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Calcium isotope ratios in the homestead and pasamonte meteorites and a devonian limestone
Geochimica et Cosmochimica Acta 1964, Vol. 28, pp. i35 to $40. Pergamon Press Ltd. Printed in Northern Ireland
Calcium isotope ratios in the Homestead and Pasamonte meteorites and a Devonian limestone MILO M. BACKUS,* W. H. Pwso~,t
L. F. HERZOG: and P. M. HURLEY~
Department of Geology snd Geophysics Massachusetts Institute of Technology, Cambridge, Massachusetts (Received Abstract-Isotope abundance were, in atom per cent:
8 Julze 1963;
results 40 42 43 1:: 48
obtained
revised 21 dqust for
96.88 0.655 0.138 2.12 0.0046 0.200
& & * & f &
“common”
1963) calcium
(Devonian
limestone)
0.05 0.006 0.002 0.04 O.OOlO] 0.006
Calcium extracted from the Homestead, Iowa chondrite was isotopically identical to “common” calcium to within &l per cent, except for a better measurement of *6Ca,‘*oCa ratio of (33 i 1) x 10m6. In contrast. 40Ca may be depleted by as much as 3 per cent in the Pasamonte achondrite, which reportedly has a Ca/K ratio of 150.
ISOTOPE ratio data for element 20, calcium, are of particular interest to cosmology, nucleogenesis and isotope geochemistry because of the unusually broad mass range covered by this element, and also to geo- and cosmochronometry because one isotope, 40Ca, is a daughter of radioactive 40K. Calcium has six stable isotopes having respectively 20, 22, 23, 24, 26 and 28 neutrons, with the relative abundances indicated in Table 1. The isotope 40Ca makes up about 9i per cent of the total; this is unfortunate from the age-determination standpoint because 4OK is a very rare isotope, making up, at present, only 0.012 per cent of total potassium. In fact, if one accepts that the per cent weight abundances of potassium and calcium in the terrestrial crust are 2.6 and 3.6 respectively (RAXKAMA, 1950), the overall increase in the abundance of 4oCa due to K-decay in 5 x 10’ yr will have been only about 0.1 per cent if one makes the simplifying (but no doubt erroneous) assumption that the crust has always been a closed system.5 On the other hand, if material has been added to the crust. from * Present address:
Texas Instruments Incorporated, Dallas, Texas. t Present address: Massachusetts Institute of Technology, Cambridge. Massachusett,s. $ Present address: The Pennsyh.ania State University, University Park, Pennsylvania. 5 The present-day 4% content of the crust, is 3.1 x 10B4 u% per cent, (2.6 x 1.2 s 10e4), while 40Ca is 3.5 wt. per cent (3.6 x 0.97); hence the ratio 4oK/40Ca is about 0.9 x 10m4. Because 40K has a half life of onlv 1.33 x 10n yr, 5.3 x 10s yr ago there would have been 16 times as much 4oK. But 4oK has two modes of decay, t,o 4oAr as well as 4oCa; however, since about 88 per cent goes to calcium, 4.1 X lows wt. per cent of 40Ca will have been produced in the crust in 5.3 x lo9 years by this trrtnsmutation, if the Kabundance assumed is correct; and, if the &-abundance assumed is correct, this amount represents about 0.1 per cent of total calcium. 735
time ta time from the mantle, the increase expected will be less because the mantle is more basic than the arust. Thus the only hope for utifisation of the K/Ca age m&hod lies in amens ah&h, a%the time of crys~~s~~~o~~ ~nce~tr~t~ K and ex&ded @a (Bat-zros, 1955). Recent demonstrations t&at differences in isotope ratio exist between terrestrial and meteoritic silver, barium, and xenon, have focused new interest tm the isotopes of calcium. The data reported herein were mostly obtained in 19&&-5 (BACEUS, L956; Hamcra et al., 19&S; E~~zoa, lQSS>, but in the interim no ~d~~~un~~ anafy3es have ~~~a~d in the ~~ra~~r~~ One reason for the seeming neglect of this important efement is the fact that its analysis by mass spectroscopy is difficult. Compounds that have a sufficiently high vapor pressure at low temperatures to make possible the study of calcium with gas-analysis isotope ratio mass spectrometers are unknown, and the element is ionised very poorly in a t~~rrn~o~c source. Further, with a th~~~~c source, ano%her technical proMem, isotope ~act~~~a~~on during va~r~t~~~ ~~~~ et al., 19s?4), Bas Emited the isotope ratio rephcation precision achievable. The problem is especially serious in the case of calcium because of its broad mass range, The present paper gives results of replicate analysis of Ca from a terrestrial limestone, a chondrite with a CafB ratio of about 11, and an achondrite with a CajK ratiu of about 150, The ~rograrn~~ is being co&rn.