Potassium-calcium dates from pegmatitic micas

EARTH AND PLANETARY SCIENCE LETTERS 12 (1971)j399-405. NORTH-HOLLAND PUBLISHING COMPANY

POTASSIUM-CALCIUM

DATES FROM PEGMATITIC

MICAS *

M.L. COLEMAN** Department of Earth Sciences, University of Leeds, Leeds, England Received 17 April 1971 Revised version received 22 September 1971

To determine radiogenic 4°Ca in micas, pure samples of Ca of 100 #g or less were separated by elution from a cation-exchange column in ammonium form: interference from Fe and AI was prevented by complexing them as citrates. The 4°Ca/44Ca ratio was determined with a precision of up to 0.03% by using a spike of 42Ca and 48Ca. K-Ca dates conform with Rb-Sr results on micas from the Scottish Highlands, while more precise determinations on samples from SW Greenland show some divergence. The latter suggest that pegmatitic material may not be suitable for comparison of decay constants.

1. Introduction The purpose of this work was to determine potassium-calcium dates on selected samples, which if sufficiently accurate could be used for comparison of the rubidium and potassium decay constants. Natural calcium consists of six isotopes whose abundances are given in table 1. Table 1 Natural abundances of calcium, Backus et al. [ 1]. 40

42

43

44

46

48

96.88

0.655

0.138

2.12

0.0046

0.200

The ability to use the beta decay of 4°K to 4°Ca as a dating method is hindered by the fact that normal calcium contains about 97 atoms% 4°Ca. This usually makes a radiogenic increment negligible in comparison. Thus samples o f very high K/Ca ratio (greater than 100) and sufficient age, must be used. Inghram et al. [2] confirmed the branching ratio of * The work presented here formed part of a doctoral thesis submitted to the University of Leeds, August 1970. ** Currently at The Department of Geology, University of Alberta, Edmonton 7, Alberta, Canada.

4°K radioactive decay by measuring radiogenic 4°Ar and 4°Ca in a sylvite sample. From spectrographic data, Ahrens [3] showed that some pegmatitic lepidolites possessed low enough concentrations of normal calcium to allow radiogenic enrichments as large as 37%. Polevaya et al. [4] measured a 9% radiogenic enrichment to 4°Ca in a Lower Cambrian sylvite. The absolute amounts of radiogenic enrichment are very small; for example a sample 1000 m y old with 10% K 2 0 contains only 6.4 ppm radiogenic 4°Ca. A survey of some published mineral analyses showed that pegmatitic micas has the lowest calcium contents. Many samples were quoted with CaO content of "0.00%" or "trace" [ 5 - 7 ] . However, subsequent isotope dilution analysis on selected samples showed very few with less than 0.1% Ca. Solid source mass-spectrometry was employed for the calcium isotope analysis using 44Ca as the standard against which increments o f 4°Ca were measured, because it is the next most abundant isotope and is not produced by any known radioactive decay series. Two main considerations control the accuracy of measurement o f the 4°Ca/44Ca ratio: signal-to-noise ratio and correction for mass-discrimination. It was found that only extremely pure samples would give a sufficiently large, stable ion beam and that even the smallest traces of iron or aluminium could reduce the

400

M.L. Coleman, Potassium-calcium dates

emission of calcium ions by a factor of 100: therefore a cation-exchange method of calcium separation was developed. Errors due to instrument mass discrimination can be minimised by rigorous standardisation of operating conditions. A more satisfactory approach is the use of a double spike as an internal standard to monitor instrument discrimination in each analysis [8, 9]. Dodson [10] showed that the error in a four isotope system is lessened by alternating masses of peaks that are mainly spike with those that are mainly normal. In a spike-normal mixture there is a small contribution from spike to normal peaks and vice versa, for which correction can be made. Hirt and Epstein [11] attempted to overcome the problem of measuring 48Ca/total calcium ratios by choosing a double spike of 43Ca and 46Ca which are the least abundant of the natural isotopes. Nevertheless, in minimising the isotopic cross-contamination they aggravated the problem of fractionation because of the small mass difference between the spike isotopes, and also had the problem of measuring the very small 48Ca abundances. Therefore, in the present work a double spike of 4ZCa and 48Ca was chosen. There is a possibility of an increase in the 4°Ca/44Ca ratio due to natural fractionation. However, the effect is likely to be small [12] and if the radiogenic increment of 4°Ca is large enough to be measured satisfactorily then the error due to fractionation will be negligible.

