 65 66 67 68 69 70 71
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.a. n.a.
n.d. n.d. n.d. n.d. n.d. ? 21.3 ? 28.0 ? 16.0 n.a.
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.a. n.a.
n.d. n.d. n.d. n.d. ? 23.1 ? 19.1 ? 18.1 n.a. n.a.
n.d. n.d. n.d. n.d. ? 28.1 n.d. ? 15.5 n.a. n.a.
? 14.1 ? 12.1. ? 7.7 n.d. n.d. ? 3.3. ? 3.3. ? 8.7 ? 8.7
? 29.2 ? 25.4 ? 7.9 n.d. n.d. ? 6.1 ? 6.1 ? 5.7 ? 6.2
n.d. n.d. n.d. n.d. ? 5.5 n.d. n.d. ? 6.5 n.d.
Eu Gd Tb Dy Ho Er Tm Yb Lu
n.a. ± not analysed; n.d. ± not detected; ? ± data near the LOD and the error is >20%.
3.2. Case studies: correlation between CL-intensity and activator element content in calcite and feldspar
mostly too low for quenching Mn2 -and REEactivated CL (see Table 1).
3.2.1. Case studies: manganese in calcite and feldspar There is a linear correlation between the activator element concentration (<1000 ppm) and the CL-intensity (Fig. 2 Fig. 3). Deviations from this linear correlation are based mainly on small variations in the Mn distribution, the statistic error of CL and PIXE analyses, dierences in the analysed spot size (micro-PIXE: £ 10 lm, QHRS-CL: £ 30 lm) and depth of penetration. Also selfquenching and quenching by Fe2 eect deviations from the linear correlation are if the concentrations are >1000 ppm respectively >2000±3000 ppm [17] (see Fig. 2(a) and (b)). Above these concentration levels the combination of both, selfquenching and quenching by Fe eects a complex correlation of CL-intensity and element concentration. Self-quenching is more eective than quenching by Fe2 as it takes place at lower concentrations levels [17]. Here, the eciency of ``Fequenching'' increases with increasing Mn-content according to decreasing average Fe2 and Mn2 ion distance [17]. This is attributed to the fact that an energy transfer between activator and quencher ions, respectively is only possible at small atomic distance (e.g. Ref. [4]). Other quencher ions e.g. Ni2 , some REE (e.g. La3 , Nd3 (also activator), Ho3 ) and Fe3 (in carbonates) are much less effective than Fe2 [1]. Additional, in most silicates Fe3 is an important activator. However, if other quenchers than Fe2 are incorporated in natural carbonates and feldspar, their concentration is
3.2.2. Case studies: REE in calcite and feldspar Most of divalent and trivalent REE (Eu2=3 , 3 Er , Tb3 , Dy3 , Sm3 , Pr3 , Ho3 and Nd3 ) shows CL-bands in the near UV-, visible- and IR spectrum. The structure of the REE related peaks in a spectrum are dierent depending on the symmetry and strength of host crystal ®eld [1]. Therefore, Ca2 substituted by REE in e.g. calcite-, aragonite-, feldspar-, apatite structure is unequivocal by detectable using the CL-spectroscopy although the peaks show nearly constant wavelengths. The micro-PIXE analyses of Sm-doped synthetic calcite (Fig. 4) indicate a linear correlation between Sm-activated CL-intensity and the Smconcentration. The CL-spectrum in Fig. 5 re¯ects the Sm-content of 79 ppm (micro-PIXE analyses: statistic error 17.49%, LOD 41.8 ppm, accumulation time 65 min.). After ®tting and ®ltering the CL-spectrum (details in Ref. [11]) the calculated LOD by QHRS-CL is 1.3 ppm (accumulation time 30 s). In many natural (also synthetic samples) the CL-spectroscopy indicate REE concentrations below the detection limit of micro-PIXE (Fig. 6). REE are mostly incorporated in natural minerals in groups. Therefore, sensibilisation, quenching and absorption by other REE are dominant eects yielding strong variations in the correlation between the REE-activated CL-intensity and the REE-content [1]. At present state, only for few REE the quantitative CL-spectroscopy is
474
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 470±477
Fig. 1. (a) Spectrum of Mn2 -activated CL in dolomite and comparable ESR-spectrum (sample: HM; analyses conditions: acceleration voltage 14 keV, beam current density 9 lA/mm2 , exposure time: 5 s). The CL-peak position at 575 and 661 nm indicates that Mg2 and Ca2 are substituted by Mn2 , this is also demonstrated by the ESR signal (sixth line of the hyper®ne structure from Mn2 ) (Mn 1100 ppm). (b) Spectrum of Mn2 , Fe3 , Sm3 , Dy3 , Tb3 and Nd3 -activated CL of feldspar (sample: spruce mine (albite: An10 ); analyses conditions: acceleration voltage 14 keV, beam current density 9 lA/mm2 , exposure time: 30 s) and comparable ESR-spectrum. The CL-peak position at 570 nm indicates the Ca2 substitution by Mn2 , this is also demonstrated by the ESR signal (sixth line of the hyper®ne structure of the central spin transition) (Mn 15.9 ppm). Fe3 is incorporated in Al3 position (tetrahedral site) while the REE are in Ca2 -position. The REE content is mostly near or below the LOD of micro-PIXE (10±40 ppm). Notice the dierent position of the Mn-signal in dolomite and feldspar in the ESR-spectrum. Analysis conditions for all samples: Microwave 9.7 GHz (X-Band); microwave power 10 mW; modulation 0.0005 T, modulation frequency 100 kHz.
