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High resolution rare-earth elements analyses of natural apatite and its application in geo-sciences: Combined micro-PIXE, quantitative CL spectroscopy and electron spin resonance analyses
Nuclear Instruments and Methods in Physics Research B 161±163 (2000) 846±851
www.elsevier.nl/locate/nimb
High resolution rare-earth elements analyses of natural apatite and its application in geo-sciences: Combined micro-PIXE, quantitative CL spectroscopy and electron spin resonance analyses D. Habermann a
a,*
, T. G otte b, J. Meijer c, A. Stephan c, D.K. Richter b, J.R. Niklas
a
Institute of Experimental Physics, University of Technology, Silbermannstrasse 1, 09599 Freiberg, Germany b Institute of Geology, Ruhr-University, Bochum, Germany c Institute of Experimental Physics III, Ruhr-University, Bochum, Germany
Abstract The rare-earth element (REE) distribution in natural apatite is analysed by micro-PIXE, cathodoluminescence (CL) microscopy and spectroscopy and electron spin resonance (ESR) spectroscopy. The micro-PIXE analyses of an apatite crystal from Cerro de Mercado (Mexico) and the summary of 20 analyses of six francolite (conodonts of Triassic age) samples indicate that most of the REEs are enriched in apatite and francolite comparative to average shale standard (NASC). The analyses of fossil francolite revealing the REE-distribution not to be in balance with the REE-distribution of seawater and ®sh bone debris. Strong inhomogenous lateral REE-distribution in fossil conodont material is shown by CL-mapping and most probably not being a vital eect. Therefore, the resulting REE-signal from fossil francolite is the sum of vital and post-mortem incorporation. The necessary charge compensation for the substitution of divalent Ca by trivalent REE being done by dierent kind of electron defects and defect ions. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 41.75.Ak; 61.18.Fs; 29.30.Aj; 83.80.Nb Keywords: Apatite; Francolite; Micro-PIXE; CL-spectroscopy; CL-microscopy; Electron spin resonance (ESR)
1. Introduction The apatite mineral and its isomorphs are characterised by complex chemical composition and in nature both the organic and inorganic ori-
* Corresponding author. Tel.: +49-3731-2670; fax: +493731-4314. E-mail address: [email protected] (D. Habermann).
gin are distinctive. The apatite general formula is Ca5 (F,Cl,OH)(PO4 )3 and two dierent symmetries of Ca-sites (Ca(I) with C3 and Ca(II) with Cs symmetry) are one important characteristic of the apatite structure. The substitution of elements occurs in all apatite sites and not all of them have the same valency as the original ion. Therefore, substituting of the constituent ions Ca2 , PO3ÿ 4 and Fÿ by, e.g., OHÿ , Fÿ , Na , REE3 , Y3 , COÿ 3, ÿ 3ÿ 3ÿ , CO OH , CO F and NaCO are CO03 , SiO4ÿ 3 3 3 4 common in apatite minerals.
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 9 9 8 - 2
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 846±851
The francolite, a carbonate±¯uorapatite with >1% F and appreciable amount of CO2 [1], is the most common phosphate mineral in phosphorites unaltered by metamorphism or weathering [2]. The francolite structure (general formula: Ca10 ÿ a ÿ b Naa Mgb (PO4 )6 ÿ x (CO3 )x ÿ y ÿ z (CO3 F)y (SO4 )z F2 ) is indicated by a complex trace element chemistry [3]. In geo-sciences many of the stable and radiogenic isotopes are used as tracers and dating devices. The francolite from fossil bones of marine organism is applied as one important tracer mineral in reconstructing the isotope composition of paleo seawater. Isotopes of the structural components O, C and S of francolite are used for reconstructing the temperature of formation, the age and the diagenetic environment, respectively. The paleo land±sea distribution and oceanography can be reconstructed by isotopic work using, e.g., neodymium. However, the chemical and structural evolution of francolite during diagenesis is controversially discussed as a major limiting factor. As the initial isotope composition of this material is unknown, it is necessary to ®nd an independent indicator determining the possible isotopic overprint. Numerous chemical analyses of phosphorite deposits (summary in [2]) verify, that the REEs re¯ect parts of the geo-chemical evolution of natural phosporites. Generally, in natural apatite Ca2 is often substituted by REE3 and charge compensation is done by (a) intrinsic electron defects, (b) depletion of the constituent ions, and (c) a coupled substitution of the constituent ions by REE3 and, e.g., OHÿ , CO3 OH3ÿ and SiO4ÿ 4 . In the latter cases ((b) and (c)) the initial isotope composition will be affected by depletion eects and incorporated ions. Consequently, the complex analyses of REE incorporation in apatite could give information about the chemical and structural alteration of biogenic apatite. It is the purpose of this work to use the combination of: · micro-PIXE ± quantitative REE analyses, · CL-spectroscopy ± analysing the REE incorporation in Ca(I) and/or Ca(II) position, · CL-microscopy ± mapping the relative REE distribution, and · ESR-spectroscopy ± determining structural defects achieved by, e.g., REE incorporation