2. Chemical methods The samples, containing between 50 and 100 gg of Ca, were decomposed in a mixture of hydrofluoric and sulphuric acids. In order to obtain hydrofluoric acid of sufficiently low calcium content, the reagent was prepared by dissolving bottled hydrogen fluoride gas in distilled water. The ion-exchange method avoided interference from iron and aluminium by complexing them as citrates. The citrate complex was stabilised by means of an ammonium citrate-citric acid buffer, ptt 3.8, which necessitated having the resin in ammonium form initially. The resin used was Biorad AG 50W-X8, graded at +400 - 2 0 0 mesh. The decomposed sample was washed quantitatively into a weighed beaker using distilled water and dilute

hydrochloric acid. An aliquot of the solution, containing approximately 50 ~g of Ca, was weighed into a weighed beaker into which a quantity of spike was weighed. Citric acid was added followed by concentrated Aristar grade ammonia solution to bring the pH to approximately 3.8, indicated by the occurrence of a greenish-yellow colour due to the formation of the ferric citrate complex. The final adjustment was accomplished by adding 1 molar ammonia solution with a narrow-range pH test paper. After passing the solution through the column and washing it through with more buffer, 0.2 molar hydrochloric acid was used to remove ammonium ions and leave the column in hydrogen form. This process does not elute calcium. The final elution used 1.75 normal hydrochloric acid and the calcium fraction was evaporated to dryness. Initially some trouble was experienced with the varying quantities of blank calcium that occurred, but careful control of the reagents used showed that the blank could be kept constant. The decomposition stage produced 4.3/~g of blank Ca per gram of sample, while the ion-exchange reagents contributed 10.0/lg per analysis. The calcium concentrations quoted later in tables 5 and 6 are those obtained by deducting the calculated blank from the apparent calcium concentration. |sotopically, the blank proved to be identical to the normal calcium standard used. There was no detectable isotopic effect on a standard sample of normal calcium processed by the extraction method. Potassium analyses were carried out by the standard method used at Leeds University for potassium-argon age determinations: flame photometry using an instrument with digital output. The sample solutions were those used for calcium analysis and the reproducibility achieved was better than 0.1% of the concentration.

3. Isotopic analysis 3.1. M e t h o d

The samples were run as calcium chloride using a triple filament system. Measurements were made on a modified A.E.I. MS5 mass-spectrometer of 30 cm radius and 90 ° sector. A peak-switching system which controlled the magnet current was used initially. Because of difficulties with hysteresis effects due to the large mass-difference in calcium analysis, a method

401

M.L. Coleman, Potassium-calcium dates

of direct field control using a flux integrating circuit was developed [13]. This proved to be successful. A separate control, allowing small positive and negative increments to be made to the accelerating voltage, was used to select a position for measurement of the baseline or to check beam centring. Ion currents were read directly using a Keithley vibrating-reed electrometer, the output of which was digitised and fed to a printer and a paper-tape punch. The ion-current of the largest peak was usually in the range 10 -1° to 10 -9 amp but the time during which the beam was satisfactorily stable varied from 30 min to 6 hr. The peaks to be measured were scanned cyclically and a baseline and tuning check were made in each scan; the latter was accomplished by observing beam centring. A typical scan was to read masses 43½, 44, 48, 40, 40+ and 42, where 43½ and 40+ are the baseline and tuning check respectively. The time required to complete one scan was approximately 60 sec. The baseline was found to be level over the whole spectrum so that only one baseline reading was necessary. Due to its relatively low mass, the resolution o f the calcium spectrum is very good and no tailing problem is involved. Johnson-Matthey "Specpure" calcium carbonate was used as a laboratory standard for chemical assay and isotopic ratios. A number of isotopic analyses were made on this material and the results are compared with previous determinations of normal calcium isotope abundances in table 2.

