- Email: [email protected]
A method for 231Pa analysis by thermal ionization mass spectrometry in silicate rocks
Chemical Geology 157 Ž1999. 147–151
A method for 231 Pa analysis by thermal ionization mass spectrometry in silicate rocks Bernard Bourdon a
a,)
, Jean-Louis Joron
b,1
, Claude J. Allegre `
a
Laboratoire de Geochimie et Cosmochimie, CNRS-UMR 7579, Institut de Physique du Globe de Paris T14-24, 4 Place Jussieu, 75252 ´ Paris Cedex 05, France b Laboratoire Pierre Sue, ¨ CE r Saclay, 91191 Gif-sur-YÕette, France Received 18 June 1998; revised 2 December 1998; accepted 2 December 1998
Abstract We describe a new technique for analyzing 231 Pa in silicate rocks by isotope dilution mass spectrometry. This technique is different from the technique developed by previous studies ŽPickett et al., 1994. and also permits the determination of down to a 100 fg of 231 Pa with a 1–2% uncertainty at the 2 s level. While its performance is comparable to the LANL technique ŽPickett et al., 1994., a number of modifications may help extend the application of 231 Pa– 235 U to a wider number of geochemistry labs: Ž1. this technique does not require 237 Np, a hazardous a-emitter for 233 Pa spike preparation, Ž2. Pa is run as a double-oxide on a tungsten filament with a sensitivity comparable to the metal method. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Protactinium; Mass spectrometry; Isotope dilution
1. Introduction In the recent years, the analysis of 231 Pa– 235 U disequilibrium by mass spectrometry has been more and more widely used in the field of volcanology and mantle geochemistry ŽGoldstein et al., 1993, 1994; Lundstrom et al., 1995; Pickett and Murrell, 1997. as well as quaternary geochronology and climate studies ŽEdwards et al., 1997; Edmonds et al., 1998.. However, the number of studies including Pa data measured by mass spectrometry is still limited. This could be related to the difficulty in implement-
ing the chemical separation of Pa from natural samples or by difficulties in obtaining 233 Pa spike. In this paper, we present an alternative technique to what has been presented by Pickett et al. Ž1994.. Our technique offer similar performances to the Pickett technique but our different approach may increase the availability of Pa analysis by ID-TIMS to a greater number of geochemical laboratories.
2. Analytical methodology 2.1. Spike preparation
) Corresponding author. Fax: q33-1-442-737-52; e-mail: [email protected] 1 E-mail: [email protected].
The tion of
233 232
Pa spike was prepared by neutron activaTh in the Orphee ´ reactor in Saclay. 233 Th
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 1 9 8 - 3
148
B. Bourdon et al.r Chemical Geology 157 (1999) 147–151
produced by neutron capture then decays to 233 Pa with a half-life of 22.3 mn. Microgram quantities of 232 Th were deposited on Al foil and then irradiated by thermal neutrons Ž1.6 = 10 13 n P cmy2 P sy1 . for an hour. Following the irradiation, the sample was left for a week to let the short-lived nuclides produced during the irradiation Ž24 Na, T s 15 h, 28Al, T s 2.5 mn. decay. The Al foil was then leached with 9 N HCl and the leach solution was dried down. Consequently, the Pa solution was separated from Al and other impurities on a 2 ml column filled with AG1X8 200–400 mesh. 233 Pa was then fixed on the column while Al and Th were eluted. After washing with 6 cv Žcolumn volume. of 9N HCl, Pa is eluted with 2 cv of 9 N HCl q 0.1 N HF. The elution solution contained fluoride ions which stabilizes Pa and prevents its hydrolysis ŽGuillaumont et al., 1968.. A solution containing approximately 1 pg P gy1 of 233 Pa was produced every 6 months for mass spectrometric analysis. The solution was checked for purity on a GeŽLi. g-counter and no other nuclides were detected. The 231 Par233 Pa ratio in the spike was measured and found to be - 7 = 10y4 at the time of the measurements, which will ultimately not affect the calculations of 231 Pa concentration in samples. The preparation of the spike solution is in general very fast Žhalf a day. and provided access to a nuclear reactor is available, is more convenient than the technique used by Pickett et al. Ž1994. where repeated purification steps are required to eliminate all 237 Np in the final 233 Pa spike solution. Because of the absence of 237 Np, there is also no requirement to recalibrate the spike with time. It is essentially similar to the technique used by Anderson and Fleer ŽAnderson and Fleer, 1982. for producing 233 Pa.
