SOME NEW METHODS FOR THE DETERMINATION OF RARE-EARTH ELEMENTS IN GEOLOGICAL MATERIALS USING THERMAL AND EPITHERMAL NEUTRON ACTIVATION

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1 Journal of Radioanalytical Chemistry, Vol. 38 (1977) SOME NEW METHODS FOR THE DETERMINATION OF RARE-EARTH ELEMENTS IN GEOLOGICAL MATERIALS USING THERMAL AND EPITHERMAL NEUTRON ACTIVATION A. O. BRUNFELT,* I. ROELANDTS,** E. STEINNES*** *Mineralogical-Geological Museum, University of Oslo (Norway) **Geology, Petrology and Geochemistry University of Libge (Belgium) ***Institutt for Atomenergi Isotope Laboratories, K/eller (Norway) (Received November 15, 1976) Three procedures are outlined for the determination of rare-earth elements in geological materials. The irradiation of the samples is carried out by either thermal or epithermal neutrons. Two of the methods, one of which is especially suitable for ultramafic rocks are based on radioehemical separations, while in the third method nondestructive analysis is applied to apatites. The w-ray activity measurements are performed by means of coaxial Ge(Li)-deteetors. Introduction Knowledge of the rare-earth contents of rocks and minerals has proved to be a valuable tool in geochemical studies aimed to determine the source and evolution of various rock types, and in certain crystallochemical studies whose object is to establish the basic principles governing element partition between mineral phases.!-3 The techniques described here have been developed to supply the need for rapid accurate analytical methods created by this strong geological interest in the rareearth elements (REE). The determination of these elements may be carried out by neutron activation analysis (NAA) by irradiation of the samples with either thermal or epithermal neutrons. The feasibility of the latter technique may be assessed by means of calculated or observed "advantage factors ''4 which for a given nuclide is defined as Fa = (Rcd)d/(RCd)D where (Rcd)d and (Rcd)D are the cadmium ratios of an interfering nuclide and the nuclide in question. This paper summarizes methods developed by the authors using thermal as well as epithermal neutron activation with application to different types of geological material. Relevant nuclear data and calculated epithermal "advantage factors" for the rare-earth nuclides of interest are given in Table 1. J. Radioanal. Chem. 38 (1977) *

2 Table 1 Nuclear properties, cadmium ratios and "advantage factors" for some nuclides of interest in this work Stable nuclide Half-life of induced radionuclide Most suitable 7-energy, kev Thermal neutron activation crosssection,t s barn Resonance activation integral,l 6 barn RCd F a 139LA 140Ce t4~nd 148Nd s ONd i s 2Sin tsleu t SSGd t SgTb t64dy 16Silo,0Er a69tm t68yb 74yb ~'6yb I "~ SLu t~6lu 40.2 h 32.8 d ll.ld 1.73 h ~ 53.t h 12m --~28 h 47.0 h 9.2h 12 y 18 h 72,1 d 139 m 26.9 h 7.5h 130 d 32 d 4,2d 1.9h 3,7h 6,7d , , , Au *Calculated values 6 assuming RCd ~ 2.8. **Relative to a nuclide following the 1/v law (Rcd= 65 under the present conditions). Silicate rocks We have developed two methods recently for the determination of REE in silicate rocks. One of those is based on 1 hr epithermal NAA followed by a chemical group separation and Ge(Li) 7-ray spectrometry, s The group separation is based on hyd- roxide and fluoride precipitation steps. The chemical yield is established by a re- activation technique. The method gives data for La, Nd, Sm, Eu, Gd, Dy, Ho, Er 452 J. RadioanaL Chem. 38 [1977)

3 A. O. BRUNFELT et al: SOME NEW METHODS ~0 s e~ S o o ~0 2 ~0 i I i~,. 100 ~ Channel number Fig. 1. Gamma-ray spectra of group separated REE fractions of basalt BCR-1, 2 hrs after the end of irradiation. Cttrve A - thermal neutron activation; curve B - epithermal neutron activation (1 kev per channel) w, o~ ~0 s...,~ - ~ ~c ~ ~ g o () 'O3- ~o o ~oo ~ ~oo 300 4oo D" Chonnel number Fig. 2. Gamma-ray spectra of group-sep~ated REE fractions of granite G-2, 2 hrs after the end of irradiation. Carve A - thermal neutron activation; curve B - epithermal neutron activation (1 kev per channel) and Lu. Gamma-spectra of two samples of basalt (BCR-1) recorded 2 hrs after the end of irradiation are shown in Fig. 1. One of the samples was irradiated under cadmium cover, while the other was activated simultaneously outside the cadmium box. A similar comparison of v-ray spectra for a granite (G-2) is shown in Fig. 2. The depression of the 3,-lines of J6 S Dy in the spectra following epithermal activation as well as the enhancement of the v-lines of ZagNd are easily seen for both BCR-1 and G-2. The v-peaks of 166Ho, ~71Er and 176ruth stand clearly above d. Radioanal. Chem. 38 (1977] 453

