11) in geological samples

Chemical Geology, 40 (1983) 353--359 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

353

A N U C L E A R METHOD FOR THE ANALYSIS OF LIGHT ELEMENTS (Z ~< 11) IN GEOLOGICAL SAMPLES

M. BASUT~U l, J.-N. BARRANDON 1, M. VOLFINGER 2 and J.-L. ROBERT 2 IGroupe de Recherche GARNAC, Service du Cyclotron, C.N.R.S., 45045 Orldans Cedex (France) 2 Centre de Recherches sur la Synth$se et Chimie des Min~raux, C.N.R.S., 45045 Orleans Cedex (France) (Received September 16, 1982; revised and accepted January 5, 1983)

ABSTRACT Basut~u, M., Barrandon, J.-N., Volfinger, M. and Robert, J.-L., 1983. A nuclear method for the analysis of light elements (Z < 11) in geological samples. Chem. Geol., 40: 353--359. A nuclear method of measurement of major as well as trace light elements in silicates is presented. This method is particularly applied to the determination of Li content in micas in order to optimize some experimental conditions. The other elements that can be reached with a good sensitivity (~ 10 ppm) are Be, B, F and Na. The particles which provoke the nuclear reactions are single-charged a-particles provided by a Van de Graaf accelerator. The possibility of obtaining a well-focused a-beam and the good accuracy of the method permit us to consider this method as a valuable complement of the electron microprobe.

INTRODUCTION

The best-known nuclear method of chemical analysis is neutron activation. Other methods exist for the analysis of elements for which neutron activation is unsuitable, or for reaching lower concentrations. For example, it is possible to irradiate the samples with charged particles. In this topic, the research group Garnac of the C.N.R.S., working at the cyclotron laboratory of Orleans, studies the possibilities of analytical applications of charged particle beams (Borderie et al., 1977, 1980; Borderie, 1978; Borderie and Barrandon, 1978). For light elements in geological samples, one of the best methods is irradiation by low-energy (3.5 MeV) single-charged particles. For these light elements, irradiation by a-particles gives compound excited nuclei which rapidly return to their ground state while emitting other particles and 7-rays. This is not an activation method; the samples are n o t radioactive when the irradiation stops; the emitted 7-rays must be detected and measured during the

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354

irradiation. The method is of special interest for Li, Be, B, F and Na. The energy of the emitted 7-rays is < 2 MeV (Table I); then, they can be analysed using ordinary spectrometers. The usual constituents of silicates, especially Si and A1, are not involved and do not interfere. The 7-spectra obtained are simple, as shown in Fig. 1. This method is particularly interesting for the non-destructive analysis of light elements in a silicate matrix. For details on the methodology, see the three references above. The above method has been checked by the same authors on geological samples. The aims of this short paper are to present this m e t h o d to geologists, and to apply it to the analysis of the Li content of synthetic Li-micas described in Robert et al. (1983) and Volfinger (in prep.). The main objective of this work is the optimization of some experimental conditions in order to TABLE ! Main characteristics of the analysis under irradiation by single-charged a-particles of 3.5 MeV Nuclear reaction

Energy of the emitted 7 radiation (keV)

Limit of detection* (ppm)

7Li(a,a'~) 7Li 9Be(~,n) 12C 10B(a,p) 13C 180(~,n) 21Ne 19F(a,n) 22Ne 23Na(a ,~'7) 23Na

478 3,417 169 350 1,275 440

0.4 1 30 1,800 10 60

*In a silicate matrix.

V'~

Ioo

No

so

Li

Na

J%Ja

s~o

I IOO0

i 1$00

i

2000

P

Eg (KEY)

Fig. 1. An example of 7-ray spectrum obtained during the irradiation of a sample of riebeckite by low-energy a-particles.

