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. however, his three other NETT and WASSERBURG (1970), Russ, BURNETT, measurements plot close to this mass fractionation LINGENFELTERand WASSERBURC(1971), Russ (1974), line around RI8 = 0.00205. We have decided to use DEPAOLO and WASSERBURG(1976), PAPANASTASSIOU, our directly measured oxygen composition of DEPAOLO and WASSERBURG (1977), and DEPAOI.O RI8 = 0.00211 and R 17 = 0.000387 in the future. This (1978). Oxygen corrections have up to nolt been made will make small changes in our reference values for with RI8 = 18O/“O = 0.002045 and the normal Nd isotopic composition. Large varia- R17 = “O/l60 = 0.0003708. This composition is one bility of oxygen isotope ratios is found in nature and of the four oxygen isotope measurements by NIER the values given above may not necessarily apply to (1950) and was chosen mainly because it was the most mass spectrometers in other laboratories. For precise determination and was the value listed by example, L. NYQUW (personal communication) LEDERER, HOLLANDER and PERLMAN (1967) (cf. obtained R,8 = 0.00216 + 4 and R17 = 0.000396 f 6 DEPAOLO, 1978). NIER (1950, p. 792) gives the follow- at the Johnson Space Center Laboratories. We have ing percentage abundances: 160, 99.759; “0, 0.0374; not been able to identify the oxygen reservoir which is 180, 0.2039. The corresponding ratios are contributing to the NdO’ molecular ions in the mass ‘sO/lhO = 0.0020439 and 170/160 = 0.0003749. spectrometer. The difference in the isotopic compo- However, since 1976 we have measured the oxygen sition measured by NIER (1950) and our measure- isotope composition during Nd isotope measurements ments of NdO’ ion beams could be due to mass in the Lunatic 1 mass spectrometer both in normal fractionation relative to air oxygen. There seems to be samples. pure tracer. and mixtures of tracer and several stages where fractionation could occur: (I) normal (cf. PAPANASTASSIOUet al., 1977; DEPAOLO, during the process of oxidizing the Nd on the Re 1978) (five measurements). This was done by measur- filament by heating in air while the sample is being ing the ratios 150Nd180/150Nd’60 and “‘Ndl’O/ loaded, (2) in the mass spectrometer during evapor- 1strNdlhO which may be measured with a precision of ation and ionization of NdzOJfs) to form 0.2 and 0.5”,, respectively. The oxygen composition NdO’(g) + O,(g). and (3) due to a very small air leak was most recently remeasured in November 1980. The oxygen in the ion source. This may, in principle, cause average value measured for 1sO/16O is R,, = 0.00211 several percent variation in ‘*O!“O. Alternatively. and all the data are strongly peaked around this value the difference could mainly be due to mass fraction- with the lowest average value being R,8 = 0.00210 ation in Nier’s mass spectrometer. NIEDER~,R. PAPA- and the highest average value being RI8 = 0.00213 NASTASSIOU and WASSERBURG (1981) obtained for individual runs. Within a single run that showed 180/160 = 0.002CO by measuring TiO+ and ScO+ the largest variation the lowest value measured is with the sample loaded on a heavily oxidized Ta fila- R,, = 0.00208 (early in a run at low intensity) and the ment and intimately mixed with a slurry of Ta205. highest is R,, = 0.00214 (late in a run at high inten- The tantalum oxide, which in this case was the major sity); however, the data are strongly clustered between oxygen reservoir, clearly gives a different oxygen com- 0.00210 and 0.00213. The average “O/“O value position in the Lunatic mass spectrometer than measured is R,, = 0.000387. The data follow a mass obtained by measuring NdO+. It is evident that large fractionation trend described by R,, = 0.09171 variations in the oxygen composition may occur in a RI8 + 0.0001935. The Nier value we have used plots mass spectrometer depending on the method used for
Table 4. THE ISOTOPIC COMPOSITIONOF NORMAL Nd
I. Measured NdO+ ion beam intensitiesfor masses 158, 159, 161, 162, 164, and 166 relative to the ion beam intensity at mass 160
------158Nd0 ls3Nd0 161Nd0 16*Nd0 '64Nd0 le6Nd0 '6ONdO 160Nd0 IGoNdO 160Nd0 160Nd0 160Nd0 1.135353 (0.510959)*0.349514 0.724495 0.243969 0.238512 These ratios ar‘ethose which would t9 +6 ?lO t6 ?6 obtain for '46Nd/'42Nd= 0.636151
II. oxide factors (calculatedfrom the above NdO ratios, leOI' = 0.00211 and '80/'60 = 0.000387)
f142/144 fl43/144 f145/144 fl46/144 fl481144 fl501144
1.00260 1.00173s 0.998403 0.999501 0.996337 1.000450
III. CalculatedNd isotope ratios
lu2Nd lq3Nd lk5Nd lu6Nd 14*Nd 150Nd Correctedfor mass Corrected for oxygen with: 'l*'+Nd lrrNd 141tNd 'lt4Nd l*'+Nd '+'+Nd discriminationwith: '8O/lbO "O/1%
1.138305 (0.511847)*0.348956 0.724134 0.243075 0.238619 146Nd/1r2NdE 0.636151 0.00211 0.000387 1.141827 (0.512638)*0.348417 SO.7219 0.241578 0.236418 'sbNd/'s4NdI 0.7219 0.00211 0.000387 1.138266 (0.511836)*0.348968 0.724109 0.243079 0.238581 's6Nd/'42NdI 0.636151 0.002045 0.000370s 1.138313 0.348948 0.724139 0.243072 0.238641 IQ6Nd/14*NdE 0.636151 ** ** t14 r7 ts +a +16