847
to examine in case studies the incorporation and distribution properties of REE3 in natural inorganic apatite and fossil francolite. 2. Experimental 2.1. Methods 2.1.1. PIXE The micro-PIXE analyses were done at the Dynamitron Tandem Laboratorium (DTL) at the Ruhr-University Bochum (method in [4]). At an average spot-size of 10 lm and a proton energy of 3 MeV the detection limit for most transition metal elements is around 10 ppm and for REEs from 20 to 300 ppm. The spectra were analysed by the GUPIX [5] software package. 2.1.2. High resolution spectroscopy of cathodoluminescence (HRS-CL) The HRS-CL based on combination of the ``hot cathode'' CL-microscope (type HC1-LM [6]) and an EG&G digital triple grating spectrograph with liquid N2 -cooled CCD camera. The spectra were accumulated within an exposure time of 30±120 s, depending on the CL-intensity. For high lateral resolution the analysed area was focused to a spot of 30 lm in diameter. To prevent charging during the CL analyses the samples are covered by a few Angstroms thick gold coating (more details in [7]). The detection limit for REE2=3 and Mn2 is around 100 ppb [7]. 2.1.3. Electron spin resonance spectroscopy (ESR) Spectra of orientated single crystals and powder samples were recorded at room temperature and at 70 K at X-band frequencies of about 9.75 GHz with a Bruker ESP 300e using 100 kHz modulation. The observed ESR signals were signed by their g value, which is de®ned by DE hm gbH ;
1
where h is the PlanckÕs constant, m the frequency of the radiation, g a parameter describing the interaction of the paramagnetic centre with the external magnetic ®eld, b the Bohr magneton and H is the intensity of the resonance magnetic ®eld.
848
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 846±851
The resonance frequency was determined by a frequency counter, and the magnetic ®eld was calibrated using DPPH (a-diphenyl-bpicryl hydrazyl), which has a g factor of 2:0036 0:0003. 2.2. Samples The samples are apatite single crystals from Cerro de Mercado (Mexico) and francolite from conodonts ± fossil bones from marine chorda-animals of not cleared systematic position (sample age: Triassic, locations: Germany, Greece). Samples from fossil organic bones of marine organism and inorganic apatite are prepared as powder sample, orientated single crystals and polished thin sections. The apatite minerals are measured by micro-PIXE, ESR-spectroscopy and HRS-CL. 3. Results and discussion 3.1. Rare-earth element chemistry of apatite and francolite Fig. 1(a) shows 4 analyses of an apatite crystal from Cerro de Mercado and the summary of 20 analyses of 6 francolite (conodonts) samples indicating that most of the REEs to be enriched in apatite and francolite comparative to average Shale standard (NASC [8]). The PIXE analyses data from the REEs: Pr, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb are near the limit of detection (LOD) of PIXE 26±295 ppm, respectively. Therefore, only elements are plotted where the REE incorporated in the apatite structure were also determined by CL-spectroscopy and where corresponding shale data were available. The Shale-normalised REE-distribution of francolite from fossil conodonts, recent ®sh bone debris (data from [9]) and seawater (data from [10]) is plotted in Fig. 1(b). The analyses of francolite show that the REE-distribution of this material is not in balance with the REE-distribution of seawater. Moreover, this should be the case, if the francolite (e.g., from ®sh bones [9]) is built up by marine organism and if the chemical composition is unaected. These data reveal that francolite showing no and also strong thermal al-
Fig. 1. (a) Shale-normalised (NASC standard [8]) REE-distribution of an apatite crystal from Cerro de Mercado (Mexico) (magmatic origin); (b) Shale-normalised REE-distribution of francolite from conodonts, seawater (data from [10]) and recent ®sh bone debris (data from [9]). The REE-distribution in conodonts are analysed by micro-PIXE. Note the enrichment of Ce and Eu compared to unaltered francolite. Grey-shaded area indicates REE which can be also analysed by HRS-CL. CAI alteration index, see Fig. 2; LOD limit of detection.
