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Determination of the 87Rb decay constant
Geochimica
et Cosmochimica
Acta,
1977. Vol. 41. pp. 1745 to 1749. Pergamon
Deter~nation
Press.
Printed
in Great
Britain
of the *‘Rb decay constant
D. W. DAVIS, J. GRAY and G. L. GUMMING Department of Physics
and H. BAADSGAARD Department of Geology, University of Alberta, Edmonton, Alberta, T6G 251, Canada (Received 21 October 1976; accepted in revised form 25 July 1977)
Abstract-The decay constant s’Rb has been r~etermin~ by m~suring the amount of radiogenic 8’S produced over a period of 19 years, in 20g samples of purified RbClO, using isotope dilution techniques. The rubidium sample was spiked with 84Sr and the nanogram quantities of strontium separated by coprecipitation with Ba(NO,),. Analyses were carried out on a 25cm, 90” sector mass spectrometer equipped with a Spiraltron electron multiplier. Measurement of three independent ratios permitted continuous monitoring of the ion beam fractionation. The average of nine determinations gives a value for the decay constant of 1.419(+0.012) x 10-r’ yr-’ (2~). [rr,* = 4.89(*0&t) x
10’0yr.]
INTRODUCTION
separation of the trace amounts of strontium from the rubidium to prevent isobaric interference in the AN ACCURATE value of the decay constant of s7Rb mass spectrum and in measuring the strontium and is essential in determining the ages of minerals and Ibiza qu~ti~tively. A purified RbCIO, sample rocks using the RbSr dating technique. The value was prepared in 1956 by MCMULL~ et al. (1966). of this constant has been in dispute for over 20 yr. About one kilogram of RbCl was converted to the The two most often quoted measurements are those perchlorate by precipitation with perchloric acid. The of ALDRICH et al. (1956) and FLYNN and GLENDENIN product was then further purified by five successive (1959). Aldrich et al. found a value of recrystallizationsin demineralized water to give batch A. 2 = 1.395 + 0.056 x lo- ” yr- ’ by adjusting the Two months later a portion of this, batch B, was Rb-Sr ages in pegmatite minerals to agree with conagain recrystallized. Two more batches, C and D, cordant U-Pb ages from the same rocks. Flynn and were taken from A eleven months later and recrystalGlendenin found a value of 1= 1.475 & 0.031 x lized slowly. lo- l1 yr- ’ by measuring the beta decay rate of *‘Rb After seven years MCMULLEW et al. (1966) with a liquid scintillator. The difference between these measured the amount of radiogenic strontium provalues is almost 6%. The Aldrich value is the most duced over this period in portions of each batch. They widely used although its precision is poor. It also infound an average value for the decay constant of volves the assumption that the U-Pb and Rb-Sr sys(1.469 + 0.012) x lo-” yr-‘. They did not monitor tems in the minerais have behaved similarly. An accumass fractionation during the measurements, howrate independent physical determination of the “Rb ever, and this value may be subject to systematic decay constant is much to be desired. However, a large error. The present experiment was performed on the fraction of the p- particles emitted are of very low same material 19 yr after its purification. By this time, energy, making the counting measurement subject to a 20 g sample of RbClO, would be expected to consystematic errors which are difficult to estimate. tain about 0.67 ng of 87Sr, assuming a decay constant One technique which is capable in principle of sup of 1.39 x 10-l’ yr-‘. It is quite feasible to measure plying both good precision and good accuracy is to such quantities using ion counting mass-spectrometric allow a sample of strontium-free rubidium to decay techniques. In the present investigation three indepenfor a known time and to measure the amount of accudent mass ratios were measured during the course mulated radiogenic *‘Sr with a mass spectrometer of each Sr-analysis in order that individual fractionusing the isotope dilution technique. For a relatively ation corrections could be made.. short decay time (ea. 19 yr) the decay may be expressed as : EXPERIMENTAL
I = [87Sr]*[87Rb]-‘t-’ where [“Sr]* is the amount of radiogenic strontium accumulated over time t by a known quantity of rubidium, [*‘Rb]. The experimental problems are in the
TECHNIQUE
Chemistry
About 20g of RbClO, was added to a weighed 250ml Teflon beaker in a hood kept at an over pressure by filtered air. The beaker was then covered and reweighed.
1745
I 746
D. W. DAVS et al.
After returning the beaker to the hood, a clean Tefloncoated magnetic stirring element was added and one of the spike vials was emptied into it and thoroughly rinsed with 1.5N HCl. Enough water was added to make about 2OOml of solution. The beaker was next covered with a quartz plate and the solution stirred magnetically and heated to just below the boiling point using a hot plate and a heat lamp until all of the RbClO, dissolved. The heat was then reduced and the cover removed to allow the solution to partiahy evaporate. When approx 5Oml remained, the solution was cooled with dry ice to precipitate as much of the perchlorate as possible. The solution was then decanted into a 100 ml Teflon beaker, evaporated under moderate heat to about 5 ml, cooled to 0°C and decanted into a 10ml Teflon beaker. The solution. containing about 25mg of RbCIO,, was evaporated to dryness to remove HCl. The residue was dissolved in about 1 ml of hot water, and the solution transferred to a small quartz centrifuge tube. The tube was cooled to 0°C and RbCIO, crystals centrifuged out. The supernatant liquid was transferred to a 5 ml quartz centrifuge and about 2 mg of purified Ba(NOs)* added to the tube which was then capped with Parafilm and inverted until the BafNO,), dissolved. Vapour distilled 957; HNO, was then addedi’~lowly until barium nitrate began to precipitate. More HNOs was added to make about 5 ml of solution. The tube was capped, inverted several times and left to stand overnight. The barium nitrate crystals, containing the strontium, were centrifuged out and the acid decanted off. Residual acid in the tip of the tube was removed using a prewashed glass capillary. The tube was then refilled with acid, capped and inverted to rinse the crystals which were then centrifuged out and the acid removed as before. A minimum amount of water was added to redissolve the barium salt and the entire precipitation and rinsing procedure repeated. Subsequently, the barium and strontium were dissolved in I ml of 2.5 N HCI and added to a Teflon column containing 2 ml Dowex 5OW-X12 cation exchange resin. 2.5 N HCI was passed through the column and the strontium was eluted in the fraction from 8 to IS ml. During the earfier analyses it was found that even after this procedure it was rather difficult to burn the repining Rb off the filament. From separation 13 onwards the fotlowing step was added. The solution was evaporated to dryness and dissolved in 1 drop of water. It was then loaded into a clean quartz micro-column containing a layer of Dowex 5OW-X12 about 20mm in length and I mm in diameter. 