natural variations in terrestrial calcium abundances would be very small, therefore, the different values for 4°Ca/44Ca reported by various workers most likely were due to mass-discrimination. The double spike was made by dissolving in 1 molar HC1 two samples of CaCO3 enriched to 85% and 90% in 42Ca and 48Ca, respectively. The spike solution was analysed isotopically and correction for mass-discrimination was made by analysing mixtures of the spike with aliquots of a gravimetrically prepared solution of the normal standard. Thus, the spike was calibrated with respect to the normal standard both for concentration and isotopic composition. Therefore the choice of the standard value for 4°Ca/44Ca does not affect the isotopic analysis o f unknown samples since the values altered by discrimination are corrected to the value of the standard (via the spike) and variation from natural fractionation is likely to be indetectably small. The spike calibration was checked periodically. The concentration of 42Ca in the spike was 0.0971 micromoles/g. The values o f the spike ratios used are given in table 3. Table 3 Isotopic ratios of the double spike. 40/42

44/42

48/42

0.256

0.01910

1.1710

1

7

8

3.2. Data reduction

Table 2 Determinations of natural calcium isotope abundances, normalised to 4°Ca/44Ca = 45.70. 42/44

43/44

46/44

48/44

Nier, 1938 [14]

0.306

0.070

0.0016

0.092

White and Cameron, 1948 [15]

0.301 7

0.062 2

0.0015

0.084 2

Backusetal.,1964[1] 0.309 6

0.0651 15

0.00156 0.094 6 3

This work

0.0619 6

0.0016 2

0.3095 9

0.093 2

The data are normalised to 4°Ca/44Ca = 45.70 to eliminate differences caused by mass discrimination. This value was chosen as a result o f the work o f Backus et al. [1]. Hirt and Epstein [ l l ] showed that the

It is necessary to correct for the small contribution of normal isotope to one that is mainly spike in a mixture of the two (and vice versa). Error magnification due to isotopic cross-contamination is small and can be minimised by calculation of the optimum spike/ normal ratio using the method developed by Dodson [101. During the course of a series o f measurements over six hr the apparent 4°Ca/44Ca ratio could vary by as much as 4%. However, the use of a double spike has given precision equivalent to 0.2% in the corrected 4°Ca/44Ca ratio generally (0.03% in the best case) compared with 1% and 0.6% in work by Letolle and Stahl respectively [16, 17] on unspiked samples. The results o f the sample-spike mixture analyses were calculated by the method of direct solution of the linear algebraic equations formulated by

402

M.L. Coleman, Potassium-calcium dates

Table 4 Description of samples. Oxford University collection no.

Mineral and loc',dity

Rb ppm

Normal Sr, ppm

20746 20747 20751

Muscovite from Laxfordian pegmatite west of Badcall, Scourie area Biotite from granitic pegmatite Badcall Biotite from granitic pegmatite west of Scourie More

31708 31721 31734

Polylithionite from pegmatitic naujaite boulder below ll{maussaq Glacier Polylithionite from pegmatite in naujaite, North-West ll{maussaq Polylithionite from southern part of II[maussaq area

6958 1439 2247 1992 6675 10025 1980

5.4 9.4 2.6 3.3 0.89 1.78 0.74

Dodson [10]. The method applies a correction for mass discrimination which is linearly dependent on mass. This method is completely satisfactory as it gave consistent results in analyses where the uncorrected ratios varied continuously during the series o f runs. Data reduction was accomplished using a computer program written in Algol 60 which accepted raw data from the mass-spectrometer output. In order to avoid a skewed result caused by the inclusion of one or two wild ratios, extreme results were rejected by an objective test developed from Chauvenet's criterion

[18a]. 4. Sample selection

5. Results The values used in the calculation are 4°K: 0.0119 atom %, XB" 4.72 X 10-1°/yr, Xk: 0.585 X 10- ~o/yr and the rubidium-strontium dates included for comparison have been recalculated to a half-life of 5.0 X 10 x° yr, where necessary. Each date is the result of a series of mass-spectrometer runs on a separate spiked sample, weighted proportionally to the inverse of the square of the standard error before calculation of the weighted mean. The quoted error in the weighted mean is given by e 2 = ~((~-xi)2 w