2.2. Sample digestion Prior to dissolution, 233 Pa spike solution was added to the sample powder to yield a 231 Par233 Pa ratio of about 1–3. About 0.5 to 2 g of powdered sample depending on sample size was digested with 10 ml of concentrated HF to which a few drops of concentrated HNO 3 were added. The mixture was left overnight in a closed Savillex beaker at a tem-
perature of 100–1108C. After evaporation, it was redissolved in 20 ml of a 9 N HCl–H 3 BO 3 mixture. The addition of boric acid avoided the formation of insoluble fluorides that would have lead to Pa loss. We have not used any perchloric acid which requires especially designed fume hoods. Any excess boric acid was discarded by centrifugation prior to column loading. Radiotracer studies with 233 Pa had shown that no significant amount of Pa Ž- 1%. is lost at this stage. The amount of boric acid to be added depended on the chemical composition of the rocks that had been dissolved and was calculated to fully complex the available fluoride ions.
2.3. Separation and purification of Pa The 9 N HCl solution Ž2 cv. containing the sample was then loaded on a 20 ml anion resin column ŽAG1-X8, 200–400 mesh.. During this first stage, Fe and U are retained on the column while Pa was eluted in 2 cv of 9 N HCl q 0.1 N HF. The fraction containing Pa was then evaporated to about half of its initial volume and could be directly loaded on a 2 ml TRU spece column. About 0.5 ml of saturated boric acid solution were added to complex fluoride ions at this stage. Avoiding to dry down the solution prevented Pa loss by hydrolysis. The column was washed with 9 N HCl Ž2 cv., HCl 8 N Ž6 cv. prior to Pa elution in 2 N HF Ž2 cv.. The Pa fraction was then further purified on a 1 ml AG1-X8 resin similar to what had been used by Pickett et al. Ž1994.. The sample was loaded with 2 cv, washed with 6 cv of 9 N HCl, 13 cv of 8 N HCl and 2.5 cv of 7.5 N HNO 3 before Pa elution with 9 N HCl q 0.1 N HF Ž2 cv.. The last HNO 3 step washed efficiently any remaining Sc. The same solution was loaded on a final 1 ml TRU spece column without having to dry down the solution. Saturated H 3 BO 3 was added to the solution to complex any free fluoride before loading onto the TRU spece column. After loading, the column was washed with 4 cv of 4 N HCl and Pa is eluted with 2 cv of 2 N HF. This column separated U from Pa during the 4 N HCl wash. This last step was very fast and removed organics that were found after the third column. An overall view of the chemical separation is shown on Fig. 1.
B. Bourdon et al.r Chemical Geology 157 (1999) 147–151
Fig. 1. Schematic view of the technique used for separation and purification of Pa.