teration ± related to the colour alteration index (CAI) classi®cation (scale: from 1 (none to low grade thermal alteration) to 5 (high grade thermal alteration), which is used in the geo-sciences ± can be chemically aected. The enrichment of Ce, Eu and some HREE (Ho±Lu) is consequently most probably the result from alteration during diagenesis and/or weathering post-mortem.
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 846±851
849
850
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 846±851
3.2. CL-microscopy and spectroscopy of apatite and fossil francolite Fig. 2. shows three representative CL-photographs and corresponding CL-spectra of conodonts from dierent locations in Germany with CAI 1, 1.5 and 4 and Greece with CAI 1. Blue CL is intrinsic, while violet to orange and pink CL are characteristic for REE3 -activation (see spectra Fig. 2(d)±(e)). The CL-spectra of conodonts are typical for apatite-like structure revealing REE3 predominantly build in the high symmetric Ca(I) position. Our spectra are quite similar to that of Gaft et al. [11] who proved that REE3 are mainly built in Ca(I) position in natural apatite. The spectrum of Fig. 2(e) indicates Pr3 and Sm3 being incorporated in the lower symmetric Ca(II) site (see also [11]). Increasing CL-intensities are positive correlated with increasing REE concentrations. There is no general trend in the CLproperties of conodonts evident but dierent kinds of lateral distribution of the REEs are recognisable. Particularly, the punctuated and inhomogeneous REE-distribution of same samples with CAI 1±1.5 (Fig. 2(a) and (b)) indicates that this could not be a vital eect and being most probably caused by REE-enrichment post-mortem. 3.3. EPR case studies on apatite and francolite In apatite minerals there are three groups of centres and of impurity ions distinguished due to the existence of tetrahedral oxyanion PO3ÿ 4 , a chain of F (or Cl) ions along the c axis, and the Ca2 cation at two non-equivalent sites [12]. Using the ESR-spectroscopy it is possible to determine paramagnetic centres in minerals which are related to intrinsic and extrinsic defects. Our case studies of francolite from conodonts and one apatite
crystal from magmatic source indicates that the charge compensation of REE3 ± mainly Ca(I) position ± is done by defect centres located at the Fÿ position and [PO4 ] complex and position (Fig. 3). The signals at around g 2.0 are attributed to the addition of lines from Oÿ centre (substitution: 3ÿ O2ÿ > Fÿ ), PO2ÿ 3 , SiO3 and dierent kinds of COradicals. The annexation of REE3 in Ca-position is combined with the formation and incorporation of e.g. O2ÿ , PO-, SiO- and CO-radicals.
4. Conclusion The combination of PIXE, CL and ESR spectroscopy gives detailed quantitative data of concentration, charge (respectively, charge compensation) and lattice position of trace elements in the minerals. However, the original chemistry of the trace elements ± particularly the REE concentration ± of conodonts is unknown. It is not clear on which scale the documented enrichment occurs, but high REE-concentrations and enrichment of Ce and some REEs (Sm, Eu) are most probably aected by the chemical composition of surface water and/or burial ground water. The ¯uid compositions can dier in a wide range. Hence, the resulting REE signal from fossil francolite used in this study is the sum of vital and post-mortem incorporation. The latter is basically controlled by local chemical and physical properties of the ¯uids. The isotope composition (elements: O, C, Nd) of fossil francolite will be aected by the combined incorporation of REE3 and CO-, SiO-radicals during diagenesis. Therefore, the REEs are possible tracer elements indicating multiple chemical modi®cations during the diagenesis of biogenic apatite. Consequently, if fossil francolite from marine organism shows the
b Fig. 2. (a)±(c) CL-photographs of conodonts from dierent locations (Germany: CAI 1 ± low grade thermal alteration, 4 ± high grade thermal alteration; Greece: CAI 1). The CL-colours of natural francolite are predominantly controlled by the intrinsic and the REE activation: blue CL is intrinsic, violet to orange and pink CL are REE-activated. CL-photographs indicating dierent kinds of inhomogenous REE-distribution. The numbers 1±3 indicate points where CL-spectra were taken. (d)±(e) Corresponding CL-spectra to conodonts (samples Germany CAI 1 and 4) are characteristic for apatite-like structure revealing REE3 build in Ca(I) and Pr3 and 3 3 3 3 Sm3 also in Ca(II)-positions (marked: Pr3 Ca II -transitions P0 ! H6 and P0 ! F2 lines of low intensity at k 634 and 646 nm; 4 6 Sm3 -transitions G ! H lines at k 651±661 nm) [11]. 5=2 9=2 Ca II
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 846±851
851
Fig. 3. First derivatives ESR-spectra from (a) apatite single crystal (C. d. Mercado) and (b) francolite powder sample (Ri140: Triassic conodonts (Greece) with CAI 1).