355

use the a-irradiation m e t h o d as a routine analytical m e t h o d on geological samples. PRINCIPLE AND SETTING

The nuclear reactions used for the analysis of the concerned elements are given in Table I. The a-particles of 3.5 MeV are produced by a Van de Graaf accelerator. The analysis was performed using the 7-MeV accelerator at the Nuclear Research Centre, Strasbourg, France. The experimental device is represented in Fig. 2. The 7-detector ( G e - L i diode connected to a 4000-channel analyser) is at 90 ° to the a-beam. The diameter of the spot can be adjusted with magnetic lenses. In the present case, the minimum diameter of the spot on the sample is 1 m m 2. In order to eliminate the possible fluctuations of intensity of the incident a-beam, the charges received by the sample holder are integrated over the measuring time of the 7-rays; therefore, the ~/-activity is given as the n u m b e r of ~/-photons received by the detector divided by the quantity of electricity received by the sample holder (7/pCb). The analysis of the sample is carried out by comparison with standards using the relation:

(Xsample/Xstandard) = (Asample/Astandard) X (Ssample/Sstandard) where Xsample and Xstandard are the Li contents (wt. %) of the sample and the standard, respectively; Asample and Astandard are the measured activities (7/pCb); Ssample and Sstandard are the "stopping powers" of the sample and the standard (MeV cm -1) which depend on their bulk composition. In effect, the penetration of a-particles in matter is weak (~ 15 pm) for an energy of 3.5 MeV. Owing to interactions between these charged particles and matter, e[ecfr o magne fl c Lens Nb coltimators

J

insu [a, ors I

~

viewing porf I

i

Fig. 2. Illustration of the device used for irradiation of the sample by a-particles and meas u r e m e n t o f t h e e m i t t e d v-rays.

356 the n u m b e r o f e x c i t e d nuclei is variable f r o m o n e m a t r i x to a n o t h e r . T h e " s t o p p i n g p o w e r " s o f t h e c o m p o s i t e t a r g e t is c a l c u l a t e d b y t h e relation: s = ~.'xis i l

w h e r e x i is the w e i g h t p r o p o r t i o n o f t h e e l e m e n t i in t h e target. APPLICATION TO Li IN SYNTHETIC MICAS T h e s a m p l e s o f s y n t h e t i c Li-micas o b t a i n e d are c o n s t i t u t e d b y small particles ~ 1 g m in size. T o be s u b m i t t e d t o t h e irradiation, t h e solid m u s t have a g o o d c o h e s i o n , w i t h a well-defined surface w h o s e p o s i t i o n o n t h e s a m p l e h o l d e r is v e r y r e p r o d u c i b l e . F r o m a p o w d e r , it is possible to p r e p a r e pellets w i t h a binding m a t e r i a l . T h e m a t e r i a l c h o s e n h e r e is graphite, w i t h d i f f e r e n t p r o p o r t i o n s f o r t h e m i c a - - g r a p h i t e m i x t u r e . T a b l e II gives results o b t a i n e d o n s o m e s a m p l e s b y c o m p a r i s o n w i t h s t a n d a r d s p r e p a r e d similarly. Several m e a s u r e m e n t s are carried o u t o n d i f f e r e n t p o i n t s o f t h e pellet t o t e s t t h e rep r o d u c i b i l i t y . C o m p a r i n g c o l u m n s 3 and 5 o f T a b l e II, o n e can see t h a t this u n c e r t a i n t y is g r e a t e r t h a n t h a t o f t h e V-counting. This is d u e to h e t e r o g e n e ities in t h e g r a p h i t e pellet, b e c a u s e o n e n o t i c e s a decrease o f r e p r o d u c i b i l i t y with increasing d i l u t i o n in graphite. Special care m u s t t h e n be t a k e n in TABLE II Measurement time, dilution rate in graphite, reproducibilityand quality of the analyses of Li by irradiationwith a-particles

1

2 3 (min.) (%)

4

5 (%)

6

7

5--95 30--70 GL2 116 118 122 123 124 138 129 139

10 10 6 90 60 30 20 25 10 ~10 ~10

1 1 2 3 3 4 4 4 5 5 5

1 1 2.4 6 6 13.4 13.4 13.4 19.6 19.6 19.6

1,250 7,600 7,510 118 522 1,244 1,800 1,760 5,220 5,970 14,900

theor. 1,300 theor. 7,450 theor. 7,600 129 n.d. 1,554 1,830 2,230 5,720 n.d. 14,150