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 470±477
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Fig. 2. (a) Correlation between CL-intensity (normalised to counts/5 s), Mn- and Fe-content in visually homogeneously luminescent calcite. The data reveal an excellent linear correlation (R 0.97) in the range between 19 and 1000 ppm. Mn-analyses below the LOD of micro-PIXE (10 ppm) were done using QHRS-CL. The calibration value (linear function) of QHRS-CL based on the analyses data of an Iceland spar (black arrow) (micro-PIXE analyses see Table 1). Two analyses points show a distinct deviation from that linear correlation indicating that Fe2 -quenching and Mn2 -self-quenching occur at 4000 ppm respectively 1100 ppm (white arrows). (b) Seven analyses points show Fe-content below the LOD of micro-PIXE and therefore not plotted in the Fig. 2(b). The trace element concentration of elements with atomic number >27 are near or below the LOD of micro-PIXE (REE: n*10 ppm, others n*1±10 ppm). Modi®ed from Ref. [18].
Fig. 3. Correlation between CL-intensity (normalised to counts/5 s) and Mn-content in feldspar (sample: spruce mine). The data reveal a linear correlation in the range between 4.8 and 27.2 ppm. The micro-PIXE analyses at 4.8 and 7.3 ppm are only two time above the LOD of micro-PIXE. The relative CLintensity is relatively high, so that Mn can be analysed below the LOD of micro-PIXE.
possible while estimating their CL-intensity is unequivocal. These elements are Dy3 , Eu3 , Sm3 and Nd3 because some of their peaks lie in a spectral region where no other REE have emission peaks, absorption bands and no or only minor
Fig. 4. Synthetic Sm-doped calcite is analysed by micro-PIXE and CL-spectroscopy (micro-PIXE data see Table 1). The LOD for micro-PIXE lies in the range of 40±70 ppm. The reference CL-spectrum (see Fig. 5).
sensibilisation occurs. The ®tting and ®lter algorithms for the other REE are still in work. 4. Conclusion The combination of micro-PIXE and QHRSCL is a powerful tool for trace element analyses in
476
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 470±477
Fig. 5. CL-spectrum of Sm-doped synthetic calcite compared to 79.6 ppm Sm, which is slightly highly than the micro-PIXE LOD of 65 ppm. The calculate LOD of the QHRS-CL is 1.3. (Analyses conditions: acceleration voltage 14 keV, beam current density 9 lA/mm2 , exposure time: 30 s).
the ppb range. Fig. 7 shows a line scan through a calcite cement generation of early stage to late stage of burial diagenesis, analysed by micro-PIXE and QHRS-CL indicating that both methods are highly comparable. It is clear that the minor deviations are attributed to small scale inhomogeneities in the Mn-distribution. Additionally, in the crystal structure incorporated elements can be analysed unambiguously using QHRS-CL. The high potential in this method is its high sensibility
Fig. 7. Comparison between micro-PIXE analysis and quantitative CL-spectroscopy QHRS-CL analysis, re¯ecting the conformity of these two methods. Cement sequence of a Triassic limestone of Hydra (Greece). The QHRS-CL analysis spots are located between the micro-PIXE analysis spots (sample: FB; micro-PIXE analyses from [9], QHRS-CL analyses from [11]).
and the very low damage of the sample. An other advantage is, that this method can also supply detailed information about the crystal structure. Quantitative CL-spectroscopy is not only applicable to carbonates. The combination of microPIXE and QHRS-CL is a very powerful method for analysing REE, which is one of the most important potentials.