1.5 N HCI was passed through the column and the strontium was eluted in the fraction from 7 to 12 drops. The solution was then evaporated to a very small drop and loaded onto an oxidized tantalum filament using a quartz capillary. This additional step produced a substantial reduction in the amount of rubidium on the filament. The filament load was treated with a small drop of phosphoric acid for runs 2-t t and with perchloric acid for runs 12- 17. The tem~rature was slowlv increased to fume off the excess acid-and the filament finally heated to a dull glow for one second. Analysis 1 was an unsuccessful attempt to separate the strontium and rubidium using cation exchange columns. Numbers S-11 were loaded onto the filament using a larger amount of phosphoric acid than usual. This unfortunately had the effect of inhibiting the emission and sometimes caused it to be erratic. The only analysis of this group which produced reliable data was number 1I. Analyses 7 and 12 did not succeed because of difficulties with the removal of rubidium. The water used was distilled in a quartz fractionation column and had a strontium blank of 0.53 ng/lOO mi. The acids were ail distilled below the boiling point. The 6 N HCI had a strontium blank of 1.94n~/i~ml. The total blank of the separation procedure was determined by per-
forming all the steps of a normal separation using only the spike. This was done twice during the course of the analyses. The Iirst measurement gave 1.10 ng and the second gave 1.05 ng. This is about the amount to be expected from the water and acids used. All equipment with which the solutions came into contact was cleaned with hot Htl for at least 12hr and rinsed with pure water. Plastic gloves were worn during most of the operations. which were all carried out in a filtered-air hood. Isotope ratio measur~mc~lts were carried out with a 2.5cm, 90: sector mass spectrometer equipped with oil diffusion pumps which maintained a pressure of about 3 x 10Ys torr during operation. Peak switching was accomplished by changing the magnetic field. Random fluctuations in the field were sensed by a set of Hail probes and compensated for by a feedback circuit controlling a Kepco operational power supply. A 0.01.5cm exit slit was used in conjunction with a 0.030 cm detector slit. This gave a resolution of more than 600, permitting the resolution of all hydrocarbon peaks in the strontium mass range. In the worst case, that of m/e 86, the hydrocarbon peak was less than 10:; of the strontium peak and was fully resolved. Hydrocarbons with half integral mass-to-charge ratios were not observed. Ion beams were detected with a Spiraltron electron multiplier operated typically at 35tZGV. The output was Fed into an Ortec 486 amplifier pulse height analyser foitowed by an Ortec 773 timer-counter. Each peak was counted for ten seconds. Peak switching and printout were controlled by a Texas Instruments 980A computer. The timing was carried out by a second Ortec timer. The strontium isotope ratios normally measured were 84/88, 87188 and 86/88. Each peak was centered by incrementing the accelerating potential at the beginning of a run. Minor adjustments to the magnetic field could be made manually. during a run, to correct for any drift. The a5Rb peak was measured before and after each 87/88 run. Normally, about 2 hr were needed to burn off the rubidium sufficiently well to begin the analysis. Strontium measurements were started when the s’Rb peak was less than 2”; of the s’Sr peak. The source was checked for contamination by periodically running a blank filament at high temperature. In no case was any strontium peak observed although occasionally a small amount of rubidium was detected at high temperatures (which required that the source assembly be cleaned). The background signal between peaks was measured at the end of each analysis. SpilKs
The spike used in the measurements was NBS SRM-988 which contains 99.89%: %r SRM-988 was diluted to a known concentration z&d stored in 1.5 N HCl in a volumetric flask. After one further dilution with 1.5 N NC1 in a TeIIon bottle, the spikes were aliquoted into quartz vials with a quartz micro-pipette and individually weighed (nanogram set 5). In order to test the procedures used in handling and measuring very small quantities of strontium, the known amount of *%r in a nonogram-sized spike was “determined” by comparison with NBS SRM-987 strontium. Ahquots of normal strontium were made up using the NBS SRM-987 Sr standard in the followina wav. The carbonate was converted to the nitrate in a weighed platinum dish and dried at 250°C overnight. A stock solution was made up from this salt and the concentration determined by transferring a weighed portion into a platinum dish. drying and converting it to the sulfate by the addition of H,SO,. The sulfuric acid was then fumed off and the sampie heated to a constant weight. The theoretical amount of strontium present as calculated from the NBS
Determination
of the s’Rb decay constant
I741
The amount of rubidium contained in the salt was determined by diluting a solution of known concentration and adding a “Rb enriched spike. The isotopic composition GlYe”value Measuredvalue Dlff. was then measured using a triple filament ion-source. Normal rubidium standards (NBS-984) were interspersed with (ng) (ng) the spiked runs to allow fractionation corrections to be made. These corrections were 0.34’%. The isotopic composition of the unspiked salt was also measured and found to be the same as that of the standards. The Cs and K concentrations in all four batches were 5.0822 5.0648 c.34 f .591% measured by concentrationdilution flame photometry using Rb-enriched standards. The water content of A, B 5.0824 5.0868 ,.09 i .l%% and D was measured by heating the samples for at least 12 hr at 300°C and reweighing. These three contaminants fail to account for the total contaminant measured by isotope dilution as seen in Table 2. The sum of their weight percentages is less than the weight percentage of the total concentration and the dilution procedure was 0.11869 g. contaminant by about 1%. We have been unable to identify The amount measured was 0.11854g. The discrepancy is the additional contaminating material. We believe that the isotope dilution measurements are accurate to within 0.13%. +0.25x and these were used to determine the correction A known portion of the diluted SRM-987 Sr stock solufactors. The amount of s’Rb contained in the samples is tion was added to an aliquot of the ‘%r spike used for calculated from the amount of normal Rb assuming an RbSr analysis in this laboratory and the mixture