- Email: [email protected]
ID-MC-ICP-MS)
Chemical Geology 157 Ž1999. 1–12
Application of isotope dilution for precise measurement of ZrrHf and 176 Hfr177 Hf ratios by mass spectrometry žID-TIMSrID-MC-ICP-MS/ K. David a
a,)
, J.L. Birck a , P. Telouk b, C.J. Allegre `
a
Laboratoire de Geochimie et Cosmochimie, CNRS–URA 1758, Institut de Physique du Globe, 4 Place Jussieu, 75252 Paris, Cedex 05, ´ France b Laboratoire de Geochimie, Ecole Normale Superieure de Lyon, 46, allee ´ ´ ´ d’Italie, 69 364 Lyon, cedex 07, France Received 4 December 1997; revised 3 November 1998; accepted 3 November 1998
Abstract This paper presents a Hf and Zr chemical purification method and calibration procedure used to measure ZrrHf ratios in terrestrial and extraterrestrial rocks by isotope dilution mass spectrometry with a mixed 96 Zr– 179 Hf spike. Sample preparation and mass spectrometric procedures for thermal ionization mass spectrometry ŽTIMS. and magnetic sector multiple-collector ŽMC-ICP-MS. analyses are described. We used a new combination of chemical procedures to achieve a highly reliable method for Hf and Zr isotopic measurements wMinster, J.F., Allegre, C.J., 1981. The isotopic composition of ` zirconium in terrestrial and extraterrestrial samples: implications for extinct 92 Nb. Geochim. Cosmochim. Acta, 46: 565–573; Barovich, K.M., Beard, B.L., Capple, J.B., Johnson, C.M., Kyser, T.K., Morgan, B.E., 1995. A chemical method for hafnium separation from high-Ti whole rock and zircon samples. Chem. Geol., 121: 303–308x. It involves two-stage anion exchange with minimal amounts of sulphuric acid. Undesirable elements for hafnium and zirconium isotopic analyses are removed and negligible blanks are obtained. For Hf and Zr isotopic measurements performed by thermal ionization mass spectrometry ŽVG 354. and magnetic sector multiple-collector ICP-MS ŽPlasma 54., two sample preparations schemes are proposed. A specific load of less than 5 mg of zirconium with 200 ng of hafnium was used in TIMS to avoid the inhibitor effect of zirconium on Hf thermal ionization. For ICP-MS analyses, storage of samples as oxalates stabilizes both elements in aqueous solution without interferences on mass spectra. ZrrHf ratios are measured with similar internal precision for both ID-TIMS and ID-MC-ICP-MS and a reproducibility of 2.5‰ ŽRSD.. This is a factor of 10 better than that obtained by classical techniques used to measure the concentrations of zirconium and hafnium. 176 Hfr177 Hf isotope ratios were obtained with an internal precision ranging from 0.04‰ to 0.1‰ and a reproducibility of 0.1‰ ŽRSD.. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Mass spectrometry; Zirconium; Hafnium; Chemical ratios;
)
Corresponding author. Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, UK. Fax: q441865-272072; e-mail: [email protected]
176
Hfr177 Hf
1. Introduction Concentration ratios of highly refractory element pairs with the same degree of incompatibility Žlow
0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 8 . 0 0 2 0 1 - 0
2
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
bulk crystal-liquid partition coefficient. during petrogenetic processes such as Zr–Hf are comparable to isotope ratios in serving as tracers of different mantle sources ŽHofmann et al., 1986.. Because these twin pairs are isovalent trace elements with very close ˚ Hf s 0.79 A; ˚ Whittaker and ionic radii ŽZr s 0.80 A, Muntus, 1970., they should display an extremely coherent behavior during igneous processes and retain their respective chondritic ratio. ZrrHf data on oceanic basalts obtained with a precision ranging from 1 to 5%, using X-ray fluorescence ŽXRF. for zirconium, instrumental neutron activation analysis ŽINAA. for hafnium or isotope dilution spark source mass spectrometry ŽID-SSMS. for both elements, do not show significant variations of the ZrrHf ratio. Data obtained on sixty nine mid-ocean ridge ŽMORB. and ocean island basalts ŽOIB. gave ZrrHf ratios equal to 36.6 " 2.9 which is similar to the chondritic value ŽZrrHf s 36.3; Jochum et al., 1986.. This suggests that the ZrrHf ratio is not fractionated between the different sources of oceanic basalts. We report here improved ZrrHf ratio measurements in order to assess its uniformity in oceanic basalts. Isotope dilution with a mixed spike of Zr and Hf in thermal ionization mass spectrometry ŽID-TIMS. and magnetic sector multiple-collector ICP-MS ŽIDMC-ICP-MS. are techniques capable of measuring concentration ratios ZrrHf with a high-precision. Previous ZrrHf ratio measurements in seawater samples have been performed by isotope dilution mass spectrometry with a precision of 3% ŽBoswell and Elderfield, 1988.. Precise Zr and Hf concentrations of around 1% and 4% ŽRSD. have already been measured by inductively coupled plasma mass spectrometer with a quadrupole mass analyser ŽID-ICPMS. where the precision is limited by the variation in plasma source transmission ŽXie and Kerrich, 1995.. Furthermore, the high sensitivity and precision of the MC-ICP-MS technique for Hf isotopic measurements allows determination of 176 Hfr177 Hf isotope ratios. Two major problems limit Hf and Zr analyses using thermal ionization mass spectrometry ŽTIMS.. First, the first ionization potentials of Hf and Zr are relatively high Ž IZr s 6.84 eV, I Hf s 7 eV; Handbook of Chemistry and Physics, 1987., resulting in poor ionization efficiencies for thermal ionization.
Second, because titanium is more abundant in geological samples Ž0.1% wt. for meteorites and 1–3% wt. for basalts. than zirconium and hafnium Žabout a hundred to a few ppm, respectively. with a slightly lower first ionization potential Ž I Ti s 6.82 eV., it tends to suppress the thermal ionization of hafnium and zirconium if present on the filament. In contrast to conventional TIMS, the magnetic sector-multiple collector ICP-MS exhibits better ionization for refractory elements such as zirconium and hafnium and matrix effects are by far less important. However, a potential difficulty with this technique is that the presence of large amounts of titanium may produce the clogging of the sample-cone orifice which significantly reduces the transmission of hafnium and zirconium and cause systematic drift in measured isotope ratios ŽBlichert-Toft et al., 1997.. Consequently, a complete separation of hafnium and zirconium from titanium and other elements, principally those interfering with the hafnium mass spectrum, is a prerequisite. Terrestrial and extraterrestrial samples are investigated by using a two-stage ion-exchange chromatography procedure. Conditions for the high-precision measurements of ZrrHf ratios by isotope-dilution in thermal ionisation mass spectrometry and magnetic sector multiple-collector ICP-MS are discussed as well as the internal precision and the reproducibility of ZrrHf and 176 Hfr177 Hf ratios.
2. Analytical procedure 2.1. Reagents and labware Hydrofluoric, hydrochloric, nitric acids and water were purified by subboiling distillation from Teflon and quartz stills. Sulphuric acid 98% and hydrogen peroxide 36% Žwrv. were commercial analytical grade reagents ŽMerck, Darmstadt, Germany; Carlo Erba Reactifs, France.. Whole rock dissolutions were performed in Savillex 026R PFA pressure vials ŽSavillex, Minnetonka, MN, USA. and Parr digestion bomb ŽParr Instrument, IL, USA.. For the chemical purification, two columns in polypropylene Ži.d.s 0.5 cm and 0.8 cm. were filled
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
with Bio-Rad AG1-X4 resin Ž200–400 mesh. in the chloride form to a height of 5 cm and with Bio-Rad AG1-X8 resin Ž200–400 mesh. in the chloride form to a height of 10.5 cm, respectively. Prior to sample loading, AG1-X4 and AG1-X8 anion-resins were washed twice with the following sequence of reagents: 4 N HFŽ10 ml. –H 2 OŽ5 ml. –6 N HCl Ž10 ml. and equilibrated with 5 ml 2 N HF and 10 ml 1 N H 2 SO4 , respectively. All the labware in contact with the samples were cleaned prior to use with a mixed solution containing commercial nitric Ž60%. and hydrofluoric Ž50%. acids, then high purity hydrochloric acid and finally double distilled water. All chemical preparations were conducted in an over-pressured clean-lab.
3
Table 1 Flow-chart for Hf extraction from a sample
2.2. Sample dissolution-matrix effects on the extraction of the IVth group elements Sample dissolution follows a modification of the standard procedure in HNO 3rHF acid mixtures of Minster and Allegre ` Ž1981.. For all rocks, the mixed tracer 96 Zr– 179 Hf is added to the powder. The mineral phases are decomposed in a 4.3-ml HFrHNO 3 Ž93r7 vrv. mixture in Savillex w vials at 1508C for 2 days. For materials containing zircons or other refractory minerals, the procedure was repeated on the residue obtained from this first stage. In the latter case, the PFA bomb is placed into a Parr digestion bomb and the dissolution is performed at 2208C for several days. To equilibrate the pressure within the two vessels, a solution HFrHNO 3 Ž93r7 vrv. is placed inside the Parr digestion bomb, outside the 026R vials. For meteorites, the second decomposition on the residue was performed in PFA vials at 1508C in order to aid solution of Zr and Hf. After the dissolution, the sample is treated following the procedure detailed in Table 1. The solution and the insoluble fluoride are separated by centrifugation. The precipitate is then washed ultrasonically twice with 5 ml H 2 O. All supernatants are combined and evaporated to dryness at 908C. The residue is suspended in 1 ml of 4 N HF, warmed and then centrifuged. The residue is then washed with 1 ml of H 2 O and 1 ml of 2 N HF. All supernatants are mixed together giving 3 ml solution, 2 N in hydrofluoric acid which is then loaded on the first-stage anion exchange column ŽAG1X4..
We used Sx and Rx to symbolize the supernatant and the residue obtained at each stage.