REE pattern of seawater, these samples are most probably chemically unaected by diagenetic processes. Acknowledgements
[3] [4]
The authors thank the referees for their helpful comments to prepare this manuscript. The microPIXE analyses were supported by a grant of the Deutsche Forschungsgemeinschaft (DFG) (Ri 216/ 16).
[5]
References
[9]
[1] D. McConnel, in: Apatite ± Its Crystal Chemistry, Mineralogy, Utilisation, and Geologic and Biological Occurrences, Springer, Berlin, 1973. [2] I. Javris, W.C. Burnet, Y. Nathan, F.S.M. Almbaydin, A.K.M. Attia, L.N. Castro, R. Flicoteaux, M.E. Hilmy, V.
[6] [7] [8]
[10] [11] [12]
Husain, A.A. Qutawnah, A. Serjani, Y.N. Zanin, Eclogae geol. Helv. 87 (3) (1994) 643. G.H. McClellan, S.J. Van Kauwenbergh, in: A.J.G. Nothold, I. Javris (Eds.), Phosphorite Research and Development, Geol. Soc. Spec. Publ., 1990, p. 23. J. Meijer, A. Stephan, J. Adamczewski, H.H. Bukow, C. Rolfs, T. Pickart, F. Bruhn, Nucl. Instr. and Meth. B 89 (1994) 229. J.A. Maxwell, W.J. Teesdale, J.L. Campbell, Nucl. Instr. and Meth. B 95 (1995) 407. R.D. Neuser, Bochumer geol. u. geotech. Arb. 44 (1995) 116. D. Habermann, J. Meijer, R.D. Neuser, D.K. Richter, C. Rolfs, A. Stephan, Nucl. Instr. and Meth. B 150 (1999) 470. L.A. Haskin, M.A. Haskin, in: Ahrens (Ed.), Origin and Distribution of Elements, Pergamon, Oxford, 1966, pp. 889±912. A.J. Fleet, in: P. Henderson (Ed.), Rare-earth element geochemistry, Elsevier, Amsterdam, 1989, p. 343. M. Bernat, Cah. ORSTOM Serie Geol., 1975, p. 65. M. Gaft, R. Reisfeld, G. Panczer, G. Boulon, S. Shoval, B. Champagnon, Opt. Mat. 8 (1997) 149. L.G. Gilinskaya, M.Y. Sherberbakova, Y.N. Zanin, Litol. Polezn. Iskop. 6 (1971) 111.