~ed (L. E. M,f with an &tempt to develop ~rn~~~~~d~ech~~q~es for isotope ratio dete~~~~~o~~ and in particular, to control the problem of fractionation during analysis, AXALYTICM,, TECRNIQVE The antips rep&ud were made zztthe Massachuse%s Institute of Technology with a 6 in. ra&us, 60’ de~~~t~on~~~~e-~~~~s~g mass s~~~~me~~r designed by one of us (L. F, If.). Ion currents ~a~~~~ a s~~~~~-be~rnc&h&w were meas~& either with a d.c. amplifier* or a dynamic capacitor amphfier? (1011$2 input resistor), with a 10 in. strip-chart recorder used for peak display, Ion acceleration was approximately 20QOV. Unfortunately, neither an electron multiplier detector or ~ip~~-~larne~~ source wes available at the time these analyses were made. The ca~c~~~ iso%opes~ct~rn BW recorded re~~t~d~~ by vsry’ing the magnetos fiefd in a ~~c~procat~ngfashion. The ana@eo trrbe ww c~~t~~~~~s~ypumped by s single, am-stage mercury diffusion pump (Priest design), and during ,thesemeasurements, the analyser pressure was between 5 x 10M7and 5 x 10-6mm Hg, as measured by an ion gauge close to the source. Calcium was separated from i&hetatai sample &x mZgss~ec~rorn~~~ analy&r by ion exchange; it was loaded or&o the therrn~o~~c source Samen% &her as oxafate fin suspension) or nitrate (in solution). The eEciency with which cabSum is ionised by the surface ionisation (thermionic) techsique is rather small because of its high first ionisation potential (649 eV); for this reason, and because of the rarity of some of the isotopes to be studied, it was not generally possible to obtain successful. Ca z~na3yseswhen Ihe sample was placed on Ta ribbon (@-001 x 9430 x o+%@ in,) &laments, suob as bad been successf~ly used for, for ~~~rn~~e~R, Rb * Essentially identical to me described by A. 0. C. NI~:R { 1947). t Applied Physics Carpnration Model 30.
Calcium isotope ratios in the Homestead and Pasamonte meteorites
735
and Sr analysis.
Therefore, a considerable effort was expended in finding a substrate which would provide greater ionisation efficiency. Eventually a substrate consisting of rough, porous platinum electrolytically deposited on tantalum was adopted; it appeared to be about ten times as efficient, as Ta ribbon on the production of Ca ions. The overall efficiency of the instrument (ions collected/atoms applied) was measured as 0.4 x 10e6 or iess for Ca on an oxidised Ta filament, compared to about 40 s 10m6for Sr. In the analyses reported herein, several types of substrate were used: oxidised tantalum, TaO plus borax, new Pt on Ta, and used HF-cleaned Pt on Ta. No influence of substrate on isotope ratio was observed. The samples analysed contained from 1 to 10 ,ug of calcium. In order to obtain a set of isotope ratios having precisions of the order of one per cent for all ratios except those involving 46Ca, a 40Ca ion current of at least 6 x lo-l2 A for a Samples were run to completion and the minimum of one hour was required. emission weighted isotope ratio averages were calculated for each run. From 20 to 60 peak sets were measured for each run. RESULTS 1. Terrestrial
calcium
(a) Devonian limestone. A bottle of Mallinkrodt “Calcium A. R., low alkali” was established as the calcium standard for the M.I.T. Laboratory. According to the manufact,urer, this calcium “ -originates in a limestone from a quarry near Jamesville, New York. This is a Devonian age limestone and comes from principally two strata: (1) Onondaga limestone, immediately above Oriskany sandstone and (2) Helderbergian or Stromatopora limestone, immediately below the Oriskany sandstone.“* Four different measurements made on this sample, using somewhat different ion sources, gave the results listed in Table 1. The first three determinations were made using the d.c. feedback amplifier, while the fourth was made with the The 46Ca/Wa ratio was determined in a separate dynamic capacitor amplifier. run as 0.000047 i O*OOOOlO. The precision of these measurements: expressed as standard deviation, appears to be of the order of one per cent; it is best for the 42140, 44140 and 42144 isotopic ratios. The absolute errors suggested in Table 1 include, in addition to the calculated precisions, allowance for an isotope fractionation of 13 per cent for the 48140 ratio, and systematically smaller fractionation effects in the case of the other ratios, in which the mass differences are less extreme. have been altered to take The ratios and abundances designated as “corrected” into account a small postulated instrumental mass discrimination effect; the relative corrections vary with the square root of the mass ratio (BACK~S~ 1955). (b) Other measurem.ents. Other modern measurements of the isotope abundances of reagent calcium have been made by NIER (1938)) SHERWIN and DEMPSTER ( 1941)? WHITE and CAMEROK (1948) and HERZOU (1951). These data are presented in Table 2. Herzog, using an instrument very similar to the one used in the present study, obtained abundances in better agreement with the present results and WHITE and CAMEROR’S than with NIER’s. * In a letter from J. C. PERRY (1955).
MILOM. BACKVS,W. H. PINSON,L. F. HERZOCand P. M. HUXLEY
738
Table l(a).