analysed isotopic abundance of 27.8346% (CATANZARO et al., 1969). on the mass spectrometer. The known value of the *%r spike was 2.4184 f 0.0005 pg. The value calculated from Dead time the normal Sr concentration is 2.4207 rt 0.0032 s. A porThe data were corrected for counter dead time using tion of the normal Sr was also mixed with an aliquot of the SRM-988 stock and analysed. The calculated value of the formula: the *%r in the SRM-988 stock was 1.1811 f 0.0005 pg F, = F,(l - F,.z)_’ using the NBS value for the concentration. The measured value using the normal Sr stock was 1.1785 k 0.0036pg. where F, is the true count rate, FoI is the measured count The errors on the measurements are expressed as ACT.In rate and T is the dead time of the detection system. Anaboth cases the measurements agree with the expected conlyses 2,3 and 13 to 17 were assumed to have a dead time centrations to within the error. of 1.01 psec (see Table 3). The standard deviation in the The dilute normal Sr stock was further diluted in a Tefdead time is about +4%. This amounts to less than k 0.2% lon bottle and aliquoted into quartz vials in the same manof any peak measured during the experiment and is usually ner as the %r spike. The contents of each of these vials much less. could then be mixed with an aliquot of *4Sr spike and Throughout analyses 4, 5, 6 and 11 a specially conused to check the concentration of the spike by measuring structed fast discriminator was used (instead of the slower the 84/88 and 86/88 ratios in the mixture. The s4Sr spikes Ortec amplifier) to reduce the dead time to an almost were individually weighed out to a precision of +O.Ol%. negligible value (0.13 psec). The 0.13 psec is an estimate The mean amount of the 84Sr spike in each vial was derived from twice the width of the output pulse. Use of 5.0814 k 0.0032 ng total Sr, while that of the normal alithe discriminator was discontinued after analysis 11 quots was 19.858 k 0.018 ng of normal Sr. Four of the because it had a relatively high firing threshold and since nanogram-sized 84Sr spikes were mixed with normal Sr the voltage output spectrum of the Spiraltron decreases aliquots and the mixture analysed. The results shown in after long use, it was possible that some of the low energy Table 1 indicate that we could-expect to be able to deterpulses would not be counted. mine nanogram quantities of Sr to somewhat better than 1% accuracy with our techniques. A small correction for Table 2. Measurement of impurities in RbC104 a filament loading blank had to be made to these data. This was determined by measuring the 84/88 peaks during an analysis of the pure s4Sr spike. The mean of four analyses was 60 f 11 pg normal Sr loading blank. This in4.4% 0.30% Batch A 2.2% 0.76% volves a correction factor of about 0.3%. A second group of spikes (nanogram set 6) was prepared 4.5% and used for analyses 16 and 17. Unfortunately the cross 4.1% checks on set 6 indicated that the spike concentrations 4.5% were about 1% lower than expected. These measurements were of good precision, and it is difficult to understand how such a large discrepancy could occur in one dilution. man: 4.4% A possible explanation for the cross checks giving an inac1.5% B 0.338 0.16% 0.14% curate answer is that the loading blank was much greater than usual, although this is quite unlikely. The results of 2.0% analyses 16 and 17 were not used in calculating the final 1.9% value for the decay constant because of this uncertainty in the spike concentration. Inclusion of these data in the Meall: 1.8% final calculation would not significantly alter the value obtained. Table 1. Cross-checks of procedures and measurement. Nanogram set 5
Rubidium-87
As mentioned, the four batches of rubidium perchlorate are designated A, B, C and D. The grain size of B is fine while that of C and D is quite coarse and A is intermediate.
c
0.18%
0.10%
D
0.17%
0.13%
1.5%
0.04%
1.5%
I748
D. W. DAVISet at.
Table 3. Dead times from Eimer & Amend Sr analyses EA-1 1.11 .90 1.28 .94 avg:
1.00
z .11
Table 4. Strontium blank originally present in RbCIO, assuming a total separation procedure blank of I ng
EA-2 p sec.
1.16
t
.lO
Maximum sample blank lnqm/gm “Rbl
% of --
total
blank
._
2
.07
.70
f
.19
3.20
88%
t
.ll
.90
f
.24
2.20
86%
t
.08
2.10
86%
1.39
82%
3.83
90%
1.20
75%
1.64
84%
1.63
84%
.70
67%
t
.a4
Overall
avg:
average:
1.01
*
.04
1.04
f
.09
u sec.
RESULTS AND DISCUSSION
Calculation of the decay constant from a single analysis requires an assumption about the fraction of 87Sr present in the blank. This will be a weighted average of the isotopic composition of the Sr blank added during the gyration process and Sr which was originally present in the salt. Unfortunately, the latter was not measured at the time the sample was purified. The total amount of strontium blank in each sample can be calculated from the 84/88 ratios listed in Table 5. This will be partly separation procedure blank and partly a blank originally present in the
sample. Assuming the minimum procedure blank to be 1 ng, the maximum sample blank per gram of “Rb may be calculated. This is listed in Table 4. It is obvious from this table that the largest proportion of blank was contained originally in the sample. As has been mentioned, the procedure taken to purify the original sample involved conversion to the perchlorate by precipitation with perchloric acid followed by at least five successive recrystallizations in demineralized water. In view of this it seems exceedingly unlikely that much of the original strontium
Table 5. Results of all isotopic analyses as well as the amount and fraction of radiogenic *‘Sr, the amount of 87Rb, the decay time and the calculated decay constant for each sample from batches A, B and C @‘Rb
?$I* Sampie 28 38 4A SA, SAl 6C II8 13B 14A ISB, I SB, I SBS ISB: l6D, 16D, l7C, 17c1; I7Cf 17c:
84/88 I.04952 +0.00147 0.66010 i@XX87 0.7593 I ~0.00160 0.9 I286 +0.00213 0.89050 *0.00173 2.12oco +0.00604 - 1.57935 *0.00434 I.06065 *0.00223 0.92381 f0.00102 1.04616 +0.oM46 I.04511 * 0.00253 0.04479 f 0.00226 1.03325 +0.00253 1.0!9525 f 0.00264 1.08537 &0.00182 I.65002 *0.00375 1.63732 +WJO285 1.62730 *o.m293 I.60949 *0.00460
87/88 0.26701 +0.ooo51 0.16258 ~O!XOS6 0.17780 f o.mO47 0.21834 +0.MW67 0.2 I702 f o.Ocms 0.38474 * omm 0.29 589 +0.00120 0.25509 *0.OtXX3 0.22332 +o.mm 0.26025 * 0.00057 0.25912 &O.o0068 0.25973 f omo64 0.25973 f0.00064 0.21565 fO.cco73 0.21553 f o.cQO49 0.22634 ~O,ooO65 0.22549 *o.w342 0.22549 *0.@XW 0.22549 + O.WO42
* Calculations are cross-correlated
86j88 0.11964 *o.w37 0.11993 ~0.00061 0.11947 f0.00043 0.12101 fOm341 0.11933 f oJm30 0.12056 rto.om33 0.12012 fOLm41 0.12013 to.cco37 0.12074 &00X26 0.12115 *0.OW48 0.12150 *0JE545 0. I 2074 &O.o0046 0.12074 f0.00046 0.12120 fO.OM)64 0.12063 *o.orKl32 0.12164 ~O.ooO58 0.12071 ~O.OW21 0.12071 &O.M)021 0.12071 +0.Mx)21
% Rad.