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
4
The dissolution scheme results in a fluoride precipitate containing the rare earth elements, alkaline earth elements and a supernatant with IVth group elements ŽZr, Hf and Ti. which form strong complexes with fluoride ions, transition elements ŽCr, Mn, Fe, Co, Ni, Zn. and also actinides ŽTh, Pa and U. ŽBlichert-Toft et al., 1997.. We have noticed that the matrix type had an effect on hafnium and zirconium extraction. A coprecipitation of hafnium and zirconium occurred with commercial calcium or magnesium fluoride while this coprecipitation does not occur from materials with a high silica content. Similar results already observed with protactinium indicated that calcium fluorides Žand those of magnesium to a lesser extent. are effective carriers for protactinium in the residue while the presence of fluosilicate complexes such as Na 2 SiF6 , K 2 SiF6 and BaSiF6 reduces the degree of coprecipitation of protactinium ŽD’yachova and Spitsyn, 1963.. The Hf and Zr yields of the dissolution and extraction procedures were calibrated using gamma counting with a GeŽLi. detector ŽPierre-Sue ¨ Laboratory of Atomic Energy Commission-Saclay.. For this work, we used the radionuclides 95 Zr and 181 Hf which were obtained from natural Zr and Hf irradiated with slow neutrons. The g-ray lines of 95 Zr and 181 Hf were 756.6 and 482.2 keV, respectively ŽAdams and Dams, 1969.. Using this procedure with an irradiated alkali basalt from Fangataufa ŽF49., Hf
and Zr yields were found to be of the order of 95% and 100%, respectively. 2.3. Chemical separations To measure ZrrHf ratios by mass spectrometry, two elution steps on anionic resins are employed Žmajor element and titanium separations—Table 2.. For the first stage, the major element separation is adapted from Minster and Allegre ` Ž1981.. The sample matrix ŽFe, Al and Mg. and residual traces of REE are removed with 4 ml of 2 N HF, followed by 1 ml of 0.5 N HFr0.5 N HCl while Hf, Zr and Ti are adsorbed on the resin as fluoride complexes and then eluted with 3 ml of 3 N HCl. After adding 0.2 ml concentrated sulphuric acid to the latter fraction, it is slowly evaporated in a clean airflow box. The solution is taken to fumes of sulfuric acid just before loading the solution on the second anionic column. This step removes fluoride ions and polynuclear species of hafnium and zirconium ŽStrelow and Bothma, 1967.. The procedure used to separate Ti from Zr and Hf on an anion exchange resin AG1X8 is adapted from Barovich et al. Ž1995.. Prior to loading, 0.46 ml of concentrated hydrogen peroxide and 13.37 ml of water are added to the sample. The solution obtained is: 1% H 2 O 2 –0.52 N H 2 SO4 . The hydrogen perox-
Table 2 Column specifications and Hf and Zr separation scheme Resin type
AG1-X4 200–400 mesh Žanion, Fy form.
AG1-X8 200–400 mesh Žanion, HSO4 y form.
Column Dimensions
Volume Žcm
5 = 0.5 cm
1
10.5 = 0.8 cm
5.28
Eluent
Volume Žcm3 .
Elements eluted
sample in 2 N HF
3Ž3 cv.
Bulk sample, Co, Ni, Cr, Zn and traces of REE
2 N HF 0.5 N HFr0.5 N HCl 3 N HCl sample in 1% H 2 O 2 r 0.52 N H 2 SO4 1% H 2 O 2 r0.52 N H 2 SO4 H 2O 1 N HFr1 N HCl 6 N HCl 6 N HCl
4Ž4 cv. 1Ž1 cv. 3Ž3 cv. 13.83 Ž2.6 cv.
3.
17Ž3.2 cv. 5Ž0.95 cv. 10Ž1.9 cv. 3Ž0.57 cv. 3Ž0.57 cv.
Hf with Ti, Zr
Ti
Hf with Zr
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
ide is used to prevent hydrolysis of titanium forming an orange brown colored complex. Titanium is eluted with 17 ml of 1% H 2 O 2 –0.52 N H 2 SO4 solution due to its weak affinity on the anionic resin in a H 2 SO4rH 2 O 2 solution Ž D - 0.5 for 0.3% H 2 O 2 –0.5 N H 2 SO4 ; Strelow and Bothma, 1967.. After the resin is rinsed with 5 ml of H 2 O, followed by 10 ml of 1 N HFr1 N HCl before eluting Hf and Zr with 3 ml of 6 N HCl. The typical elution curves of IVth group elements are illustrated in Fig. 1. In the procedure proposed by Barovich et al. Ž1995., zirconium and hafnium are eluted together with traces of titanium in a relatively large volume of sulphuric acid. In our case, the intermediate solution of 1 N HFr1 N HCl is used to remove traces of sulfuric acid without eluting zirconium and hafnium. The elution scheme proposed here allows zirconium and hafnium to be collected in a smaller volume Ž3 ml. compared to 10 ml obtained by Barovich et al. Ž1995.. 2.4. Mass spectrometry 2.4.1. VG 354 The ID-TIMS measurements were made on Re single filaments, using a modified VG 354 with five Faraday cups and a Daly detector.
5
In order to avoid the suppressor effect of zirconium on the ionization of hafnium generally encountered in TIMS when the both elements are not cleanly separated, the aliquot obtained after the second-stage anion exchange column separation must contain less than 5 mg of zirconium with 100–200 ng of hafnium ŽCorfu and Noble, 1992.. After the Hf–Zr residue dissolution in 1 ml of 0.1 N HF, the sample is loaded and dried on a zone-refined rhenium filament Ž25 mm Žthick. = 0.75 mm Žwide... A solution containing 20 mg of silica gel, 5 mg of boric acid with 30 mg of suspended rhenium metal and 15 mg of suspended boron metal Ž99.99% of purity for both metals. is then loaded. The addition of boron metal to the load has already been used to measure hafnium by TIMS ŽSalters and Hart, 1991.. The total load is dried on the filament with a current of 1.5 A. After drying, the load would sometimes chip off slightly from the filament. However, subsequent addition of silica gel usually cements the load to the filament. If the boron metal is used alone to improve the work function of rhenium filament, the latter would fuse at a temperature about 1600– 16508C. To avoid this effect, it is necessary to strengthen the filament with the addition of rhenium metal powder in the load.
Fig. 1. Elution scheme for the separation of titanium from zirconium and hafnium on AG1X8 column. The loading solution contains 40 mg of TiŽIV., 600 mg of 95 ZrŽIV. with the presence of 181 Hf traces in 0.52 N H 2 SO4r1% H 2 O 2 solution. The Ti yield was determined by using a semi-quantitative colorimetric method with a hydrogen peroxide solution diluted to 3%Žvrv.. Hafnium and zirconium are detected with 0.2% pyrocatechol violet to 0.2% by adjusting to pH 5 with 5 M ammonium acetate buffer. This method requires removal of fluoride ions by fuming H 2 SO4 to prevent decolorisation of the zirconiumŽhafnium.rpyrocatechol violet complex. The interference of titanium ions which react similarly with pyrocatechol violet is eliminated by adding EDTA to the solution.
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K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
After introduction into the mass spectrometer, a three-step heating filament procedure is employed. During the first stage the filament is heated to 4 A in 40 min, silica gel and boric acid are evaporated. The current is then raised to 4.5 A in 50 min where the Zrq beam can be focused on Daly detector. Finally, the filament current is increased at a rate of 100 mA per min until a stable zirconium ion beam is observed on collector Žthe optimal ionization temperature is around 19008C Ž I s 5–5.2 A... Zirconium acquisition is performed using a single Faraday cup. The magnetic field is switched by the computer through the following sequence 90–90.5– 91–94–96–98 where peaks at masses 90 and 90.5 are the reference peak and baseline, respectively. Zirconium isotope ratio measurements for spiked samples were performed by TIMS with 90 Zr signals of 0.3 = 10y1 1 A on average while 187 Re ranges from 0.006 = 10y1 1 A to 0.070 = 10y1 1 A. Because Hfq emission occurs at a higher temperature, a Hfq beam appears on the Daly detector after the data collection of zirconium. Data acquisition on the Daly detector can be obtained at this temperature or with higher current Ž I s 5.6 A.. A similar procedure for hafnium ratio measurements is adopted for masses 175.5–176–177–178–179–180 with the background at 175.5. Hafnium isotope ratios were measured on Daly detector with 179 Hf beams ranging from 0.0065 = 10y1 1 A to 0.015 = 10y1 1 A. The 187 Re signals obtained at the beginning of the run were of the order of 0.1 = 10y1 1 –0.2 = 10y1 1 A. The integration time is 4 s for zirconium and hafnium peaks and the baseline. During a normal run, 160 ratios in 20 blocks are collected. By using a mixed standard solution of hafnium and zirconium ŽHf 1 mg–Zr 5 mg. for which the ZrrHf ratio does not reflect that of the typical sample, we noticed that the presence of Zr did not have deleterious effect on in-run precision of hafnium isotope ratios and hafnium sensitivity. Two data acquisitions of 10 blocks have resulted in 179 Hfr 178 Hf ratios measurements with internal precisions of 0.3‰ for 1 mg of Hf and 0.4‰ for 1 mg of Hf with 5 mg of Zr. The Mo isobaric interferences on the Zr mass spectrum at masses 92, 94 and 96 are estimated by measuring 98 Mor90 Zr ratio. All potential interferences such as Lu, Yb, W and Ta for the Hf mass
spectrum were not observed. On a few occasions, we observed the appearance of interferences on 178 Hf and 180 Hf due to rhenium ions Ž‘ghost peaks’. diffused by the shielding plate and could be avoided by setting the shielding plate potential outside the interval where they occur. The isotope ratios 96 Zrr90 Zr and 179 Hfr178 Hf used in isotopic dilution have been corrected for mass discrimination using a linear law and the spike-sample mixing equation. As fractionation was always less than 0.5% per amu, this procedure is valid to the 10y5 level of precision.