www.elsevier.nl/locate/nimb
High resolution rare-earth elements analyses of natural apatite and its application in geo-sciences: Combined micro-PIXE, quantitative CL spectroscopy and electron spin resonance analyses D. Habermann a
a,*
, T. G otte b, J. Meijer c, A. Stephan c, D.K. Richter b, J.R. Niklas
a
Institute of Experimental Physics, University of Technology, Silbermannstrasse 1, 09599 Freiberg, Germany b Institute of Geology, Ruhr-University, Bochum, Germany c Institute of Experimental Physics III, Ruhr-University, Bochum, Germany
Abstract The rare-earth element (REE) distribution in natural apatite is analysed by micro-PIXE, cathodoluminescence (CL) microscopy and spectroscopy and electron spin resonance (ESR) spectroscopy. The micro-PIXE analyses of an apatite crystal from Cerro de Mercado (Mexico) and the summary of 20 analyses of six francolite (conodonts of Triassic age) samples indicate that most of the REEs are enriched in apatite and francolite comparative to average shale standard (NASC). The analyses of fossil francolite revealing the REE-distribution not to be in balance with the REE-distribution of seawater and ®sh bone debris. Strong inhomogenous lateral REE-distribution in fossil conodont material is shown by CL-mapping and most probably not being a vital eect. Therefore, the resulting REE-signal from fossil francolite is the sum of vital and post-mortem incorporation. The necessary charge compensation for the substitution of divalent Ca by trivalent REE being done by dierent kind of electron defects and defect ions. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 41.75.Ak; 61.18.Fs; 29.30.Aj; 83.80.Nb Keywords: Apatite; Francolite; Micro-PIXE; CL-spectroscopy; CL-microscopy; Electron spin resonance (ESR)
1. Introduction The apatite mineral and its isomorphs are characterised by complex chemical composition and in nature both the organic and inorganic ori-
* Corresponding author. Tel.: +49-3731-2670; fax: +493731-4314. E-mail address: [email protected] (D. Habermann).
gin are distinctive. The apatite general formula is Ca5 (F,Cl,OH)(PO4 )3 and two dierent symmetries of Ca-sites (Ca(I) with C3 and Ca(II) with Cs symmetry) are one important characteristic of the apatite structure. The substitution of elements occurs in all apatite sites and not all of them have the same valency as the original ion. Therefore, substituting of the constituent ions Ca2 , PO3ÿ 4 and Fÿ by, e.g., OHÿ , Fÿ , Na , REE3 , Y3 , COÿ 3, ÿ 3ÿ 3ÿ , CO OH , CO F and NaCO are CO03 , SiO4ÿ 3 3 3 4 common in apatite minerals.
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 9 9 8 - 2
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 846±851
The francolite, a carbonate±¯uorapatite with >1% F and appreciable amount of CO2 [1], is the most common phosphate mineral in phosphorites unaltered by metamorphism or weathering [2]. The francolite structure (general formula: Ca10 ÿ a ÿ b Naa Mgb (PO4 )6 ÿ x (CO3 )x ÿ y ÿ z (CO3 F)y (SO4 )z F2 ) is indicated by a complex trace element chemistry [3]. In geo-sciences many of the stable and radiogenic isotopes are used as tracers and dating devices. The francolite from fossil bones of marine organism is applied as one important tracer mineral in reconstructing the isotope composition of paleo seawater. Isotopes of the structural components O, C and S of francolite are used for reconstructing the temperature of formation, the age and the diagenetic environment, respectively. The paleo land±sea distribution and oceanography can be reconstructed by isotopic work using, e.g., neodymium. However, the chemical and structural evolution of francolite during diagenesis is controversially discussed as a major limiting factor. As the initial isotope composition of this material is unknown, it is necessary to ®nd an independent indicator determining the possible isotopic overprint. Numerous chemical analyses of phosphorite deposits (summary in [2]) verify, that the REEs re¯ect parts of the geo-chemical evolution of natural phosporites. Generally, in natural apatite Ca2 is often substituted by REE3 and charge compensation is done by (a) intrinsic electron defects, (b) depletion of the constituent ions, and (c) a coupled substitution of the constituent ions by REE3 and, e.g., OHÿ , CO3 OH3ÿ and SiO4ÿ 4 . In the latter cases ((b) and (c)) the initial isotope composition will be affected by depletion eects and incorporated ions. Consequently, the complex analyses of REE incorporation in apatite could give information about the chemical and structural alteration of biogenic apatite. It is the purpose of this work to use the combination of: · micro-PIXE ± quantitative REE analyses, · CL-spectroscopy ± analysing the REE incorporation in Ca(I) and/or Ca(II) position, · CL-microscopy ± mapping the relative REE distribution, and · ESR-spectroscopy ± determining structural defects achieved by, e.g., REE incorporation
847
to examine in case studies the incorporation and distribution properties of REE3 in natural inorganic apatite and fossil francolite. 2. Experimental 2.1. Methods 2.1.1. PIXE The micro-PIXE analyses were done at the Dynamitron Tandem Laboratorium (DTL) at the Ruhr-University Bochum (method in [4]). At an average spot-size of 10 lm and a proton energy of 3 MeV the detection limit for most transition metal elements is around 10 ppm and for REEs from 20 to 300 ppm. The spectra were analysed by the GUPIX [5] software package. 2.1.2. High resolution spectroscopy of cathodoluminescence (HRS-CL) The HRS-CL based on combination of the ``hot cathode'' CL-microscope (type HC1-LM [6]) and an EG&G digital triple grating spectrograph with liquid N2 -cooled CCD camera. The spectra were accumulated within an exposure time of 30±120 s, depending on the CL-intensity. For high lateral resolution the analysed area was focused to a spot of 30 lm in diameter. To prevent charging during the CL analyses the samples are covered by a few Angstroms thick gold coating (more details in [7]). The detection limit for REE2=3 and Mn2 is around 100 ppb [7]. 2.1.3. Electron spin resonance spectroscopy (ESR) Spectra of orientated single crystals and powder samples were recorded at room temperature and at 70 K at X-band frequencies of about 9.75 GHz with a Bruker ESP 300e using 100 kHz modulation. The observed ESR signals were signed by their g value, which is de®ned by DE hm gbH ;
1
where h is the PlanckÕs constant, m the frequency of the radiation, g a parameter describing the interaction of the paramagnetic centre with the external magnetic ®eld, b the Bohr magneton and H is the intensity of the resonance magnetic ®eld.