Composition of calcium from a Devonian limestone (raw data)
Primary ion sonroe CaC,O, on TaO Ca(NO,), borax on TaO
Ca(NO,)a on used Pt plated filament Ca(NO& on new Pt plated filament Average *
Total no. of peak sots
42
Z
43 0
44 a0
48 a0
48 Z
20
0~00678
0.00144
0.0223
0.00212
0.312
23
0.00696*
0.00144
0.0222
0.00208
0.298*
57
0.00682
0.00142
0.0222
0.00212
0.310
53
0.00697 0.00680
0.00142 0.00143,
0.0223 0.0222,
0~00210 0~00210,
0.308 0.310,
* Ratios involving Ca42in the second rnn appear to be anomalous and were not included in the average. This rnn was the poorest in the group from the standpoint of emission stability. Table l(b). Composition of calcium from a Devonian limestone (corrected for instrumental mass discrimination) Isotope Ratios
Isotope Abundances (atom per cent)
48 42
0.306 & 0.007
40
96.88 5 0.05
42 40
0.00676 i 0.00006
42
0.655 + 0.006
0*00142 + 0.00002
43
0.138 + 0.002
0.0219 i- 0.0004
44
2.12 * 0.04
0~000047 5 0~000010
46
0.0046 & 0.0010
0.00206 & 0.00006
48
0.200 & 0.006
43 ;i-d 44 0 46 40 48 a0
The stated standard deviations may be considered a measure of the absolute accuracy of the stated values, except in the case of 40Ca. Apparently, all the earlier analyses were of reagent calcium. Thus the ratios involving 40Ca measured by these workers might be expected to be identical only if, fortuitously, the same samples were analysed, or if the radiogenic contribution in all cases is (as it is believed to be) negligibly small. However, 48Ca/42Ca and 48Ca/Wa are both higher in the analyses made in the present study, by 9-11 per cent, than in the analyses made by NIER and WHITE and CAMERON. The observed differences (with the exception of NIER’S value for 43/40) are consistent with the hypothesis that the values of NIER and WHITE and CANERON show a slight depletion in the heavier isotopes due to mass discrimination. This may be due to the fact that only the first fraction of the sample evaporated was in general measured. The possibility of such an effect was suggested by NIER (1938). In our measurements the apparent isotope ratios changed by up to 17 per cent during evaporation of a single sample with the early fraction depleted in heavier isotopes.
and method
etal. (1954)
. __ ._ -
._ -
+ %a t 44% $ Wa
Pasamonte achondrite
-3.4
f
3
AO.00677 0.00008 0.0070 ~0~00016
Homestead
chondrite
0.00680 f 0.00006
CaCI,
Reagent
.
.-
--
_I
f
6
-2.2
f
0.0224 &0.0004 0.0229 *0.0006
0.00142 * 0.0003 0.00153 f 0.0006
-7.7
0.0223 -f 0.0004
000143 f0~00002
44140
0.0213 f0.0007 0.0220 *0~0002 0.0226 & 0.0003
43140
5
46140
0~000033 ~0~000001 --
0~000047 -&0~000010
ratio data (as measured)
0~00150 &0~00004 0.00136 *0~00004 -
isotope
0.0066 *0~0002 0.0066 *0~0001 0.00675 f 0~00008
42140
Reagent Ca from Devonian limestone
Ca
Reagent
Ca metal
__-
Sample
2. Calcium
was measured in 31 sets from 3 runs. measurable in 17 sets was measured in a separate run using a large sample.
Indicated depletion in ‘OCa, Pasamonte vs. Homestead (Oh)
BACKUR
vaporisation, El3 ionisation WHITE and CAMERON (1948) thermionic HERZOG(1951) thermionic, Pt filament BACKIJ~ etal. (1955) thermionic, Pt and TaO filaments BACKUS etal. (1955)
NIER (1938)crucible
Analyst
Table
-4.3
f
7
0.00208 *O.OOOOF 0.00217 &0~00008
0~00210 ~0~00006
0.00191 &0.00006 0.00185 *0~00002 -
48140
1
14
122$
1537
4
1
81*
Total no. of sets
5
runs
No. of
740
MILOM. BACKUS,W. H. PINSON,L. F. HERZOCand P. M. HURLEY
The results reported represent an emission total evaporation history of the sample.