2.4.2. Plasma 54 The measurements by ID-MC-ICP-MS were performed on a plasma source mass spectrometer wmagnetic sector multiple-collector inductively coupled plasma mass spectrometer ŽEcole Normale Superieure ´ de Lyon.x equipped with nine Faraday cups and a Daly detector with ion counting. Detailed descriptions of the instrumentation can be found in different publications ŽWalder and Freedman, 1992; Walder et al., 1993; Halliday et al., 1995; Blichert-Toft et al., 1997.. Because zirconium and hafnium compounds in aqueous solution are characterized by their high degree of hydrolysis and adsorption, and by their tendency to form various complex ions and polymer compounds, it is recommended that a special procedure is used ŽElinson and Petrov, 1969.. In our case, the aliquot obtained after the chemical purification and containing approximatively 1 mg of hafnium with zirconium was evaporated to dryness with 10 ml of 1 M H 2 C 2 O4 . This procedure allows storage of zirconium and hafnium in the form of oxalates which are easily soluble in water. Furthermore, the formation of strong complexes between oxalate ions and zirconium and hafnium is used to stabilize zirconium and hafnium in aqueous solution. We also noticed that an oxalic acid solution having a concentration of 10y3 M, similar to that of running solutions, does not produce interfering species on the hafnium and zirconium mass spectra ŽFig. 2.. Sample solutions Ž c s 500 ppb–1 ppm. were prepared on the day of analysis by dissolution in 0.5–1 ml of 0.05 N HNO 3 . According to the procedure detailed by BlichertToft et al. Ž1997., sample solutions were introduced
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
7
94 and 96 were corrected by monitoring the 95 Zrr Zr ratio. Potential interferences on Hf mass spectra coming from Lu and Yb at masses 176, from Ta and W at masses 180 but also from diatomic species such as Ba40Ar, La40Ar, Ce40Ar, Dy16 O, Er16 O observed at masses 176, 177, 178, 179 and 180 ŽXie and Kerrich, 1995. have never been detected due to the Hf purification prior to sample analysis. The presence of memory effects generally encountered in plasma source mass spectrometry are not observed on Hf and Zr mass spectra due to washing between each sample with: 0.05% HF then 3% HNO 3 . Mass fractionation is corrected according to an exponential law ŽRussell et al., 1978. where the mass fractionation correction factor deduced from standard measurements is generally f s y2 ŽBlichert-Toft et al., 1997.. For each run, 2 s screening of the data is done. The high sensitivity of the magnetic sector-multiple collector ICP-MS in Lyon w0.01 nA for a 50 ppb Hf solution; Blichert-Toft et al., 1997. for Hf isotopic allows us to measure Hf and Zr isotope ratios with total Hf and Zr signals ranging from 10 = 10y1 1 A to 15 = 10y1 1 A for 500 ppb–1 ppm solutions. This is a sensitivity a factor of 10 higher than that obtained in the best TIMS experiments with 1 mg of hafnium ŽPatchett and Tatsumoto, 1980; Salters and Hart, 1991; Corfu and Noble, 1992.. 90
Fig. 2. Spectra for H 2 C 2 O4 Ž cs10y3 M. blank solution on Zr and Hf mass ranges.
into the plasma using the free aspiration mode. With the plasma source configuration proposed in Lyon which associates the glass-expansion nebulizer with the Fassel type spray chamber and capillary teflon tubing, the natural solution uptake rate in the freerunning mode of operation is typically 50 mlrmin ŽP. Telouk, manuscript in preparation.. Measurements were performed in the static mode. The Faraday detectors are corrected for differences in ion-collection efficiency at the beginning of each day with standard solutions. Generally, cup efficiency factors are calculated relative to the values recommended for the isotopic composition of Hf ŽStevenson and Patchett, 1990. and for that of Zr ŽMinster and Ricard, 1981. reported in Table 3. During hafnium and zirconium data acquisition, 60 and 40 ratios are collected in 20 and 12 min, respectively. The baseline is the average of the measurement at y0.5 and q0.5 amu for each analysed masses. The interferences from Mo on the Zr mass spectra at masses 92,
Table 3 Isotopic composition of Hf and Zr in mixed spikes and standard solutions Solution
Hafnium ratios 176r178 177r178 179r178 180r178
Spike
Normal
Plasma 54
TIMSrPlasma 54
0.052329"44 0.339746"04 26.23247"32 2.564253"35
0.192316 0.681585 0.499261 1.285924
a
Zirconium ratios b 91r90 0.188432"20 92r90 0.286442"21 94r90 0.397611"18 96r90 4.927253"69 a
0.21799 0.33338 0.3381 0.05439
Stevenson and Patchett Ž1990., Blichert-Toft et al. Ž1997.. Minster and Ricard Ž1981.. Errors for enriched isotopic ratios are in-run precision Ž2 s ..
b
8
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
2.5. Hf and Zr concentrations and isotopic compositions of double spikes
Table 4 Determination of Hf and Zr concentrations in mixed spikes 179 Hf solutions by isotope dilution mass spectrometry
Reference solutions of these two elements were purchased from commercial sources. Although specifications of commercial standard solutions are within a few permille there is not always an overlap of uncertainties with solutions from the different manufacturers. This bias can reach eight permille. We preferred therefore to use a common calibration procedure for both metals to warrant a precise knowledge of the ZrrHf ratio relative to possible systematic biases due to uncontrolled impurities in pure commercial metals. Standard solutions used to calibrate the mixed spike 96 Zr– 179 Hf by isotope dilution mass spectrometry were obtained from Carlo Erba wCG HF1 for Hf Ž1007 ppm., CG ZR1 for Zr Ž1004 ppm.x and National Institute of Standards and Technology wSRM 3122 for Hf Ž9363 ppm with a density equal to 1.068., SRM 3169 for Zr Ž9479 ppm with a density equal to 1.055.x. Standard Hf and Zr concentrations were determined by direct titration with 0.01 M EDTA solution using 0.1% Xylenol Orange in aqueous solution as colorimetric reagent ŽPilkington and Wilson, 1965.. Preliminary titration of EDTA concentration was determined with a zinc solution prepared gravimetrically from pure zinc metal of 99.99% purity and a 0.1% Xylenol Orange. The red zinc–xylenol orange complex formed at pH s 4 in 7 M ammonium acetate solution is titrated with EDTA to the first permanent lemon yellow color. The average concentrations obtained after duplicate titrations are: NIST—Hf s 9342 ppm Ž9342– 9342 ppm., Zr s 9565 ppm Ž9560–9569 ppm.; Carlo Erba—Hf s 1013 ppm Ž1012–1014 ppm., Zr s 1011 ppm Ž1010–1012 ppm.. There is a good agreement between our values and the certified concentrations, except for NIST Zr concentration for which a difference of eight permille is observed with the certified value. Although some precautions have been taken to prevent the hydrolysis of Zr and Hf in solution Žmoderately acid solutions, heated in the pH 0–1.5 region., this inconsistency can only be explained by minor but significant loss of solvent by evaporation. In order to determine the hafnium and zirconium concentrations of the two mixed spike solutions by isotope dilution mass spectrometry, we have gravi-
Standard
No. Double spike 1 Double spike 2 replicate Zr Žppm. Hf Žppm. Zr Žppm. Hf Žppm.
Carlo Erba
1 2 3 4 1 2
NBS Average error
5.349 5.345 5.345 5.349 5.348 5.347 0.002
0.557 0.556 0.556 0.559 0.560 0.560 0.558 0.002
5.011 5.007 5.008 5.011 5.011 5.009 0.002
96
Zr–
0.581 0.58 0.58 0.583 0.585 0.584 0.582 0.002
All errors are one standard deviation.
metrically prepared different mixed solutions containing standard solutions from Carlo Erba and NIST. Average mixed spike hafnium and zirconium concentrations shown in Table 4, and are known with a precision of 3.5‰ and 0.4‰, respectively. The isotopic composition of the mixed spike 96 Zr– 179 Hf obtained by plasma source mass spectrometry is reported in Table 3. With reference to the high stability of the instrumental fractionation encountered in plasma source mass spectrometry, zirconium and hafnium isotope ratios are corrected for this fractionation within 1‰.