*Present-dayaverage chondriticvalue (JACOBSENand WASSERBURG, 1980). **Measuredas Nd+ at uru (~PAOLO, 1981). 2314 G. J. WASSERINIRC;er trl. analysis. Care must therefore always be taken to In practice it may be easier to caiculatc the ~s~lt~~pl<. establish the possible range of variations and their composition of Nd from the follouing II~c:~I cqua- effect on the isotopic composition to be measured if it tions sequentially is desirable to measure oxide species rather than metal species. 143Nd = 15’Nd0 - “‘NdRI. Nd oxide ion beam intensity ratios arc calculated 144Nd = lh[)NdO _ “=NdK ‘A’Ndi(,.l relative to the ion beam intensity measured at mass y;; 1 y;;g I w;;:” 1 w.iir: 160 ( 16’Nd0 = 144NdlhO + 1‘%3Nd’70 + t42Nd’80), The peak intensities are measured in the sequence: 148Nd = ‘h4NdO _ ‘4”N&; - mass 160, 159, 158, 166, 164, 162 and 161. Data are “‘Nd = lh6Nd0 - lJHNdRIH taken in sets of 10 mass scans, averaged, corrected for contributions from Nd”0 and Nd180 species, and rather than carrying out the matrix m\crsloii Ihchc then corrected for mass fractionation. In the first part two methods are, of course. equivalent and 111~mvcr\c of Table 4 we give the average ratios we measure for matrix may be obtained by carrying through ihc suh- NdO+ ratios (part I) in the Lunatic mass spec- stitutions in the above equations such thaw on/c thz trometers uncorrected for species containing “0 or relative abundances of NdO ’ rnolcct~Iar illn\ I_(, 180, The relative abundances of NdO’ molecular and RI8 appear on the right hand side. ions with masses 158. 159, 160. 161. 162. 164 and 166 The inverse matrix has a relatively simple form if uc ignore terms that are less than 10 ”
1 0 --R,- 1 -R,u -R,, +2R,,R,x -Ri8 I 1 +R:s +=,,R,B -Ris -R,T 1 0 !I / 0 0 +R:s +R,,R,s - R,x I ii i 0 0 0 0 +R:, R ah i -- __.-...---.~._.___ _ _.._.______. The inverse matrix can also be used to define ‘(INI& may be related to the Nd isotopic composition factors’.,tij (which are functions of R , - and R Lxi swh through seven linear equations with seven unknowns. that These may be conveniently written in matrix nota- tion. Let ND0 = (’ ‘*NdO. ’ sQNdO. ‘hoNdO. 16’Nd0, 162Nd0, ““NdO, ‘“‘NdO) be the vector giving the relative abundances of NdO’ molecular which will give us a direct relatmnship hetwch:n. for ions (which we calculate relative to ‘(‘ONdO) and let example, (159/160) and (143Nd “4JNd). ‘I%< oidc ND = (142Nd, 143Nd, ld4Nd, ‘S”Nd. 14(>Nd, 148Nd factors’ are as follows: and ““Nd) be the vector giving the Nd isotopic com- position. We then have - I’[1 - (158/160)R,H -- (159.‘lhO)R.-. / ::::r::: 1 [1 - (158,‘159)R,,]f,,, 14., NDO=_A*ND .f145:144 = [l - (160/161)R,: (159:161)H,, t 2R,;R,8(158;‘161)]1’,,, Ia., where 4 is a 7 x 7 matrix .f‘,46 144 = [l ~- (161/‘162)R,- (lCfi, 162)R,, + 2R1,R18(159,1621 + (158/162)R:“l,!.,,, IfJ f 148,144 = [1 - (162/164)R,s 1 R,~R,,[lhi lb41 +- R:8(160/164)1,1,,, l&J .f‘150~144 = [l - (164/166)R,, + (162:‘166)R:,]f‘,,, ,114 Here (159/160) is an abbrevlatlon for i’.“NdO ‘“ONdO) which is defined above in the detinrllon ol the NdO vector; the other abbreviations arc defined The Nd isotopic composition can thus be calculated similarly. from From these oxide factors. it follows that ;i j”, ND = _A-‘*ND0 increase in RI8 and a corresponding S’,, ~ncrcasc l”‘h[(, !‘llp.Jd. In R1, will change the ratios if R,, and RI8 are known. The final step is to divide 143Nd/‘44Nd, ‘45Nd/‘44Nd. !J”Ndil~‘Nd. ““Vd the values obtained for 14’Nd, ‘43Nd, 14’Nd, 146Nd. 144Nd and ‘5”Nd,/144 Nd by -t 0.50. t-0.64. - 0.20. 148Nd and ““Nd with the value obtained for 144Nd. +0.50,-0.37 and +2.43 e-units respectiveI>. Al’both Samarium and neodymium in standard solutions 7315 compositions are normalized to the same lated from the oxide data as discussed above and then 15hNd/‘42Nd isotope ratio. The maximum variation are corrected for instrumental mass fractionation in RI8 of about 1.5”,, between individual runs using the equation obtained by direct measurements suggests that effects due to variability in the oxygen composition are at most 0.2 t-units for all isotope ratios except for ““Nd IJ4Nd. which may vary by at most 0,7+units. This is confirmed bl other data as discussed below. Based on average values obtained in 1976 fol- about Numerical values for the oxide factors are given in 10Nd mass spectrometer runs an average value of
