The rubidium-strontium age of the Bishopville aubrite and its component enstatite and feldspar

Ckochlmica et Ccmnochimics Aeta 1085,Vol. ‘29,pp. 1085to 1099.PergamonPressLtd. Printedin NorthernIreland

The rubidium-strontium age of the Bishopville aubrite and its component enstatite and feldspar W. COMPSTON,J. F. LOVERING Department

of Geophysics, Australian

and M. J. VERNON

National University,

Canberra

(Received 24 Auguat 1964) Ah&&-Replicate measurements have been made of the Sr and Rb concentrations and Sr isotopic compositions of two total-meteorite samples of the Bishopville aubrite, and of the component enstatite and plagioclase from a third sample. These data provide sn “internal” age for the meteorite. They apparently define an isochron whose equivalent age lies between 3.5 x lo@ and 3.9 x lo9 years at the 96 per cent confidence level, suggesting that this aubrite may have formed at a signilicantly later time than the major period of meteoritic fractionation some 4.6 to 4.6 x lo0 years ago. Other interpretations for the apparent isochron are also discussed. Comparative data are also given for the Moore County eucrite which agree well with analyses reported elsewhere after the necessary normalization of isotopic ratios is carried out.

INTERPRETATION of Sr isotopic analysis of material believed to originate directly from the Earth’s upper mantle depends very critically upon the value one assumes for the initial Sr*’ abundance of the Earth. An accurate estimation of this parameter solely from measurements of terrestrial material is difficult on present knowledge: for a beginning it seems desirable to fall back on the meteorite-Earth analogy, snd to use Sr isotopic analyses of relevant meteorites to provide a datum point for the evolution of terrestrial strontium. A number of such analyses are already available. We choose to repeat this kind of study primarily because it appeared at the time that mass-spectrometers may not be sufficiently absolute in their measurements of isotopic ratios to permit close comparison of results between different laboratories. We therefore required the meteoritic data as measured by our machine. As a second objective, we wished to secure the age and initial Sr *’ abundance wholly from measurements on a single meteorite, by the use of different total-rock samples and also by the use of its separated mineral constituents. This has not, to our knowledge, been attempted previously although leaching experiments may approximate to it. Earlier Rb-Sr age determinations on meteorites have been made by grouping the total-rock data of two or more different meteorites together and employing the assumption that both the age and initial Sr *’ abundance of these meteorites were the same. This procedure was necessary because the low ratio of Rb/Sr in most meteorites has produced only a small enrichment in Srs’, so that the estimation of age is particularly dependent on the choice of the value for primordial Srs7 abundance. However if a range in Rb-Sr ratios can be found between the components of a single body sufficient to generate accurately-measurable differences in Sr8’ abundance by the present time, then both the age and initial abundance of the one meteorite can be THE

1085

1086

W.