4 .a3 0~3 0 + Q~ LJO0~O N rn~o 0 rn~ -.t 'u5 ~r X N I000 / a) 10 ~ N m ~ ~,~- ~=o~o 500 I000 ~I~00 Chonnel number o ~,/I,//////~ ~oo~ ~ b) ~ looo lo 500 I Channel number ID- c ~ 10000( Nr ~. ~ t. 9 ~'~ v. I,~ ooo ~ o~ N c) ~ loo( u loo I I Channel number Fig. 3. Gamma-ray spectra from an irradiated mangerite rock sample. Curve a - without radiochemical separation; curve b - group separated REE fraction; curve c - group separated Rb, Cs fraction 454 J. Radioanal. Chem. 38 (1977)

5 A. O. BRUNFELT et al;: SOME NEW METHODS 7,~I='M HNO3-CH3oH H~ -- ~ f(15.b5 v/v ) ~I00 --,-- HN03-7F~M HNO3-CH3OH s H-~ OH I(45.55 v/v) 80 (--10"~ 90 v/v ) ~ Nd 8!i " ~ 60- I: o Sr 40 :: ';,'- Y- ii I~'1 '~Eo I :: #, % ~m I Volume etuted/ml Fig. 4. Elution of REE from Dowex l-x8 with mixtures of CH~OH/HNO3; 10 ml elution volumes were measured. The peak area for each element corresponds to 100% of the total amount eluted the background in the 7-spectrum of the epithermally activated BCR-1. For G-2, which has a much lower content of the heavy REE, however, these peaks are quite small. The other method is based on 5 days' irradiation of the samples with thermal neutrons. 7 After decomposition of the samples with alkali followed by leaching with water, Rb and Cs are determined in the supernatalxt liquor. The hydroxide fraction, which contains most of the Sr, Ba and REE, is dissolved in hydrochloric acid and extracted with tri-n-butyl phosphate in order to remove the major part of S9Fe and 46Sc. Then the aqueous phase before the counting is passed through an anion exchange column in order to separate a number of other interfering radionuclides, the most important being 6~ 122Sb and 1248b. The chemical yield has been determined by reactivation on more than forty different rock samples. From those experiments, no significant fractionation of the REE during these steps were evident. The chemical recovery varied from 75 to 85 per cent, while the variation of the individual REE carriers in the same sample was less than 3% in each case. The method yields data for 9 REE, namely La, Ce, Nd, Sm, Eu, Gd, Tb, Yb and Lu. The 7-spectra corresponding to the different fractions from an irradiated rock sample are shown in Fig. 3 and compared with the 7-spectrum of the same irradiated rock sample without this radiochemical treatment. This method has been extensively used in our REE studies (e.g. anorthositic rocks 7'8 and sediments) J. RadioanaL Chem. 38 [1977) 455

6 Ultramafic rocks Ultramafic rocks show very low concentrations of alkalies, alkaline earths and REE, and therefore require more extensive separations. For this purpose we have extended the second procedure described above with a mixed solvent anion-exchange separation which splits the REE into three groups. 12 Methanol/nitric acid is used as eluant. Fig. 4 shows the elution curves obtained in an initial experiment carried out to establish the working conditions. In Fig. 5 the 7-spectra of the eluted fractions for BCR-1 are shown. On application to the USGS standard rock dunite DTS-1, our data agree very closely with the average values of previously published NAA results. Apatites As the REE content of apatites is of the order of tenths of a per cent or higher, the REE can be determined non-destructively by neutron activation using a short irradiation. Independent portions of apatites were activated with thermal and epithermal reactor neutrons for 2 and 15 rain, respectively, followed by Ge(Li)7-ray spectrometry. 13 As expected from calculated advantage factors Fa (Table 1), it appeared that Nd, Sm, Gd, Ho and Er are favourable cases for determination with the epithermal technique whereas La, Eu and Dy are not. Ce, Tb, and Yb were also most conveniently determined by thermal NAA while Pr and Tm were the only elements which could not be determined either by thermal or by epithermal NAA. In addition to the REE data, abundance values of Na, Mn, Th were obtained. This very simple approach has already been used for a large number of apatite samples. ~ Accuracy of the REE analyses The quality of REE data for rocks and minerals may be judged on the basis of comparison diagrams with chondritic meteorites 14 (i.e., the ratio of each REE content in ppm to the average content of the same element in chondritic materials, plotted against atomic number or ionic radius). Owing to the nature of the partial separations within the REE group that accompany rock formation processes, such comparison diagrams are likely to exhibit smooth curves, except for Ce and Eu which may have been selectively separated from the other REE by natural processes 456 J. Radioanal. Chem. 38 (1977)