In order to test the method, material was chosen that had been dated previously by the rubidiumstrontium method. Muscovite and biotite samples from the Scottish Highlands which had been analysed by Giletti et al. [19] had shown themselves to be potentially suitable [20]. They were chosen not because of their geochronological interest but because they have very high Rb/Sr ratios and it was hoped that the K/Ca ratios would be similarly high. Some polylithionite samples from southwest Greenland, examples of extreme differentiation, were thought to be ideal for analysis since they contain less than 2 ppm normal strontium, allowing the radiogenic proportion to be greater than 90% [21 ]. This material was chosen, despite its rarity, in the hope that it might produce dates sufficiently accurate to be compared precisely with strontium dates. The samples are described in table 4. The polylithionites were available in several mesh sizes which were treated separately.

X wi)

(n -- 1 ) X ~

Wi

(from ref. [18b] )

where x i is a result, w i its weighting, n the number of results, 2 the weighted mean and e w its error. Earlier work with long runs showed that between-run variation was sometimes greater than within-run errors, So that these variations (which may have been due to long term shifts in peak tuning) might become more apparent, each run was limited to 15 to 20 scans after which the tuning was checked. Thus more than one run would be made on each sample loading. The results are presented in tables 5 and 6. The aliquot taken for analysis usually contained 50 to 100 #g Ca, so that the ideal spike/normal ratio could be achieved without using excessive amounts of spike, The calcium concentration quoted is corrected for blank calcium contribution. The radiogenic increment is given as a fraction of the total measured calcium, including blank. All samples except 20746/1 were washed briefly

403

M.L. Coleman, Potassium-calcium dates Table 5 Highland mica results (40* --- radiogenic 4°Ca). Sample

20746/1 20746/2 20746/3 20747 20751

No. of runs made

Ca

40*/40 %

40* (ppm)

K, %

(ppm)

6 2 3 2 8

2380 1720 1510 3920 640

0.50 0.54 0.60 0.37 1.12

12.9 10.7 11.2 15.4 10.4

8.29 8.29 8.29 4.39 7.67

± 1.3 ± 1.2 ± 0.6 ± 4.4 ± 0.6

K-Ca age (my)

1700 1500 1550 2700 1550

± ± ± ± ±

180 ] 170 80 800 100

Rb-Sr age (my) [ 19] Mica

Feldspar

1590 ± 35

1740 -+40 1640 _+40

1570 -+ 30 1640 -+ 35 1580 +- 30

2270 _+60 2460 _+60

Table 6 Southwest Greenland polylithionite results (40* = radiogenic 4°Ca). Sample

Mesh

No. of runs made

Ca (ppm)

31708 A/1 A/2 B C

+80-40 +80-40 +40 hand picked +4

3 12 2

162 144 590

1

31721 A/1 A/2 B/1 B/2 B/3

+80-40 +80-40 +40 +40 +40

31734/1 /2

+40-20 +40-20

40"/40 %

40* (ppm)

K, %

K-Ca age (my)

3.9 6.1 1.7

6.7 + 0.9 9.4 ± 0.5 10.4 ± 1.3

9.20 9.20 9.40

990 ± 140] 1270 + 7 0 | 1350 + 170

440

1.9

8.6 ± 0.6

7.89

1330 + 9 0 J

3 6 10 2 1

140 60 280 109 30

3.4 9.33 2.63 6.08 16.5

5.1 6.2 7.92 7.1 6.9

± 0.6 +-0.14 -+ 0.14 ±0.7 -+0.9

9.52 9.52 9.51 9.51 9.51

770-+ 952+ 1090 + 1010+ 980+

11 6

67 39

8.60 13.29

6.72 ± 0.07 6.83 -+ 0.14

9.69 9.69

in 0.5 n o r m a l h y d r o c h l o r i c acid, t o r e m o v e surface dust. This process r e m o v e d c o n s i d e r a b l e a m o u n t s o f n o r m a l c a l c i u m w i t h o u t a f f e c t i n g the r a d i o g e n i c cont e n t or p o t a s s i u m c o n c e n t r a t i o n .