3. Results and discussion 3.1. RecoÕery and procedural blanks Total Hf and Zr yields have been determined by measuring zirconium and hafnium concentrations of the alkali basalt F49 from Fangataufa by isotope dilution. Total Hf and Zr recoveries for whole rocks through the chemistry procedure were 98% and 90%, respectively. For Hf-rich samples such as basalts, continental rocks and sediments, total procedural chemistry blanks were about 260–350 pg for hafnium and 6 ng for zirconium. For Hf-poor samples such as meteorites, the blank level was 160 pg for hafnium and 2.4 ng for zirconium. In all cases, blank levels are a factor of 10,000 and 1000 lower than the amount of Hf and Zr in Hf-rich and poor samples, respectively. Previous works have shown that Hf contamination is principally related to hydrofluoric acid with a minor
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
contribution carried by sulphuric acid ŽPatchett and Tatsumoto, 1980; Barovich et al., 1995.. The contamination originating from the ion exchange columns and FEP beakers was significantly reduced by alternating cleaning with dilute hydrofluoric acid and hydrochloric acid ŽPatchett and Tatsumoto, 1980; Minster and Allegre, 1981.. ` 3.2. Zr r Hf and
17 6
9
3.3. Analytical reproducibility and accuracy Reproducibility of ZrrHf and 176 Hfr177 Hf ratio measurements by isotope dilution mass spectrometry Ždefined as relative standard deviation of the mean or RSD and calculated from the standard deviation or 2 s . were determined with replicate analyses of the alkali basalt F49 from Fangataufa. Five aliquots were decomposed in HF–HNO 3 while the other three were decomposed in HF–HBr. The data reported in Table 5 show that both dissolution schemes give similar reproducibility for ZrrHf ratio measurements. The reproducibility of the ID-TIMS technique deduced from results of six replicate analyses wZrrHf s 40.06 " 0.2 Ž2 s .x is estimated at 2.5‰ ŽFig. 3.. Furthermore, ZrrHf ratio measurements of two aliquots made by ID-MC-ICPMS are consistent within analytical uncertainty with those obtained with ID-TIMS ŽFig. 3.. Seven aliquots analysed by ID-MC-ICP-MS yielded an average 176 Hfr177 Hf value of 0.282995 " 0.000052 Ž2 s . ŽFig. 4.. The accuracy of the analytical results obtained in this study was investigated by analyses of two well defined geological standard reference materials, the basalt BE-N and the granite AC-E distributed by the International Working Group and by comparison to results of other laboratories employing different analytical techniques. Although isotope dilution mass spectrometry is judged to be an analytical technique of high accuracy, we compared zirconium and hafnium concentration measurements to those obtained with INAA, XRF and ID-SSMS reported in
Hf r17 7Hf internal precision
When mixed Zr–Hf spikes are used, the precision of ZrrHf ratio is independent of sample and spike weights. The ZrrHf internal precision is only function of zirconium and hafnium isotope ratios in samples, mixed spike and spiked samples. The 179 Hfr178 Hf and 96 Zrr90 Zr ratios were measured in TIMS with an internal precision between 0.2–2‰ for hafnium and 0.4–0.7‰ for zirconium. The precision of 179 Hfr178 Hf ratios obtained by multiple collector ICP-MS were between 0.02– 0.06‰ and 0.1–0.7‰ for 96 Zrr90 Zr ratios. Therefore, ZrrHf ratio measurements by TIMS have an in-run precision ranging from 0.6‰ to 2‰ and for the magnetic sector multiple collector ICP-MS from 0.2‰ to 1‰ Moreover, the high precision of Hf isotope ratios obtained with the Plasma 54 allows us to measure 176 Hfr177 Hf ratios from spiked samples analyses with an internal precision ranging from 0.04‰ to 0.1‰. The relation used to determine the uncertainty on 176 Hfr177 Hf ratio measurements includes the correction for the spike contribution using isotope ratios corrected for mass fractionation and associated internal precision. These isotope ratios are known with an in-run precision better than 0.1‰.
Table 5 Zr, Hf concentrations and ZrrHf measurements of the alkali basalt F49 from Fangataufa by ID-TIMS and ID-MC-ICP-MS Techniques
Procedure of dissolution
No. aliquot
Zr Žppm.
Hf Žppm.
ZrrHf
VG 54
HFrHNO3
1 2 3 4 5 6 all aliquots
177.78 175.65 176.92 176.17 175.37 175.78
4.426 4.392 4.405 4.405 4.372 4.401
40.17 39.99 40.16 39.99 40.11 39.94
HFrHBr
Plasma 54 a b
HFrHNO3
3 7
b
Errors are two times the standard deviation Ž2 s .. Duplicate measurement on the same aliquot.
176.88 176.21
4.409 4.388
40.12 40.16
Average
Error a
40.11
0.20
40.01
0.17
40.06
0.20
10
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
Fig. 3. ZrrHf ratio measurements performed by ID-MS ŽTIMS and MC-ICP-MS.. Error bars for each measurement represent the in-run precision Ž2 s ..
Table 6. These techniques do not give Zr and Hf concentrations with a better precision but are adequate methods to measure both elements on the same aliquot. We avoided comparison of ZrrHf ratio measurements by isotope dilution mass spectrometry with Zr ŽXRF.rHf ŽINAA. ratios, as the latter are ob-
tained by two different analytical techniques. The average Hf and Zr concentrations for BE-N ŽHf: 5.53 " 0.3Ž1 s .; Zr: 270.22 " 35.51Ž1 s .; Govindaraju and Roelandts, 1993. compiled from 17 and 32 values obtained by INAA and XRF, respectively, agree within 1 s with our results. The zirconium
Fig. 4. 176 Hfr177 Hf ratio obtained from ZrrHf ratio measurements by ID-MC-ICP-MS. Error bars for each measurement represent the in-run precision Ž2 s ..
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
11
Table 6 Comparison of results by Plasma 54, INAA and XRF for Hf and Zr in geochemical reference samples BE-N and AC-E Reference material Hf Žppm.
BE-N Žbasalt. AC-E a Žgranite.
Plasma 54
INAA
This work
Govindaraju and Roelandts Ž1993.
5.841 28.014 28.011
5.53 " 0.3Ž1 s .
Govindaraju Ž1995.
ID-SSMS
INAA
Jochum et al. Ž1990.
Korotev Ž1996.
5.37 27.9 " 3.4Ž1 s .
5.85 28.7
Zr Žppm.
BE-N AC-E a
Plasma 54 b
XRF
This work
Govindaraju and Roelandts Ž1993.
278.45 " 0.25 831.87 " 0.38 836.17 " 1.51
270.22 " 35.51Ž1 s .
Govindaraju Ž1995.
ID-SSMS
XRF
Jochum et al. Ž1990.
Stork et al. Ž1987.
270
278
780 " 81Ž1 s .
a
Duplicate analysis. Errors for Zr concentrations measured by Plasma 54 are in-run precision Ž2 s .. The Hf concentrations are determined with in-run precision ranging from 0.02‰ to 0.06‰. All other errors are one standard deviation. Data determined by isotope dilution-spark-source mass spectrometry are given with precision 2–5% relative ŽJochum et al., 1990.. b
concentrations determined by XRF and ID-SSMS confirm the accuracy of our measurements performed by isotope dilution mass spectrometry. Similarly, we can observe a good agreement between our Hf concentrations and the compiled values, except for that obtained by ID-SSMS with a difference of 8%. Both reference rocks determined by INAA are in excellent agreement with our data ŽKorotev, 1996.. Unfortunately, because a data compilation for AC-E obtained only by INAA and XRF does not exist, we have compared Zr concentrations of our two replicates with recommended data, i.e., average Zr concentrations obtained by several techniques including the XRF wZr: 780 " 81 Ž1 s . Ž n s 79. ŽGovindaraju, 1995.. In this case, good agreement within 1 s is obtained for AC-E.
4. Conclusion The results presented here demonstrate that ZrrHf ratios measured by isotope dilution mass spectrometry Žthermal ionization mass spectrometry and magnetic sector-multiple collector ICP-MS. are improved in precision by a factor of at least 10 relative to other methods.
The analytical procedure has been applied to a wide variety of geological samples ranging from Hf-poor materials Žmeteorites. to Hf-rich rocks Žbasalts, continental rocks and sediments., the results of which will be presented elsewhere ŽDavid et al., in preparation.. During this investigation the separation scheme proved reliable.
Acknowledgements The authors wish to thank Michel Treuil of the Earth Sciences department of Pierre Sue ¨ laboratory ŽAtomic Energy Commission-Saclay. for permitting us to use the ion counting for chemistry calibration. Jean-Louis Joron is thanked for his help during the first part of this work. We are grateful to Francis Albarede ` for giving us the opportunity to use the Plasma 54 at ‘Ecole Normale Superieure’ of Lyon to ´ measure Hf isotope ratios in terrestrial and extraterrestrial samples. We would like to thank Pierre Schiano in particular for his constructive help and interest in this study. Constructive reviews by V.J.M. Salters and M.H. Thirlwall have greatly improved the manuscript. [MB]
12
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
References Adams, F., Dams, R., 1969. Compilation of gamma-transition energies. J. Radioanal. Chem. 3, 105–123. Barovich, K.M., Beard, B.L., Cappel, J.B., Jonhson, C.M., Kyser, T.K., Morgan, B.E., 1995. A chemical method for hafnium separation from high-Ti whole rock and zircon samples. Chem. Geol. 121, 303–308. Blichert-Toft, J., Chauvel, C., Albarede, ` F., 1997. Separation of Hf and Lu high-precision isotope analysis of rock samples by magnetic sector-multiple collector ICP-MS. Contrib. Mineral. Petrol. 127, 248–260. Boswell, S.M., Elderfield, H., 1988. The determination of zirconium and hafnium in natural waters by isotope dilution mass spectrometry. Mar. Chem. 15, 197–209. Corfu, F., Noble, S.R., 1992. Genesis of the southern Abitibi greenstone belt, superior Province: evidence from zircon Hf isotope analyses using a single filament technique. Geochim. Cosmochim. Acta 56, 2081–2097. D’yachova, R.A., Spitsyn, V.I., 1963. Concentration of protactinium in materials with a high content of silicic acid. Radiokhimiya 5, 106–110. Elinson, S.V., Petrov, K.I., 1969. Analytical Chemistry of Zirconium and Hafnium, Humphrey Scientific Publ., Ann Arbor, MI. Govindaraju, K., 1995. Working values with confidence limits for twenty-six CRPG, ANRT and IWG-GIT geostandards. Geostand. Newslett. 19, Special Issue. Govindaraju, K., Roelandts, I., 1993. Second report Ž1993. on the first three GIT-IWG rock reference samples: anorthosite from Greenland, AN-G; Basalt D’Essey-la-Cote, ˆ BE-N; Granite de Beauvoir, MA-N. Geostand. Newslett. 17, 227–294. Halliday, A.N., Lee, D.C., Christensen, J.N., Walder, A.J., Freedman, P.A., Jones, C.E., Hall, C.M., Yi, W., Teagle, D., 1995. Recent developments in inductively coupled plasma magnetic sector multiple collector mass spectrometry. Int. J. Mass Spectrom. Ion Proc. ŽNier Vol. 146r147, 21–23. Handbook of Chemistry and Physics, 1987–1988, CRC Press, FL. Hofmann, A.W., Jochum, K.P., Seufert, M., White, W.M., 1986. Nb and Pb in oceanic basalts: new constraints on mantle evolution. Earth Planet. Sci. Lett. 79, 33–45. Jochum, K.P., Seufert, H.M., Spettel, B., Palme, H., 1986. The solar system abundances of Nb, Ta and Yb and the relative abundances refractory elements in differentiated planetary bodies. Geochim. Cosmochim. Acta 50, 1173–1183. Jochum, K.P., Seufert, H.M., Thirlwall, M.F., 1990. Multi-element analysis of 15 international standard rocks by isotope-dilution spark source mass spectrometry. Geostand. Newslett. 14, 469–473.