Fig. 6. CL-spectrum of a natural Mn2 -, Dy3 , Sm3 and Tb3 activated calcite (sample DB14-2). The Mn and REE content is below the detection limit of micro-PIXE (Mn 10 ppm, Dy 16 ppm). Using QHRS-CL 5 ppm Mn and 13 ppm Dy are analysed. (Analyses conditions: acceleration voltage 14 keV, beam current density 9 lA/mm2 , exposure time: 30 s).
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 470±477
Acknowledgements This study was ®nancially supported by the Deutsche Forschungsgemeinschaft (DFG-Projects: Ri 216/13-1 and 2). Thanks to M. Pl otze for carrying out the carbonate ESR analyses. Thanks to F. Bruhn for oering the sample Fb5 and part of it micro-PIXE analyses. References [1] [2] [3] [4] [5]
A.S. Marfunin, Springer, Berlin, 1979, p. 352. H. Machel, Geoscience Canada 12 (1985) 139. R.A. Mason, Chem. Geol. 64 (1987) 209. D.J. Marshall, Unwin-Hyman, Boston, 1988, p. 146. G. Walker, O.E. Abumere, B. Kamaluddin, Mineral. Mag. 53 (1989) 201. [6] R.A. Mason, A.N. Mariano, Chem. Geol. 88 (1990) 191. [7] D.K. Richter, U. Zinkernagel, Geol. Rdsch. 70 (1981) 1276.
477
[8] M.M. Savard, J. Veizer, R. Hinton, J. Sediment. Res. A 65 (1995) 208. [9] F. Bruhn, Diss. Ruhr-Universit at, Bochum, 1995, p.172. [10] R.D. Neuser, Bochumer geol. u. geotech. Arb. 44 (1995) 116. [11] D. Habermann, Diss. Ruhr-Universit at, Bochum, 1997, p. 152. [12] N.P.-O. Homman, C. Yang, K.G. Malmquist, Nucl. Instr. and Meth. A 353 (1994) 610. [13] J.A. Maxwell, W.J. Teesdale, J.L. Campbell, Nucl. Instr. and Meth. B 95 (1995) 407. [14] J. Meijer, A. Stephan, J. Adamczewski, H.H. Bukow, C. Rolfs, T. Pickart, F. Bruhn, Nucl. Instr. and Meth. B 89 (1994) 229. [15] D.N. Lumdsen, R.V. Lloyd, Geochim. Cosmochim. Acta 48 (1984) 1861. [16] A. El Ali, V. Barbin, G. Calas, B. Cervelle, K. Ramseyer, J. Bouroulec, Chem. Geol. 104 (1993) 189. [17] D. Habermann, R.D. Neuser, D.K. Richter, F. Bruhn, V.M Goldschmidt Conference 1996 (Heidelberg), J. Conference Abstracts 1 (1) (1996) 226. [18] D. Habermann, R.D. Neuser, D.K. Richter, Sed. Geol. 116 (1998) 13.