COMPSTON,

J. I?. LOVERIN@and M. J. VERNON

estimated without reference to the data of other meteorites, and with no assumptions concerning their genesis and evolution. On the other hand, the assumption that the system analyzed has remained closed with respect to the parent and daughter elements which is common to all strontium age determinations, must now be applied down to the mineral level. This is a disadvantage as it involves in turn definite assumptions concerning the thermal history of the meteorite and of the diffusive behaviour of radiogenio Sr. A third objective arises from current ideas on the origin of the enstatite chondrites or aubrites themselves. It has been suggested that these bodies have evolved by the melting of enstatite chondrite material together with the gravitational segregation of most of the metal and sulphide phases (e.g. MORGAN and LOVERING, 1964), so that the aubrites should be younger than the chondrites. Two independent Rb-Sr studies now indicate that the various chondritic meteorites solidified at least 4.6 x IO9 years ago (GAST,1962; PINSON et d., 1964). We therefore wished to examine whether Rb-Sr measurements on the Bishopville aubrite might show a significantly younger value than this for the time of melting of their parent chondritic material. The technical success of Rb-Sr age measurements on meteorites depends upon obtaining low contamination levels, and upon securing preoise meas~ements of the Srs7/Sra0 ratios. We therefore give a detailed account of our experimental methods. PREPARATION OF MATERIAL The composition of the various phases in the Bishopville aubrite has been studied Details will be reported elsewhere using an electron-probe X-ray microanalyzer. (LOVERING, 1964). The aubrite consists mostly of enstatite fragments with smaller amounts of o~go~lase-feldspar and minor amounts of clino-enstatite, kamacite, taenite, sohreibersite, Ti-bearing troilite and several other unusual sulphide phases. For the present study, two separate portions of the (solid) meteorite each weighing about 2 g were taken as the total-meteorite samples TR-1 and TR-2. Each was crushed to -100 mesh and each split into two aliquots, A and B. Two further solid fragments (samples I and II) were also taken, each crushed lightly and the metallic particles removed by a hand-magnet while the largest sulphide aggregates were removed by hand picking. The silicate portions of both samples I and II were then crushed to -80 mesh and then separated into “light, ~~rrne~ate (sp. gr. 2.96) and heavy” fractions using heavy liquids. Acetylene tetrabromide fraction and a suswas used first to separate a floating “light”, oligoclase-rich pended “intermediate” fraction from the “heavy” pyroxene and remnant sulphide fraction. ~ethylene iodide (sp. gr. 3.3) was then used to separate suspended pyroxene (mostly enstatite with some olino-pyroxene) from the heavier sulphide impurities. The relative abundances of the various separated fractions from both samples I and II are shown in Table I. The overall metal content of the Bishopville aubrite is about 0.5per cent (by weight) while the oligoclase content is about f 1 per cent (calculated from the Na,O content of the meteorite and its constituent oligoclase feldspar). The fractions analyzed represent the oligoclase-rich and enstatite-rich portions of sample I and the enstatite-rich portion of sample II. The oligoclase fraction of I

The rubidium+trontium

1087

age of the Bishopville aubrite

Table 1. Heavy liquid separations of Bishopville aubrite samples I and II Sample I Metal phases Sulphide phases Impure metal-sulphide mixtures Oligoclase-rich fraction (light) Intermediate fraction Enstatite-rich fraction (heavy) Total weight

0.8 (wt.%) 0.3% 8.7% 7.9% 11.3% 79.8% 0.381 g

Sample 11 0.3 (wt.%) 0.3% 9.9% 17.7% 02.9% 14567 g

contains about 50 per cent (by weight) of pyroxene contamination while the enstatite fraction of I is relatively pure. The enstatite fraction of II contains about 2 per cent oligoclase contamination. DETERMINATION OF Rb, K AND Sr BY ISOTOPE DILUTION Approximately O-5 g of the powered sample was weighed into a 100 ml. platinum dish, wet with demineralized water, 10 ml. of 48 per cent hydrofluoric acid added, and this was taken to dryness on a water-bath. 5 ml. of hydrofluoric acid and 5 ml. of 70 per cent perchloric acid were added and then taken to dryness on a hot-plate. The residue was dissolved in 2.5 N hydrochloric acid. This solution was transferred without loss to a, tared 100 ml. Pyrex glass beaker with Parafilm cover, and warmed to ensure complete mixing. When cool, the solution was weighed, and aliquots weighed into 10 or 30 ml. Pyrex glass beakers. Separate aliquots were usually taken for each element. The aliquot sizes were chosen to contain about 8pg of (common) Sr, 15 ,ug of Rb, and at least 300 ,ug of K. The tracer solutions, respectively containing about 3.5 pg Srs6 and Srs4, 6 ,ug Rbs7 and 80 lug K41, were then added, and the “spiked” aliquots acid, transtaken to dryness. The residues were dissolved in 5 ml. hydrochloric ferred to cation exchange columns, eluted with hydrochloric acid and the required fraction collected. This was evaporated to dryness, and the sample transferred in one drop of demineralized water to the mass-spectrometer filaments, using a Pyrex glass pipette. Owing to the low abundances of the elements in certain of the meteorite samples, the preferred amounts and proportions of sample and tracers could not be maintained, nor were approximate prior estimates of the abundances available. For these samples, we used the minimum amount of tracer Sr which would yield a satisfactory run on the mass-spectrometer, which was about 0.5 ,ug, and occasionally resorted to adding all three tracers to the whole of the sample. The ion-exchange columns are of Pyrex glass, 30 cm x l-2 cm i.d., fitted with a stopcock with a P.T.F.E. key at the bottom and with a B14 socket at the top. The resin is retained by an acid-washed glass wool plug. The resin used is Dowex 5OW-X8, 200-400 mesh and each column contains 8 g of dry resin. Above the column is a pressure head consisting of a 150 ml. reservoir joined by 50 cm of 2 mm i.d. tube to a B14 cone fitted with a P.T.F.E. sleeve. Columns for Rb separation eluted with 1.00 N HCl (K cut 110-135 ml., Rb cut 130-160 ml.) and for strontium with 2.50 N HCl (K cut 40-55 ml., Sr cut 90-160 ml.), the flow rate