Korotev, R.L., 1996. A self consistent compilation of elemental concentration data for 93 geochemical reference samples. Geostand. Newslett. 20, 217–245. Minster, J.F., Allegre, C.J., 1981. The isotopic composition of ` zirconium in terrestrial and extraterrestrial samples: implications for extinct 92 Nb. Geochim. Cosmochim. Acta 46, 565– 573. Minster, J.F., Ricard, L.P., 1981. The isotopic composition of zirconium. Int. J. Mass Spectrom. Ion Phys. 37, 259–272. Patchett, P.J., Tatsumoto, M., 1980. A routine high-precision method for Lu–Hf isotope geochemistry and chronology. Contrib. Mineral. Petrol. 75, 263–267. Pilkington, E.S., Wilson, W., 1965. The influence of polynuclear zirconium species on direct titration of zirconium and hafnium with EDTA. Anal. Chim. Acta 33, 577–585. Russell, W.A., Papanastassiou, D.A., Tombrello, T.A., 1978. Ca isotope fractionation on the Earth and other solar system materials. Geochim. Cosmochim. Acta 42, 1075–1090. Salters, V.J.M., Hart, V.J.M., 1991. The mantle sources of ocean ridges, islands and arcs: the Hf-isotope connection. Earth Planet. Sci. Lett. 104, 364–380. Stevenson, R.K., Patchett, P.J., 1990. Implications for the evolution of continental crust from Hf isotope systematics of Archean detrital zircons. Geochim. Cosmochim. Acta 54, 1683–1697. Stork, A.L., Smith, D.K., Gill, J.B., 1987. Evaluation of geochemical reference standards by X-ray fluorescence analysis. Geostand. Newslett. 11, 107–113. Strelow, F.W.E., Bothma, C.J.C., 1967. Anion exchange and a selectivity scale for elements in sulfuric acid media with a strongly basic resin. Anal. Chem. 6, 595–599. Walder, J.W., Freedman, P.A., 1992. Isotopic ratio measurement using a double focusing magnetic sector mass analyser with an inductively coupled plasma as an ion source. J. Anal. At. Spectrom. 7, 571–575. Walder, J.A., Platzner, I., Freedman, P.A., 1993. Isotope ratio measurement of lead, neodymium and neodymium–samarium mixtures, hafnium and hafnium–lutetium with a double focusing multiple collector inductively coupled plasma mass spectrometer. J. Anal. At. Spectrom. 8, 19–23. Whittaker, E.J.W., Muntus, R., 1970. Ionic radii for use in geochemistry. Geochim. Cosmochim. Acta 34, 945–956. Xie, Q., Kerrich, R., 1995. Application of isotope dilution for precise measurement of Zr and Hf in low-abundance samples and international reference materials by inductively coupled plasma mass spectrometry: implications for ZrŽHf.rREE fractionations in komatites. Chem. Geol. 123, 17–27.
Application of isotope dilution for precise measurement of ZrrHf and 176 Hfr177 Hf ratios by mass spectrometry žID-TIMSrID-MC-ICP-MS/ K. David a
a,)
, J.L. Birck a , P. Telouk b, C.J. Allegre `
a
Laboratoire de Geochimie et Cosmochimie, CNRS–URA 1758, Institut de Physique du Globe, 4 Place Jussieu, 75252 Paris, Cedex 05, ´ France b Laboratoire de Geochimie, Ecole Normale Superieure de Lyon, 46, allee ´ ´ ´ d’Italie, 69 364 Lyon, cedex 07, France Received 4 December 1997; revised 3 November 1998; accepted 3 November 1998
Abstract This paper presents a Hf and Zr chemical purification method and calibration procedure used to measure ZrrHf ratios in terrestrial and extraterrestrial rocks by isotope dilution mass spectrometry with a mixed 96 Zr– 179 Hf spike. Sample preparation and mass spectrometric procedures for thermal ionization mass spectrometry ŽTIMS. and magnetic sector multiple-collector ŽMC-ICP-MS. analyses are described. We used a new combination of chemical procedures to achieve a highly reliable method for Hf and Zr isotopic measurements wMinster, J.F., Allegre, C.J., 1981. The isotopic composition of ` zirconium in terrestrial and extraterrestrial samples: implications for extinct 92 Nb. Geochim. Cosmochim. Acta, 46: 565–573; Barovich, K.M., Beard, B.L., Capple, J.B., Johnson, C.M., Kyser, T.K., Morgan, B.E., 1995. A chemical method for hafnium separation from high-Ti whole rock and zircon samples. Chem. Geol., 121: 303–308x. It involves two-stage anion exchange with minimal amounts of sulphuric acid. Undesirable elements for hafnium and zirconium isotopic analyses are removed and negligible blanks are obtained. For Hf and Zr isotopic measurements performed by thermal ionization mass spectrometry ŽVG 354. and magnetic sector multiple-collector ICP-MS ŽPlasma 54., two sample preparations schemes are proposed. A specific load of less than 5 mg of zirconium with 200 ng of hafnium was used in TIMS to avoid the inhibitor effect of zirconium on Hf thermal ionization. For ICP-MS analyses, storage of samples as oxalates stabilizes both elements in aqueous solution without interferences on mass spectra. ZrrHf ratios are measured with similar internal precision for both ID-TIMS and ID-MC-ICP-MS and a reproducibility of 2.5‰ ŽRSD.. This is a factor of 10 better than that obtained by classical techniques used to measure the concentrations of zirconium and hafnium. 176 Hfr177 Hf isotope ratios were obtained with an internal precision ranging from 0.04‰ to 0.1‰ and a reproducibility of 0.1‰ ŽRSD.. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Mass spectrometry; Zirconium; Hafnium; Chemical ratios;
)
Corresponding author. Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, UK. Fax: q441865-272072; e-mail: [email protected]
176
Hfr177 Hf
1. Introduction Concentration ratios of highly refractory element pairs with the same degree of incompatibility Žlow
0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 8 . 0 0 2 0 1 - 0
2
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
bulk crystal-liquid partition coefficient. during petrogenetic processes such as Zr–Hf are comparable to isotope ratios in serving as tracers of different mantle sources ŽHofmann et al., 1986.. Because these twin pairs are isovalent trace elements with very close ˚ Hf s 0.79 A; ˚ Whittaker and ionic radii ŽZr s 0.80 A, Muntus, 1970., they should display an extremely coherent behavior during igneous processes and retain their respective chondritic ratio. ZrrHf data on oceanic basalts obtained with a precision ranging from 1 to 5%, using X-ray fluorescence ŽXRF. for zirconium, instrumental neutron activation analysis ŽINAA. for hafnium or isotope dilution spark source mass spectrometry ŽID-SSMS. for both elements, do not show significant variations of the ZrrHf ratio. Data obtained on sixty nine mid-ocean ridge ŽMORB. and ocean island basalts ŽOIB. gave ZrrHf ratios equal to 36.6 " 2.9 which is similar to the chondritic value ŽZrrHf s 36.3; Jochum et al., 1986.. This suggests that the ZrrHf ratio is not fractionated between the different sources of oceanic basalts. We report here improved ZrrHf ratio measurements in order to assess its uniformity in oceanic basalts. Isotope dilution with a mixed spike of Zr and Hf in thermal ionization mass spectrometry ŽID-TIMS. and magnetic sector multiple-collector ICP-MS ŽIDMC-ICP-MS. are techniques capable of measuring concentration ratios ZrrHf with a high-precision. Previous ZrrHf ratio measurements in seawater samples have been performed by isotope dilution mass spectrometry with a precision of 3% ŽBoswell and Elderfield, 1988.. Precise Zr and Hf concentrations of around 1% and 4% ŽRSD. have already been measured by inductively coupled plasma mass spectrometer with a quadrupole mass analyser ŽID-ICPMS. where the precision is limited by the variation in plasma source transmission ŽXie and Kerrich, 1995.. Furthermore, the high sensitivity and precision of the MC-ICP-MS technique for Hf isotopic measurements allows determination of 176 Hfr177 Hf isotope ratios. Two major problems limit Hf and Zr analyses using thermal ionization mass spectrometry ŽTIMS.. First, the first ionization potentials of Hf and Zr are relatively high Ž IZr s 6.84 eV, I Hf s 7 eV; Handbook of Chemistry and Physics, 1987., resulting in poor ionization efficiencies for thermal ionization.
Second, because titanium is more abundant in geological samples Ž0.1% wt. for meteorites and 1–3% wt. for basalts. than zirconium and hafnium Žabout a hundred to a few ppm, respectively. with a slightly lower first ionization potential Ž I Ti s 6.82 eV., it tends to suppress the thermal ionization of hafnium and zirconium if present on the filament. In contrast to conventional TIMS, the magnetic sector-multiple collector ICP-MS exhibits better ionization for refractory elements such as zirconium and hafnium and matrix effects are by far less important. However, a potential difficulty with this technique is that the presence of large amounts of titanium may produce the clogging of the sample-cone orifice which significantly reduces the transmission of hafnium and zirconium and cause systematic drift in measured isotope ratios ŽBlichert-Toft et al., 1997.. Consequently, a complete separation of hafnium and zirconium from titanium and other elements, principally those interfering with the hafnium mass spectrum, is a prerequisite. Terrestrial and extraterrestrial samples are investigated by using a two-stage ion-exchange chromatography procedure. Conditions for the high-precision measurements of ZrrHf ratios by isotope-dilution in thermal ionisation mass spectrometry and magnetic sector multiple-collector ICP-MS are discussed as well as the internal precision and the reproducibility of ZrrHf and 176 Hfr177 Hf ratios.