Micro-PIXE and quantitative cathodoluminescence spectroscopy: Combined high resolution trace element analyses in minerals D. Habermann
a,c,*
, J. Meijer b, R.D. Neuser a, D.K. Richter a, C. Rolfs b, A. Stephan
b
a
Institute of Geology, Ruhr-University Bochum, D-44780 Bochum, Germany Institute of Physics, Ruhr-University Bochum, D-44780 Bochum, Germany Institute of Experimental Physics, TU-Bergakademie Freiberg, D-09596 Freiberg, Germany b
c
Abstract We combined high resolution Cathodoluminescence (CL)-spectroscopy and micro-PIXE to study the correlation of the activator concentration and the CL-intensity. Based on these results the Quantitative High Resolution Spectral analysis of Cathodoluminescence (QHRS-CL) is developed. Micro-PIXE and the new method (QHRS-CL) have been used to investigate trace elements in minerals. Using micro-PIXE and related methods the crystal lattice site and charge state of the analysed elements cannot be determined. This can be analysed exactly by using QHRS-CL. So the combination of micro-PIXE and QHRS-CL is a powerful tool for analysing trace element concentration above 100 ppb, the charge state and the lattice site of these elements in crystal structures. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Since cathodoluminecence (CL) is used to detect trace element distribution in minerals a possible quantitative analysis of activator elements by their CL-intensity was the incentive to numerous investigations. The CL of minerals is predominantly controlled by the activator ions Mn2 , Fe3 , Cr3 and rare earth elements (REE2=3 ) and the quencher ions like Fe2 . There is a general agreement in literature about the dominant role of the Mn2 -ion as the most important activator element and Fe2 as the most ecient quencher el-
* Corresponding author. E-mail: [email protected]
ement in minerals (e.g. Refs. [1±6]). The correlation of CL-intensity and activator concentration and the low limit of activator concentrations, above which CL occurs, is often controversially discussed [7,5]. There is a widespread assumption in geosciences, that a concentration level of 10±30 ppm is needed before Mn2 activation in calcite is present (e.g. Refs. [8, 9]). As these authors based their de®nition on visible determination the CL-intensity, this interpretation is not objective. However, the CL is very sensitive to low variations in the activator and quencher elements concentrations of minerals. In this paper spectroscopic investigation of CL combined with micro-PIXE will be used to analyse the correlation between CL-intensity and activator element concentration. The powerful combination
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 9 2 6 - 4
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 470±477
of quantitative CL-spectroscopy and micro-PIXE for trace element analyses in minerals will be demonstrated. 2. Method The samples are calcite, dolomite and feldspar minerals from sedimentary, hydrothermal or magnetic source. Polished thin sections of the samples are measured by micro-PIXE and Quantitative High Resolution Spectroscopy of CLemission (QHRS-CL). The QHRS-CL-device developed at the RuhrUniversity Bochum based on the combination of 1. the ``hot cathode'' CL-microscope (type HC1LM) [10], 2. an EG&G digital triple grating spectrograph with a LN2 -cooled CCD-camera, 3. a noncommercial software for quantitative CLspectral analyses. For the ®rst time the QHRS-CL allows to apply CL for quantitative trace element analyses in minerals. This method is calibrated e.g. for calcite by using an island spar analysed by micro-PIXE and CL-spectroscopy (Table 1) [11]. CL-Spectra were accumulated within an exposure time of 5±120 s, depending on the CLintensity. For high lateral resolution the analysed area was focused to a spot of 30 lm in diameter. To prevent charging during the CL analyses thick gold the samples are covered by a few A coating. To avoid variations in the coating thickness and on that attributed variations in CL-intensity the coating was done at standard conditions. The CL-intensity is also controlled by other physical processes ± e.g. thermal quenching, generating of lattice defects ± depending on the physical properties of the mineral and the analyses conditions. For each mineral group we de®ned standard analyses conditions in experiments. More details are described in Ref. [11]. At present, the calculated detection limit of QHRS-CL is in the range of 0.1 ppm. To keep the calculated detection limit of micro-PIXE, QHRSCL and moreover the quantitative ionoluminescence [12] comparable this is calculated on the
471
same way basing on the LOD calculation of the GUPIX [13] software package. At a discriminated CL-peak with a con®dence level of 99.86% the smallest p acceptable CL-peak area in a spectrum is 3 Nb , where Nb is the background at the peak position. The micro-PIXE analyses were done at the Dynamitron Tandem Laboratorium (DTL) at the Ruhr-University Bochum (method in Ref. [14]). At an average spot-size of 10 lm and a proton energy of 3 MeV, the detection limit for Mn is 10 ppm and for most REE >n*10 ppm. The spectra were analysed by the GUPIX [13] software package. The lattice site of Mn2 in calcite, dolomite and feldspar samples are analysed by Electron Spin Resonance (ESR) with a Bruker ESP 300e (XBand) at the Institute of Experimental Physics at the University of Mining and Technology Freiberg. 3. Experiment results and discussion 3.1. Correlation between CL-spectroscopy and electron spin resonance spectroscopy The wavelength of Mn2 -activated CL is controlled by symmetry and strength of the crystal ®eld. Small variations either in symmetry and/or the atomic distance ± between the central metal ion and the surrounding oxygen-ligands ± caused a shift in wavelength of Mn2 -activated CL. For example, in the dolomite structure the Mn2 -ion can be incorporated in Ca- as well as Mg-position (e.g. Ref. [15]). From there, the Mn2 -peaks emit at 575 nm (Mn2 in Ca-position) and 660 nm (Mn2 in Mg-position) (e.g. Refs. [16,11]) (Fig. 1(a)). The CL-data of Mn-activated dolomite (Fig. 1(a)) and feldspar (Fig. 1(b)) are comparable with the data of ESR spectroscopy, where Mn2 incorporation in the dierent lattice positions can be documented. Only powder sample and orientated single crystals can be analysed by ESR. So, the advantage of CL-spectroscopy compared to ESR is, that Mn2 -partition in the crystal lattice can be analysed in thin sections with a point size of 30 lm.