1088

W. COMPSTON, J. F. LOVERINGand M. J. VERNON

being approximately 1 ml. per min. They were calibrated by using 10 mg amounts of the respective elements and weighing the residues of 5 ml. fractions. After each separation the columns are washed with 100 ml. 6.0 N HCI, 50 ml. demineralized water and then with 100 ml. of the appropriate acid. The columns are never allowed to run dry between separations and 5 ml. of acid is left above the resin surfaces. This acid is run out before loading a sample and ensures that any contaminating ions that have diffused out of the resin after the washing cycle are shifted down the column ahead of the sample cut. The performance of the ion-exchange columns was found to depend considerably upon the size of the sample. With aliquots equivalent to about O-5 g of the original mineral or greater, the alkalis may be excessively diluted by iron. Bulky samples were therefore converted back to perchlorates after spiking and heated strongly to convert most of the iron and aluminium to oxides. The alkalis were dissolved in hot de~neralized water, any suspended oxides centrifuged out using 5 ml. polypropylene tubes and a second, much smaller residue obtained after evaporation which could be then effioiently separated by the ion-exchange process. CONTAMINATION LEVELS

The initial alkali determinations at low level were particularly prone to contamination, which originated not only in the general chemistry but also in the mass-spectrometer, which employed the triple filament teohnique. Contamination in the mass-spectrometer was later eliminated by cleaning all parts of the ion-source exposed directly to the filaments after each sample, by running the centre filament at low rather than high temperature, and by increasing the side-filament temperature until the beam intensity was of the order 10-l@ A for Rba’ and IO-@ A for K3@. If a small beam was present with only the centre Glament burning, the isotopic ratio was measured at this level to ensure that it was closely the same as with the intense beams. Samples were loaded on both side filaments and each filament was heated separately to obtain two observations of the isotopic ratio. In the absence of contamination, these should not differ by more than about one per cent which is an upper limit for isotopic fractionation effects experienced with this mass-spectrometer. Rhenium filaments were used because of the lower alkali content of rhenium, rather than tantalum and tungsten, The filaments were decontaminated in the mass spectrometer before loading by running at very high temperatures. New filaments were used for each sample. Contamination was monitored by running several blank samples through the entire procedure, using the minimum amounts of tracers. In addition, the sources of contamination in the chemistry were investigated by analyzing each of the reagents, and by measuring the separate effect of the ion-exchange process. The data for the reagents enables us to estimate the contamination from this source alone. They are shown in Table 2. If reagents were the only source of contamination, then strict control of the amounts used should produce a known estimates of the amounts of contamination from the and reproducible blank, reagents in the various blanks are shown in Table 2, together with the amounts actually measured. Blanks 1 and 2 were made up from different batches of reagents than those analyzed in Table 2, whose analyses showed apparently greater

1089

The rubidium-strontium age of the Bishopville aubrite

concentrations of rubidium and strontium. However, owing to the over-estimate of contamination in these blanks which is evident from Table 3, we now suspect that one or more of the earlier measurements on reagents may have been incorrect. With the exception of these earliest blanks, the strontium measured in the other three exceeds the amount estimated from reagents alone by an average of O-07 ,ug, which must be attributed to some other source. The latter is evidently the ionexchange process, which was shown-by direct measurements (Table 3) to contribute on the average precisely the strontium excess found in the blanks. Thus, all the Table 2. Analyses of reagents Bb rglg 0.52 0.34 0.41 3.10 180

Hydrochloric acid, 6.2 N Demineralized water Hydrochloric acid, 2.6 N Perchloric acid, 72% Hydrofluoric acid, 48% English Pyrex glass American Pyrex glass Pyrex glass, WASSERBURG et cd. (1964)