part II of Table 4 using RI7 = 0.000387 and (14’Nd ‘14* Nd), = 0.636151 was obtained. This \due R ,H = 0.00211. is used to correct for mass fractionation. This value The exact mathematical form of the fractionation was obtained using the old (Nier) oxygen compo- lag which describes instrumental mass fractionation sition, but we have decided to keep this ratio lixed during thermal ionization is not well known (cf. Rus- and let the other isotope ratios change v hen using SI I 1.. PAPANASTASSIOI.and TOMRRELLO. 1978). We our new oxygen composition since the absolute iho- have chosen for Nd and Sm the commonly used topic abundances are not precisely known. The a\cr- power law. In the past we have also made mass frac- age mass fractionation obtained for several hundred tionation corrections with an iterative procedure on a runs since that time is r = +0.0003 with a total range quasi-linear law that is numerically equivalent to the from -0.002 to +0.002. The extreme limits arc rare power law for all isotopes to within 1 part in IO’ for and even within individual runs most data cluster cvcn the largest mass fractionations we have ob- between -0.0005 and iO.001. To avoid artifacts due served. As discussed below the power law has limita- to inadequacies in the mass fractionation lau. high tions and is probably not the optimum choice. How- quality data are collected with a restricted range of ever. for the small fractionations which were obtained mass fractionation (-0.001 < 3 < +O.OOl 1. Some here under very controlled conditions, this law intro- laboratories use different values of the normalizing duces no significant error. ratio for mass fractionation corrections corresponding The isotopes chosen for mass fractionation correc- to a value of x = +0.0015 (cf. O’NIONS cv c/l.. 1977). tion have been chosen to both maximize the mass However, MOORE (1978) at the National Bureau of difference between them, and at the same time have a Standards concludes from prelimmary expermicnts of ratio relatively close to unity, thus maximizing the NdCI, vaporization that such a value must be a tov,er precision of the instrumental fractionation factor. and limit. His best estimate of the ‘absolute value 01 ultimately, maximizing the precision of the measured 14hNd/‘42Nd is 0.6369 (2 = ~0.0003). corresponding “?Nd “‘Nd. For Nd the best choice is ‘5”Nd,~1’*Nd more closely to the average value that wc measure of (largest mass spread) and the second best is 0.636151 rather than the average value of 0.63223 ‘lhNd ‘“‘Nd. In the past we have used 1’“Nd;‘J2Nd measured in some other laboratortes. The choice of and ‘s’Sm,‘s4Sm for normalization of unspiked Nd an isotope pair for normaliration and the actual value and Sm runs. However. we use “‘Nd and “‘Sm as chosen for this ratio cannot cause discrepancies tracers so WC have decided not to use these isotope between data from various laboratorlez. It should ratios in the future for normalization. We always used always be possible to compare the data directly after I~(~Nd:lJ’Nd and IaXSm, “‘Srn for normalization of an appropriate (I priori data transformation. unless spiked runs and have decided to also use in the future errors have been made in the actual measurement of thcsc ratios for normalization of unspiked runs. Other isotope ratios. Thus, terrestrial t( I431 values should laboratories hube used different ratios. For example, be essentially independent of the normalization tmlcss lJXNd la4Nd is used at La Jolla (LUGMAIR. MARTY, data are taken with large mass fractionation relative K~:KI‘/ and %HFININ. 1976) and Johnson Space to the standard values reported here in Table 5. This (‘enter (NYQUIST (II trl.. 1979). while O’NtoNs. HAMIL- is an important point and should be welt understood. TON and EVLSSFN (1977) used ‘JhNd;.144Nd. Using Part III of Table 4 gives, in the lirst rot+. the Nd ““Nd “:Nd and calling the Nd isotope fractionation isotopic composition calculated with our new oxy- factor per atomic mass unit x, we have z delined by: gen isotope composition normahlcd to 14hNd:‘4’Nd = 0.636151. In the second row this Nd 1.‘4_ , isotopic composition has been rcnormali/cd to 14(‘Ndj ‘14Nd = 0.7219. another comment! used nor- I malizition (O’NIONS YI t/l., 1977). In the third rou the where (“(‘Nd,’ 14’Nd)l, denotes the choice of normal- Nd isotope composition calculated with Nier’s izing ratio. If the light isotopes are enhanced then oxygen values is given normalized to x > 0, while if the heavy isotopes are enhanced then ‘4hNdj’42Nd = 0.636151. It is seen that the x < 0. Normally a goes from positive values in the 143Nd;14’Nd isotope ratio calculated using our Ned beginning of the run to negative values at the end of oxygen isotope composition and the same value for the run. In processing data the measured average Nd 14hNd,142Nd is 0.21 parts in IO5 higher than that isotope ratios for each set of IO mass scans are calcu- obtained using the old values. Thus Table 5. ISOTOPIC COMPOSITION OF NORMAL Nd AND Sm AND TRACERS USED IN THIS LABORATORY :5i)Nd I. Neodymium -----~"'LNd lq3Nd lg5Nd "'6Nd lQ8Nd 14"VJd lQ4Nd lb4Nd lb4Nd lG4Nd L'.'.,Vd Normal 1.138305 (0.511847)* 0.348956 0.724134 0.243075 0.?18blY t9 t6 t10 26 'h iioNd-tracer 0.86932 0.44014 0.39180 O.Y6197 0.74804 110.98'1 i’tx+Sm “tJSm ISOS, :a’Sm 11. Samarium __------‘4BSm 149sm I5"% is% '5~sm '54% '54Sm i /~//qrn Normal 0.13516 0.65918 0.49419 0.60750 0.32440 l.,l',Ji !I t2 t2 t2 '4 'u7Sm-tracrr 0.28037 467.29 4.08879 2.43458 0.82243 l.hlLYU -- -_ *Present-day chondritic value (JACOBSEN and WASSERBURG, 1980) recalculated with iaO/1sO = 0.00211 and '7O/'6O = 0.000387 and normalized to 1'6Nd/'q2Nd - 0.636151. values previously reported by this laboratory should unchanged and subsequent e(k) determinatiou~~ c~tr/cr~- be incrrnsed by 0.21 parts in IO’. Much of our earlier Lrted with the neck Nd isotopic, r~rrio.