weighted
average
obtained
from the
2. Homestead chondrite We have already reported an Rb-Sr age study of the Homestead chondrite (HERZOG and PINSON, 1956) and this plus K-Ar data point to a crystallisation age of about 4.5 x log yr for this meteorite. The Ca/K ratio in Homestead is reported to be about 11 (UREY and CRAIG, 1953) so that one will expect only a 0.01 per cent increase in its *OCa in 4.5 x log yr. Ta.ble 3. Calcium isotope ratios not involving calcium-40 Sample
Analyst
Ca metal NIER, 1938 WHITEand CAMERON, Reagent Ca 1948 Reagent CaCl, HERZ(JG,1951 Staasfurt sylvite HERZOG,1951 Homestead chondrite BACKTJS et aZ., 1955 BA~KUSet al., 1955 Pasamonte achondrite Bikita lepidolite BACKUSet al., 1955 Devonian limestone (raw) Devonian limestone (corrected)
44142
48142
48143
44143
42143
3.24
0.298
1.28
14.2
4.40
3.34 3.34 3.26 3.31 3.27 -
0.280 _ 0.307 0.312 -
1.36 _ 1.47 1.42 -
16.1 _ 15.8 15.0 15.8
4.85 _ 4.77 4.58 -
3.27
0.310
1.47
15.6
4.75
3.24
0.306
1.45
15.5
4.77
An excellent run was obtained on Homestead calcium; 122 sets of peaks were obtained for all isotopes except the rare *Va, for which just 1’7 measurements were made. The ratios measured (Table 3) all agree to within &l per cent with those of the Devonian limestone, except possibly in the case of *Va, but in this case the limestone analysis carries a large error. For this isotope, the Homestead abundance, which has a standard deviation of 13 per cent, is in good agreement with NIER’S analysis of a terrestrial sample. 3. Pasamonte achondrite We chose this meteorite for analysis in an earlier Rb-Sr study because it was reported (UREY and CRAIG, 1953) to have a very low K/Ca ratio (l/150). This indicated that the Rb/Sr ratio should also be so low as to preclude any measurable change in a7Sr relative abundance during 10 x 10Qyr. In the earlier paper already mentioned (HERZOG and PINSON, 1956) we solved for the time at which the relative isotopic abundance of 87Sr would have been the same in both Homestead and Pasamonte, and found it to be roughly (3 f 1) x 109 yr. Unfortunately only a mediocre run was had on Pasamonte Ca, and it was not possible to repeat it due to exhaustion of sample. Fourteen peak sets (exclusive of *%a) were recorded. Isotope ratios not involving *OCa are in agreement, within
Calcium isotope ratios in the H0mestea.d and Pasamonte
meteorites
541
the experimental precision, with those found for the limestone, but ratios involving “OCa appear to be up to 3 per cent lower in Pasamonte. It is difficult to reconcile such abundances with the concept of a common origin time of the two meteorites about 5 x 10Q yr ago, although, because of the large limits of error of the measurements, this is not impossible. Because of the implica,tions of this result, the Pasamonte calcium should be remeasured with greater precision. 4.
Variations in, tlzr abundance ratios of Ca isotopes other than. 40Ca
Ss discussed above, the limits of error of the single-filament technique for the isotope ratio determination of calcium are so large as to make impossible a search for small differences in ratio. But the present data do indicate that any such differences must at least be less than 3 per cent for ra.tios involving 42Ca, 44Ca, and 4sCa for these samples. SCWMARY The relative abundances of the six isotopes of calcium are the same for calcium from the Homestead, Iowa chondrite as for reagent Ca derived from a terrestrial Devonian limestone, within the limits of error of the measurements; but for Ca derived from the Pasamonte achondrite, while the other relative abundances are “normal”, the partly radiogenic isotope 4OCa mav be depleted by as much as 3 per cent,. Using accepted potassium and calcium abundances (2.6 and 3.6 per cent), and the isotope ratios measured in the limestone, isolated material of the composition of the t,errestrial crust could only have had the Pasamonte 40Ca relative abundance about 12 x log yr ago. Further, if the K/Ca ratio reported for Homestead by others is correct, this meteorite would have had the abundance found in Pasamonte about 16 x log yr ago. Both results are incompatible with the K-Ar, Rb-Sr and other ages of the objects and suggests that further work on calcium abundances should be carried out. Acknowledgements-This work was supported in large part by funds provided Energy Commission under grant AT(30-I)-1381.
by the Atomic
REFERENCES BACKUS M. M. (1955) Xa.ss Spectrmetric Determination of the Relutiae Isotopic Abundances Ph.D. Thesis, Massachusetts Instibute of of Calcium and the Determination of Geologic Age. Technology. HERZOG L. F. (1951) Unpublished report. HERZOG L. F., PISSOK W. H.. BACKUS M. M., STRICKLAND L. and HERLEY P. M. (1954) i-a&tiom in Isotopic Abundances of Sr, Ca and ,4 and Relakd Topics. V.S.A.E.C. Document N.P.O. 3934-11 (4). HERZOG L. F., PINSOP; W. H., BACKUS M. M., STRICKLAND L., CROMIER R. F., GREENEWALT D. and HTJRLEY I’. M. (1955) Variations in. Isotopic Abundances of ST, Ca and 9 and Related Topics. U.S.A.E.C. Document N.P.O.-3935-11 (4). X.R.C. Kucl. Sci. Ser. Report 19, HERZOG L. F. (1956) Rb-Sr and K-Ca Analyses and agea. 114-129. HERZOG L. F. and PINSON W. H. (1956) Rb/Sr age, elemental and isotopic abundance studies of stony meteorites. Amer. J. Sci. 224, 555-566. NIER A. 0. C. (1938) Isotopic composition of Sr, Ba, Bi, Tl and Hg. Phys. Rev. 53, 282.
742
MILO 51. BACKUS, W. H. PINSON, L. F. HERZOG and P. M. HTJRLEY
NIER A. 0. C. (1947) A mass spectrometer for isotope and gas analysis. Rev. Sci. Iwtwm. 18, 398-411. RANEAMA K. and SAHAMATH. G. (1950) Geochemktry. University Chicago Press, Chicago. SHERWIN C. W. and DEMPSTERA. J. (1941) The relative abundance of the calcium isotopes. Phya. Rev. 59, 114. UREY H. C. and CRAIG H. (1953) The composition of the stone meteorites and the origin of the meteorites. Geochim. et Comwchim. Acta 4, 36. WHITE J. R. and CAMERONA. E. (1948) The natural abundance of isotopes of stable elements. Phya. Rev. 74, 991.