This chemical separation was calibrated by using a basaltic rock that had been irradiated by a neutron flux. The chemical yields for Pa were controlled by g-spectrometry on a GeŽLi. detector and were found to be around 80%. In order to further improve the chemical separation of Ta and Hf from Pa, the digestion mixture was also doped with neutronactivated tracer solutions of those elements. The separation factor for these elements was thus estimated to be better than 10 4 . The chemistry yields for Pa were routinely checked on a NaI detector. Total procedural blanks were also estimated to - 0.3 fg, which was well below the amount of Pa loaded on the filament Ž100 to 1000 fg.. 2.4. Mass spectrometry The clean Pa fraction was then redissolved in about 1 ml of concentrated HF and loaded on a W
149
filament followed by 3 ml of silica-gel Ž12.5 mg P mly1 of SiO 2 .. Prior to loading, the W filaments Ž0.025 mm = 0.75 mm. were outgassed for several hours with a applied 2 kV voltage with a 4 A DC current. Measurements were made on a FINNIGAN MAT 262 equipped with an ETP electron multiplier connected to an ion counting system. The sample was slowly preheated for about 1 h and brought to a temperature of about 1350–14008C. Pa was ionized as a double oxide as described by Hall and Walter Ž1956.. About 100–1000 cps on the 231 PaOq 2 peak were obtained for about 1 to 2 h with several hundred femtograms of Pa. The overall ionization efficiency Ž2–5‰. was similar to what was obtained by Pickett et al. Ž1994.. The 263 and 265 masses were measured by peak switching for five cycles with 8 s integration times. The ion yields were a factor of ten greater with silica-gel compared to graphite loaded on W. The oxygen correction from 231 Pa16 O18 O and 231 Pa17O 2 on mass 265 amounts to f 4‰. A 5% variation in the oxygen isotope composition only affected the final result by - 1‰. Isobaric interferences reaching initially thousands of counts per second around mass 263 amu rapidly decayed to less than 1 cps above 2.4 A ŽT ; 12008C. after 1–1.5 h of preheating. The baseline around PaO 2 masses were monitored before and after the runs. Baseline at 263.5 amu was also measured during the mass spectrometric runs and the 263.5:265 ratio was found to be less than 5 = 10y4 , corresponding to approximately 0.05 " 5 cps for a 100 cps signal at mass 265. As 233 Pa decays to 233 U, there may be an isobaric contribution of 233 U on the 265 amu peak. The Pa measurements were done less than 3 days after the final UrPa separation on the TRU spece column. Additionally, the emission of U occurred at a lower temperature than Pa and monitoring of 238 U generally showed less counts than 231 Pa Ž10–50 cps.. To check for a possible contribution of 233 U on the 265 amu peak, we ran 238 U– 231 Pa mixture and showed that the measured 231 Par238 U ratio normalized to the actual ratio ranged from 400 to 1200 at running temperatures. For a measurement that would take place 3 days after the last UrPa separation, the contribution of 233 U on the 265 amu peak would then be less than 10y4 . Estimated 2 s uncertainties for Ž231 Par235 U. were 1–2%. Mass fractionation
B. Bourdon et al.r Chemical Geology 157 (1999) 147–151
150
Table 1 Pa measurements by TIMS in volcanic rocks
231
Karthala 85KA08 85KA08R Hekla. Iceland Hek-8) Hek-8-1 Hek-8-2
Age
Ž231 Par235 U.
"2 s
Ž230 Thr238 U.
Th ppm
U ppm
Pa Žfg gy1 .
1918 1918
1.899 1.890
0.026 0.026
1.444 –
4.37 –
1.11 –
691 688
1.191 1.205 1.210
0.012 0.009 0.010
1.10 " 2 – –
7.74 – –
2.32 – –
900 901 905
)Data from Pickett and Murrell Ž1997.. Uncertainties are 2 s error bars of the mean estimated based on all the sources of uncertainties Žsee text.. Activity ratios are calculated using l235 s 9.8485 = 10y1 0 ay1 and l231 s 2.116 = 10y5 ay1 . Replicate from powder of 85KA08.
obtained for U samples in similar running conditions ŽC. Gopel, pers. comm.. amounts to 1.5‰ per amu. ¨ If one takes a similar figure for Pa, it will not greatly affect the measured ratios, given the overall uncertainty of 1–2%. It is difficult to give a more precise assessment of mass fractionation of Pa isotope during a run because the external reproducibility of a given block is higher than the predicted effect of mass fractionation. Reproducibility was tested by replicate analysis of a Grande Comore basalt and a Icelandic rock ŽHek8. as shown on Table 1. Fig. 2 shows data obtained for a typical run of a basaltic sample containing approximately 180 fg of 231 Pa.