2. Analytical procedure 2.1. Reagents and labware Hydrofluoric, hydrochloric, nitric acids and water were purified by subboiling distillation from Teflon and quartz stills. Sulphuric acid 98% and hydrogen peroxide 36% Žwrv. were commercial analytical grade reagents ŽMerck, Darmstadt, Germany; Carlo Erba Reactifs, France.. Whole rock dissolutions were performed in Savillex 026R PFA pressure vials ŽSavillex, Minnetonka, MN, USA. and Parr digestion bomb ŽParr Instrument, IL, USA.. For the chemical purification, two columns in polypropylene Ži.d.s 0.5 cm and 0.8 cm. were filled
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
with Bio-Rad AG1-X4 resin Ž200–400 mesh. in the chloride form to a height of 5 cm and with Bio-Rad AG1-X8 resin Ž200–400 mesh. in the chloride form to a height of 10.5 cm, respectively. Prior to sample loading, AG1-X4 and AG1-X8 anion-resins were washed twice with the following sequence of reagents: 4 N HFŽ10 ml. –H 2 OŽ5 ml. –6 N HCl Ž10 ml. and equilibrated with 5 ml 2 N HF and 10 ml 1 N H 2 SO4 , respectively. All the labware in contact with the samples were cleaned prior to use with a mixed solution containing commercial nitric Ž60%. and hydrofluoric Ž50%. acids, then high purity hydrochloric acid and finally double distilled water. All chemical preparations were conducted in an over-pressured clean-lab.
3
Table 1 Flow-chart for Hf extraction from a sample
2.2. Sample dissolution-matrix effects on the extraction of the IVth group elements Sample dissolution follows a modification of the standard procedure in HNO 3rHF acid mixtures of Minster and Allegre ` Ž1981.. For all rocks, the mixed tracer 96 Zr– 179 Hf is added to the powder. The mineral phases are decomposed in a 4.3-ml HFrHNO 3 Ž93r7 vrv. mixture in Savillex w vials at 1508C for 2 days. For materials containing zircons or other refractory minerals, the procedure was repeated on the residue obtained from this first stage. In the latter case, the PFA bomb is placed into a Parr digestion bomb and the dissolution is performed at 2208C for several days. To equilibrate the pressure within the two vessels, a solution HFrHNO 3 Ž93r7 vrv. is placed inside the Parr digestion bomb, outside the 026R vials. For meteorites, the second decomposition on the residue was performed in PFA vials at 1508C in order to aid solution of Zr and Hf. After the dissolution, the sample is treated following the procedure detailed in Table 1. The solution and the insoluble fluoride are separated by centrifugation. The precipitate is then washed ultrasonically twice with 5 ml H 2 O. All supernatants are combined and evaporated to dryness at 908C. The residue is suspended in 1 ml of 4 N HF, warmed and then centrifuged. The residue is then washed with 1 ml of H 2 O and 1 ml of 2 N HF. All supernatants are mixed together giving 3 ml solution, 2 N in hydrofluoric acid which is then loaded on the first-stage anion exchange column ŽAG1X4..
We used Sx and Rx to symbolize the supernatant and the residue obtained at each stage.
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
4
The dissolution scheme results in a fluoride precipitate containing the rare earth elements, alkaline earth elements and a supernatant with IVth group elements ŽZr, Hf and Ti. which form strong complexes with fluoride ions, transition elements ŽCr, Mn, Fe, Co, Ni, Zn. and also actinides ŽTh, Pa and U. ŽBlichert-Toft et al., 1997.. We have noticed that the matrix type had an effect on hafnium and zirconium extraction. A coprecipitation of hafnium and zirconium occurred with commercial calcium or magnesium fluoride while this coprecipitation does not occur from materials with a high silica content. Similar results already observed with protactinium indicated that calcium fluorides Žand those of magnesium to a lesser extent. are effective carriers for protactinium in the residue while the presence of fluosilicate complexes such as Na 2 SiF6 , K 2 SiF6 and BaSiF6 reduces the degree of coprecipitation of protactinium ŽD’yachova and Spitsyn, 1963.. The Hf and Zr yields of the dissolution and extraction procedures were calibrated using gamma counting with a GeŽLi. detector ŽPierre-Sue ¨ Laboratory of Atomic Energy Commission-Saclay.. For this work, we used the radionuclides 95 Zr and 181 Hf which were obtained from natural Zr and Hf irradiated with slow neutrons. The g-ray lines of 95 Zr and 181 Hf were 756.6 and 482.2 keV, respectively ŽAdams and Dams, 1969.. Using this procedure with an irradiated alkali basalt from Fangataufa ŽF49., Hf
and Zr yields were found to be of the order of 95% and 100%, respectively. 2.3. Chemical separations To measure ZrrHf ratios by mass spectrometry, two elution steps on anionic resins are employed Žmajor element and titanium separations—Table 2.. For the first stage, the major element separation is adapted from Minster and Allegre ` Ž1981.. The sample matrix ŽFe, Al and Mg. and residual traces of REE are removed with 4 ml of 2 N HF, followed by 1 ml of 0.5 N HFr0.5 N HCl while Hf, Zr and Ti are adsorbed on the resin as fluoride complexes and then eluted with 3 ml of 3 N HCl. After adding 0.2 ml concentrated sulphuric acid to the latter fraction, it is slowly evaporated in a clean airflow box. The solution is taken to fumes of sulfuric acid just before loading the solution on the second anionic column. This step removes fluoride ions and polynuclear species of hafnium and zirconium ŽStrelow and Bothma, 1967.. The procedure used to separate Ti from Zr and Hf on an anion exchange resin AG1X8 is adapted from Barovich et al. Ž1995.. Prior to loading, 0.46 ml of concentrated hydrogen peroxide and 13.37 ml of water are added to the sample. The solution obtained is: 1% H 2 O 2 –0.52 N H 2 SO4 . The hydrogen perox-
Table 2 Column specifications and Hf and Zr separation scheme Resin type
AG1-X4 200–400 mesh Žanion, Fy form.
AG1-X8 200–400 mesh Žanion, HSO4 y form.
Column Dimensions
Volume Žcm
5 = 0.5 cm
1
10.5 = 0.8 cm
5.28
Eluent
Volume Žcm3 .
Elements eluted
sample in 2 N HF
3Ž3 cv.
Bulk sample, Co, Ni, Cr, Zn and traces of REE
2 N HF 0.5 N HFr0.5 N HCl 3 N HCl sample in 1% H 2 O 2 r 0.52 N H 2 SO4 1% H 2 O 2 r0.52 N H 2 SO4 H 2O 1 N HFr1 N HCl 6 N HCl 6 N HCl
4Ž4 cv. 1Ž1 cv. 3Ž3 cv. 13.83 Ž2.6 cv.
3.
17Ž3.2 cv. 5Ž0.95 cv. 10Ž1.9 cv. 3Ž0.57 cv. 3Ž0.57 cv.
Hf with Ti, Zr
Ti
Hf with Zr
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
ide is used to prevent hydrolysis of titanium forming an orange brown colored complex. Titanium is eluted with 17 ml of 1% H 2 O 2 –0.52 N H 2 SO4 solution due to its weak affinity on the anionic resin in a H 2 SO4rH 2 O 2 solution Ž D - 0.5 for 0.3% H 2 O 2 –0.5 N H 2 SO4 ; Strelow and Bothma, 1967.. After the resin is rinsed with 5 ml of H 2 O, followed by 10 ml of 1 N HFr1 N HCl before eluting Hf and Zr with 3 ml of 6 N HCl. The typical elution curves of IVth group elements are illustrated in Fig. 1. In the procedure proposed by Barovich et al. Ž1995., zirconium and hafnium are eluted together with traces of titanium in a relatively large volume of sulphuric acid. In our case, the intermediate solution of 1 N HFr1 N HCl is used to remove traces of sulfuric acid without eluting zirconium and hafnium. The elution scheme proposed here allows zirconium and hafnium to be collected in a smaller volume Ž3 ml. compared to 10 ml obtained by Barovich et al. Ž1995.. 2.4. Mass spectrometry 2.4.1. VG 354 The ID-TIMS measurements were made on Re single filaments, using a modified VG 354 with five Faraday cups and a Daly detector.
5
In order to avoid the suppressor effect of zirconium on the ionization of hafnium generally encountered in TIMS when the both elements are not cleanly separated, the aliquot obtained after the second-stage anion exchange column separation must contain less than 5 mg of zirconium with 100–200 ng of hafnium ŽCorfu and Noble, 1992.. After the Hf–Zr residue dissolution in 1 ml of 0.1 N HF, the sample is loaded and dried on a zone-refined rhenium filament Ž25 mm Žthick. = 0.75 mm Žwide... A solution containing 20 mg of silica gel, 5 mg of boric acid with 30 mg of suspended rhenium metal and 15 mg of suspended boron metal Ž99.99% of purity for both metals. is then loaded. The addition of boron metal to the load has already been used to measure hafnium by TIMS ŽSalters and Hart, 1991.. The total load is dried on the filament with a current of 1.5 A. After drying, the load would sometimes chip off slightly from the filament. However, subsequent addition of silica gel usually cements the load to the filament. If the boron metal is used alone to improve the work function of rhenium filament, the latter would fuse at a temperature about 1600– 16508C. To avoid this effect, it is necessary to strengthen the filament with the addition of rhenium metal powder in the load.
Fig. 1. Elution scheme for the separation of titanium from zirconium and hafnium on AG1X8 column. The loading solution contains 40 mg of TiŽIV., 600 mg of 95 ZrŽIV. with the presence of 181 Hf traces in 0.52 N H 2 SO4r1% H 2 O 2 solution. The Ti yield was determined by using a semi-quantitative colorimetric method with a hydrogen peroxide solution diluted to 3%Žvrv.. Hafnium and zirconium are detected with 0.2% pyrocatechol violet to 0.2% by adjusting to pH 5 with 5 M ammonium acetate buffer. This method requires removal of fluoride ions by fuming H 2 SO4 to prevent decolorisation of the zirconiumŽhafnium.rpyrocatechol violet complex. The interference of titanium ions which react similarly with pyrocatechol violet is eliminated by adding EDTA to the solution.