Calcium isotope ratios in the Homestead and Pasamonte meteorites and a Devonian limestone MILO M. BACKUS,* W. H. Pwso~,t
L. F. HERZOG: and P. M. HURLEY~
Department of Geology snd Geophysics Massachusetts Institute of Technology, Cambridge, Massachusetts (Received Abstract-Isotope abundance were, in atom per cent:
8 Julze 1963;
results 40 42 43 1:: 48
obtained
revised 21 dqust for
96.88 0.655 0.138 2.12 0.0046 0.200
& & * & f &
“common”
1963) calcium
(Devonian
limestone)
0.05 0.006 0.002 0.04 O.OOlO] 0.006
Calcium extracted from the Homestead, Iowa chondrite was isotopically identical to “common” calcium to within &l per cent, except for a better measurement of *6Ca,‘*oCa ratio of (33 i 1) x 10m6. In contrast. 40Ca may be depleted by as much as 3 per cent in the Pasamonte achondrite, which reportedly has a Ca/K ratio of 150.
ISOTOPE ratio data for element 20, calcium, are of particular interest to cosmology, nucleogenesis and isotope geochemistry because of the unusually broad mass range covered by this element, and also to geo- and cosmochronometry because one isotope, 40Ca, is a daughter of radioactive 40K. Calcium has six stable isotopes having respectively 20, 22, 23, 24, 26 and 28 neutrons, with the relative abundances indicated in Table 1. The isotope 40Ca makes up about 9i per cent of the total; this is unfortunate from the age-determination standpoint because 4OK is a very rare isotope, making up, at present, only 0.012 per cent of total potassium. In fact, if one accepts that the per cent weight abundances of potassium and calcium in the terrestrial crust are 2.6 and 3.6 respectively (RAXKAMA, 1950), the overall increase in the abundance of 4oCa due to K-decay in 5 x 10’ yr will have been only about 0.1 per cent if one makes the simplifying (but no doubt erroneous) assumption that the crust has always been a closed system.5 On the other hand, if material has been added to the crust. from * Present address:
Texas Instruments Incorporated, Dallas, Texas. t Present address: Massachusetts Institute of Technology, Cambridge. Massachusett,s. $ Present address: The Pennsyh.ania State University, University Park, Pennsylvania. 5 The present-day 4% content of the crust, is 3.1 x 10B4 u% per cent, (2.6 x 1.2 s 10e4), while 40Ca is 3.5 wt. per cent (3.6 x 0.97); hence the ratio 4oK/40Ca is about 0.9 x 10m4. Because 40K has a half life of onlv 1.33 x 10n yr, 5.3 x 10s yr ago there would have been 16 times as much 4oK. But 4oK has two modes of decay, t,o 4oAr as well as 4oCa; however, since about 88 per cent goes to calcium, 4.1 X lows wt. per cent of 40Ca will have been produced in the crust in 5.3 x lo9 years by this trrtnsmutation, if the Kabundance assumed is correct; and, if the &-abundance assumed is correct, this amount represents about 0.1 per cent of total calcium. 735
time ta time from the mantle, the increase expected will be less because the mantle is more basic than the arust. Thus the only hope for utifisation of the K/Ca age m&hod lies in amens ah&h, a%the time of crys~~s~~~o~~ ~nce~tr~t~ K and ex&ded @a (Bat-zros, 1955). Recent demonstrations t&at differences in isotope ratio exist between terrestrial and meteoritic silver, barium, and xenon, have focused new interest tm the isotopes of calcium. The data reported herein were mostly obtained in 19&&-5 (BACEUS, L956; Hamcra et al., 19&S; E~~zoa, lQSS>, but in the interim no ~d~~~un~~ anafy3es have ~~~a~d in the ~~ra~~r~~ One reason for the seeming neglect of this important efement is the fact that its analysis by mass spectroscopy is difficult. Compounds that have a sufficiently high vapor pressure at low temperatures to make possible the study of calcium with gas-analysis isotope ratio mass spectrometers are unknown, and the element is ionised very poorly in a t~~rrn~o~c source. Further, with a th~~~~c source, ano%her technical proMem, isotope ~act~~~a~~on during va~r~t~~~ ~~~~ et al., 19s?4), Bas Emited the isotope ratio rephcation precision achievable. The problem is especially serious in the case of calcium because of its broad mass range, The present paper gives results of replicate analysis of Ca from a terrestrial limestone, a chondrite with a CafB ratio of about 11, and an achondrite with a CajK ratiu of about 150, The ~rograrn~~ is being co&rn.~ed (L. E. M,f with an &tempt to develop ~rn~~~~~d~ech~~q~es for isotope ratio dete~~~~~o~~ and in particular, to control the problem of fractionation during analysis, AXALYTICM,, TECRNIQVE The antips rep&ud were made zztthe Massachuse%s Institute of Technology with a 6 in. ra&us, 60’ de~~~t~on~~~~e-~~~~s~g mass s~~~~me~~r designed by one of us (L. F, If.). Ion currents ~a~~~~ a s~~~~~-be~rnc&h&w were meas~& either with a d.c. amplifier* or a dynamic capacitor amphfier? (1011$2 input resistor), with a 10 in. strip-chart recorder used for peak display, Ion acceleration was approximately 20QOV. Unfortunately, neither an electron multiplier detector or ~ip~~-~larne~~ source wes available at the time these analyses were made. The ca~c~~~ iso%opes~ct~rn BW recorded re~~t~d~~ by vsry’ing the magnetos fiefd in a ~~c~procat~ngfashion. The ana@eo trrbe ww c~~t~~~~~s~ypumped by s single, am-stage mercury diffusion pump (Priest design), and during ,thesemeasurements, the analyser pressure was between 5 x 10M7and 5 x 10-6mm Hg, as measured by an ion gauge close to the source. Calcium was separated from i&hetatai sample &x mZgss~ec~rorn~~~ analy&r by ion exchange; it was loaded or&o the therrn~o~~c source Samen% &her as oxafate fin suspension) or nitrate (in solution). The eEciency with which cabSum is ionised by the surface ionisation (thermionic) techsique is rather small because of its high first ionisation potential (649 eV); for this reason, and because of the rarity of some of the isotopes to be studied, it was not generally possible to obtain successful. Ca z~na3yseswhen Ihe sample was placed on Ta ribbon (@-001 x 9430 x o+%@ in,) &laments, suob as bad been successf~ly used for, for ~~~rn~~e~R, Rb * Essentially identical to me described by A. 0. C. NI~:R { 1947). t Applied Physics Carpnration Model 30.