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Table 1 PIXE-analyses data of an (a) Island spar crystal (sample: I-spar) and (b) feldspar crystal (sample: spruce mine S-M) for calculating the Mn-calibration value of the QHRS-CL method. In both cases the quencher (e.g. Fe, Ni) and sensitiser elements (e.g. Yb) are in low concentration respectively not detected by micro-PIXE (average detection limit 10 ppm). Co-activators like most REE are only in the feldspar sample analysed by micro-PIXE and QHRS-CL. (c) The data of the synthetic Sm3 -doped calcite show that only Sm is incorporated in sucient quantities. Therefore quenching and sensitising by other trace elements can be excluded Part a
I-SPAT-1
I-SPAT-2
I-SPAT-3
I-SPAT-4
I-SPAT-5
I-SPAT-6
I-SPAT-7
25 26 28 38 39 56
159 241 n.d. 660 n.d. n.d.
159 203 n.d. 658 n.d. n.d.
160 177 n.d. 614 n.d. n.d.
198 248 n.d. 458 n.d. n.d.
173 128 n.d. 352 n.d. n.d.
134 46 n.d. 377 n.d. n.d.
118 42 n.d. 299 n.d. n.d.
Part b
S-M1
S-M2
S-M3
S-M4
S-M5
S-M6
S-M7
S-M8
25 26 28 38 39 56 57 58 59 60 61 62 63 59 65 66 67 68 69 70 71
Mn Fe Ni Sr Y Ba La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
1.3 99.1 n.d. ? 13.2 ? 7.8 n.a. 61.8? 122.4 n.d. 39.8 ? 12.9 n.d. n.d. n.d. n.d. n.d. ? 5.9 ? 4.1 n.d. ? 8.8 ? 2.2
7.3 93.9 n.d. 20.9 ? 9.5 n.a. ? 87.9 131.9 ? 49.1 ? 38.1 ? 9.3 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
27.2 90.8 n.d. 16.1 7.4 n.a. ? 59.8 93.9 ? 32.2 ? 36.3 ? 6.1 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.a. ? 2.8
? 4.8 108.9 n.d. 19.6 7.4 n.a. ? 82.0 133.8 ? 43.6 ? 31.3 ? 7.7 ? 18.3 ? 13.8 ? 19.0 n.d. n.d. n.d. ? 4.5 n.d. ? 3.8 n.d.
15.9 120.2 n.d. 12.7 7.1 n.a. 95.7 140.0 ? 40.0 ? 33.5 ? 9.4 ? 8.8 ? 5.9 n.d. n.d. n.d. n.d. ? 3.4 n.d. ? 2.0. n.d.
? 6.2 101.3 n.d. ? 9.2 ? 11.8 n.a. ? 159.6 163.7 ? 37.6 ? 79.3 ? 61,7 ? 58.3 ? 24.7 ? 11.7 ? 22.4 n.d. ? 22.9 ? 7.4 n.d. n.d. n.d.
18.3 123.9 n.d. ? 16.6 ? 8.4 n.a. ? 126.4 168.1 ? 70.3 ? 63.9 n.d. ? 23.3 ? 14.0 ? 25.6 ? 10.0 ? 11.7 ? 5.5 n.d. n.d. n.d. n.d.
? 6.3 117.8 n.d. ? 23.3 n.d. n.a. ? 111.1 ? 141.4 ? 70.4 ? 79.1 ? 97.5 ? 115.8 ? 29.1 ? 48.3 n.d. ? 99.6 ? 40.3 ? 50.0 ? 22.3 ? 43.8 n.d.