(B)
0.8227
68.34
3.2312
17.970
0.6177
47.89
2.3 130
18.890
0.6483
52.45
2.3381
19.205
0.7843
61.04
2.8270
19.274
0.7830
61.08
2.8270
19.274
0.7463
78.00
2.9 I78
18.339
0.7044
71.41
2.5582
19.263
0.8523
66.77
3.1324
19.364
0.8012
61.96
2.9059
19.567
0.8988
67.31
3.2565
19.383
0.8972
67.12
3.2565
19.383
0.8934
67.30
3.2565
19.383
0.9035
67.30
3.2565
19.383
0.6245
60.54
2.3757
IS.682
0.6262
60.62
2.3757
IS.682
0.4483
62.38
1.7247
18.690
0.4449
62.39
1.7247
18.690
0.4477
62.39
1.7247
18.690
0.4527
62.39
1.7247
18,690
f&
I.417 +0.01 I 1.437 +0.021 1.444 kO.016 1.439 f0.016 1.437 io.013 1.395 +0.014 1.429 iO.018 1.405 *0.014 I.409 + 0.008 1.424 +0.015 1.422 *0.016 I.415 kO.016 1.432 iO.016 I.407 + 0.027 I.411 f0.019 I.391 f 0.024 1.380 f0.016 I .389 *o.oici I.404 *0.01s
1.438 fO.O1O
I.423 +0,009
1.410 f0.016
1.388 +0.015
Determination
of the “‘Rb decay constant
in the RbCl would have remained, especially since the water used would probably have had a rather high strontium blank as evidenced by the results in Table 4. It seems quite likely therefore that the remaining blank would have an isotopic composition characteristic of that in the water used for the recrystallization. If all of the data are plotted on a graph of 87Sr/86Sr vs 87Rb/86Sr the best fit straight line is found to have an intercept of 0.731 f 0.020 (2a). This is quite high for a normal strontium blank and has a rather large uncertainty attached to it. It may also be affected by systematic errors since there is only a 20% probability that the x2 value of this data set would be exceeded by chance in a random distribution. The errors on the individual analyses are calculated from the precision of the counting measurements on each peak. Possible sources of systematic error which may contribute to a non-random distribution are error in the chemical contaminant, which is not likely to exceed 0.25%; error in the spike calibration, which is only significant for analyses 16 and 17 which were rejected on this account; error produced by the dead time, which is unlikely to exceed 0.1%; and an error from a non-linear fractionation trend. The main instrumental difficulty encountered during the earlier analyses involved the field stabilizer. All runs within an analysis were selected on a basis of low x2 values and several runs were rejected in analyses 3, 5 and 6. Difficulties with the stabilizer had been resolved by the time number 13 was analysed. The data from number 2 were rerun after this. The analyses which are considered most likely to be free of systematic error are 2, 13, 14 and 15. These all had steady emission, no instrumental difficulties, several replicate measurements on each ratio and they are all from batches A or B in which the contaminant is well known. Plotting these data on a graph of 87Sr/86Sr vs 87Rb/86Sr gives an intercept of 0.683 f 0.032 with a 60% probability of randomness. Thus the direct determination of the initial blank composition is uncertain and we consider it better to choose the initial ratio from physical considerations, since the data are probably not free from systematic error. The initial value of 87Sr/86Sr chosen was 0.709. This is the isotopic composition of the blank in the reagents used in this laboratory. The ratios in Table 5 were corrected for fractionation assuming a true 86Sr/88Sr ratio of 0.1194. The decay constants for the individual analyses were derived from these data and a weighted mean was calculated. The weights are determined from the inverse present
1749
square of the standard deviations which were determined according to the formula: %a =
(%~7,8 - @5%a6,8)/R
+ 2%06,8
+ o/;rc4 8
where %a is the total percentage standard deviation; %c7,8r %a4,8 and %usi8 are the percentage standard deviations for the 87Sr/88Sr, 84Sr/88Sr and 86Sr/88Sr ratios, respectively, and R is the fraction of the “Sr that was radiogenic. A further percentage error of *0.25% was added to the mean decay constant from each batch to allow for an error in the Rb concentration. The value found for the decay constant is 1 = 1.419(*0.012) x 10-l’ yr-‘. Theerror quoted is a two sigma error. It does not include any error in the initial blank Sr composition. The average of 2, 13, 14 and 15 (considered the most accurate analyses) is 1.414(f0.015) x lo- l1 yr-’ (20). This is statistically indistinguishable from the overall mean. We consider therefore that the overall mean 1 = 1.419(+0.012) x 10-l’ yr-’ represents the most reliable measurement of the decay constant of “Rb. This value compares favorably with the two most recent measurements of the decay constant which are that of NEUMANNand HUSTER (1974) who found I = 1.42(+0.03) x lo-” yr-’ by fi counting experiments and AFANASS’YEV et al. (1974), who found I = 1.42 x lo-” yr- ’ from geochronological comparisons. Acknowledgements-We wish to thank Drs. THOMLINSON, FRITZEand MCMULLENfor allowing us to analyse batches of the RbClO, purified 19 yr ago. Without their generosity this work could not have been carried out. This work was supported in part by grants from the National Research Council of Canada. REFERENCES AFANASS’YEV G. D., ZYKOR S. I., GOROKOVI. M. and SHAININ L. L. (1974) Correlativity of geochronometric ages yielded by coexisting minerals with different 4K, “‘Rb and some other radioactive element decay constants used (Abstract). International Meeting for Geochronology, Cosmochronology and Isotope Geology, Paris, August. ALDRICHL. T., WETHERILL G. W., TILTONG. R. and DAVIS G. L. (1956) The half-life of *‘Rb. Phys. Reo. 104, 1045-1047. CATANZAROE. J., MURPHY T. J., GARNER E. L. and SHIELDSW. R. (1969) Absolute isotopic abundance ratio and atomic weight of terrestrial rubidium. J. Rex Nat. Bur. Std. 13A, No. 5, 511~516. FLYNN K. F. and GLENDENIN L. E. (1959) Half-life and beta spectrum of *‘Rb. Phys. Rev. 116, 744-748. MCMULLENC. C., FRITZEK. and TOMLIN~CBN R. H. (1966) The half-life of *‘Rb. Can. J. Phys. 44, 3033-3038. NEUMANNW. and HUSTERE. (1974) The half-life of s’Rb measured as difference between the isotopes “Rb and “‘Rb. Z. Physik 270, 121-127.
et Cosmochimica
Acta,
1977. Vol. 41. pp. 1745 to 1749. Pergamon
Deter~nation
Press.