Fig. 2. Measured 231 PaO 2 r233 PaO 2 during a typical mass spectrometer run. The mean value is shown by the solid line while the dashed lines indicate the 2 s error bar on the mean.
3. Standard measurements and spike calibration The 233 Pa spike Žhalf life s 26.967 days. was calibrated by replicate analysis of a 1-Ma old rhyolitic glass from Long Valley ŽLV18. given to us by G. Davies. This glass was dated by 87 Rb– 87 Sr and yielded an age of 1 Ma ŽG. Davies, pers. comm... For the purpose of this study, it was assumed that 231 Pa and 235 U are in secular equilibrium in this rock. As a way of cross-calibrating our rock standard, we also analyzed an Icelandic dacite ŽHek8. that had already been analyzed by D. Pickett at LANL ŽPickett and Murrell, 1997.. To calculate the 231 Par235 U ratio, we used the U concentration given by Pickett and Murrell Ž1997. assuming that the powder was homogeneous. To calculate the uncertainty on Ž231 Par235 U., we took an uncertainty of 1% for the U concentration. The calculated Ž231 Par235 U. agrees with the value given by D. Pickett to better than 1% at the 2 s level ŽSee Table 1.. The two results given in Table 1 which are replicate from powder also agree within error. Our data set is then both internally consistent and consistent with the Los Alamos data. A statistical test for comparing the mean values of 231 Par235 U for Hek8 shows that there is only a 5% risk that the our results and Pickett’s results are distinct. Table 1 also gives replicate analysis for two alkali basalts from the Grande Comore Island, which shows that our external reproducibility including uncertainties in U concentrations is about 1%. If we assume a conservative
B. Bourdon et al.r Chemical Geology 157 (1999) 147–151
151
1% uncertainty on the U concentrations, the overall uncertainty obtained by error propagation in the Ž231 Par235 U. ratios for this sample is 1.4%.
and an anonymous reviewer have greatly helped improving the manuscript. This is IPGP contribution a1570. [NA]
4. Conclusions
References
The first technique using mass spectrometry for analyzing 231 Pa was published by Pickett et al. Ž1994.. The technique we developed here for 231 Pa analysis by ID-TIMS is largely modified from the original technique. Firstly, we do not use the aemitter 237 Np for producing the 233 Pa spike. This is significant because handling of 237 Np requires lab facilities that are especially designed for such purposes. Our technique only involves 233 Pa which represents a much smaller radiological hazard. Then our technique based on column separation rather than solvent extraction using the actinide-specific resin TRU spec from EiChrome. Lastly, we analyze Pa as a double oxide on a tungsten filament. The sensitivity we obtain is comparable to what was claimed by Pickett et al. Ž1994.. The use of tungsten as a substitute for Re may be useful at labs where RerOs are also analyzed on the same mass spectrometer by negative ions.
Acknowledgements Discussions with Gerard Manhes, ´ ` Jean-Louis Birck and Tim Elliott were helpful. Olgeir Sigmarsson and Gareth Davies are thanked for providing samples. Careful reviews by S. Goldstein ŽLANL.