Column Column Column Column Column Column Column

1.4 0.8 1 3.9 1.8 1.5 1.5 1.3 2.0 ~2 ~2

1: sample number; samples 116--129 are those described in Robert et al. (1983). 2: measuring and irradiationtime for a current of 50 nA. 3: 2a per cent of uncertainty on the counting rate. 4: dilution rate of the mica in graphite. 5: 2a per cent of reproducibility. 6: Li content of the mica (in p p m ) --a-irradiationmethod. 7: Li content of the mica (in p p m ) -- flame spectrometry.

357

preparing the samples. A second technique has been used on our synthetic micas, consisting of preparing pellets of the pure product without any b i n d ing material (pressure 7000 bar). The results obtained are given in Table II, column 6, LiF being used as a standard. They show good precision and good reproducibility. From the sensitivity viewpoint for Li, on three samples containing 16 ppm Li (flame spectrometry), diluted with t w o parts of graphite, the uncertainty on the 7-counting (2a) was 2.3%. This means that on a pure sample, pelleted w i t h o u t graphite, or on a polished single crystal or rock, one can reach some ppm of Li with good precision. For low concentrations, the duration of irradiation has to be long (~ 100 min.) under the conditions offered by the accelerator used, to reach the indicated precision and sensitivity. The lower detection limits in a silicate matrix are given in the third column of Table I. The background on a pellet of pure graphite is 1.115 7//~Cb with 2a' = 16.1%. The uncertainty on the measurements on pellets having graphite as a binding material is close to 3.2%. The reproducibility of the sample--graphite mixture and the definition of the pellet surface are less good than this value. Thus, it is recommended that work be done on pure pelleted material. OTHER APPLICATIONS AND CONCLUSION

Lithium and fluorine have been measured in international standards b y this m e t h o d (Basut~u, 1980). These standards are those provided by the Centre de Recherches P~trographiques et G~ochimiques (C.R.P.G., Nancy, France) and distributed by the Association Nationale de la Recherche Technique (A.N.R.T., Paris, France). The measurements were performed on pure, pelleted p o w d e r y materials, w i t h o u t any binding material, as r e c o m m e n d e d above. Results are given in Table III. Those concerning the measurements of Li contents are in good agreement with the recommended values. However, for the measurements of fluorine, there are some discrepancies: the contents T A B L E III

Measurement with a - i r r a d i a t i o n o f t h e Li and F contents in International S t a n d a r d s International Standard

Basalt B R

Biotite Mica-Fe Phlogopite Mica-Mg Serpentine U B - N Diorite D R - N Bauxite B X - N

Li content ( p p m ± 30)

16 1,172 118 33 44 46

± 3 ± 36 ± 6 ± 6 ± 3 ± 3

Recommended

F content

Recommended

value o f Li content* 1

( p p m ± 30)

value o f F content*

13 1,400 120 30 44 54

1,770 17,900 32,500 260 870 960

1,000 15,600 28,500 (290) *2 ( 4 0 0 - - 6 4 0 ) *2

, l F r o m several m e a s u r e m e n t s b y various authors and m e t h o d s . ,2 N o value is recommended.