Printed
in Great
Britain
of the *‘Rb decay constant
D. W. DAVIS, J. GRAY and G. L. GUMMING Department of Physics
and H. BAADSGAARD Department of Geology, University of Alberta, Edmonton, Alberta, T6G 251, Canada (Received 21 October 1976; accepted in revised form 25 July 1977)
Abstract-The decay constant s’Rb has been r~etermin~ by m~suring the amount of radiogenic 8’S produced over a period of 19 years, in 20g samples of purified RbClO, using isotope dilution techniques. The rubidium sample was spiked with 84Sr and the nanogram quantities of strontium separated by coprecipitation with Ba(NO,),. Analyses were carried out on a 25cm, 90” sector mass spectrometer equipped with a Spiraltron electron multiplier. Measurement of three independent ratios permitted continuous monitoring of the ion beam fractionation. The average of nine determinations gives a value for the decay constant of 1.419(+0.012) x 10-r’ yr-’ (2~). [rr,* = 4.89(*0&t) x
10’0yr.]
INTRODUCTION
separation of the trace amounts of strontium from the rubidium to prevent isobaric interference in the AN ACCURATE value of the decay constant of s7Rb mass spectrum and in measuring the strontium and is essential in determining the ages of minerals and Ibiza qu~ti~tively. A purified RbCIO, sample rocks using the RbSr dating technique. The value was prepared in 1956 by MCMULL~ et al. (1966). of this constant has been in dispute for over 20 yr. About one kilogram of RbCl was converted to the The two most often quoted measurements are those perchlorate by precipitation with perchloric acid. The of ALDRICH et al. (1956) and FLYNN and GLENDENIN product was then further purified by five successive (1959). Aldrich et al. found a value of recrystallizationsin demineralized water to give batch A. 2 = 1.395 + 0.056 x lo- ” yr- ’ by adjusting the Two months later a portion of this, batch B, was Rb-Sr ages in pegmatite minerals to agree with conagain recrystallized. Two more batches, C and D, cordant U-Pb ages from the same rocks. Flynn and were taken from A eleven months later and recrystalGlendenin found a value of 1= 1.475 & 0.031 x lized slowly. lo- l1 yr- ’ by measuring the beta decay rate of *‘Rb After seven years MCMULLEW et al. (1966) with a liquid scintillator. The difference between these measured the amount of radiogenic strontium provalues is almost 6%. The Aldrich value is the most duced over this period in portions of each batch. They widely used although its precision is poor. It also infound an average value for the decay constant of volves the assumption that the U-Pb and Rb-Sr sys(1.469 + 0.012) x lo-” yr-‘. They did not monitor tems in the minerais have behaved similarly. An accumass fractionation during the measurements, howrate independent physical determination of the “Rb ever, and this value may be subject to systematic decay constant is much to be desired. However, a large error. The present experiment was performed on the fraction of the p- particles emitted are of very low same material 19 yr after its purification. By this time, energy, making the counting measurement subject to a 20 g sample of RbClO, would be expected to consystematic errors which are difficult to estimate. tain about 0.67 ng of 87Sr, assuming a decay constant One technique which is capable in principle of sup of 1.39 x 10-l’ yr-‘. It is quite feasible to measure plying both good precision and good accuracy is to such quantities using ion counting mass-spectrometric allow a sample of strontium-free rubidium to decay techniques. In the present investigation three indepenfor a known time and to measure the amount of accudent mass ratios were measured during the course mulated radiogenic *‘Sr with a mass spectrometer of each Sr-analysis in order that individual fractionusing the isotope dilution technique. For a relatively ation corrections could be made.. short decay time (ea. 19 yr) the decay may be expressed as : EXPERIMENTAL
I = [87Sr]*[87Rb]-‘t-’ where [“Sr]* is the amount of radiogenic strontium accumulated over time t by a known quantity of rubidium, [*‘Rb]. The experimental problems are in the
TECHNIQUE
Chemistry
About 20g of RbClO, was added to a weighed 250ml Teflon beaker in a hood kept at an over pressure by filtered air. The beaker was then covered and reweighed.
1745
I 746
D. W. DAVS et al.
After returning the beaker to the hood, a clean Tefloncoated magnetic stirring element was added and one of the spike vials was emptied into it and thoroughly rinsed with 1.5N HCl. Enough water was added to make about 2OOml of solution. The beaker was next covered with a quartz plate and the solution stirred magnetically and heated to just below the boiling point using a hot plate and a heat lamp until all of the RbClO, dissolved. The heat was then reduced and the cover removed to allow the solution to partiahy evaporate. When approx 5Oml remained, the solution was cooled with dry ice to precipitate as much of the perchlorate as possible. The solution was then decanted into a 100 ml Teflon beaker, evaporated under moderate heat to about 5 ml, cooled to 0°C and decanted into a 10ml Teflon beaker. The solution. containing about 25mg of RbCIO,, was evaporated to dryness to remove HCl. The residue was dissolved in about 1 ml of hot water, and the solution transferred to a small quartz centrifuge tube. The tube was cooled to 0°C and RbCIO, crystals centrifuged out. The supernatant liquid was transferred to a 5 ml quartz centrifuge and about 2 mg of purified Ba(NOs)* added to the tube which was then capped with Parafilm and inverted until the BafNO,), dissolved. Vapour distilled 957; HNO, was then addedi’~lowly until barium nitrate began to precipitate. More HNOs was added to make about 5 ml of solution. The tube was capped, inverted several times and left to stand overnight. The barium nitrate crystals, containing the strontium, were centrifuged out and the acid decanted off. Residual acid in the tip of the tube was removed using a prewashed glass capillary. The tube was then refilled with acid, capped and inverted to rinse the crystals which were then centrifuged out and the acid removed as before. A minimum amount of water was added to redissolve the barium salt and the entire precipitation and rinsing procedure repeated. Subsequently, the barium and strontium were dissolved in I ml of 2.5 N HCI and added to a Teflon column containing 2 ml Dowex 5OW-X12 cation exchange resin. 2.5 N HCI was passed through the column and the strontium was eluted in the fraction from 8 to IS ml. During the earfier analyses it was found that even after this procedure it was rather difficult to burn the repining Rb off the filament. From separation 13 onwards the fotlowing step was added. The solution was evaporated to dryness and dissolved in 1 drop of water. It was then loaded into a clean quartz micro-column containing a layer of Dowex 5OW-X12 about 20mm in length and I mm in diameter. 