~are directly com- reported data for unspiked samples were normalized parable with the earliet ones [eo,,,)(I\) (with t0 15”Ndi142Nd = 0.2096. while our new (‘Nd/‘44Nd)or.U) = e,rw(k) (with (iNd,S”4Nd)w*it,,)I. ‘50Nd/‘42 Nd value of 0.209627 (normalized IO ‘The grand mean values of terrestrial normals for ‘46Nd/142Nd = 0.636151) is 1.3 parts in i04 higher nonradiogenic Nd isotopes (Tables 4 and 51. wh~ctr than the old value. are here used. were recalculated from ttrosl: ilt The difference in relative isotopic composition e(k) MK~LLO~H and WASSEKHLIRGI 1978). w h(ch Table 6. ISO'IUPICCOMPOSITION OF AMES Nd METAL NORMALS, THE USGS STANDARD ROCK BCR-1, AND THE LA JOLLA Nd OXIDE, ALL NORMALIZED TO 1s6Nd/"'2Nd : 0.636151 AND EXPRESSED AS DEVIATIONS IN PARTS IN lo4 (e-VALUES) FROM THE REFERENCE VALUES IN TABLE 5.* i(142) *(143) .(145) I (148) c(15U) llNdA II 1 -0.02tO.26 -13.73'0.37 -0.63tO.37 -0.75kO.58 -0.13+0.i8 #2 +0.31t0.53 -13.61r0.61 +1.00~0.49 -0.lht1.27 -1.ORtl.i') 113 -0.23+0.7') -i3.91+0.35 t0.72r0.74 -0.21f1.36 $14 -0.13t0.49 -13.73r0.58 -0.03t0.86 -0.45t0.86 -0.02+0.33 -14.03t0.37 co.28ro.52 +0.36+0.59 +0.38t0.9'1 -0.25'0.43 -14.46t0.45 -0.56iO.69 +0.31+0.99 +0.3e1.20 -0.06t0.32 -14.19t0.32 -0.llt0.46 -0.49t0.64 +0.58?0.9h tO.19~0.48 -14.22'0.73 -0.05+0.59 +0.20?0.77 +1.56'1.4') -0.03t0.42 -14.05e0.45 -0.16+0.56 -0.39t0.90 -1.40?1.4h nNdB jr1 -0.05kO.43 -13.a7to.59 +0.45t1.15 +0.20t1.03 -0.13?1.2(? I/2 c0.01t0.74 -13.79t1.27 -0.06+1.03 -0.31i1.82 113 -0.12+0.23 -13.87t0.35 +0.36?0.45 +0.29+0.75 04 +0.06t0.21 -13.79t0.23 +0.24*0.47 -0.16r0.59 -0.71?0.75 it5 +0.34+0.28 -13.97t0.35 -0.63t0.65 -0.65'0.65 -1.09tl.4ll BCR-1 ill t0.33t0.47 +0.08+0.43 -0.24*0.57 +0.27?0.74 +O.12?1.33 t2 +0.24?0.29 +0.02c0.35 -0.3420.49 -0.55t0.66 +1.20~1.26 il3 -0.13f0.35 -0.ozt0.43 +0.34t0.54 -0.28?0.78 +0.53tl.oP La Jolla NdzO3 t 1 +0.22t0.35 -15.15t0.43 +0.13+0.85 -0.06?1.19 +0.65'1.30 Average values and standard ~TTOTS of the mean for nonradiogenic isorupe:, calculated from data ylven above +0.04+0.09 ti.Olt0.21 -0.36'0.17 +0.06+0.47 es(k) = [(kNd~'44Nd),As/(kNd/14"Nd)NoR~-l]x10r Samarium and neodymium in standard solutions obtained on a mass spectrometer which has com- pletely different components from the Caltech spec- trometers (Lunatic I and III), it appears that the accu- racy of the Nd ratios given is comparable to the precision. For Sm we use 14*Sm/154Sm = 0.49419 to correct for mass fractionation. The grand mean values of terrestrial normals for Sm isotopes (Table 5) used are those of Russ (1974) which are updated from those of Russ et (II. (1971). These updated values for Nd and Sm are essentially indistinguishable from the original values of DEPAOLO and WASSERBURG (1976) dnd Russ rr a/. (1971). We have also measured the isotopic composition of Nd and Sm metals from the Ames Laboratory, and 4 the results for Nd are given in Table 6 as c-values relative to the reference values in Table 5 for the three 024304 024306 024 dilferent chunks of metal called nNdA, nNda, and 14*Nd, nNd/l. The average values and the standard errors of Fig. 2. ‘4ZNd;‘J4Nd and l4’Nd, ISANd ratios measured (as the mean calculated from all the data on nonradioge- NdO’) on Nd separated from rock samples. normalized nit isotopes given in Table 6 are given in the last row the same way as the data in Fig. I. With this normalization of this table. These results show good agreement with the values of “ZNd:‘44Nd and 14RNd, IJ4Nd are 1.138266 the grand mean values for terrestrial samples given in and 0.243079 respectively. Circles represent the same mass Table 5. One analysis of Sm metal from Ames also spectrometer runs as shown in Fig. I. squares are data obtained later. The data do not follow the trend expected for variable oxygen isotopic ratios in the NdO* ion beam, a (x 103) but can be explained by exponential mass fractionation. -3 -2 -1 0 +1 t2 1 I I I I I Variations in Nd’“O’;Nd’hO’, indicated by the tick marks on the diagonal line, appear to be ltmited to i I”,, for all runs. Numbers on the vertical line gtvc the calcu- lated mass fractionation per atomic mass untt according to the power law. shows good agreement with the Sm isotopic compo- sition given in Table 5. The Nd isotopic composition of rock standard BCR-1 which w’e have analyzed regularly and a La Jolla Nd203 standard arc also given in Table 6 as these standards have been used extensively for interlaboratory comparisons. It was shown by DEPAOLO and WASSRBURG (1976) that If3810 ,’ / I I 1 I BCR-1 has ~(143) essentially indistinguishable from 0976 0 984 0992 1000 ! 