848
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 846±851
The resonance frequency was determined by a frequency counter, and the magnetic ®eld was calibrated using DPPH (a-diphenyl-bpicryl hydrazyl), which has a g factor of 2:0036 0:0003. 2.2. Samples The samples are apatite single crystals from Cerro de Mercado (Mexico) and francolite from conodonts ± fossil bones from marine chorda-animals of not cleared systematic position (sample age: Triassic, locations: Germany, Greece). Samples from fossil organic bones of marine organism and inorganic apatite are prepared as powder sample, orientated single crystals and polished thin sections. The apatite minerals are measured by micro-PIXE, ESR-spectroscopy and HRS-CL. 3. Results and discussion 3.1. Rare-earth element chemistry of apatite and francolite Fig. 1(a) shows 4 analyses of an apatite crystal from Cerro de Mercado and the summary of 20 analyses of 6 francolite (conodonts) samples indicating that most of the REEs to be enriched in apatite and francolite comparative to average Shale standard (NASC [8]). The PIXE analyses data from the REEs: Pr, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb are near the limit of detection (LOD) of PIXE 26±295 ppm, respectively. Therefore, only elements are plotted where the REE incorporated in the apatite structure were also determined by CL-spectroscopy and where corresponding shale data were available. The Shale-normalised REE-distribution of francolite from fossil conodonts, recent ®sh bone debris (data from [9]) and seawater (data from [10]) is plotted in Fig. 1(b). The analyses of francolite show that the REE-distribution of this material is not in balance with the REE-distribution of seawater. Moreover, this should be the case, if the francolite (e.g., from ®sh bones [9]) is built up by marine organism and if the chemical composition is unaected. These data reveal that francolite showing no and also strong thermal al-
Fig. 1. (a) Shale-normalised (NASC standard [8]) REE-distribution of an apatite crystal from Cerro de Mercado (Mexico) (magmatic origin); (b) Shale-normalised REE-distribution of francolite from conodonts, seawater (data from [10]) and recent ®sh bone debris (data from [9]). The REE-distribution in conodonts are analysed by micro-PIXE. Note the enrichment of Ce and Eu compared to unaltered francolite. Grey-shaded area indicates REE which can be also analysed by HRS-CL. CAI alteration index, see Fig. 2; LOD limit of detection.
teration ± related to the colour alteration index (CAI) classi®cation (scale: from 1 (none to low grade thermal alteration) to 5 (high grade thermal alteration), which is used in the geo-sciences ± can be chemically aected. The enrichment of Ce, Eu and some HREE (Ho±Lu) is consequently most probably the result from alteration during diagenesis and/or weathering post-mortem.