Calcium isotope ratios in the Homestead and Pasamonte meteorites
735
and Sr analysis.
Therefore, a considerable effort was expended in finding a substrate which would provide greater ionisation efficiency. Eventually a substrate consisting of rough, porous platinum electrolytically deposited on tantalum was adopted; it appeared to be about ten times as efficient, as Ta ribbon on the production of Ca ions. The overall efficiency of the instrument (ions collected/atoms applied) was measured as 0.4 x 10e6 or iess for Ca on an oxidised Ta filament, compared to about 40 s 10m6for Sr. In the analyses reported herein, several types of substrate were used: oxidised tantalum, TaO plus borax, new Pt on Ta, and used HF-cleaned Pt on Ta. No influence of substrate on isotope ratio was observed. The samples analysed contained from 1 to 10 ,ug of calcium. In order to obtain a set of isotope ratios having precisions of the order of one per cent for all ratios except those involving 46Ca, a 40Ca ion current of at least 6 x lo-l2 A for a Samples were run to completion and the minimum of one hour was required. emission weighted isotope ratio averages were calculated for each run. From 20 to 60 peak sets were measured for each run. RESULTS 1. Terrestrial
calcium
(a) Devonian limestone. A bottle of Mallinkrodt “Calcium A. R., low alkali” was established as the calcium standard for the M.I.T. Laboratory. According to the manufact,urer, this calcium “ -originates in a limestone from a quarry near Jamesville, New York. This is a Devonian age limestone and comes from principally two strata: (1) Onondaga limestone, immediately above Oriskany sandstone and (2) Helderbergian or Stromatopora limestone, immediately below the Oriskany sandstone.“* Four different measurements made on this sample, using somewhat different ion sources, gave the results listed in Table 1. The first three determinations were made using the d.c. feedback amplifier, while the fourth was made with the The 46Ca/Wa ratio was determined in a separate dynamic capacitor amplifier. run as 0.000047 i O*OOOOlO. The precision of these measurements: expressed as standard deviation, appears to be of the order of one per cent; it is best for the 42140, 44140 and 42144 isotopic ratios. The absolute errors suggested in Table 1 include, in addition to the calculated precisions, allowance for an isotope fractionation of 13 per cent for the 48140 ratio, and systematically smaller fractionation effects in the case of the other ratios, in which the mass differences are less extreme. have been altered to take The ratios and abundances designated as “corrected” into account a small postulated instrumental mass discrimination effect; the relative corrections vary with the square root of the mass ratio (BACK~S~ 1955). (b) Other measurem.ents. Other modern measurements of the isotope abundances of reagent calcium have been made by NIER (1938)) SHERWIN and DEMPSTER ( 1941)? WHITE and CAMEROK (1948) and HERZOU (1951). These data are presented in Table 2. Herzog, using an instrument very similar to the one used in the present study, obtained abundances in better agreement with the present results and WHITE and CAMEROR’S than with NIER’s. * In a letter from J. C. PERRY (1955).
MILOM. BACKVS,W. H. PINSON,L. F. HERZOCand P. M. HUXLEY
738
Table l(a).