008 1016 (‘50Nd/‘42Nd)N/(‘50Nd/‘42 Nd) M zero. Repeated analysis of this sample has conlirmed Fig. I. lJZNd, lJ’Nd ratios measured on Nd (as NdO+) this result so that BCR-1 may possibly serve as a separated from rock samples. corrected for mass fraction- reference sample. The La Jolla sample (cf. LKMAIR ation by normalizing to (150Nd/‘42Nd)N = 0.209600 using and CARLSON, 1978) was obtained through the cour- the power law and corrected for oxygen using the NIER tesy of G. Lugmair. (1950) oxygen isotope composition. With this normaliz- ation the value of ‘42Nd,‘144Nd is 1.138266. Each point represents the grand mean for one mass spectrometer run CHOICE OF FRACTIONATION LAW (2OG300 ratios) plotted against the mean value of the mass fractionation for the run expressed Existing data provide information on the suitability of as (‘ioNd/‘J2Nd)U~(‘5”Nd/‘JZNd)M along the scale at the different mass fractionation laws for Nd. DI-PAOLO the hottom. The top scale gives the fractionation (1978) showed that in six tmspiked Nd mass spectrometer per atomic mass unit expressed runs the average mass fractionation per atomic mass unit as x = [(‘5”Nd/142Nd)U/(‘soNd/14ZNd)M - 1]/8. The (2) was as large as +0.0015 to ~0.002. These Nd isotope “‘Nd ‘44Nd ratio is invariant in the samples analyzed, so data were corrected for mass fractionation using a power the apparent variation must be an artifact of the measure- law with ““Nd:‘“‘Nd as the normalizing ratio. He noted ment procedure. The solid line shown depicts the trend that the mean corrected value of ‘JZNd:‘“JNd for these six expected if the isotopes of Nd were fractionating according highly fractionated runs was displaced from the accepted to an exponential law, but the data were being corrected value by a small but signilicant amount. All other Nd ISO- using a power law. Similarly. the dashed line corresponds topic ratios in these runs were within error of the accepted to the effect expected for correction using a linear law and values. The data for these six runs are shown in Fig. I Nd were actually fractionating according to an exponential together with data for another 23 unspiked Nd runs with law. If the data were corrected using an exponential law. more typical values of mass fractmnation. The no correlation of ‘42Nd/‘44 Nd with mass fractionation r4’Nd,“‘“Nd ratios for these 23 runs are all wlithin error of would he observed. Data from DEPAOLO and WASS~RBURG the accepted value and do not show any significant corre- (1976) and DFPAOLO (1978). lation wjith the average Nd mass fractionation for each run. (148Nd/‘44Nd)N/(‘48Nd/‘44Nd)M where the subscripts L, P. and I: rcitl- to the lrnc,tr. po\+~t. 0.984 0.992 1000 1.008 1.016 and exponential ‘laws’ for mass fractionation. rcspc:cttc4?. Q, +2--l-r --r- I .-I- 1 mmmm-! WC note that if the light isotopes arc enhanced II! the ioil heama relative to the ‘standard \tutc‘ then 7 1 (I. :\hilc 11. the heavy isotopes are enhanced then Y _. 0 I !lt. I~IIW\ obtained for I,. z,,~and CX,may he used tn cast I CCCI SI ::I measured kaluea RF: of the isotupeh i. / anti (‘01 t a~11hit respectively give a set of values cortccted (C) f01- lit235 frr,k tionation R,,“‘I . RE”“. and Rc”’ usmg the f~llttv !tl;i L’~U,I lions where i/i_L- ~~I- -1 ? ! &68 0.984 1.000 1d16 ~1 ’ 03: (‘50Nd/142Nd)N/(‘50Nd/(42Nd)M and Fig. 3. C’omparison of mabb fractionatton L,orrccted Nd isotopic ratios using the power la\+ and the erponentl;il law. The calculation assumes that there exists a common ‘standard state’ set of Nd isotopic ratios ‘Nd “‘Nd fog both laws corresponding to zzp = Y, = 0. The scale at th< bottom gikes the normal ’ 'ONd,“‘Nd as ;I fraction of the measured value, and corresponds to a mas, fractionation I ranging from about -0.004 to +0.004 left to right. The curves also apply to a good approximation if ‘“HNd lJJNd is used for normalization (top scales. The curve shown fclr ‘42Nd”“4Nd is also plotted on Fig. I. The dashed cu~-\t‘ shows the comparison of the linear I;IH with the exponen- tial laa for ‘42Nd;‘44Nd. Si mt‘1, at cur\es could he drawn for other ratio\ where I, 5 x,(u. I,). x,, s zp(ll. 1.1.and I, -- ji ,,il I ;,I \uttl However. including the data fl-om the six htghly fractkl- ciently small values of r. these three ditkrcnt la\*.\ (111g~x, I~ nated runs DEPAOI.O (1978) noted that the “?Nd, ‘41Nd the same corrected values to the required :ICC~I;~~\. I~CJU ratios appear to be correlated with the aver;tge Nd ma\< C~PI-, when x becomes sufficiently large. each gjf ~hcbc IJV,, fractionation 2 for individual runs. \\III give significantly different corrected c alum> ~lzl~nti;n~i DEPAOLO (1978) considered this effect to bc caused hj on the choice of isotopes (II. 1’1 used to calcular~ E .I’~c variations in RI8 and RI7 which correlated with 1. How- choice among these laws to correct for mass I’t-acuon;ltt(lrl ever, variatkons in RI 8 and R ,, should also affect other NJ will depend on which law best rcpraduces the ‘~~:and:~ld isotopic ratios. Figure 2 sho\vs corrected ‘42Nd:‘J”Nd state’ of isotopic reference value\ K$ independent ,li’ the, values plotted against corrected “‘Nd, “*Nd for the runs choice of isotopic pair (u, I>) used ~CIc;dculatc .r .