1.5 N HCI was passed through the column and the strontium was eluted in the fraction from 7 to 12 drops. The solution was then evaporated to a very small drop and loaded onto an oxidized tantalum filament using a quartz capillary. This additional step produced a substantial reduction in the amount of rubidium on the filament. The filament load was treated with a small drop of phosphoric acid for runs 2-t t and with perchloric acid for runs 12- 17. The tem~rature was slowlv increased to fume off the excess acid-and the filament finally heated to a dull glow for one second. Analysis 1 was an unsuccessful attempt to separate the strontium and rubidium using cation exchange columns. Numbers S-11 were loaded onto the filament using a larger amount of phosphoric acid than usual. This unfortunately had the effect of inhibiting the emission and sometimes caused it to be erratic. The only analysis of this group which produced reliable data was number 1I. Analyses 7 and 12 did not succeed because of difficulties with the removal of rubidium. The water used was distilled in a quartz fractionation column and had a strontium blank of 0.53 ng/lOO mi. The acids were ail distilled below the boiling point. The 6 N HCI had a strontium blank of 1.94n~/i~ml. The total blank of the separation procedure was determined by per-
forming all the steps of a normal separation using only the spike. This was done twice during the course of the analyses. The Iirst measurement gave 1.10 ng and the second gave 1.05 ng. This is about the amount to be expected from the water and acids used. All equipment with which the solutions came into contact was cleaned with hot Htl for at least 12hr and rinsed with pure water. Plastic gloves were worn during most of the operations. which were all carried out in a filtered-air hood. Isotope ratio measur~mc~lts were carried out with a 2.5cm, 90: sector mass spectrometer equipped with oil diffusion pumps which maintained a pressure of about 3 x 10Ys torr during operation. Peak switching was accomplished by changing the magnetic field. Random fluctuations in the field were sensed by a set of Hail probes and compensated for by a feedback circuit controlling a Kepco operational power supply. A 0.01.5cm exit slit was used in conjunction with a 0.030 cm detector slit. This gave a resolution of more than 600, permitting the resolution of all hydrocarbon peaks in the strontium mass range. In the worst case, that of m/e 86, the hydrocarbon peak was less than 10:; of the strontium peak and was fully resolved. Hydrocarbons with half integral mass-to-charge ratios were not observed. Ion beams were detected with a Spiraltron electron multiplier operated typically at 35tZGV. The output was Fed into an Ortec 486 amplifier pulse height analyser foitowed by an Ortec 773 timer-counter. Each peak was counted for ten seconds. Peak switching and printout were controlled by a Texas Instruments 980A computer. The timing was carried out by a second Ortec timer. The strontium isotope ratios normally measured were 84/88, 87188 and 86/88. Each peak was centered by incrementing the accelerating potential at the beginning of a run. Minor adjustments to the magnetic field could be made manually. during a run, to correct for any drift. The a5Rb peak was measured before and after each 87/88 run. Normally, about 2 hr were needed to burn off the rubidium sufficiently well to begin the analysis. Strontium measurements were started when the s’Rb peak was less than 2”; of the s’Sr peak. The source was checked for contamination by periodically running a blank filament at high temperature. In no case was any strontium peak observed although occasionally a small amount of rubidium was detected at high temperatures (which required that the source assembly be cleaned). The background signal between peaks was measured at the end of each analysis. SpilKs
The spike used in the measurements was NBS SRM-988 which contains 99.89%: %r SRM-988 was diluted to a known concentration z&d stored in 1.5 N HCl in a volumetric flask. After one further dilution with 1.5 N NC1 in a TeIIon bottle, the spikes were aliquoted into quartz vials with a quartz micro-pipette and individually weighed (nanogram set 5). In order to test the procedures used in handling and measuring very small quantities of strontium, the known amount of *%r in a nonogram-sized spike was “determined” by comparison with NBS SRM-987 strontium. Ahquots of normal strontium were made up using the NBS SRM-987 Sr standard in the followina wav. The carbonate was converted to the nitrate in a weighed platinum dish and dried at 250°C overnight. A stock solution was made up from this salt and the concentration determined by transferring a weighed portion into a platinum dish. drying and converting it to the sulfate by the addition of H,SO,. The sulfuric acid was then fumed off and the sampie heated to a constant weight. The theoretical amount of strontium present as calculated from the NBS
Determination
of the s’Rb decay constant
I741
The amount of rubidium contained in the salt was determined by diluting a solution of known concentration and adding a “Rb enriched spike. The isotopic composition GlYe”value Measuredvalue Dlff. was then measured using a triple filament ion-source. Normal rubidium standards (NBS-984) were interspersed with (ng) (ng) the spiked runs to allow fractionation corrections to be made. These corrections were 0.34’%. The isotopic composition of the unspiked salt was also measured and found to be the same as that of the standards. The Cs and K concentrations in all four batches were 5.0822 5.0648 c.34 f .591% measured by concentrationdilution flame photometry using Rb-enriched standards. The water content of A, B 5.0824 5.0868 ,.09 i .l%% and D was measured by heating the samples for at least 12 hr at 300°C and reweighing. These three contaminants fail to account for the total contaminant measured by isotope dilution as seen in Table 2. The sum of their weight percentages is less than the weight percentage of the total concentration and the dilution procedure was 0.11869 g. contaminant by about 1%. We have been unable to identify The amount measured was 0.11854g. The discrepancy is the additional contaminating material. We believe that the isotope dilution measurements are accurate to within 0.13%. +0.25x and these were used to determine the correction A known portion of the diluted SRM-987 Sr stock solufactors. The amount of s’Rb contained in the samples is tion was added to an aliquot of the ‘%r spike used for calculated from the amount of normal Rb assuming an RbSr analysis in this laboratory and the mixture analysed isotopic abundance of 27.8346% (CATANZARO et al., 