K rglg

Sr pg/g

x x x x

10-4 104 104 IO-4 x 10-a

1.11 o-14 o-53 22.0 3.0

10-4 10” 104 104 x 10-a 3.3 12.0 14.0

-

x x x x

28,

x 10-s -.. _--

Table 3. Analysis of blank runs Bb (rg)

Blank Blank Blank Blank Blank

1 2 3 4 5

Sr (rg)

estimated

measured

O-029 o-029 0.012 0.012 0.004

0~012+ 0*019* 0.012 0.014 0.007

Ion-exchange process

-

o-00 O-01 0.03

estimated

B (rg)

measured

measured

0.17 0.17

o-13* o-13*

-

0.035 0.035 0.015

0.09 0.15 0.054

6.1 5.0 -

0.13 O-06 0.07 0.08

0.5 -

-

* Without the ion-exchange operation.

observed contamination can be accounted for in terms of the reagents and the ion-exchange process. The strontium cont~bu~d by the reagents was iso~pi~ally analyzed without tracer: the value found for Sr87fSrs6was 0.705. The oontamination picked up in the ion-exdhange process was likewise found to be common strontium. Nevertheless, to avoid the possibility of cross-contamination, we reserved separate ion-exchange columns for the “spiked” and “unspiked” samples, and glassware exposed to spiked material was not reused. The Sr contamination associated by WASSERBUR~ et al. (1964) with the effect of hot solutions of some silicates upon glassware has not been observed in this study. This is shown by the reasonable agreements between duplicate Sr determinations in Table 4, and between unspiked and calculated measurements of the sample’s

1090

W. COMPSTON,J. F. LOVERINQ and M. J, VERNON

Srs7/SP ratio in Table 4. On the other hand, PINSON et al. (1965) imply that such contamination is the major source of error in their Sr determinations. We have no explanation for its apparent absence here, but it may be significant that the Sr content of our glassware (Table 2, English Pyrex) is almost five times lower than the value observed by WASSERBUR~ for his glassware (American Pyrex). Table 4. Common Sr concentrations of the Bishopville and Moore County meteorites (common Sr aa fig/g taken as 886.7 x SrSa aa p moles/g)

Bishopville anbrite Total-meteorite Total-meteorite Total-meteorite Total-meteoric

1-A 1-B 2-A 2-B

Tracer Sr handled (trgf -_ 4.7 2.2 2.5 2.8

Concentration Sr in sample @g/g) 10.2, 10.1, 14.4, 14.2,

1.7 1.7 1.2 1.1

1.6 3.8

39.2,

3.0

2.1 1.1

77.0

Sample Sr handled @g) 1.4 2.4 1.5 l-8

Blank correotion (%)

Oligoclase concentrate I-A Oligoclase ooncentrate I-B Oligoclase, corrected for enstatite contamination

0.9

-

-

En&at&e concentrate I-A Enstatite concentrate I-B Enstatite concentrate II-A

0.3 0.1 1.5

0.3 0.3 3.7

1.57 1.62 3.11,

6.9 55 2.8

Enstatite II, corrected for oligoclase contamination

-

-

1.61,

--

38.1,

-

Retxgent correction

Moore County achondrite

(%) Total-meteorite Totalmeteorite GAIT (1962)

1-A I-B

3.6 8.8 -

2.3 2-4 -

71.9 71.7 79.5

0.2 0.2 -

PRECISE MEASUREMENT OF ISOTOPIC RATIOS

The mass-sp~~trome~r used is a Metropolitan-Vickers type MS2-SG. This is a six-inch, ninety-degree sector machine with a source vacuum look. The source was operated at 2 kV. The standard high voltage supply was modified to allow positive and negative variation of about 10 V between the centre-filament and the two side-~laments, and a variation of the voltage between the filament-block and “D-plates” from approximately O-300 V. These adjustments allowed greater control over the sensitivity and resolution than with the standard supply; source performance in these respects was now closely as described by CRAIU (1959). Resolution was further improved by rest~oting the length of the beam-divergence slit in the ion-gun to 0.125 in. With a O-004in. defining slit, .the beam width at the collector is about 0.011 in. for 99.9 per cent collection. For common Sr, the tail-correction due to gas-scatter is less than 0.1 per cent under mass 87 after 2 hr pumping, and less than 0.02 per cent overnight. Resolution is slowly impaired by