~III.~~nctc from Fig. I plus additional Iruns. The diagonal line in Fig. 2 pendent of the magnitude of 1. Kvwt I C( d I i’i7X1 tir.\t shows the trend expected for no Nd mass fractionation discussed the use of the exponential Ia\\ I hci dcmoti (r = 0) and with the oxygen Isotope composition varying strated that for Ca isotopes neither- the lineal I,,\% n,>~ the from run to run. Thus if the observed variations in power law adequately describes the large I~IYIIumentai ‘42Nd/‘44Nd were due to a widely varying oxygen compo- mass fractionation effects in C‘a and that the c~p~~ncnt~:~I sition from run to run. we would expect a strong negative law provided a good lit to their dat,t, correlation with the ‘IENd: 14“Nd ratios on the same Using the above equations the sxpectcti d:ticrcnc~~~ samples. The data in Fig. 2 show that this IL not obser\cd hctween the power and exponential laws have IXYI~ GLICII. The relatively tight clustering of the “‘Ntl ‘JJNd Iratio in latcd using ‘“‘Nd 14’Nd as the normaliGg ratI*) tI”ig, II fact confirms that the variation of RIH from ~run to run is of The graph shows the ‘error‘ ((R$“‘,Rz”l I I 10’ lll Table 7. ATOMIC MASSES OF Nd AND Sm ISOTOPES* Neodymium 14*Nd lk3Nd j+"Nd li15Nd lk6Nd lgaNd '=Nd Atomif mass 141.907731 142.909823 143.910096 144.912582 145.913126 147.916901 149.920900 +6 t6 +6 f6 +6 r6 ?6 Atomic mass 143.912009 146.914907 147.914832 148.917193 149.917285 151.919741 153.922218 t6 t6 k6 t6 t6 t5 t6 *From WAPSTRA and BOS (1977). 2320 G. J. WASSERHLRC; cr d. use this ratio for calculating 1’4Nd concentrations, if may produced by the mass spectrometer. Thus when ihc tr;iLci appear that this could cause substantial interlaboratory Nd was measured, a procedure was used wjhich b\;th idcnr- discrepancies in 14’Sm, ‘41Nd ratios. Assume we have a ical to that used for the measurement of the normal Nd gravimetrically calibrated NJ normal solution with a given 1x 200 ng): the same amount of Nd was loadcti ~>nto tbc concentration in /cg;‘g. Assume also that we have normal- filament and data were taken at the same beam I~~ISI~~II\ iLed our Nd isotopic data to two different values for and filament temperature. ““Nd,“““Nd called A and B and these are 0.724134 and In this way the mass discrimination could be rs~~ma~cd 0.7219. respectively (set Table 4, part III). Ignoring the within about &0.02”,, to -&0.03”,, per mass unit. rhis r\ti. trivial differences in atomic weight calculated with these mate is sufficient for the tracer because it conl,un> oni! two isotopic compositions 15 parts in IO’), we get for the small amounts of the isotopes other than ‘50Nd. The ;~h- concentration of “ONd (in nmol!g) in the normal solution solute maximum uncertainty in the tracer composition due calculated with these Iwo ditferent isotopic compositions to mass fractionation is probably about 24,,, per rnas\‘ unit. that The net correction to ‘43Nd/‘J4Nd in spiked >arnplc\ (where ““Nd tracer added is twice ““Nd samplcj is ap- [‘““Nd] ;(IRMAI _ 1.008792 ,iji!Nd];;“‘R”“l proximately O.OC014with an associated error of %0.00000 This reflects the difference in (““Nd:Nd),t,,,,r for the due to the uncertainty in the tracer composition. I he err-c,r two different normalizations. in this correction is therefore insignificant. The concentration of 15”Nd in a tracer calibrated with The “‘Nd tracer also contains rsC’Sm. Meahurmg the this normal will thus depend on whether we use the iso- Sm element composition in the Nd tracer (during ;I Nti topic composition called A or the one called B. We have that --- for MIX, TRACER, and NORMAL. -______.__..__._.__.._. ~_. Here MIX denotes the mixture of tracer and normal and M the mass of tracer and normal used to calibrate the tracer. As the discrimination corrections for all terms in the numerator and denominator are taken to be the same, it tracer run) gives ‘5”Sm’154Sm = 1I2 If. 1. This 1s ,tn uppc>r follows that the concentrations of the tracer determined limit to the true valud as the presence of ls”Nd should with either isotopic composition B or A are related by increase this ratio. During the Nd tracer composi[rcln run ‘54Sm’h0 was monitored and the inference from ‘coSm’hO to ‘5”Nd’60 was less than 2 parts in IO’ l‘l>r samples spiked with lsoNd and ‘J7Sm, the e&c1 oi the ‘“Sm cross-contamination is to give an incorrect Srn con- centration using the ““Sm isotope. This is not miportan! as concentrations can be calculated from the oitbcr SK Now in the same manner using the spike equation above isotopes. However, for lunar samples where neutron cup- for determining the concentration in a sample we get ture information is sometimes required from qxhcti samples, the neutron induced enrichmenl in ““Sm ~112mr~ l--.