Part c
Sm-p1
Sm-p2
Sm-p3
Sm-p4
Sm-p5
Sm-p6
Sm-p7
Sm-p8
25 26 28 38 39 56 57 58 59 60 61 62
n.d. n.d. n.d. ? 6.4 n.d. n.d. n.a. n.a. n.d. n.d. n.d. 79.6
n.d. ? 18.1 ? 9.0 ? 3.2 n.d. n.d. n.a. n.a. n.d. n.d. n.d. 523.0
? 19.2 ? 14.6 n.d. ? 9.7 n.d. n.d. n.a. n.a. n.d. n.d. n.d. 202.4
? 75.8 ? 8.1 n.d. ? 16.4 n.d. n.d. n.a. n.a. n.d. n.d. n.d. 60.4
n.d. ? 19.9 n.d. ? 3.0 n.d. ? 203.0 n.a. n.a. n.d. n.d. n.d. 132.5
? 5.1 ? 7.0 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 83.4
n.d. ? 9.7 n.d. n.d. n.d. n.d. ? 78.8 ? 58.8 n.d. n.d. n.d. 167.2
? 5.9 ? 8.9 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. ? 14.3 104.9
Mn Fe Ni Sr Y Ba
Mn Fe Ni Sr Y Ba La Ce Pr Nd Pm Sm
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 470±477
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Table 1 (Continued) Part c
Sm-p1
Sm-p2
Sm-p3
Sm-p4
Sm-p5
Sm-p6
Sm-p7
Sm-p8
63 59 65 66 67 68 69 70 71
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.a. n.a.
n.d. n.d. n.d. n.d. n.d. ? 21.3 ? 28.0 ? 16.0 n.a.
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.a. n.a.
n.d. n.d. n.d. n.d. ? 23.1 ? 19.1 ? 18.1 n.a. n.a.
n.d. n.d. n.d. n.d. ? 28.1 n.d. ? 15.5 n.a. n.a.
? 14.1 ? 12.1. ? 7.7 n.d. n.d. ? 3.3. ? 3.3. ? 8.7 ? 8.7
? 29.2 ? 25.4 ? 7.9 n.d. n.d. ? 6.1 ? 6.1 ? 5.7 ? 6.2
n.d. n.d. n.d. n.d. ? 5.5 n.d. n.d. ? 6.5 n.d.
Eu Gd Tb Dy Ho Er Tm Yb Lu
n.a. ± not analysed; n.d. ± not detected; ? ± data near the LOD and the error is >20%.
3.2. Case studies: correlation between CL-intensity and activator element content in calcite and feldspar
mostly too low for quenching Mn2 -and REEactivated CL (see Table 1).
3.2.1. Case studies: manganese in calcite and feldspar There is a linear correlation between the activator element concentration (<1000 ppm) and the CL-intensity (Fig. 2 Fig. 3). Deviations from this linear correlation are based mainly on small variations in the Mn distribution, the statistic error of CL and PIXE analyses, dierences in the analysed spot size (micro-PIXE: £ 10 lm, QHRS-CL: £ 30 lm) and depth of penetration. Also selfquenching and quenching by Fe2 eect deviations from the linear correlation are if the concentrations are >1000 ppm respectively >2000±3000 ppm [17] (see Fig. 2(a) and (b)). Above these concentration levels the combination of both, selfquenching and quenching by Fe eects a complex correlation of CL-intensity and element concentration. Self-quenching is more eective than quenching by Fe2 as it takes place at lower concentrations levels [17]. Here, the eciency of ``Fequenching'' increases with increasing Mn-content according to decreasing average Fe2 and Mn2 ion distance [17]. This is attributed to the fact that an energy transfer between activator and quencher ions, respectively is only possible at small atomic distance (e.g. Ref. [4]). Other quencher ions e.g. Ni2 , some REE (e.g. La3 , Nd3 (also activator), Ho3 ) and Fe3 (in carbonates) are much less effective than Fe2 [1]. Additional, in most silicates Fe3 is an important activator. However, if other quenchers than Fe2 are incorporated in natural carbonates and feldspar, their concentration is
3.2.2. Case studies: REE in calcite and feldspar Most of divalent and trivalent REE (Eu2=3 , 3 Er , Tb3 , Dy3 , Sm3 , Pr3 , Ho3 and Nd3 ) shows CL-bands in the near UV-, visible- and IR spectrum. The structure of the REE related peaks in a spectrum are dierent depending on the symmetry and strength of host crystal ®eld [1]. Therefore, Ca2 substituted by REE in e.g. calcite-, aragonite-, feldspar-, apatite structure is unequivocal by detectable using the CL-spectroscopy although the peaks show nearly constant wavelengths. The micro-PIXE analyses of Sm-doped synthetic calcite (Fig. 4) indicate a linear correlation between Sm-activated CL-intensity and the Smconcentration. The CL-spectrum in Fig. 5 re¯ects the Sm-content