1969). on the mass spectrometer. The known value of the *%r spike was 2.4184 f 0.0005 pg. The value calculated from Dead time the normal Sr concentration is 2.4207 rt 0.0032 s. A porThe data were corrected for counter dead time using tion of the normal Sr was also mixed with an aliquot of the SRM-988 stock and analysed. The calculated value of the formula: the *%r in the SRM-988 stock was 1.1811 f 0.0005 pg F, = F,(l - F,.z)_’ using the NBS value for the concentration. The measured value using the normal Sr stock was 1.1785 k 0.0036pg. where F, is the true count rate, FoI is the measured count The errors on the measurements are expressed as ACT.In rate and T is the dead time of the detection system. Anaboth cases the measurements agree with the expected conlyses 2,3 and 13 to 17 were assumed to have a dead time centrations to within the error. of 1.01 psec (see Table 3). The standard deviation in the The dilute normal Sr stock was further diluted in a Tefdead time is about +4%. This amounts to less than k 0.2% lon bottle and aliquoted into quartz vials in the same manof any peak measured during the experiment and is usually ner as the %r spike. The contents of each of these vials much less. could then be mixed with an aliquot of *4Sr spike and Throughout analyses 4, 5, 6 and 11 a specially conused to check the concentration of the spike by measuring structed fast discriminator was used (instead of the slower the 84/88 and 86/88 ratios in the mixture. The s4Sr spikes Ortec amplifier) to reduce the dead time to an almost were individually weighed out to a precision of +O.Ol%. negligible value (0.13 psec). The 0.13 psec is an estimate The mean amount of the 84Sr spike in each vial was derived from twice the width of the output pulse. Use of 5.0814 k 0.0032 ng total Sr, while that of the normal alithe discriminator was discontinued after analysis 11 quots was 19.858 k 0.018 ng of normal Sr. Four of the because it had a relatively high firing threshold and since nanogram-sized 84Sr spikes were mixed with normal Sr the voltage output spectrum of the Spiraltron decreases aliquots and the mixture analysed. The results shown in after long use, it was possible that some of the low energy Table 1 indicate that we could-expect to be able to deterpulses would not be counted. mine nanogram quantities of Sr to somewhat better than 1% accuracy with our techniques. A small correction for Table 2. Measurement of impurities in RbC104 a filament loading blank had to be made to these data. This was determined by measuring the 84/88 peaks during an analysis of the pure s4Sr spike. The mean of four analyses was 60 f 11 pg normal Sr loading blank. This in4.4% 0.30% Batch A 2.2% 0.76% volves a correction factor of about 0.3%. A second group of spikes (nanogram set 6) was prepared 4.5% and used for analyses 16 and 17. Unfortunately the cross 4.1% checks on set 6 indicated that the spike concentrations 4.5% were about 1% lower than expected. These measurements were of good precision, and it is difficult to understand how such a large discrepancy could occur in one dilution. man: 4.4% A possible explanation for the cross checks giving an inac1.5% B 0.338 0.16% 0.14% curate answer is that the loading blank was much greater than usual, although this is quite unlikely. The results of 2.0% analyses 16 and 17 were not used in calculating the final 1.9% value for the decay constant because of this uncertainty in the spike concentration. Inclusion of these data in the Meall: 1.8% final calculation would not significantly alter the value obtained. Table 1. Cross-checks of procedures and measurement. Nanogram set 5
Rubidium-87
As mentioned, the four batches of rubidium perchlorate are designated A, B, C and D. The grain size of B is fine while that of C and D is quite coarse and A is intermediate.
c
0.18%
0.10%
D
0.17%
0.13%
1.5%
0.04%
1.5%
I748
D. W. DAVISet at.
Table 3. Dead times from Eimer & Amend Sr analyses EA-1 1.11 .90 1.28 .94 avg:
1.00
z .11
Table 4. Strontium blank originally present in RbCIO, assuming a total separation procedure blank of I ng
EA-2 p sec.
1.16
t
.lO
Maximum sample blank lnqm/gm “Rbl
% of --
total
blank
._
2
.07
.70
f
.19
3.20
88%
t
.ll
.90
f
.24
2.20
86%
t
.08
2.10
86%
1.39
82%
3.83
90%
1.20
75%
1.64
84%
1.63
84%
.70
67%
t
.a4
Overall
avg:
average:
1.01
*
.04
1.04
f
.09
u sec.
RESULTS AND DISCUSSION
Calculation of the decay constant from a single analysis requires an assumption about the fraction of 87Sr present in the blank. This will be a weighted average of the isotopic composition of the Sr blank added during the gyration process and Sr which was originally present in the salt. Unfortunately, the latter was not measured at the time the sample was purified. The total amount of strontium blank in each sample can be calculated from the 84/88 ratios listed in Table 5. This will be partly separation procedure blank and partly a blank originally present in the
sample. Assuming the minimum procedure blank to be 1 ng, the maximum sample blank per gram of “Rb may be calculated. This is listed in Table 4. It is obvious from this table that the largest proportion of blank was contained originally in the sample. As has been mentioned, the procedure taken to purify the original sample involved conversion to the perchlorate by precipitation with perchloric acid followed by at least five successive recrystallizations in demineralized water. In view of this it seems exceedingly unlikely that much of the original strontium
Table 5. Results of all isotopic analyses as well as the amount and fraction of radiogenic *‘Sr, the amount of 87Rb, the decay time and the calculated decay constant for each sample from batches A, B and C @‘Rb
?$I* Sampie 28 38 4A SA, SAl 6C II8 13B 14A ISB, I SB, I SBS ISB: l6D, 16D, l7C, 17c1; I7Cf 17c:
84/88 I.04952 +0.00147 0.66010 i@XX87 0.7593 I ~0.00160 0.9 I286 +0.00213 0.89050 *0.00173 2.12oco +0.00604 - 1.57935 *0.00434 I.06065 *0.00223 0.92381 f0.00102 1.04616 +0.oM46 I.04511 * 0.00253 0.04479 f 0.00226 1.03325 +0.00253 1.0!9525 f 0.00264 1.08537 &0.00182 I.65002 *0.00375 1.63732 +WJO285 1.62730 *o.m293 I.60949 *0.00460
87/88 0.26701 +0.ooo51 0.16258 ~O!XOS6 0.17780 f o.mO47 0.21834 +0.MW67 0.2 I702 f o.Ocms 0.38474 * omm 0.29 589 +0.00120 0.25509 *0.OtXX3 0.22332 +o.mm 0.26025 * 0.00057 0.25912 &O.o0068 0.25973 f omo64 0.25973 f0.00064 0.21565 fO.cco73 0.21553 f o.cQO49 0.22634 ~O,ooO65 0.22549 *o.w342 0.22549 *0.@XW 0.22549 + O.WO42
* Calculations are cross-correlated
86j88 0.11964 *o.w37 0.11993 ~0.00061 0.11947 f0.00043 0.12101 fOm341 0.11933 f oJm30 0.12056 rto.om33 0.12012 fOLm41 0.12013 to.cco37 0.12074 &00X26 0.12115 *0.OW48 0.12150 *0JE545 0. I 2074 &O.o0046 0.12074 f0.00046 0.12120 fO.OM)64 0.12063 *o.orKl32 0.12164 ~O.ooO58 0.12071 ~O.OW21 0.12071 &O.M)021 0.12071 +0.Mx)21
% Rad.