--_.l[‘50W.2’M”‘.r = l,oo8792 be determined. The neutron capture rates can still be C~ICLI- [ls”Nd];AMPI.f lated from the depletion of ‘49Sm. The concentration of 14“Nd in the sample calculated with these two methods is thus 1 II08792 999487 1.009310 It follows that the discrepancy caused in the concentration -- of 14“Nd by using such very different ‘soNd/i44Nd ratios is only about O.OS:;, which reflects directly the difference in RESULTS OF ISOTOPE DILUTION l’“‘+Nd,‘NdL,T,M,, for the two different normalizations. CALIBRATIONS THE ISOTOPIC COMPOSITION OF Table 8 shows the concentrations of the stock sol- THE TRACERS utions of 147Sm and lsoNd tracers determined by gra- The measured isotopic compositions of the Nd and Sm vimetry in 1975 and later by isotope dilution using tracers are given in Table 5. The atomic fraction of ““Nd three different sets of Sm (nSmA, nSma, nSm/j) and in the Nd tracer is (15”Nd,‘Nd),,o,,c = 0.96177 and the Nd (nNdA, nN&, nNd/l) normals made from differ- atomic fraction of ““Sm in the Sm tracer is (‘4’SmiSm),,o,,c = 0.97844. The largest uncertainty in ent metal chunks. The gravimetric concen&atlon of determining the tracer composition is mass fractionation “‘Nd in the tracer is within error (3”~ ,J that Samarium and neodymium in standard solutions 2321 Table 8. CJNCSNTRATIONS IN CONIZNTRATED TRACER SOLUTIONS (T Sm147 AND T Nd150) DETERMINED BY DIRECT CALIBRATIONS WITH CIJNCENTRATEDNORMAL SOLUTION GIVEN IN TABLE 2 Normal Time '~'Sm(nm/g) '50Nd("m/g) 147Sm/150Nd Gravimetry*: Aug-75 61.42tO.12 29.0850.08 2.112lk99 Isotope dilution: "NdA,"SmA Mar-79 61.336'0.012 29.007'0.004 2.1145?7 "Nda."Sma~. Feb-79 61.394t0.006 29.056+0.004 2.1130t5 "NdB,"SmO Ott-79 61.388+0.006 29.072+0.004 2.1116f5 "NdB,nSmS Ott-80 61.421'0.008 29.080'0.004 2.112lt6 "(Sm/Nd)E Nor80 61.408+0.018 29.062+0.004 2.1130'9 *Calculated from the data in Table 3, using a molecular weight for the Nd?O?L, tracer of 347.435 am" and a molecular weight of the Sm203 tracer of 341.936 am". obtained by isotope dilution calibrations with Nd and Nd normals and tracers were prepared the same metal normals. Assuming the tracer is of good way so errors tend to cancel out. It is clear from quality, this suggests that the value of Table 8 that we can control the L4’Sm/‘“4Nd ratios ‘soNd/1’4Nd = 0.238619 which we used to calculate to -OS”,,,, over long periods of time. In 1980 we reca- the concentration of “‘Nd in the solutions is the librated the tracers with nNdfi and nSmp and then appropriate value to use and not the 9.3”,,,, lower made 2 I. of a mixed Sm/Nd normal from these nor- value used by some other laboratories (see previous mal solutions (see Table 1). The tracers were then section). In 1979 the concentration in the stock sol- recalibrated with the mixed normal solution and the utions of the tracers T Sm147 and T Ndl50 were results agree to within 0.5”,,,, of those obtained in pre- found to have changed to l-24,,, higher values than vious calibrations. These results clearly confirm that those determined in 1976 by isotope dilution, presum- the SmjNd ratio in this mixed normal solution is ably due to evaporation. Two new sets of Sm and Nd known to within 0.5”,,,,. normals were made in 1979 (c( and 8) and used to Table 9 shows the results of calibrations of dilute recalibrate the stock tracer solutions. These results all tracers made from T Sm147 and T Nd150 with dilute agree to - 0.5”,,,,. The ratio of the concentrations of normals made from nNdA, nNdr, nSmA, and nSmr. 14’Smi’50 Nd in the two stock solutions appears to be These data were used to calculate back the concen- more constant than the concentrations as shown in tration in the stock solutions of tracers (T Sm147 and Table 8. This is probably due to the fact that both Sm T Nd150). The first isotope dilution calibration in Table 9. ODNENTRATIONS IN U)NCENTRATED TRACER SOLUTIONS (T Sal47 AND T Nd150) DETERMINED BY ?IIXINCDILUTE TRACERS AND DILUTE NORMALS Normal Time Dilution factor 147Sm(nm/g) 150Nd(nm/g) 'q7Sm/'50Nd of tracer Sm Nd nNdA,nSmA Jan-76 l/95 l/81 61.240'0.016 28.979tO.016 2.1133 nNda,nSmu Jan-79 l/98 l/73 61.366'0.029 29.048+0.014 2.1126 nNdu,nSmu Jan-79 l/98 l/73 61.415t0.020 29.062tO.037 2.1133 nSma Jan-79 l/98 - 61.455'0.092 nNda,nSma Jan-79 l/20 l/9 61.381'0.012 29.052'0.006 2.1123 nNda,nSma Jan-79 l/3700 l/1340 61.434t0.036 29.064t0.012 2.1137 nNda,nSma Jan-79 l/25 l/15 61.427t0.023 29.061'0.009 2.1137 Table 10. RECOMMENDED VALUES FOR THE Sm/Nd MIXED NORMAL SOLUTION n(Sm/Nd)B* CIT Sm/Nd standard Nd = 434.070 ppm by weight ; 144Nd = 715.757"m/gram Sm = 141.140 ppm by weight ; 147% = 140.756nmlgram li*‘Sm = 0.31192 ; lltltNd = 0.19665 i-) ATOMIC = 0.511138 normalized t" = 0.636151 ?8 *Atomic ratios calculated from weight ratios, using atomic weights of 144.2474 and 150.3656 for Nd and Sm metals from Ames Laboratory, respectively. The normal Nd and Sm isotopic compositions used are those give" in Table 5. The lk3Nd/lQ4Nd ratio given for this mixed normal is the average value for nNdB obtained from the data in Table 6. The atomic fractions of lb7.Smand 144Nd in the Ames Sm and Nd metals are (147Sm/Sm)ATOMIC= 0.149957 and ("'Nd/Nd)ATOMIC = 0.237856, respectively. 