6
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
After introduction into the mass spectrometer, a three-step heating filament procedure is employed. During the first stage the filament is heated to 4 A in 40 min, silica gel and boric acid are evaporated. The current is then raised to 4.5 A in 50 min where the Zrq beam can be focused on Daly detector. Finally, the filament current is increased at a rate of 100 mA per min until a stable zirconium ion beam is observed on collector Žthe optimal ionization temperature is around 19008C Ž I s 5–5.2 A... Zirconium acquisition is performed using a single Faraday cup. The magnetic field is switched by the computer through the following sequence 90–90.5– 91–94–96–98 where peaks at masses 90 and 90.5 are the reference peak and baseline, respectively. Zirconium isotope ratio measurements for spiked samples were performed by TIMS with 90 Zr signals of 0.3 = 10y1 1 A on average while 187 Re ranges from 0.006 = 10y1 1 A to 0.070 = 10y1 1 A. Because Hfq emission occurs at a higher temperature, a Hfq beam appears on the Daly detector after the data collection of zirconium. Data acquisition on the Daly detector can be obtained at this temperature or with higher current Ž I s 5.6 A.. A similar procedure for hafnium ratio measurements is adopted for masses 175.5–176–177–178–179–180 with the background at 175.5. Hafnium isotope ratios were measured on Daly detector with 179 Hf beams ranging from 0.0065 = 10y1 1 A to 0.015 = 10y1 1 A. The 187 Re signals obtained at the beginning of the run were of the order of 0.1 = 10y1 1 –0.2 = 10y1 1 A. The integration time is 4 s for zirconium and hafnium peaks and the baseline. During a normal run, 160 ratios in 20 blocks are collected. By using a mixed standard solution of hafnium and zirconium ŽHf 1 mg–Zr 5 mg. for which the ZrrHf ratio does not reflect that of the typical sample, we noticed that the presence of Zr did not have deleterious effect on in-run precision of hafnium isotope ratios and hafnium sensitivity. Two data acquisitions of 10 blocks have resulted in 179 Hfr 178 Hf ratios measurements with internal precisions of 0.3‰ for 1 mg of Hf and 0.4‰ for 1 mg of Hf with 5 mg of Zr. The Mo isobaric interferences on the Zr mass spectrum at masses 92, 94 and 96 are estimated by measuring 98 Mor90 Zr ratio. All potential interferences such as Lu, Yb, W and Ta for the Hf mass
spectrum were not observed. On a few occasions, we observed the appearance of interferences on 178 Hf and 180 Hf due to rhenium ions Ž‘ghost peaks’. diffused by the shielding plate and could be avoided by setting the shielding plate potential outside the interval where they occur. The isotope ratios 96 Zrr90 Zr and 179 Hfr178 Hf used in isotopic dilution have been corrected for mass discrimination using a linear law and the spike-sample mixing equation. As fractionation was always less than 0.5% per amu, this procedure is valid to the 10y5 level of precision.
2.4.2. Plasma 54 The measurements by ID-MC-ICP-MS were performed on a plasma source mass spectrometer wmagnetic sector multiple-collector inductively coupled plasma mass spectrometer ŽEcole Normale Superieure ´ de Lyon.x equipped with nine Faraday cups and a Daly detector with ion counting. Detailed descriptions of the instrumentation can be found in different publications ŽWalder and Freedman, 1992; Walder et al., 1993; Halliday et al., 1995; Blichert-Toft et al., 1997.. Because zirconium and hafnium compounds in aqueous solution are characterized by their high degree of hydrolysis and adsorption, and by their tendency to form various complex ions and polymer compounds, it is recommended that a special procedure is used ŽElinson and Petrov, 1969.. In our case, the aliquot obtained after the chemical purification and containing approximatively 1 mg of hafnium with zirconium was evaporated to dryness with 10 ml of 1 M H 2 C 2 O4 . This procedure allows storage of zirconium and hafnium in the form of oxalates which are easily soluble in water. Furthermore, the formation of strong complexes between oxalate ions and zirconium and hafnium is used to stabilize zirconium and hafnium in aqueous solution. We also noticed that an oxalic acid solution having a concentration of 10y3 M, similar to that of running solutions, does not produce interfering species on the hafnium and zirconium mass spectra ŽFig. 2.. Sample solutions Ž c s 500 ppb–1 ppm. were prepared on the day of analysis by dissolution in 0.5–1 ml of 0.05 N HNO 3 . According to the procedure detailed by BlichertToft et al. Ž1997., sample solutions were introduced
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
7
94 and 96 were corrected by monitoring the 95 Zrr Zr ratio. Potential interferences on Hf mass spectra coming from Lu and Yb at masses 176, from Ta and W at masses 180 but also from diatomic species such as Ba40Ar, La40Ar, Ce40Ar, Dy16 O, Er16 O observed at masses 176, 177, 178, 179 and 180 ŽXie and Kerrich, 1995. have never been detected due to the Hf purification prior to sample analysis. The presence of memory effects generally encountered in plasma source mass spectrometry are not observed on Hf and Zr mass spectra due to washing between each sample with: 0.05% HF then 3% HNO 3 . Mass fractionation is corrected according to an exponential law ŽRussell et al., 1978. where the mass fractionation correction factor deduced from standard measurements is generally f s y2 ŽBlichert-Toft et al., 1997.. For each run, 2 s screening of the data is done. The high sensitivity of the magnetic sector-multiple collector ICP-MS in Lyon w0.01 nA for a 50 ppb Hf solution; Blichert-Toft et al., 1997. for Hf isotopic allows us to measure Hf and Zr isotope ratios with total Hf and Zr signals ranging from 10 = 10y1 1 A to 15 = 10y1 1 A for 500 ppb–1 ppm solutions. This is a sensitivity a factor of 10 higher than that obtained in the best TIMS experiments with 1 mg of hafnium ŽPatchett and Tatsumoto, 1980; Salters and Hart, 1991; Corfu and Noble, 1992.. 90
Fig. 2. Spectra for H 2 C 2 O4 Ž cs10y3 M. blank solution on Zr and Hf mass ranges.
into the plasma using the free aspiration mode. With the plasma source configuration proposed in Lyon which associates the glass-expansion nebulizer with the Fassel type spray chamber and capillary teflon tubing, the natural solution uptake rate in the freerunning mode of operation is typically 50 mlrmin ŽP. Telouk, manuscript in preparation.. Measurements were performed in the static mode. The Faraday detectors are corrected for differences in ion-collection efficiency at the beginning of each day with standard solutions. Generally, cup efficiency factors are calculated relative to the values recommended for the isotopic composition of Hf ŽStevenson and Patchett, 1990. and for that of Zr ŽMinster and Ricard, 1981. reported in Table 3. During hafnium and zirconium data acquisition, 60 and 40 ratios are collected in 20 and 12 min, respectively. The baseline is the average of the measurement at y0.5 and q0.5 amu for each analysed masses. The interferences from Mo on the Zr mass spectra at masses 92,
Table 3 Isotopic composition of Hf and Zr in mixed spikes and standard solutions Solution
Hafnium ratios 176r178 177r178 179r178 180r178
Spike
Normal
Plasma 54
TIMSrPlasma 54
0.052329"44 0.339746"04 26.23247"32 2.564253"35
0.192316 0.681585 0.499261 1.285924
a
Zirconium ratios b 91r90 0.188432"20 92r90 0.286442"21 94r90 0.397611"18 96r90 4.927253"69 a
0.21799 0.33338 0.3381 0.05439
Stevenson and Patchett Ž1990., Blichert-Toft et al. Ž1997.. Minster and Ricard Ž1981.. Errors for enriched isotopic ratios are in-run precision Ž2 s ..
b
8
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
2.5. Hf and Zr concentrations and isotopic compositions of double spikes
Table 4 Determination of Hf and Zr concentrations in mixed spikes 179 Hf solutions by isotope dilution mass spectrometry
Reference solutions of these two elements were purchased from commercial sources. Although specifications of commercial standard solutions are within a few permille there is not always an overlap of uncertainties with solutions from the different manufacturers. This bias can reach eight permille. We preferred therefore to use a common calibration procedure for both metals to warrant a precise knowledge of the ZrrHf ratio relative to possible systematic biases due to uncontrolled impurities in pure commercial metals. Standard solutions used to calibrate the mixed spike 96 Zr– 179 Hf by isotope dilution mass spectrometry were obtained from Carlo Erba wCG HF1 for Hf Ž1007 ppm., CG ZR1 for Zr Ž1004 ppm.x and National Institute of Standards and Technology wSRM 3122 for Hf Ž9363 ppm with a density equal to 1.068., SRM 3169 for Zr Ž9479 ppm with a density equal to 1.055.x. Standard Hf and Zr concentrations were determined by direct titration with 0.01 M EDTA solution using 0.1% Xylenol Orange in aqueous solution as colorimetric reagent ŽPilkington and Wilson, 1965.. Preliminary titration of EDTA concentration was determined with a zinc solution prepared gravimetrically from pure zinc metal of 99.99% purity and a 0.1% Xylenol Orange. The red zinc–xylenol orange complex formed at pH s 4 in 7 M ammonium acetate solution is titrated with EDTA to the first permanent lemon yellow color. The average concentrations obtained after duplicate titrations are: NIST—Hf s 9342 ppm Ž9342– 9342 ppm., Zr s 9565 ppm Ž9560–9569 ppm.; Carlo Erba—Hf s 1013 ppm Ž1012–1014 ppm., Zr s 1011 ppm Ž1010–1012 ppm.. There is a good agreement between our values and the certified concentrations, except for NIST Zr concentration for which a difference of eight permille is observed with the certified value. Although some precautions have been taken to prevent the hydrolysis of Zr and Hf in solution Žmoderately acid solutions, heated in the pH 0–1.5 region., this inconsistency can only be explained by minor but significant loss of solvent by evaporation. In order to determine the hafnium and zirconium concentrations of the two mixed spike solutions by isotope dilution mass spectrometry, we have gravi-
Standard
No. Double spike 1 Double spike 2 replicate Zr Žppm. Hf Žppm. Zr Žppm. Hf Žppm.