Composition of calcium from a Devonian limestone (raw data)
Primary ion sonroe CaC,O, on TaO Ca(NO,), borax on TaO
Ca(NO,)a on used Pt plated filament Ca(NO& on new Pt plated filament Average *
Total no. of peak sots
42
Z
43 0
44 a0
48 a0
48 Z
20
0~00678
0.00144
0.0223
0.00212
0.312
23
0.00696*
0.00144
0.0222
0.00208
0.298*
57
0.00682
0.00142
0.0222
0.00212
0.310
53
0.00697 0.00680
0.00142 0.00143,
0.0223 0.0222,
0~00210 0~00210,
0.308 0.310,
* Ratios involving Ca42in the second rnn appear to be anomalous and were not included in the average. This rnn was the poorest in the group from the standpoint of emission stability. Table l(b). Composition of calcium from a Devonian limestone (corrected for instrumental mass discrimination) Isotope Ratios
Isotope Abundances (atom per cent)
48 42
0.306 & 0.007
40
96.88 5 0.05
42 40
0.00676 i 0.00006
42
0.655 + 0.006
0*00142 + 0.00002
43
0.138 + 0.002
0.0219 i- 0.0004
44
2.12 * 0.04
0~000047 5 0~000010
46
0.0046 & 0.0010
0.00206 & 0.00006
48
0.200 & 0.006
43 ;i-d 44 0 46 40 48 a0
The stated standard deviations may be considered a measure of the absolute accuracy of the stated values, except in the case of 40Ca. Apparently, all the earlier analyses were of reagent calcium. Thus the ratios involving 40Ca measured by these workers might be expected to be identical only if, fortuitously, the same samples were analysed, or if the radiogenic contribution in all cases is (as it is believed to be) negligibly small. However, 48Ca/42Ca and 48Ca/Wa are both higher in the analyses made in the present study, by 9-11 per cent, than in the analyses made by NIER and WHITE and CAMERON. The observed differences (with the exception of NIER’S value for 43/40) are consistent with the hypothesis that the values of NIER and WHITE and CANERON show a slight depletion in the heavier isotopes due to mass discrimination. This may be due to the fact that only the first fraction of the sample evaporated was in general measured. The possibility of such an effect was suggested by NIER (1938). In our measurements the apparent isotope ratios changed by up to 17 per cent during evaporation of a single sample with the early fraction depleted in heavier isotopes.
and method
etal. (1954)
. __ ._ -
._ -
+ %a t 44% $ Wa
Pasamonte achondrite
-3.4
f
3
AO.00677 0.00008 0.0070 ~0~00016
Homestead
chondrite
0.00680 f 0.00006
CaCI,
Reagent
.
.-
--
_I
f
6
-2.2
f
0.0224 &0.0004 0.0229 *0.0006
0.00142 * 0.0003 0.00153 f 0.0006
-7.7
0.0223 -f 0.0004
000143 f0~00002
44140
0.0213 f0.0007 0.0220 *0~0002 0.0226 & 0.0003
43140
5
46140
0~000033 ~0~000001 --
0~000047 -&0~000010
ratio data (as measured)
0~00150 &0~00004 0.00136 *0~00004 -
isotope
0.0066 *0~0002 0.0066 *0~0001 0.00675 f 0~00008
42140
Reagent Ca from Devonian limestone
Ca
Reagent
Ca metal
__-
Sample
2. Calcium
was measured in 31 sets from 3 runs. measurable in 17 sets was measured in a separate run using a large sample.
Indicated depletion in ‘OCa, Pasamonte vs. Homestead (Oh)
BACKUR
vaporisation, El3 ionisation WHITE and CAMERON (1948) thermionic HERZOG(1951) thermionic, Pt filament BACKIJ~ etal. (1955) thermionic, Pt and TaO filaments BACKUS etal. (1955)
NIER (1938)crucible
Analyst
Table
-4.3
f
7
0.00208 *O.OOOOF 0.00217 &0~00008
0~00210 ~0~00006
0.00191 &0.00006 0.00185 *0~00002 -
48140
1
14
122$
1537
4
1
81*
Total no. of sets
5
runs
No. of
740
MILOM. BACKUS,W. H. PINSON,L. F. HERZOCand P. M. HURLEY
The results reported represent an emission total evaporation history of the sample.
