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Precise Determination of Sm/Nd Ratios, Sm and Nd Isotopic Abundances in Standard Solutions*

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Precise Determination of Sm/Nd Ratios, Sm and Nd Isotopic Abundances in Standard Solutions* Precise determination of Sm/Nd ratios, Sm and Nd isotopic abundances in standard solutions* G. J. WASS~KBUKC;.S. B. JA(.OBStNt, D. J. D~PAoLo:. M. T. MCCULLOCH~ and T. WEN The Lunatic Asylum of the Charles Arms Laboratory. Division of Geological and Planetary Sciences. California Institute of Technology. Pasadena. CA 91125. IJ.S.A. Abstract -The methods used for precise calibrations of Sm,,Nd ratios and the average isotopic abun- dances obtained for normal Sm and Nd are given. A mixed Sm-Nd normal solution with a precisely known ‘47Sm;‘44Nd ratio close to the nominal average chondritic value is described and the calibration discussed. Aliquots of this standard.solution are available on request and may be useful for precise interlaboratory calibration of Sm and Nd INTRODUCTION and may not be stoichiometric. They demonstrated that PHILPOTTSand SCHNETZLER(1970) had calibrated FURTHCR DEVELOPMENTof the 147Smp143Nd method both Sm and Nd spikes erroneously by about ll”,, for understanding early lunar, terrestrial, and meteor- due to nonstoichiometry. MASUDA, NAKAMURA and ite evolution must rely on precise determination of TANAKA (1973) have also noted similar problems with the age and the initial ‘43Nd;‘44Nd ratio. One critical spike calibrations for determinations of REE in chon- factor is the determination of the ratio 147Sm/‘44Nd. dritic meteorites. NAKAMURA and TA~SUMOX) (1980) A difference of l”,, in the 147Sm/‘44Nd between two reported recently that the Sm/Nd ratios they samples will result in a difference of 1.1 e-units in measured for the achondrites Moama, Moore ‘43Nd/‘44Nd over the age of the earth (1 E-unit = 1 County, and Pasamonte were too low by 1.3”,, due to part in 104). To adequately measure the 147Sm/144Nd problems in spike calibrations and revised their pub- ratio by isotope dilution thus requires tracer solutions lished ages by -6OMyr. To avoid such problems whose concentrations are known accurately. Determi- which may result from using rare earth salts as stan- nation of the concentration of a Nd tracer solution is dards we have instead always used ultrapure Sm and done by mixing a measured mass of the tracer sol- Nd metals from the Ames Laboratory (cf. DEPAOLO ution with a measured mass of a solution containing and WASSERBURC;.1976; BEAUDRY and GSCHNEIDNER. isotopically normal Nd in known concentration and 1978; DEPAOL~, 1978). This procedure has also been then measuring the isotopic composition of the mix- adopted by NYQUIST, SHIH, WOODEN, BANSAL. and ture. Thus independent of mass spectrometric errors. WIESMANN(1979) and the results by these two labora- the precise determination of Sm/Nd ratios ultimately tories appear to be in good agreement as discussed by depends on absolute gravimetric standards for Sm JACOBSENand WASSERBURG(1980). Significant discre- and Nd. We have therefore expended a major effort pancies are apparent between the meteorite data that on preparing and calibrating solutions using absolute we now use for describing average chondritic standards. The results obtained are described in this 147Smm143Nd evolution and previously published paper. Aliquots of these standards are available to the meteorite data (see JACOBSENand WASSFRHLIKG,1980). scientific community for the comparison of isotopic The present paper should fully document the pro- abundances and the calibration of Sm and Nd iso- cedures used in this laboratory and may give a basis topic tracers. This will hopefully reduce the propa- for resolving some of the observed discrepancies in gation of errors in this rapidly developing field. the future. GAST, HUBBAKI) and WIESMANN (1970) have pointed out that spike calibration for the rare-earth elements (REE) is a difficult problem because the nor- PREPARATION OF NORMAL SOLUTIONS mal suprapure standard salts contain CO2 or HZ0 The Sm and Nd normal solutions were prepared * Division Contribution No. 3512 (369). from ultrapure chunks of metal obtained from the t Present address: Department of Geological Sciences. Ames Laboratory (see BEAUDRY and GSCHNIJIINFK, Harvard Ilniversity. Cambridge. MA 02138. U.S.A. 1978, for extensive discussion). The first batch of : Present address: Department of Earth and Space metals was obtained in 1975 and is called NdA and Sciences. University of California. Los Angeles, Los SmA (Table 1). The second batch of metals was Angclcs. CA 90024, (J.S.A. S Present address: Research School of Earth Sciences, obtained in 1978 and is called Ndcc, Nd/I. Smr and The :2ustralian National Ilniversity. Canberra. Australia. Smfi (Table 1). The Sm and Nd metals contain less 2311 2312 c;. j. WASSFRHI1RG Cf tl/ Table 1. Sm AND Nd NORMALS AND TRACERS Table 2. WEIGHTS OF METALS AND GRAVIMETRil CONCENTRATIONSOF NORMAL SOLUTIONS Neodymium Samarium AND Sm/Nd MIXED NORMAL Weight (grams) Concentration:: Normal metals NdA, a, S SrnA,a. d AMES CIT (llR/R, (- 1 gram each) --..