6. Discussion

6.1. Highland mica results The a c c u r a c y o f d e t e r m i n a t i o n o f the r a d i o g e n i c c o n t e n t is generally a f u n c t i o n o f the degree o f enr i c h m e n t . Despite the small radiogenic i n c r e m e n t s , the dates c a l c u l a t e d agree well w i t h t h o s e f r o m rubid i u m - s t r o n t i u m data. D a t e s b a s e d o n a half-life o f 4.7 X 10 l ° yr for SYRb w o u l d be 6% l o w e r a n d the a g r e e m e n t w o u l d n o t be significantly worse.

Rb-Sr age (my) [21]

/

1095 +_24

90 19 17 100 ] 130 !

950-+ 10 962 -+ 20

J

1077 -+ 24

1086 ± 20

6.2. Southwest Greenland polylithionites The radiogenic e n r i c h m e n t was far greater t h a n in the S c o t t i s h samples, w h i c h allowed m o r e precise det e r m i n a t i o n s to be m a d e . Despite some i n t e r n a l inc o n s i s t e n c y , t h e a p p a r e n t c a l c i u m ages differ signific a n t l y f r o m the s t r o n t i u m o n e s in some cases. Careful e x a m i n a t i o n o f the e x p e r i m e n t a l m e t h o d does n o t reveal a n y p r o b a b l e cause for d o u b t i n g the v a l i d i t y o f the data. This is c o n f i r m e d to some e x t e n t b y the results f o r t o t a l c a l c i u m c o n c e n t r a t i o n a n d a b s o l u t e radiogenic 4°Ca c o n t e n t . B o t h are d e t e r m i n e d in t h e same i s o t o p i c analysis a n d while t h e f o r m e r varies wildly, t h e l a t t e r is r e a s o n a b l y c o n s t a n t , suggesting t h a t t h e y are i n d e p e n d e n t o f e a c h o t h e r . Since rep e a t e d d e t e r m i n a t i o n s o f t h e b l a n k gave s a t i s f a c t o r y r e p r o d u c i b i l i t y , t h e surprisingly large v a r i a t i o n s in c a l c i u m c o n t e n t m u s t be real. This m i g h t b e e x p l a i n e d

404

M.L. Coleman, Potassium-calcium dates

by i n h o m o g e n e o u s distribution possibly caused by calcium being present as discrete inclusions of a calcium-rich phase. The data suggest that there may be a relationship between grain-size and tire apparent age, in that the smaller grains may have less radiogenic calcium, but the scatter is too great to confirm this. It is interesting to note that the potassium c o n t e n t of 31708 C differs considerably from that of the others, again suggesting a correlation between grain-size and geochemical history. The quality of the r u b i d i u m - s t r o n t i u m analyses quoted is very good and the apparent ages are very accurately determined, particularly due to the high rubidium c o n t e n t s of the samples. However, because of the high r u b i d i u m and the pegmatitic origin of the samples, the results may be geochronologically invalid. hr work on the Harney granite, South Dakota, Riley [22] showed that the apparent Rb-Sr ages on high rubidium pegmatitic micas were slightly lower than that of the granite, and that muscovites separated front the granite showed "age loss", proportional to their rubidium c o n t e n t , of up to 14% when compared with the age from the whole-rock isochron. In their work on tire polylithionite samples, Moorbath el al. [21] presented a potassium-argon age of 1 180 m y for sample 31708, b u t u n f o r t u n a t e l y no estimate of the u n c e r t a i n t y was possible. hr the light of these considerations it is possible that neither the s t r o n t i u m nor calcium apparent ages represents the date of crystallisation. The discrepancies may be explained by differences in geochemical reaction to the post-crystallisation history. It appears that the only samples which will give sufficiently precise results, because of thetr e n r i c h m e n t in alkalis and depletion in alkaline earths, are likely to be pegmatitic. Under these conditions the mother-daughter isotope relationship may not truly reflect the age of the sample. If this assumption is true then it precludes the use of pegmatitic material for the comparison of decay constants.