Anderson, R.F., Fleer, A.P., 1982. Determination of natural actinides and plutonium in marine particulate material. Anal. Chem. 54, 1142–1147. Edmonds, H.N., Moran, S.B., Hoff, J.A., Smith, J.N., Edwards, R.L., 1998. Protactinium-231 and Thorium-230 abundances and high scavenging rates in the Western Arctic Ocean. Science 280, 405–407. Edwards, R.L., Cheng, H., Murrell, M.T., Goldstein, S.J., 1997. Protactinium 231 dating of carbonates by thermal ionization mass spectrometry: implications for Quaternary climate change. Science 276, 782–785. Goldstein, S.J., Murrell, M.T., Williams, R.W., 1993. 231 Pa and 230 Th chronology of mid-ocean ridge basalts. Earth Planet. Sci. Lett. 115, 151–159. Goldstein, S.J., Perfit, M.R., Batiza, R., Fornari, D.J., Murrell, M.T., 1994. Off-axis volcanism at the East Pacific Rise detected by uranium-series dating of basalts. Nature 367, 157– 159. Guillaumont, R., Bouissieres, G., Muxart, R., 1968. Chimie du ` Protactinium. I: Solutions aqueuses de protactinium penta- et tetravalent. Actinides Rev. 1, 135–163. ´ Hall, G.R., Walter, A.J., 1956. The mass spectrometry of the actinide elements. Can. J. Chem. 34, 246–252. Lundstrom, C.C., Gill, J., Williams, Q., Perfit, M.R., 1995. Mantle melting and basalt extraction by equilibrium porous flow. Science 270, 1958–1961. Pickett, D.A., Murrell, M.T., 1997. Observations of 231 Par235 U disequilibrium in volcanic rocks. Earth Planet. Sci. Lett. 148, 259–271. Pickett, D.A., Murrell, M.T., Williams, R.W., 1994. Determination of femtogram quantities of protactinium in geological samples by thermal ionization mass spectrometry. Anal. Chem. 66, 1044–1049.
A method for 231 Pa analysis by thermal ionization mass spectrometry in silicate rocks Bernard Bourdon a
a,)
, Jean-Louis Joron
b,1
, Claude J. Allegre `
a
Laboratoire de Geochimie et Cosmochimie, CNRS-UMR 7579, Institut de Physique du Globe de Paris T14-24, 4 Place Jussieu, 75252 ´ Paris Cedex 05, France b Laboratoire Pierre Sue, ¨ CE r Saclay, 91191 Gif-sur-YÕette, France Received 18 June 1998; revised 2 December 1998; accepted 2 December 1998
Abstract We describe a new technique for analyzing 231 Pa in silicate rocks by isotope dilution mass spectrometry. This technique is different from the technique developed by previous studies ŽPickett et al., 1994. and also permits the determination of down to a 100 fg of 231 Pa with a 1–2% uncertainty at the 2 s level. While its performance is comparable to the LANL technique ŽPickett et al., 1994., a number of modifications may help extend the application of 231 Pa– 235 U to a wider number of geochemistry labs: Ž1. this technique does not require 237 Np, a hazardous a-emitter for 233 Pa spike preparation, Ž2. Pa is run as a double-oxide on a tungsten filament with a sensitivity comparable to the metal method. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Protactinium; Mass spectrometry; Isotope dilution
1. Introduction In the recent years, the analysis of 231 Pa– 235 U disequilibrium by mass spectrometry has been more and more widely used in the field of volcanology and mantle geochemistry ŽGoldstein et al., 1993, 1994; Lundstrom et al., 1995; Pickett and Murrell, 1997. as well as quaternary geochronology and climate studies ŽEdwards et al., 1997; Edmonds et al., 1998.. However, the number of studies including Pa data measured by mass spectrometry is still limited. This could be related to the difficulty in implement-
ing the chemical separation of Pa from natural samples or by difficulties in obtaining 233 Pa spike. In this paper, we present an alternative technique to what has been presented by Pickett et al. Ž1994.. Our technique offer similar performances to the Pickett technique but our different approach may increase the availability of Pa analysis by ID-TIMS to a greater number of geochemical laboratories.
2. Analytical methodology 2.1. Spike preparation
) Corresponding author. Fax: q33-1-442-737-52; e-mail: [email protected] 1 E-mail: [email protected].