7 >-~-u~ a,.^ u, ' : ' u-o,_ ]ooo(,:',~ ~ I,:IJ / 1.~162 ~,,~ ~.o ~,..~x 'l 17-o ~ ~ ~ ~ ~, o) o ',, eo, 500 ] Channel number / / / '/ " I t = ~-,zo<;oc, a ~ 1000(.- d, I/I //,I, / /.I i :'.:~'1... ~o~... = ~,= Pll ~'//,,,',... : I, '~7;TSr ~) '//" I!~.:i,' ~ i" ~" looo I' '" ' ",o 500 I Channel number l~,~ - o o o ~u', -., o-" oooo ~^ ~0000 I ~- ~k4 5 ~>~,~<", i i c) E g IO0 ~0 I 500 I i Channel number Fig. 5. Gamma-ray spectra of isolated eluted fractions from basalt BCR-1. a ml of 7.8M HNO 3 -CH3OH (15+85 v/v) (flow-rate 0.6 ml min -t ): b ml of 7.8M HNO s -CH3OH (45+55 v/v) (flow-rate 0.5 mlmin"); c ml of H~O (flow-rate 0.7 ml rain-' ) J. Radioanal. Chem. 38 (1977) 457

8 c~... ~'. -'~ E... %, ",, ~... ~.~..% '.~V ;".~,. ~,~ V--.~t.<"*~ L(a Ce Nd SmEuGd TbDgHoEr YbLu J I I Ill ILl t II , Ionic rodius~ ~, Fig. 6. Chondrite-normalized patterns for 3 apatites. A - JO000 w - 5ooo o u._c 1000 n- 500 o_."z_ 100 o.c_ 5O Durango (Mexico); B - fluoroapatite from Hesnes (Norway); C - 0degaarden (Norway) 12: o. fluoroapatite from Cerro de Mercado, chloroapatite from ',:_ -o c O o t~ I:: 0.1 ~- o ~ a 9 f o ~ o ~ o o c_ W 49 La CePr Nd Sm EuGd i ii 1 1 ]1 t to6 O. 98 Ionic radius 7 "~' TbDyHo Er TmYbLu I11 I ill , B6 E o_ Fig. 7. Chondrite-normalized pattern for the dunite DTS-1. Analytical workers: 9 - BRUNFELT, ROELANDTS, STEINNES, o - HIGUCHI, TOMURA, ONUMA, HAMAGUCHI, 9 - REY, WAKITA, SCHMITT, [] - GARMANN, BRUNFELT, F1NSTAD, HEIER 458 J. Radioanal. Chem. 38 (1977)

9 owing to their ability to exist under natural conditions in more than one oxidation state. Comparison diagrams of three apatites are shown in Fig. 6. The distribution patpern of REE in DTS-1 relative to chondrites is plotted in Fig. 7. The smooth curves indicate that the accuracy of the results from the present work is satisfactory. References 1. P. GAST, Geochim. Cosmochim. Acta, 32 (1968) M.J. DRAKE, Geochim. Cosmochim. Acta, 39 (1975) B.B. JEN'SEN, Geochim. Cosmochim. Acta, 37 (1973) D. BRUNE, K. JIRLOV, Nukleonik, 6 (1964) A.O. BRUNFELT, E. STEINNES, J. Radioanal. Chem., 13 (1973) E. STEINNES, in Activation Analysis in Geochemistry and Cosmochemistry, A.O. BRUNFELT, E. STEINNES (Eds.), Universitetsforlaget, Oslo, 1971, p I. ROELANDTS, Ph. D. Thesis, University of Liege, J.C. DUCHESNE, I. ROELANDTS, D. DEMAIFFE, J. HERTOGEN, R. GIJBELS, J. DE WINTER, Earth Planet. Sci. Letters, 24 (1975) H. DYPVIK,,Cand. Real. Thesis, University of Oslo, H. DYPVIK, A.O. BRUNFELT, Sedimentology, 23 (1976) R.K.A. MYRHAUG, Cand. Real. Thesis, University of Oslo, A.O. BRUNFELT, I. ROELANDTS, E. STEINNES, Analyst, 99 (1974) A.O. BRUNFELT, I. ROELANDTS, Talanta, 21 (1974) L.A. HASKIN, R.L. KOROTEV, in Analysis and Application of Rare Earth Materials, O.B. MICHELSEN (Ed.) Universitetsforlaget, Oslo, I963, p R. SHER, in Handbook on Nuclear Activation Cross-Sections, IAEA Techn. Rept. Set. No. 156 (1974) H. ALBINSSON, in Handbook on Nuclear Activation Cross-Sections, IAEA Techn. Rept. Ser. No (1974) 15. dr. Radioanal. Chem. 38 (1977J 459