The rubidium-strontium age of the Bishopville aubrite

1091

the development of charged surfaces in the analyzer tube, which is consequently chemically cleaned after several months’ operation. The usual operating currents for Sr loaded as chloride are 4-O A centre filament and about 1-O A for each side filament. For rubidium and potassium chlorides, the corresponding values are 2.5 A and about 0.8 A. Filaments are of rhenium, 0.030 in. wide and 0.001 in. thick. Strong Sr beams do not appear until the Rb and K beams are falling rapidly. This appears to be due not merely to an increased evaporation rate of SrCl, from the side-filaments but more to a suppressing effect of alkalis on the production of Sr ions. It is seen also in the production of Ca ions, and we suspect in addition that the evaporation of relatively large amounts of CaCl, has also a suppressing effect on the Sr beam. The machine is used without an electron-multiplier. The original two d.c. amplifiers were replaced by a single Gary Model 31 electrometer, with turret-switch selection of input resistors and with critical damping. Strontium beams were measured using Victoreen Hi-Meg loll and 1012 ohm resistors, and Rb with lOfo and lo9 ohms. Different isotopes were collected and beam intensities compared by rapid switching of the magnetic field, the latter being effected by push-button selection of preset values for the reference voltage of the magnet power supply. Field hysteresis was negligible after a number of cycles between two particular isotopes. With the collector slit set at O-015 in., the peaks were sufficiently “flattopped” for the beam intensity to be insensitive to drift in the ion-accelerating voltage or magnetic field. Normally no adjustment of the latter wasneeded over a set of ten comparisons, which was the usual number taken for the measurement of one isotopic ratio. The electrometer output was digitized to obtain maximum sensitivity and preThe digitizing system comprises cision, and also monitored by a chart recorder. a Dymec Model 22llB voltage-to-frequency converter, a Hewlett-Packard Model 522B counter and Hewlett-Packard Model 560A digital recorder. The beam voltage was “counted” for one second, immediately switched, and the second beam counted for the same time after a fixed delay of about 5 sec. The latter is the time required for full response of the amplifier and magnetic field. For unspiked Sr**, the operating beam intensity was about 0.9 of full scale on the 3 or 10 V ranges of the electrometer. This would be recorded as 90,000 cps, and masses 87 and 86 in this example would be respectively 7600 and 10,800 cps. Since zero point noise is about 1 count per set using the loll ohm resistor, it is evident that sensitivity for measurements of the ratios 88/86 and 87186 is better than 0.05 per cent. Measurements of this precision were routinely obtained. The linearity of this measurement system (excluding input resistors) was found to be about 0.01 per cent using a precision potentiometer. The linearity of the input resistors was investigated by comparing their input voltages relative to that of the lo9 ohm resistor over a wide range of beam intensities. Results were similar to those of WHITTLES (1960). Accordingly, voltage differences greater than 3 V when using the lOi ohm resistor, and greater than 10 V using the 1011 ohm resistor, were avoided. Many replicate measurements have shown that the accuracy of this system is controlled more by isotopic fractionation of the samples during analysis and by