The tion of
233 232
Pa spike was prepared by neutron activaTh in the Orphee ´ reactor in Saclay. 233 Th
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 1 9 8 - 3
148
B. Bourdon et al.r Chemical Geology 157 (1999) 147–151
produced by neutron capture then decays to 233 Pa with a half-life of 22.3 mn. Microgram quantities of 232 Th were deposited on Al foil and then irradiated by thermal neutrons Ž1.6 = 10 13 n P cmy2 P sy1 . for an hour. Following the irradiation, the sample was left for a week to let the short-lived nuclides produced during the irradiation Ž24 Na, T s 15 h, 28Al, T s 2.5 mn. decay. The Al foil was then leached with 9 N HCl and the leach solution was dried down. Consequently, the Pa solution was separated from Al and other impurities on a 2 ml column filled with AG1X8 200–400 mesh. 233 Pa was then fixed on the column while Al and Th were eluted. After washing with 6 cv Žcolumn volume. of 9N HCl, Pa is eluted with 2 cv of 9 N HCl q 0.1 N HF. The elution solution contained fluoride ions which stabilizes Pa and prevents its hydrolysis ŽGuillaumont et al., 1968.. A solution containing approximately 1 pg P gy1 of 233 Pa was produced every 6 months for mass spectrometric analysis. The solution was checked for purity on a GeŽLi. g-counter and no other nuclides were detected. The 231 Par233 Pa ratio in the spike was measured and found to be - 7 = 10y4 at the time of the measurements, which will ultimately not affect the calculations of 231 Pa concentration in samples. The preparation of the spike solution is in general very fast Žhalf a day. and provided access to a nuclear reactor is available, is more convenient than the technique used by Pickett et al. Ž1994. where repeated purification steps are required to eliminate all 237 Np in the final 233 Pa spike solution. Because of the absence of 237 Np, there is also no requirement to recalibrate the spike with time. It is essentially similar to the technique used by Anderson and Fleer ŽAnderson and Fleer, 1982. for producing 233 Pa.
2.2. Sample digestion Prior to dissolution, 233 Pa spike solution was added to the sample powder to yield a 231 Par233 Pa ratio of about 1–3. About 0.5 to 2 g of powdered sample depending on sample size was digested with 10 ml of concentrated HF to which a few drops of concentrated HNO 3 were added. The mixture was left overnight in a closed Savillex beaker at a tem-
perature of 100–1108C. After evaporation, it was redissolved in 20 ml of a 9 N HCl–H 3 BO 3 mixture. The addition of boric acid avoided the formation of insoluble fluorides that would have lead to Pa loss. We have not used any perchloric acid which requires especially designed fume hoods. Any excess boric acid was discarded by centrifugation prior to column loading. Radiotracer studies with 233 Pa had shown that no significant amount of Pa Ž- 1%. is lost at this stage. The amount of boric acid to be added depended on the chemical composition of the rocks that had been dissolved and was calculated to fully complex the available fluoride ions.
2.3. Separation and purification of Pa The 9 N HCl solution Ž2 cv. containing the sample was then loaded on a 20 ml anion resin column ŽAG1-X8, 200–400 mesh.. During this first stage, Fe and U are retained on the column while Pa was eluted in 2 cv of 9 N HCl q 0.1 N HF. The fraction containing Pa was then evaporated to about half of its initial volume and could be directly loaded on a 2 ml TRU spece column. About 0.5 ml of saturated boric acid solution were added to complex fluoride ions at this stage. Avoiding to dry down the solution prevented Pa loss by hydrolysis. The column was washed with 9 N HCl Ž2 cv., HCl 8 N Ž6 cv. prior to Pa elution in 2 N HF Ž2 cv.. The Pa fraction was then further purified on a 1 ml AG1-X8 resin similar to what had been used by Pickett et al. Ž1994.. The sample was loaded with 2 cv, washed with 6 cv of 9 N HCl, 13 cv of 8 N HCl and 2.5 cv of 7.5 N HNO 3 before Pa elution with 9 N HCl q 0.1 N HF Ž2 cv.. The last HNO 3 step washed efficiently any remaining Sc. The same solution was loaded on a final 1 ml TRU spece column without having to dry down the solution. Saturated H 3 BO 3 was added to the solution to complex any free fluoride before loading onto the TRU spece column. After loading, the column was washed with 4 cv of 4 N HCl and Pa is eluted with 2 cv of 2 N HF. This column separated U from Pa during the 4 N HCl wash. This last step was very fast and removed organics that were found after the third column. An overall view of the chemical separation is shown on Fig. 1.