of 79 ppm (micro-PIXE analyses: statistic error 17.49%, LOD 41.8 ppm, accumulation time 65 min.). After ®tting and ®ltering the CL-spectrum (details in Ref. [11]) the calculated LOD by QHRS-CL is 1.3 ppm (accumulation time 30 s). In many natural (also synthetic samples) the CL-spectroscopy indicate REE concentrations below the detection limit of micro-PIXE (Fig. 6). REE are mostly incorporated in natural minerals in groups. Therefore, sensibilisation, quenching and absorption by other REE are dominant eects yielding strong variations in the correlation between the REE-activated CL-intensity and the REE-content [1]. At present state, only for few REE the quantitative CL-spectroscopy is
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Fig. 1. (a) Spectrum of Mn2 -activated CL in dolomite and comparable ESR-spectrum (sample: HM; analyses conditions: acceleration voltage 14 keV, beam current density 9 lA/mm2 , exposure time: 5 s). The CL-peak position at 575 and 661 nm indicates that Mg2 and Ca2 are substituted by Mn2 , this is also demonstrated by the ESR signal (sixth line of the hyper®ne structure from Mn2 ) (Mn 1100 ppm). (b) Spectrum of Mn2 , Fe3 , Sm3 , Dy3 , Tb3 and Nd3 -activated CL of feldspar (sample: spruce mine (albite: An10 ); analyses conditions: acceleration voltage 14 keV, beam current density 9 lA/mm2 , exposure time: 30 s) and comparable ESR-spectrum. The CL-peak position at 570 nm indicates the Ca2 substitution by Mn2 , this is also demonstrated by the ESR signal (sixth line of the hyper®ne structure of the central spin transition) (Mn 15.9 ppm). Fe3 is incorporated in Al3 position (tetrahedral site) while the REE are in Ca2 -position. The REE content is mostly near or below the LOD of micro-PIXE (10±40 ppm). Notice the dierent position of the Mn-signal in dolomite and feldspar in the ESR-spectrum. Analysis conditions for all samples: Microwave 9.7 GHz (X-Band); microwave power 10 mW; modulation 0.0005 T, modulation frequency 100 kHz.
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 470±477
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Fig. 2. (a) Correlation between CL-intensity (normalised to counts/5 s), Mn- and Fe-content in visually homogeneously luminescent calcite. The data reveal an excellent linear correlation (R 0.97) in the range between 19 and 1000 ppm. Mn-analyses below the LOD of micro-PIXE (10 ppm) were done using QHRS-CL. The calibration value (linear function) of QHRS-CL based on the analyses data of an Iceland spar (black arrow) (micro-PIXE analyses see Table 1). Two analyses points show a distinct deviation from that linear correlation indicating that Fe2 -quenching and Mn2 -self-quenching occur at 4000 ppm respectively 1100 ppm (white arrows). (b) Seven analyses points show Fe-content below the LOD of micro-PIXE and therefore not plotted in the Fig. 2(b). The trace element concentration of elements with atomic number >27 are near or below the LOD of micro-PIXE (REE: n*10 ppm, others n*1±10 ppm). Modi®ed from Ref. [18].
Fig. 3. Correlation between CL-intensity (normalised to counts/5 s) and Mn-content in feldspar (sample: spruce mine). The data reveal a linear correlation in the range between 4.8 and 27.2 ppm. The micro-PIXE analyses at 4.8 and 7.3 ppm are only two time above the LOD of micro-PIXE. The relative CLintensity is relatively high, so that Mn can be analysed below the LOD of micro-PIXE.
possible while estimating their CL-intensity is unequivocal. These elements are Dy3 , Eu3 , Sm3 and Nd3 because some of their peaks lie in a spectral region where no other REE have emission peaks, absorption bands and no or only minor
Fig. 4. Synthetic Sm-doped calcite is analysed by micro-PIXE and CL-spectroscopy (micro-PIXE data see Table 1). The LOD for micro-PIXE lies in the range of 40±70 ppm. The reference CL-spectrum (see Fig. 5).