Acknowledgements I am i n d e b t e d to Dr. M.H. Dodson for his helpful advice and encouragement throughout tire project and for Iris critical reading of the typescript, and to Dr. S. Moorbath who provided all the samples. Thanks

are due to Mr. F. Buckley for the useful discussions of the chemical problems and to the operators of the computer installation at Leeds University for their helpful service. The N.E.R.C. provided financial support by a Research Studentship, which is gratefully acknowledged. The mass-spectrometer was bought through N.E.R.C. grant G R 3/361.

References {1 ] M.M.Backus, W.H.Pinson, L.F.Herzog and P.M.Hurley, Calcium isotope ratios in the Homestead and Pasamonte meteorites and a Devonian limestone, Geochim. Cosmochim. Acta 28 (1964) 735. [21 M.G.Inghram, It.Brown, C.Patterson and D.C.ftess, The branching ratio of K 40 radioactive decay, Phys. Rev. 80 (1950) 916. [3] L.H.Ahrens, ]'he feasibility of a calcium method in the determination of geological age, Gepchim. Cosmochim. Acta 1 (1950) 312. [4] N.I.Polevaya, N.E.Titov, V.S.Belyaev and V.D.Sprintsson, Application of the Ca method in the absolute age determination of sylvites, Geochemistry 8 (1958) 897. [5] W.A.Deer, R.A.Howie and J.Zussman, Rock Forming Minerals, vot. 3 (Longmans, London, 1962). [6] M.D.Foster, Interpretation of the composition of trioctahedral micas, U.S.G.S. Prof. Paper 345-B (1960) 41. [71 P.Quensel, The paragenesis of the Varutr~isk pegmatite, including a review of its mineral assemblage, Arkiv Min. (;eol. (Stockholm) 2, 2 (1957)69. [8] L.A.Dietz, C.F.Pachucki and G.A.Land, Internal standard technique for precise isotopic abundance measurements in thermal ionisation mass spectrometry, Anal. Chem. 34 (1962)709. [9] M.ft.Dodson, A theoretical study of the use of internal standards for precise isotopic analysis by the surface ionisation technique, Part 1. General first order algebraic solutions, I. Sci. Instrum. 40 (1963) 289. [ 10] M.tt.Dodson, A theoretical study of the use of internal standards for precise isotopic analysis by the surface ionisation technique, Part I1. Error relationships, J. Sci. Instrum. Ser. 2, 2 (1969)490. [ 11 ] B.Hirt and S.Epstein, A search for isotopic variations in some terrestrial and meteoritic calcium, Trans. Am. Geophys. Un. 45 (1964) 113. [ 12] J.T.Corless, Determination of calcium-48 in natural calcium using neutron activation analysis, Anal. Chem. 38 11966) 810. [ 13] M.tl.Dodson, Stabilization of magnetic field using a flux integrating circuit, 1. Phys. E: Sci. Instrum. 3 (1970) 708.

[14] A.O.Nier, The isotopic constitution of calcium, titanium, sulphur and argon, Phys. Rev. 53 (1938) 282. [ 15] J.R.White and A.1;.Cameron, The natural abundance of isotopes of stable elements, Phys. Rev. 74 (1948) 991.

M.L. Coleman, Potassium-calcium dates

[16] R.Letolle, Sur la composition isotopique du calcium des echantillons naturels, Earth Planet. Sci. Letters 5 (1968) 207. [17] W.Stahl, Search for natural variations in calcium isotope abundances, Earth Planet. Sci. Letters 5 (1968) 171. [ 18] W.M.Smart, Combination of observations, Cambridge University Press (1958) (a) 190 (b) 101. [19] B.J.Giletti, S.Moorbath and R.St.J.Lambert, A geochronological study of the metamorphic complexes of

405

the Scottish Highlands, Quart. J. Geol. Soc. 117 (1961) 242. [20] M.L.Coleman, Radiogenic calcium in micas, unpublished M.Sc. dissertation, University of Leeds (1967). [ 21] S.Moorbath, R.K.Webster and J.W.Morgan, Absolute age determination in South-West Greenland, Medd. om Gr~mland 162 (1960) no. 9. [22] G.H.Riley, Isotopic discrepancies in zoned pegmatites, Black Hills, South Dakota, Geochim. Cosmochim. Acta 34 (1970) 713.