B. Bourdon et al.r Chemical Geology 157 (1999) 147–151
Fig. 1. Schematic view of the technique used for separation and purification of Pa.
This chemical separation was calibrated by using a basaltic rock that had been irradiated by a neutron flux. The chemical yields for Pa were controlled by g-spectrometry on a GeŽLi. detector and were found to be around 80%. In order to further improve the chemical separation of Ta and Hf from Pa, the digestion mixture was also doped with neutronactivated tracer solutions of those elements. The separation factor for these elements was thus estimated to be better than 10 4 . The chemistry yields for Pa were routinely checked on a NaI detector. Total procedural blanks were also estimated to - 0.3 fg, which was well below the amount of Pa loaded on the filament Ž100 to 1000 fg.. 2.4. Mass spectrometry The clean Pa fraction was then redissolved in about 1 ml of concentrated HF and loaded on a W
149
filament followed by 3 ml of silica-gel Ž12.5 mg P mly1 of SiO 2 .. Prior to loading, the W filaments Ž0.025 mm = 0.75 mm. were outgassed for several hours with a applied 2 kV voltage with a 4 A DC current. Measurements were made on a FINNIGAN MAT 262 equipped with an ETP electron multiplier connected to an ion counting system. The sample was slowly preheated for about 1 h and brought to a temperature of about 1350–14008C. Pa was ionized as a double oxide as described by Hall and Walter Ž1956.. About 100–1000 cps on the 231 PaOq 2 peak were obtained for about 1 to 2 h with several hundred femtograms of Pa. The overall ionization efficiency Ž2–5‰. was similar to what was obtained by Pickett et al. Ž1994.. The 263 and 265 masses were measured by peak switching for five cycles with 8 s integration times. The ion yields were a factor of ten greater with silica-gel compared to graphite loaded on W. The oxygen correction from 231 Pa16 O18 O and 231 Pa17O 2 on mass 265 amounts to f 4‰. A 5% variation in the oxygen isotope composition only affected the final result by - 1‰. Isobaric interferences reaching initially thousands of counts per second around mass 263 amu rapidly decayed to less than 1 cps above 2.4 A ŽT ; 12008C. after 1–1.5 h of preheating. The baseline around PaO 2 masses were monitored before and after the runs. Baseline at 263.5 amu was also measured during the mass spectrometric runs and the 263.5:265 ratio was found to be less than 5 = 10y4 , corresponding to approximately 0.05 " 5 cps for a 100 cps signal at mass 265. As 233 Pa decays to 233 U, there may be an isobaric contribution of 233 U on the 265 amu peak. The Pa measurements were done less than 3 days after the final UrPa separation on the TRU spece column. Additionally, the emission of U occurred at a lower temperature than Pa and monitoring of 238 U generally showed less counts than 231 Pa Ž10–50 cps.. To check for a possible contribution of 233 U on the 265 amu peak, we ran 238 U– 231 Pa mixture and showed that the measured 231 Par238 U ratio normalized to the actual ratio ranged from 400 to 1200 at running temperatures. For a measurement that would take place 3 days after the last UrPa separation, the contribution of 233 U on the 265 amu peak would then be less than 10y4 . Estimated 2 s uncertainties for Ž231 Par235 U. were 1–2%. Mass fractionation
B. Bourdon et al.r Chemical Geology 157 (1999) 147–151
150
Table 1 Pa measurements by TIMS in volcanic rocks
231
Karthala 85KA08 85KA08R Hekla. Iceland Hek-8) Hek-8-1 Hek-8-2
Age
Ž231 Par235 U.
"2 s
Ž230 Thr238 U.
Th ppm
U ppm
Pa Žfg gy1 .