weighted
average
obtained
from the
2. Homestead chondrite We have already reported an Rb-Sr age study of the Homestead chondrite (HERZOG and PINSON, 1956) and this plus K-Ar data point to a crystallisation age of about 4.5 x log yr for this meteorite. The Ca/K ratio in Homestead is reported to be about 11 (UREY and CRAIG, 1953) so that one will expect only a 0.01 per cent increase in its *OCa in 4.5 x log yr. Ta.ble 3. Calcium isotope ratios not involving calcium-40 Sample
Analyst
Ca metal NIER, 1938 WHITEand CAMERON, Reagent Ca 1948 Reagent CaCl, HERZ(JG,1951 Staasfurt sylvite HERZOG,1951 Homestead chondrite BACKTJS et aZ., 1955 BA~KUSet al., 1955 Pasamonte achondrite Bikita lepidolite BACKUSet al., 1955 Devonian limestone (raw) Devonian limestone (corrected)
44142
48142
48143
44143
42143
3.24
0.298
1.28
14.2
4.40
3.34 3.34 3.26 3.31 3.27 -
0.280 _ 0.307 0.312 -
1.36 _ 1.47 1.42 -
16.1 _ 15.8 15.0 15.8
4.85 _ 4.77 4.58 -
3.27
0.310
1.47
15.6
4.75
3.24
0.306
1.45
15.5
4.77
An excellent run was obtained on Homestead calcium; 122 sets of peaks were obtained for all isotopes except the rare *Va, for which just 1’7 measurements were made. The ratios measured (Table 3) all agree to within &l per cent with those of the Devonian limestone, except possibly in the case of *Va, but in this case the limestone analysis carries a large error. For this isotope, the Homestead abundance, which has a standard deviation of 13 per cent, is in good agreement with NIER’S analysis of a terrestrial sample. 3. Pasamonte achondrite We chose this meteorite for analysis in an earlier Rb-Sr study because it was reported (UREY and CRAIG, 1953) to have a very low K/Ca ratio (l/150). This indicated that the Rb/Sr ratio should also be so low as to preclude any measurable change in a7Sr relative abundance during 10 x 10Qyr. In the earlier paper already mentioned (HERZOG and PINSON, 1956) we solved for the time at which the relative isotopic abundance of 87Sr would have been the same in both Homestead and Pasamonte, and found it to be roughly (3 f 1) x 109 yr. Unfortunately only a mediocre run was had on Pasamonte Ca, and it was not possible to repeat it due to exhaustion of sample. Fourteen peak sets (exclusive of *%a) were recorded. Isotope ratios not involving *OCa are in agreement, within
Calcium isotope ratios in the H0mestea.d and Pasamonte
meteorites
541
the experimental precision, with those found for the limestone, but ratios involving “OCa appear to be up to 3 per cent lower in Pasamonte. It is difficult to reconcile such abundances with the concept of a common origin time of the two meteorites about 5 x 10Q yr ago, although, because of the large limits of error of the measurements, this is not impossible. Because of the implica,tions of this result, the Pasamonte calcium should be remeasured with greater precision. 4.
Variations in, tlzr abundance ratios of Ca isotopes other than. 40Ca
Ss discussed above, the limits of error of the single-filament technique for the isotope ratio determination of calcium are so large as to make impossible a search for small differences in ratio. But the present data do indicate that any such differences must at least be less than 3 per cent for ra.tios involving 42Ca, 44Ca, and 4sCa for these samples. SCWMARY The relative abundances of the six isotopes of calcium are the same for calcium from the Homestead, Iowa chondrite as for reagent Ca derived from a terrestrial Devonian limestone, within the limits of error of the measurements; but for Ca derived from the Pasamonte achondrite, while the other relative abundances are “normal”, the partly radiogenic isotope 4OCa mav be depleted by as much as 3 per cent,. Using accepted potassium and calcium abundances (2.6 and 3.6 per cent), and the isotope ratios measured in the limestone, isolated material of the composition of the t,errestrial crust could only have had the Pasamonte 40Ca relative abundance about 12 x log yr ago. Further, if the K/Ca ratio reported for Homestead by others is correct, this meteorite would have had the abundance found in Pasamonte about 16 x log yr ago. Both results are incompatible with the K-Ar, Rb-Sr and other ages of the objects and suggests that further work on calcium abundances should be carried out. Acknowledgements-This work was supported in large part by funds provided Energy Commission under grant AT(30-I)-1381.
by the Atomic
REFERENCES BACKUS M. M. (1955) Xa.ss Spectrmetric Determination of the Relutiae Isotopic Abundances Ph.D. Thesis, Massachusetts Instibute of of Calcium and the Determination of Geologic Age. Technology. HERZOG L. F. (1951) Unpublished report. HERZOG L. F., PISSOK W. H.. BACKUS M. M., STRICKLAND L. and HERLEY P. M. (1954) i-a&tiom in Isotopic Abundances of Sr, Ca and ,4 and Relakd Topics. V.S.A.E.C. Document N.P.O. 3934-11 (4). HERZOG L. F., PINSOP; W. H., BACKUS M. M., STRICKLAND L., CROMIER R. F., GREENEWALT D. and HTJRLEY I’. M. (1955) Variations in. Isotopic Abundances of ST, Ca and 9 and Related Topics. U.S.A.E.C. Document N.P.O.-3935-11 (4). X.R.C. Kucl. Sci. Ser. Report 19, HERZOG L. F. (1956) Rb-Sr and K-Ca Analyses and agea. 114-129. HERZOG L. F. and PINSON W. H. (1956) Rb/Sr age, elemental and isotopic abundance studies of stony meteorites. Amer. J. Sci. 224, 555-566. NIER A. 0. C. (1938) Isotopic composition of Sr, Ba, Bi, Tl and Hg. Phys. Rev. 53, 282.
742
MILO 51. BACKUS, W. H. PINSON, L. F. HERZOG and P. M. HTJRLEY
NIER A. 0. C. (1947) A mass spectrometer for isotope and gas analysis. Rev. Sci. Iwtwm. 18, 398-411. RANEAMA K. and SAHAMATH. G. (1950) Geochemktry. University Chicago Press, Chicago. SHERWIN C. W. and DEMPSTERA. J. (1941) The relative abundance of the calcium isotopes. Phya. Rev. 59, 114. UREY H. C. and CRAIG H. (1953) The composition of the stone meteorites and the origin of the meteorites. Geochim. et Comwchim. Acta 4, 36. WHITE J. R. and CAMERONA. E. (1948) The natural abundance of isotopes of stable elements. Phya. Rev. 74, 991.