-..-____ Normal solutions nNdA insmA IlNdA 1.06230 1.06240 1174.14 (- 1 or 2 liters each) nNda nSma IlSmA 0.307298 0.30726 333.411 nNdS nsmf? nNdu 0.91893 0.91892 999.:32 nSma 0.96209 0.96216 504.Ulr. Mixed normal - 1.5 liters nNdS + - U.45 liters nS@ nNdS 1.07015 1.07008 565.087 solution IlSUlS 1.12380 1.12351 607.77.i Oxide tracers 150Nd203(- 10 mS) "'Sln203(.~20 mg) Mixed Sm-Nd normal solution n(Sm/Nd)D: [Ndl = 434.070 up/g; [Sml = 141.140 l>R/**, Tracer solutions T Nd150 ? Sml47 (Sm/NdjWEIGHT= 0.32515 ______~~.---- than 100 ppm of cation impurities (Ames analysis). chosen because it is a low abundance ~sotopc m The amount of anion impurities (H. 0, N. c’. F. Cl) in nature and is not isobaric with any Nd isotope lSJSn~ the metal was measured to be less than 100 ppm at is the least abundant samarium isotope. b11t ma) the Ames Laboratory. It is however possible that the interfere with ‘44Nd if chemical separation of Sm and metal could have taken up some anion impurities Nd is not perfect. ‘49Sm and ‘s”Sm arc \ar~,&lc in during subsequent handling. The different metal lunar samples and meteorites due to neutron capture chunks (A, a, /?) had been weighed and sealed in eva- effects and ‘52Sm and 154Sm are the most abundant cuated Pyrex vials at the Ames Laboratory. Upon Sm isotopes. The ratio ‘48Sm, is4Sm can bc used to removal from the vials the metal chunks were re- provide a precise estimate of mass discrimination. By weighed with a semi-micro balance (precision and ac- adding about twice the 14’Srn already present m the curacy + 10 pg) prior to dissolution. The balance was sample a precise Sm concentration may be obtained checked with Class S weights calibrated against NBS and precise corrections can be made for other iso- weights. These weights agreed mostly within weighing topes such that ‘4gSm/‘54Sm and i50Sm,“54Sm in the error of the values given by the Ames Laboratory sample can be measured to monitor neutron capture (Table 2). The metals were transferred to I 2 I. effects if desired. polyethylene bottles, dissolved in 2.5 N HCI, and the Stock solutions of lsoNd and ‘“‘Sm tracers were resulting gravimetric concentrations are given in prepared in 1975 from oxide powders obtained from Table 2. The concentrations were obtained by weigh- Oak Ridge (DEPAOLO, 1978), Following usual pra\i- ing these bottles on a double pan balance (precision metric procedures these were repeatedly heated to and accuracy _t 10 mg). No corrections were made for 800 C in a furnace until the change in weight between buoyancy since these effects are negligible. two successive weighings was less than 0.05”,, of the The normal solutions are called nNdA. nNda. etc. total weight of the powder. The total weight loss was (see Table 1). From the normal solutions nNd/l and about 15”;. Since only - 10-20 mg of these powders nSm/l we made a mixed normal solution with an were weighed (Table 3), the concentrations of the Sm/Nd ratio very close to the average chondritic tracer solution were considered to be correct to about ratio. The gravimetric concentrations and the Sm/Nd 0.3”,,, however. no precise determination of the stoi- weight ratio for this mixed normal solution (called chiometry was done. The tracer solutions are called ‘T’ CIT Sm,/Nd standard) are also given in Table 2. Nd150 and T Sm147 respectively and their nominal gravimetric concentrations are given in Table 3 SELECTION AND PREPARATION OF TRACER SOLUTIONS THE ISOTOPIC COMPOSITION OF NORMAL Sm AND Nd The selection of tracers is important and they have been chosen from available tracers to optimize the Nd and Sm isotopic compositions were measured measurement of both “‘Ndi’44Nd ratios and Sm on the Lunatic I and III mass spectrometers (W.~SSEK- and Nd concentrations on totally spiked samples. For BURG, PAPANASTASSIOU, NENOW and BAukwh. 1969) Nd, a 961, pure i’“Nd tracer was chosen because as NdO+ and Sm’, respectively. following the pro- ““Nd is a low-abundance isotope in nature The small amounts of the other isotopes present in this Table 3. WEIGHT OF TRACER OXIDE POWDERS AND tracer cause only small changes in the relative abun- GRAVINETRIC CDNBNTRATIONS OF TRACERS dances of all the other Nd isotopes for which precise Weight rkxlcentrat;ons corrections can be made. Thus a precise determi- nation of the Nd concentration and all isotopic ratios T Nd150 10.58 mg IsaNd 5.253 US '53Nd$'3ii: to.03 tn.015 except 150Nd/i44Nd can be obtained by adding a similar amount of lsoNd tracer as the amount present T Sm147 20.42 mg '47Sm*03 10.732 ,,g'"iFmir@jlg kO.04 to.021 in the sample. For Sm a 98”,, pure 14’Sm tracer was Samarium and neodymium in standard solutions 2313 cedures described in detail by EUGSTER, TERA, BUR- clearly below this trend.
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