(B)
0.8227
68.34
3.2312
17.970
0.6177
47.89
2.3 130
18.890
0.6483
52.45
2.3381
19.205
0.7843
61.04
2.8270
19.274
0.7830
61.08
2.8270
19.274
0.7463
78.00
2.9 I78
18.339
0.7044
71.41
2.5582
19.263
0.8523
66.77
3.1324
19.364
0.8012
61.96
2.9059
19.567
0.8988
67.31
3.2565
19.383
0.8972
67.12
3.2565
19.383
0.8934
67.30
3.2565
19.383
0.9035
67.30
3.2565
19.383
0.6245
60.54
2.3757
IS.682
0.6262
60.62
2.3757
IS.682
0.4483
62.38
1.7247
18.690
0.4449
62.39
1.7247
18.690
0.4477
62.39
1.7247
18.690
0.4527
62.39
1.7247
18,690
f&
I.417 +0.01 I 1.437 +0.021 1.444 kO.016 1.439 f0.016 1.437 io.013 1.395 +0.014 1.429 iO.018 1.405 *0.014 I.409 + 0.008 1.424 +0.015 1.422 *0.016 I.415 kO.016 1.432 iO.016 I.407 + 0.027 I.411 f0.019 I.391 f 0.024 1.380 f0.016 I .389 *o.oici I.404 *0.01s
1.438 fO.O1O
I.423 +0,009
1.410 f0.016
1.388 +0.015
Determination
of the “‘Rb decay constant
in the RbCl would have remained, especially since the water used would probably have had a rather high strontium blank as evidenced by the results in Table 4. It seems quite likely therefore that the remaining blank would have an isotopic composition characteristic of that in the water used for the recrystallization. If all of the data are plotted on a graph of 87Sr/86Sr vs 87Rb/86Sr the best fit straight line is found to have an intercept of 0.731 f 0.020 (2a). This is quite high for a normal strontium blank and has a rather large uncertainty attached to it. It may also be affected by systematic errors since there is only a 20% probability that the x2 value of this data set would be exceeded by chance in a random distribution. The errors on the individual analyses are calculated from the precision of the counting measurements on each peak. Possible sources of systematic error which may contribute to a non-random distribution are error in the chemical contaminant, which is not likely to exceed 0.25%; error in the spike calibration, which is only significant for analyses 16 and 17 which were rejected on this account; error produced by the dead time, which is unlikely to exceed 0.1%; and an error from a non-linear fractionation trend. The main instrumental difficulty encountered during the earlier analyses involved the field stabilizer. All runs within an analysis were selected on a basis of low x2 values and several runs were rejected in analyses 3, 5 and 6. Difficulties with the stabilizer had been resolved by the time number 13 was analysed. The data from number 2 were rerun after this. The analyses which are considered most likely to be free of systematic error are 2, 13, 14 and 15. These all had steady emission, no instrumental difficulties, several replicate measurements on each ratio and they are all from batches A or B in which the contaminant is well known. Plotting these data on a graph of 87Sr/86Sr vs 87Rb/86Sr gives an intercept of 0.683 f 0.032 with a 60% probability of randomness. Thus the direct determination of the initial blank composition is uncertain and we consider it better to choose the initial ratio from physical considerations, since the data are probably not free from systematic error. The initial value of 87Sr/86Sr chosen was 0.709. This is the isotopic composition of the blank in the reagents used in this laboratory. The ratios in Table 5 were corrected for fractionation assuming a true 86Sr/88Sr ratio of 0.1194. The decay constants for the individual analyses were derived from these data and a weighted mean was calculated. The weights are determined from the inverse present
1749
square of the standard deviations which were determined according to the formula: %a =
(%~7,8 - @5%a6,8)/R
+ 2%06,8
+ o/;rc4 8
where %a is the total percentage standard deviation; %c7,8r %a4,8 and %usi8 are the percentage standard deviations for the 87Sr/88Sr, 84Sr/88Sr and 86Sr/88Sr ratios, respectively, and R is the fraction of the “Sr that was radiogenic. A further percentage error of *0.25% was added to the mean decay constant from each batch to allow for an error in the Rb concentration. The value found for the decay constant is 1 = 1.419(*0.012) x 10-l’ yr-‘. Theerror quoted is a two sigma error. It does not include any error in the initial blank Sr composition. The average of 2, 13, 14 and 15 (considered the most accurate analyses) is 1.414(f0.015) x lo- l1 yr-’ (20). This is statistically indistinguishable from the overall mean. We consider therefore that the overall mean 1 = 1.419(+0.012) x 10-l’ yr-’ represents the most reliable measurement of the decay constant of “Rb. This value compares favorably with the two most recent measurements of the decay constant which are that of NEUMANNand HUSTER (1974) who found I = 1.42(+0.03) x lo-” yr-’ by fi counting experiments and AFANASS’YEV et al. (1974), who found I = 1.42 x lo-” yr- ’ from geochronological comparisons. Acknowledgements-We wish to thank Drs. THOMLINSON, FRITZEand MCMULLENfor allowing us to analyse batches of the RbClO, purified 19 yr ago. Without their generosity this work could not have been carried out. This work was supported in part by grants from the National Research Council of Canada. REFERENCES AFANASS’YEV G. D., ZYKOR S. I., GOROKOVI. M. and SHAININ L. L. (1974) Correlativity of geochronometric ages yielded by coexisting minerals with different 4K, “‘Rb and some other radioactive element decay constants used (Abstract). International Meeting for Geochronology, Cosmochronology and Isotope Geology, Paris, August. ALDRICHL. T., WETHERILL G. W., TILTONG. R. and DAVIS G. L. (1956) The half-life of *‘Rb. Phys. Reo. 104, 1045-1047. CATANZAROE. J., MURPHY T. J., GARNER E. L. and SHIELDSW. R. (1969) Absolute isotopic abundance ratio and atomic weight of terrestrial rubidium. J. Rex Nat. Bur. Std. 13A, No. 5, 511~516. FLYNN K. F. and GLENDENIN L. E. (1959) Half-life and beta spectrum of *‘Rb. Phys. Rev. 116, 744-748. MCMULLENC. C., FRITZEK. and TOMLIN~CBN R. H. (1966) The half-life of *‘Rb. Can. J. Phys. 44, 3033-3038. NEUMANNW. and HUSTERE. (1974) The half-life of s’Rb measured as difference between the isotopes “Rb and “‘Rb. Z. Physik 270, 121-127.