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 846±851
849
850
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 846±851
3.2. CL-microscopy and spectroscopy of apatite and fossil francolite Fig. 2. shows three representative CL-photographs and corresponding CL-spectra of conodonts from dierent locations in Germany with CAI 1, 1.5 and 4 and Greece with CAI 1. Blue CL is intrinsic, while violet to orange and pink CL are characteristic for REE3 -activation (see spectra Fig. 2(d)±(e)). The CL-spectra of conodonts are typical for apatite-like structure revealing REE3 predominantly build in the high symmetric Ca(I) position. Our spectra are quite similar to that of Gaft et al. [11] who proved that REE3 are mainly built in Ca(I) position in natural apatite. The spectrum of Fig. 2(e) indicates Pr3 and Sm3 being incorporated in the lower symmetric Ca(II) site (see also [11]). Increasing CL-intensities are positive correlated with increasing REE concentrations. There is no general trend in the CLproperties of conodonts evident but dierent kinds of lateral distribution of the REEs are recognisable. Particularly, the punctuated and inhomogeneous REE-distribution of same samples with CAI 1±1.5 (Fig. 2(a) and (b)) indicates that this could not be a vital eect and being most probably caused by REE-enrichment post-mortem. 3.3. EPR case studies on apatite and francolite In apatite minerals there are three groups of centres and of impurity ions distinguished due to the existence of tetrahedral oxyanion PO3ÿ 4 , a chain of F (or Cl) ions along the c axis, and the Ca2 cation at two non-equivalent sites [12]. Using the ESR-spectroscopy it is possible to determine paramagnetic centres in minerals which are related to intrinsic and extrinsic defects. Our case studies of francolite from conodonts and one apatite
crystal from magmatic source indicates that the charge compensation of REE3 ± mainly Ca(I) position ± is done by defect centres located at the Fÿ position and [PO4 ] complex and position (Fig. 3). The signals at around g 2.0 are attributed to the addition of lines from Oÿ centre (substitution: 3ÿ O2ÿ > Fÿ ), PO2ÿ 3 , SiO3 and dierent kinds of COradicals. The annexation of REE3 in Ca-position is combined with the formation and incorporation of e.g. O2ÿ , PO-, SiO- and CO-radicals.
4. Conclusion The combination of PIXE, CL and ESR spectroscopy gives detailed quantitative data of concentration, charge (respectively, charge compensation) and lattice position of trace elements in the minerals. However, the original chemistry of the trace elements ± particularly the REE concentration ± of conodonts is unknown. It is not clear on which scale the documented enrichment occurs, but high REE-concentrations and enrichment of Ce and some REEs (Sm, Eu) are most probably aected by the chemical composition of surface water and/or burial ground water. The ¯uid compositions can dier in a wide range. Hence, the resulting REE signal from fossil francolite used in this study is the sum of vital and post-mortem incorporation. The latter is basically controlled by local chemical and physical properties of the ¯uids. The isotope composition (elements: O, C, Nd) of fossil francolite will be aected by the combined incorporation of REE3 and CO-, SiO-radicals during diagenesis. Therefore, the REEs are possible tracer elements indicating multiple chemical modi®cations during the diagenesis of biogenic apatite. Consequently, if fossil francolite from marine organism shows the
b Fig. 2. (a)±(c) CL-photographs of conodonts from dierent locations (Germany: CAI 1 ± low grade thermal alteration, 4 ± high grade thermal alteration; Greece: CAI 1). The CL-colours of natural francolite are predominantly controlled by the intrinsic and the REE activation: blue CL is intrinsic, violet to orange and pink CL are REE-activated. CL-photographs indicating dierent kinds of inhomogenous REE-distribution. The numbers 1±3 indicate points where CL-spectra were taken. (d)±(e) Corresponding CL-spectra to conodonts (samples Germany CAI 1 and 4) are characteristic for apatite-like structure revealing REE3 build in Ca(I) and Pr3 and 3 3 3 3 Sm3 also in Ca(II)-positions (marked: Pr3 Ca II -transitions P0 ! H6 and P0 ! F2 lines of low intensity at k 634 and 646 nm; 4 6 Sm3 -transitions G ! H lines at k 651±661 nm) [11]. 5=2 9=2 Ca II
D. Habermann et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 846±851
851
Fig. 3. First derivatives ESR-spectra from (a) apatite single crystal (C. d. Mercado) and (b) francolite powder sample (Ri140: Triassic conodonts (Greece) with CAI 1).
REE pattern of seawater, these samples are most probably chemically unaected by diagenetic processes. Acknowledgements
[3] [4]
The authors thank the referees for their helpful comments to prepare this manuscript. The microPIXE analyses were supported by a grant of the Deutsche Forschungsgemeinschaft (DFG) (Ri 216/ 16).
[5]
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