sensibilisation occurs. The ®tting and ®lter algorithms for the other REE are still in work. 4. Conclusion The combination of micro-PIXE and QHRSCL is a powerful tool for trace element analyses in
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D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 470±477
Fig. 5. CL-spectrum of Sm-doped synthetic calcite compared to 79.6 ppm Sm, which is slightly highly than the micro-PIXE LOD of 65 ppm. The calculate LOD of the QHRS-CL is 1.3. (Analyses conditions: acceleration voltage 14 keV, beam current density 9 lA/mm2 , exposure time: 30 s).
the ppb range. Fig. 7 shows a line scan through a calcite cement generation of early stage to late stage of burial diagenesis, analysed by micro-PIXE and QHRS-CL indicating that both methods are highly comparable. It is clear that the minor deviations are attributed to small scale inhomogeneities in the Mn-distribution. Additionally, in the crystal structure incorporated elements can be analysed unambiguously using QHRS-CL. The high potential in this method is its high sensibility
Fig. 7. Comparison between micro-PIXE analysis and quantitative CL-spectroscopy QHRS-CL analysis, re¯ecting the conformity of these two methods. Cement sequence of a Triassic limestone of Hydra (Greece). The QHRS-CL analysis spots are located between the micro-PIXE analysis spots (sample: FB; micro-PIXE analyses from [9], QHRS-CL analyses from [11]).
and the very low damage of the sample. An other advantage is, that this method can also supply detailed information about the crystal structure. Quantitative CL-spectroscopy is not only applicable to carbonates. The combination of microPIXE and QHRS-CL is a very powerful method for analysing REE, which is one of the most important potentials.
Fig. 6. CL-spectrum of a natural Mn2 -, Dy3 , Sm3 and Tb3 activated calcite (sample DB14-2). The Mn and REE content is below the detection limit of micro-PIXE (Mn 10 ppm, Dy 16 ppm). Using QHRS-CL 5 ppm Mn and 13 ppm Dy are analysed. (Analyses conditions: acceleration voltage 14 keV, beam current density 9 lA/mm2 , exposure time: 30 s).
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 470±477
Acknowledgements This study was ®nancially supported by the Deutsche Forschungsgemeinschaft (DFG-Projects: Ri 216/13-1 and 2). Thanks to M. Pl otze for carrying out the carbonate ESR analyses. Thanks to F. Bruhn for oering the sample Fb5 and part of it micro-PIXE analyses. References [1] [2] [3] [4] [5]
A.S. Marfunin, Springer, Berlin, 1979, p. 352. H. Machel, Geoscience Canada 12 (1985) 139. R.A. Mason, Chem. Geol. 64 (1987) 209. D.J. Marshall, Unwin-Hyman, Boston, 1988, p. 146. G. Walker, O.E. Abumere, B. Kamaluddin, Mineral. Mag. 53 (1989) 201. [6] R.A. Mason, A.N. Mariano, Chem. Geol. 88 (1990) 191. [7] D.K. Richter, U. Zinkernagel, Geol. Rdsch. 70 (1981) 1276.
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[8] M.M. Savard, J. Veizer, R. Hinton, J. Sediment. Res. A 65 (1995) 208. [9] F. Bruhn, Diss. Ruhr-Universit at, Bochum, 1995, p.172. [10] R.D. Neuser, Bochumer geol. u. geotech. Arb. 44 (1995) 116. [11] D. Habermann, Diss. Ruhr-Universit at, Bochum, 1997, p. 152. [12] N.P.-O. Homman, C. Yang, K.G. Malmquist, Nucl. Instr. and Meth. A 353 (1994) 610. [13] J.A. Maxwell, W.J. Teesdale, J.L. Campbell, Nucl. Instr. and Meth. B 95 (1995) 407. [14] J. Meijer, A. Stephan, J. Adamczewski, H.H. Bukow, C. Rolfs, T. Pickart, F. Bruhn, Nucl. Instr. and Meth. B 89 (1994) 229. [15] D.N. Lumdsen, R.V. Lloyd, Geochim. Cosmochim. Acta 48 (1984) 1861. [16] A. El Ali, V. Barbin, G. Calas, B. Cervelle, K. Ramseyer, J. Bouroulec, Chem. Geol. 104 (1993) 189. [17] D. Habermann, R.D. Neuser, D.K. Richter, F. Bruhn, V.M Goldschmidt Conference 1996 (Heidelberg), J. Conference Abstracts 1 (1) (1996) 226. [18] D. Habermann, R.D. Neuser, D.K. Richter, Sed. Geol. 116 (1998) 13.