Carlo Erba
1 2 3 4 1 2
NBS Average error
5.349 5.345 5.345 5.349 5.348 5.347 0.002
0.557 0.556 0.556 0.559 0.560 0.560 0.558 0.002
5.011 5.007 5.008 5.011 5.011 5.009 0.002
96
Zr–
0.581 0.58 0.58 0.583 0.585 0.584 0.582 0.002
All errors are one standard deviation.
metrically prepared different mixed solutions containing standard solutions from Carlo Erba and NIST. Average mixed spike hafnium and zirconium concentrations shown in Table 4, and are known with a precision of 3.5‰ and 0.4‰, respectively. The isotopic composition of the mixed spike 96 Zr– 179 Hf obtained by plasma source mass spectrometry is reported in Table 3. With reference to the high stability of the instrumental fractionation encountered in plasma source mass spectrometry, zirconium and hafnium isotope ratios are corrected for this fractionation within 1‰.
3. Results and discussion 3.1. RecoÕery and procedural blanks Total Hf and Zr yields have been determined by measuring zirconium and hafnium concentrations of the alkali basalt F49 from Fangataufa by isotope dilution. Total Hf and Zr recoveries for whole rocks through the chemistry procedure were 98% and 90%, respectively. For Hf-rich samples such as basalts, continental rocks and sediments, total procedural chemistry blanks were about 260–350 pg for hafnium and 6 ng for zirconium. For Hf-poor samples such as meteorites, the blank level was 160 pg for hafnium and 2.4 ng for zirconium. In all cases, blank levels are a factor of 10,000 and 1000 lower than the amount of Hf and Zr in Hf-rich and poor samples, respectively. Previous works have shown that Hf contamination is principally related to hydrofluoric acid with a minor
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
contribution carried by sulphuric acid ŽPatchett and Tatsumoto, 1980; Barovich et al., 1995.. The contamination originating from the ion exchange columns and FEP beakers was significantly reduced by alternating cleaning with dilute hydrofluoric acid and hydrochloric acid ŽPatchett and Tatsumoto, 1980; Minster and Allegre, 1981.. ` 3.2. Zr r Hf and
17 6
9
3.3. Analytical reproducibility and accuracy Reproducibility of ZrrHf and 176 Hfr177 Hf ratio measurements by isotope dilution mass spectrometry Ždefined as relative standard deviation of the mean or RSD and calculated from the standard deviation or 2 s . were determined with replicate analyses of the alkali basalt F49 from Fangataufa. Five aliquots were decomposed in HF–HNO 3 while the other three were decomposed in HF–HBr. The data reported in Table 5 show that both dissolution schemes give similar reproducibility for ZrrHf ratio measurements. The reproducibility of the ID-TIMS technique deduced from results of six replicate analyses wZrrHf s 40.06 " 0.2 Ž2 s .x is estimated at 2.5‰ ŽFig. 3.. Furthermore, ZrrHf ratio measurements of two aliquots made by ID-MC-ICPMS are consistent within analytical uncertainty with those obtained with ID-TIMS ŽFig. 3.. Seven aliquots analysed by ID-MC-ICP-MS yielded an average 176 Hfr177 Hf value of 0.282995 " 0.000052 Ž2 s . ŽFig. 4.. The accuracy of the analytical results obtained in this study was investigated by analyses of two well defined geological standard reference materials, the basalt BE-N and the granite AC-E distributed by the International Working Group and by comparison to results of other laboratories employing different analytical techniques. Although isotope dilution mass spectrometry is judged to be an analytical technique of high accuracy, we compared zirconium and hafnium concentration measurements to those obtained with INAA, XRF and ID-SSMS reported in
Hf r17 7Hf internal precision
When mixed Zr–Hf spikes are used, the precision of ZrrHf ratio is independent of sample and spike weights. The ZrrHf internal precision is only function of zirconium and hafnium isotope ratios in samples, mixed spike and spiked samples. The 179 Hfr178 Hf and 96 Zrr90 Zr ratios were measured in TIMS with an internal precision between 0.2–2‰ for hafnium and 0.4–0.7‰ for zirconium. The precision of 179 Hfr178 Hf ratios obtained by multiple collector ICP-MS were between 0.02– 0.06‰ and 0.1–0.7‰ for 96 Zrr90 Zr ratios. Therefore, ZrrHf ratio measurements by TIMS have an in-run precision ranging from 0.6‰ to 2‰ and for the magnetic sector multiple collector ICP-MS from 0.2‰ to 1‰ Moreover, the high precision of Hf isotope ratios obtained with the Plasma 54 allows us to measure 176 Hfr177 Hf ratios from spiked samples analyses with an internal precision ranging from 0.04‰ to 0.1‰. The relation used to determine the uncertainty on 176 Hfr177 Hf ratio measurements includes the correction for the spike contribution using isotope ratios corrected for mass fractionation and associated internal precision. These isotope ratios are known with an in-run precision better than 0.1‰.
Table 5 Zr, Hf concentrations and ZrrHf measurements of the alkali basalt F49 from Fangataufa by ID-TIMS and ID-MC-ICP-MS Techniques
Procedure of dissolution
No. aliquot
Zr Žppm.
Hf Žppm.
ZrrHf
VG 54
HFrHNO3
1 2 3 4 5 6 all aliquots
177.78 175.65 176.92 176.17 175.37 175.78
4.426 4.392 4.405 4.405 4.372 4.401
40.17 39.99 40.16 39.99 40.11 39.94
HFrHBr
Plasma 54 a b
HFrHNO3
3 7
b
Errors are two times the standard deviation Ž2 s .. Duplicate measurement on the same aliquot.
176.88 176.21
4.409 4.388
40.12 40.16
Average
Error a
40.11
0.20
40.01
0.17
40.06
0.20
10
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
Fig. 3. ZrrHf ratio measurements performed by ID-MS ŽTIMS and MC-ICP-MS.. Error bars for each measurement represent the in-run precision Ž2 s ..
Table 6. These techniques do not give Zr and Hf concentrations with a better precision but are adequate methods to measure both elements on the same aliquot. We avoided comparison of ZrrHf ratio measurements by isotope dilution mass spectrometry with Zr ŽXRF.rHf ŽINAA. ratios, as the latter are ob-
tained by two different analytical techniques. The average Hf and Zr concentrations for BE-N ŽHf: 5.53 " 0.3Ž1 s .; Zr: 270.22 " 35.51Ž1 s .; Govindaraju and Roelandts, 1993. compiled from 17 and 32 values obtained by INAA and XRF, respectively, agree within 1 s with our results. The zirconium
Fig. 4. 176 Hfr177 Hf ratio obtained from ZrrHf ratio measurements by ID-MC-ICP-MS. Error bars for each measurement represent the in-run precision Ž2 s ..
K. DaÕid et al.r Chemical Geology 157 (1999) 1–12
11
Table 6 Comparison of results by Plasma 54, INAA and XRF for Hf and Zr in geochemical reference samples BE-N and AC-E Reference material Hf Žppm.
BE-N Žbasalt. AC-E a Žgranite.
Plasma 54
INAA
This work
Govindaraju and Roelandts Ž1993.
5.841 28.014 28.011
5.53 " 0.3Ž1 s .
Govindaraju Ž1995.
ID-SSMS
INAA
Jochum et al. Ž1990.
Korotev Ž1996.
5.37 27.9 " 3.4Ž1 s .
5.85 28.7
Zr Žppm.
BE-N AC-E a
Plasma 54 b
XRF
This work
Govindaraju and Roelandts Ž1993.
278.45 " 0.25 831.87 " 0.38 836.17 " 1.51
270.22 " 35.51Ž1 s .
Govindaraju Ž1995.
ID-SSMS
XRF
Jochum et al. Ž1990.
Stork et al. Ž1987.
270
278
780 " 81Ž1 s .
a
Duplicate analysis. Errors for Zr concentrations measured by Plasma 54 are in-run precision Ž2 s .. The Hf concentrations are determined with in-run precision ranging from 0.02‰ to 0.06‰. All other errors are one standard deviation. Data determined by isotope dilution-spark-source mass spectrometry are given with precision 2–5% relative ŽJochum et al., 1990.. b
concentrations determined by XRF and ID-SSMS confirm the accuracy of our measurements performed by isotope dilution mass spectrometry. Similarly, we can observe a good agreement between our Hf concentrations and the compiled values, except for that obtained by ID-SSMS with a difference of 8%. Both reference rocks determined by INAA are in excellent agreement with our data ŽKorotev, 1996.. Unfortunately, because a data compilation for AC-E obtained only by INAA and XRF does not exist, we have compared Zr concentrations of our two replicates with recommended data, i.e., average Zr concentrations obtained by several techniques including the XRF wZr: 780 " 81 Ž1 s . Ž n s 79. ŽGovindaraju, 1995.. In this case, good agreement within 1 s is obtained for AC-E.
4. Conclusion The results presented here demonstrate that ZrrHf ratios measured by isotope dilution mass spectrometry Žthermal ionization mass spectrometry and magnetic sector-multiple collector ICP-MS. are improved in precision by a factor of at least 10 relative to other methods.
The analytical procedure has been applied to a wide variety of geological samples ranging from Hf-poor materials Žmeteorites. to Hf-rich rocks Žbasalts, continental rocks and sediments., the results of which will be presented elsewhere ŽDavid et al., in preparation.. During this investigation the separation scheme proved reliable.
Acknowledgements The authors wish to thank Michel Treuil of the Earth Sciences department of Pierre Sue ¨ laboratory ŽAtomic Energy Commission-Saclay. for permitting us to use the ion counting for chemistry calibration. Jean-Louis Joron is thanked for his help during the first part of this work. We are grateful to Francis Albarede ` for giving us the opportunity to use the Plasma 54 at ‘Ecole Normale Superieure’ of Lyon to ´ measure Hf isotope ratios in terrestrial and extraterrestrial samples. We would like to thank Pierre Schiano in particular for his constructive help and interest in this study. Constructive reviews by V.J.M. Salters and M.H. Thirlwall have greatly improved the manuscript. [MB]
12
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