1918 1918
1.899 1.890
0.026 0.026
1.444 –
4.37 –
1.11 –
691 688
1.191 1.205 1.210
0.012 0.009 0.010
1.10 " 2 – –
7.74 – –
2.32 – –
900 901 905
)Data from Pickett and Murrell Ž1997.. Uncertainties are 2 s error bars of the mean estimated based on all the sources of uncertainties Žsee text.. Activity ratios are calculated using l235 s 9.8485 = 10y1 0 ay1 and l231 s 2.116 = 10y5 ay1 . Replicate from powder of 85KA08.
obtained for U samples in similar running conditions ŽC. Gopel, pers. comm.. amounts to 1.5‰ per amu. ¨ If one takes a similar figure for Pa, it will not greatly affect the measured ratios, given the overall uncertainty of 1–2%. It is difficult to give a more precise assessment of mass fractionation of Pa isotope during a run because the external reproducibility of a given block is higher than the predicted effect of mass fractionation. Reproducibility was tested by replicate analysis of a Grande Comore basalt and a Icelandic rock ŽHek8. as shown on Table 1. Fig. 2 shows data obtained for a typical run of a basaltic sample containing approximately 180 fg of 231 Pa.
Fig. 2. Measured 231 PaO 2 r233 PaO 2 during a typical mass spectrometer run. The mean value is shown by the solid line while the dashed lines indicate the 2 s error bar on the mean.
3. Standard measurements and spike calibration The 233 Pa spike Žhalf life s 26.967 days. was calibrated by replicate analysis of a 1-Ma old rhyolitic glass from Long Valley ŽLV18. given to us by G. Davies. This glass was dated by 87 Rb– 87 Sr and yielded an age of 1 Ma ŽG. Davies, pers. comm... For the purpose of this study, it was assumed that 231 Pa and 235 U are in secular equilibrium in this rock. As a way of cross-calibrating our rock standard, we also analyzed an Icelandic dacite ŽHek8. that had already been analyzed by D. Pickett at LANL ŽPickett and Murrell, 1997.. To calculate the 231 Par235 U ratio, we used the U concentration given by Pickett and Murrell Ž1997. assuming that the powder was homogeneous. To calculate the uncertainty on Ž231 Par235 U., we took an uncertainty of 1% for the U concentration. The calculated Ž231 Par235 U. agrees with the value given by D. Pickett to better than 1% at the 2 s level ŽSee Table 1.. The two results given in Table 1 which are replicate from powder also agree within error. Our data set is then both internally consistent and consistent with the Los Alamos data. A statistical test for comparing the mean values of 231 Par235 U for Hek8 shows that there is only a 5% risk that the our results and Pickett’s results are distinct. Table 1 also gives replicate analysis for two alkali basalts from the Grande Comore Island, which shows that our external reproducibility including uncertainties in U concentrations is about 1%. If we assume a conservative
B. Bourdon et al.r Chemical Geology 157 (1999) 147–151
151
1% uncertainty on the U concentrations, the overall uncertainty obtained by error propagation in the Ž231 Par235 U. ratios for this sample is 1.4%.
and an anonymous reviewer have greatly helped improving the manuscript. This is IPGP contribution a1570. [NA]
4. Conclusions
References
The first technique using mass spectrometry for analyzing 231 Pa was published by Pickett et al. Ž1994.. The technique we developed here for 231 Pa analysis by ID-TIMS is largely modified from the original technique. Firstly, we do not use the aemitter 237 Np for producing the 233 Pa spike. This is significant because handling of 237 Np requires lab facilities that are especially designed for such purposes. Our technique only involves 233 Pa which represents a much smaller radiological hazard. Then our technique based on column separation rather than solvent extraction using the actinide-specific resin TRU spec from EiChrome. Lastly, we analyze Pa as a double oxide on a tungsten filament. The sensitivity we obtain is comparable to what was claimed by Pickett et al. Ž1994.. The use of tungsten as a substitute for Re may be useful at labs where RerOs are also analyzed on the same mass spectrometer by negative ions.
Acknowledgements Discussions with Gerard Manhes, ´ ` Jean-Louis Birck and Tim Elliott were helpful. Olgeir Sigmarsson and Gareth Davies are thanked for providing samples. Careful reviews by S. Goldstein ŽLANL.
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