2322 G. J. WASSERBURC;ef tr/ 1976 (see Table 9) agreed to within 3”(,,,with the gravi- variations and petrogenetic models. Gco/~hr~. Rrs. Lerr. metric values using the normals nNdA and nSmA. 3. 249%252. This is considered to be good agreement as the tracers EUGSTER O., TERA F.. BURNETT D. S. and WASSERBURG G. J. (1970) The isotopic composition of gadolinium and were prepared from oxide powders and the gravi- neutron-capture effects in some meteorites. J. Geophvs. metric uncertainty is x3”<,,,. The results all agree to Rex 75. 2753 -2768. within about O.Yq,,,of the values obtained from direct GAST P. W.. HUBBARD N. J. and WIE.CMANN H. (1970) calibrations with concentrated normals even for Chemical composition and petrogenesis of basalts from Tranquility Base. Prw. Apollo I I hmw SC;.Conf:. pp. tracer dilutions up to a factor of 1:3700 (see Table 9). 1143~1163. As most of our work is done with tracers and normals HAMILTON P. J.. O’NIONS R. K.. Evt~:& N. M.. BRIIX- much more dilute than those in the stock solution, WATER D. and ALL AART J. (1978) Sm Nd isotopic inves- this confirms that no significant errors are introduced tigations of the Isua supracrustals. West Greenland: im- by the dilutions from the concentrated solutions. plications for mantle evolution. Ntrrlrrc 272, 41 43. JACOBSEN S. B. and WASSEKRUR(; G. J. (1980) Sm -Nd iso- The recommended values for the CIT Sm/Nd stan- topic evolution of chondrites. Etrrrh Ploncr. SC,/. L.rtr. 50. dard n(SmjNd)P are given in Table 10. The values 139-155. given for the concentrations and the Sm/Nd ratio by LCIIERER C. M.. HOLLANIXR J. M. and PI.KI MA\ 1. (1967) weight are calculated only from gravimetry and do Tub/r 01 I.sotqws. 594 pp. Wiley. LUGMAIR G. W. and CARLSON R. W. (1978) The Sm -Nd not include any of the isotope dilution calibrations. history of KREEP. Proc. Yrh L~~ntrr Pltrwt Sci. Conf.. pp. The (Sm/Nd),,,,,, ratio is thus only limited by 689%704. weighing uncertainties and purity of the metals. Using LUC;MAIRG. W., MARTI K.. KLJRT~ J. P. and SCHEININ N. the Nd and Sm isotopic compositions in Table 5, the B. (1976) History and genesis of Lunar troctolite 76535 or: How old is old? Proc. 7th Ll~,ltrr P/trrw/ Sci. Cod.. DD. ‘47Sm/‘44Nd isotope ratio was calculated and the 1L 2009%2033. ‘43Nd,“44Nd value given is from the data in Table 6. MASUDA A., NAKAMURA N. and TANAKA T. (1973) Fine Aliquots of this standard and the unmixed standards structures of mutually normalized rare-earth patterns of nNda. nNdfl. and nNdA are available upon request. chondrites. Grochim. Cosmochim. Ac,ro 37, 239 248. MCCULLOCH M. T. and WASSERBURC;G. J. (1978) Barium Although the absolute concentrations may be some- and neodymium isotopic anomalies In the Allende what uncertain in small aliquots of n(Sm/‘Nd)P due to meteorite. A.wwphys. J. 220, L15 ~L19. possible evaporation during shipping and storing, the MOORE L. J. (1978) Isotopic fractionation phenomena and Sm/Nd ratio is precisely known and will not change high precision neodymium analyses. In Short Pqwrs o/ due to evaporation. rhr 4th Inttwurtionrrl Confrrmw. Guoc~kronok~y~. Cos- moc~hrono/oy~~. I.sotop Grdogy (ed. R. E. Zartman), pp. 301-302. U.S. Geol. Surv. Open-File Rep. 78 ~701. .4c,linon,/rtlyen1ents This work has been supported by NAKAMURA N. and TATS~MOTO M. (1980) A 4.0 b.y. impact NSF grant EAR 76-22494 and NASA grant NGL metamorphism age of the Modoc L6 chondrite deter- 05-002-188. mined by the Srn-Nd method. Mefrorirics 15, 334-335. We wish to thank L. NYQUISTfor a very thorough review NIEDERER. F. R., PAPANASTASSIOUD. A. and WASSERBURG of this paper which resulted in several substantial improve- G. J. (1981) The isotopic composition of titanium in the ments. We also thank R. K. O’NIONSfor his review of this Allende and Leoville meteorites. Guoc,hrm. C‘n.srnr,c~hinl. paper and his genuine concern for interlaboratory stan- Acrtr 45, 1017~1031. dards. A third anonymous reviewer who thought that the NIER A. 0. (1950) A redetermination of the relative abun- paper should not be published is thanked for making us dances of the isotopes of carbon, nitrogen. oxygen, aware that the “seemingly rigorous attention to detail in argon, and potassium. Phrs. Rcr. 77, 789 793. the described nrocedures” which are here Dresented “is NYCXJISTL. E.. SHIH C-Y.. W(K)DEN L. J.. BANSAL B. M. probably comAon practice in laboratories in;olved in this and WIESMANN H. (1979) The Sr and Nd isotopic record tvne of w*ork”. We thank G. LUGMAIR for sending us a La of Apollo 12 basalts: implications for lunar geochemical jcilla Nd isotope standard and for discussing his-views on evolution. Proc. I fkh Lww Pltrnul. .Qi. Conf.. pp. Nd isotope analysis with us. Support for some of the time 77-I 14. and computing necessary for writing this paper was O’NIONSR. K., HAMILTON P. J. and Evf.Nst N N. M. 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