± 220 ± 2,000 ± 3,100 ± 38 ± 100 ± 195

358 o b t a i n e d w i t h t h e a - m e t h o d are g r e a t e r t h a n t h e r e c o m m e n d e d values. T h e s e are d r a w n f r o m a large set o f d i f f e r e n t values a n d it s e e m s t h a t t h e w e t c h e m ical m e t h o d s generally used can be t h e s o u r c e o f an e v e n t u a l loss o f fluorine. A n o t h e r v e r y interesting a p p l i c a t i o n is t h e d i r e c t d e t e r m i n a t i o n o f t h e ratio b e t w e e n t w o e l e m e n t s (e.g., F / N a , F / L i , N a / O , etc.) in a single c r y s t a l smaller t h a n t h e s e c t i o n o f t h e a - b e a m . T h e a - m e t h o d is s u f f i c i e n t l y a c c u r a t e to verify or find the s t o i c h i o m e t r y o f a m a t e r i a l , w i t h o u t a n y d e s t r u c t i o n o f t h e crystal. Such an a p p l i c a t i o n is t h e n p a r t i c u l a r l y a d a p t e d t o c o m p l e m e n t r a d i o c r y s t a l l o g r a p h i c studies. S o m e a c c u r a t e m e a s u r e m e n t s have b e e n p e r f o r m e d b y Basut~u {1980) o n polished sections o f spilites u n d e r t h e e x p e r i m e n t a l c o n d i t i o n s i n d i c a t e d above, t h e results o f w h i c h are given in T a b l e IV. T h e m e t h o d is suitable f o r a c c u r a t e m e a s u r e m e n t s , since t h e localization o f Li in t h e f e l d s p a r p h a s e (50 p p m w i t h a p r e c i s i o n o f 4%), and its a b s e n c e in t h e associated c a r b o n a t e , are verified. T h e p e n e t r a t i o n o f a - p a r t i c l e s in m a t t e r being weak, t h e q u a l i t y o f t h e irradiated surface is f u n d a m e n t a l , t o g e t h e r w i t h t h e r e p r e s e n t a t i v i t y o f the irradiated v o l u m e , just like w i t h t h e e l e c t r o n m i c r o p r o b e . With g o o d focusing ( s o m e p m 2) o f t h e a - b e a m , o n e can have an i n s t r u m e n t able to c o m p l e t e t h e e l e c t r o n p r o b e , analysing t h e lightest e l e m e n t s . T h e facilities required s e e m extensive, b u t t h e i r cost is less t h a n t h a t o f an e l e c t r o n m i c r o p r o b e . T h e o n l y c o n s t r a i n t is access t o a particle a c c e l e r a t o r able to p r o v i d e l o w - e n e r g y a-particles. TABLE IV Results of measurements of Li on a Na-feldspar and an associated carbonate in a spilite Mineral

Li content (ppm)

Feldspar (1) Calcite ( 1 ) Feldspar (2) Calcite ( 2 ) Feldspar (3)

58 ± 2 ~<4 52 ± 2 ~<4 52 ± 2

REFERENCES Basutqu, M., 1980. Contribution ~ l'analyse par r~actions nucl~aires ~ l'aide de particules alpha de basse ~nergie (< 3.5 MeV). Thesis, University of Paris VI, Paris. Borderie, B., 1978. Analyse par activation ~ l'aide de tritons, particules alpha et ions lourds. Int~r~t des faisceaux de basse ~nergie (~ I MeV/nucl~on) pour l'analyse "imm4diate". Thesis, University of Paris, Sud-Orsay. Borderie, B. and Barrandon, J.-N., 1978. New analytical developments in prompt 7-ray spectrometry with low energy tritons and ~ particles. Nucl. Instrum. and Methods, 156: 483--492.

359

Borderie, B., Pinault, J.-L. and Barrandon, J.-N., 1977. Nouvelles possibilit~soffertes par la spectrom~trie 7 sous irradiation.Analusis, 5 (6): 280--283. Borderie, B., Basut~u, M., Barrandon, J.-N. and Pinault, J.-L.,1980. Accurate determination of lithium, boron, fluorine and sodium in some matrices using low energy ~ particlesinduced ~ rays. J. Radioanal. Chem., 56: 185--198. Robert, J.-L., Volfinger, M., Barrandon, J.-N. and Basut~u, M., 1983. Lithium in the interlayer space of synthetic trioctahedral micas. Chem. Geol., 40:337--351 (this issue). Volfinger, M., in preparation. Crystal-chemical interpretation of thermodynarnical parameters for the formation of non-ideal solid solutions by exchange equilibrium of Li between micas and hydrothermal solutions.