1092

W. COMPSTON, J. F. LOVERIN~and M. J. VERNON

variable mass-discrimination in the mass-spectrometer, rather than by random errors due to amplifier noise of beam-intensity variations. These effects have been corrected by the use of a double Sr *6, Srs4 tracer which provides in effect an internal standard isotope-ratio. They have been corrected in unspiked Sr measurements by using the normalization procedure developed by FAURE and HURLEY (1963). Some 200 independent Sr determinations using the double-tracer have shown that the standard deviation for a single measurement of the internal standard Sr*e/Sr*4 ratio from its gravimetric value is 0.15 per cent. Nearly the same value is found for the standard deviation for a single measurement of the unspiked Sr**/Sr*6 ratio for natural Sr (from many sources) relative to our mean value of 8.340, indiaating that machine effects are responsible for at least the major variations seen in this ratio as measured on this machine. This supports the observations of FAURE and HURLEY (1963) and HEDGE and WALTHALL (1963) towards the general validity of the normalization procedure. We have normalized our data to the value 8.340 for Sr8*/SrS6 rather than 8.375 (0.1194 Sr8s/Sr88) as used by FAURE and HURLEY (1963) on the grounds that 8.340 represents the mean value measured in this laboratory over the two years ending middle 1962. Values as high as 8.375 were rarely observed. Furthermore, the mean value for observations from 1962 to the present which have been made completely with the digitized readout system will be lower than 8.340. RESULTS: TECHNICAL ASPECTS Tables 4, 5 and 6 contain the analytical results for two different total-meteorite samples of the Bishopville aubrite, each measured in duplicate (lA, 1B and 2A, 2B), and for oligoclase and enstatite concentrates taken from other samples of the meteorite. The Tables also contain our data for duplicate total-meteorite analysis of the Moore County eucrite, which may be compared with those of GAST (1962). The values shown for elemental concentrations have been corrected for the contamination previously described, and the resulting percentage decrease is shown for each value. Except for two of the enstatites, all these corrections are less than 3 per cent so that variations of up to 30 per cent in the size of the reagent- and ion exchange-blank would introduce errors of only 1 per cent in the indicated concentration. From the reproducibility of the blank determinations shown in Table 3, it is unlikely that variations of this magnitude will occur. On the other hand, there is no doubt that an occasional large error has occurred in the alkali determinations due to contamination other than from reagents and the ion exchange process. This is evident in Table 6 from replicate analyses of Rb made both on splits of the same (powered) solid sample (TRl-A and -B, TR2-A and -B), and on aliquots of the same dissolution (TRS-A). The two values for TRl-A represent measurements on two successive fractions from the ion exchange columns, the Rb peak which gave 1.83 lug/g, and the K peak which gave 1.71 ,uglg. This difference is real and originated in the chemistry, since each fraction was measured twice in the massspectrometer using a second loading on new filaments with identical results. We conclude that it was caused by contamination introduced during the handling of The possibility that it was the result of failure to collect the the first fraction. correct Rb fraction from the column seems to be excluded by the intense ion beams

1093

The rubidium-strontium. age, of the Bishopville aubrite

available from both fractions. Such an effect has been observed with other samples but was alw&ys associated with weak and r&pi~y-f&lling beams under the usual operating conditions. The irregulsr occurrence of additional contamination makes replication of alkali analyses essential. It also provides 8 basis for the rejection of any determination which is more than s, few per cent greater than its duplicate, especially if the Table 5. Sr isotopic comp~itions of the Bishopville and Moore County meteorites sompk3 SF handled (!Jg) Bishopville aubrite:

SrBB/Srs@ (aa measured)

SP/SrBS (normalized) 0.7257 0.7257 0.7251I 0.7260 0.72551 0.7214 0.7214) 0.7212 0.7207I 0.7216 0.7214” 0.7568* o-7459+ 0.7401 0,7426*

1-A

1.2

Total-meteorite 1-B

2.7

Total-meteorite 2-A

4.3

Total-meteorite 2-B

37

Oligoclrsseconcentrate 1-A Oligoclaae concentrate 1-B Oligoclaae 1 (corrected for enstatite) Enstatite 1-A Enstatite 1-B Enstatite conc~ntr&te 2-A

1.2 -

8.339 8-338 8.332 8.356 8.359I 8.345 8.336I 8.344 8,3401 8.344 not memnred

16

not measured not measured 8,330

Total-rne~ri~

En&at&e 2-A (corrected for plagioclase) Moore Connty achondrite:

-

Tot&l-rn~~ori~ 1-A

7.0

Tot&meteorite 1-B

98

GAST (1902)

-

-

8.340 8.336 8-323 8-325i

0.6963 0.6979 0.6971 0.6972I

8.338 8.321 8.333I 8.432

06961 0.6971 0.6970I 0.6975

Mean SrE7fS$6 (corrected for reagents)

0.7260 0.7257 0.7217 0.7213 0.7218 0.7216 0.7210 0.7605 07680 0.7410 0.7606 0.6971

0.6967

* Calcule;ted from double spike measurements. Sr determinations on the same duplicates are in agreement. Thus, in Table 6, the Rb data for oligoclase I-B and for enstatite I-A have been rejected. Further

replicates were not possible owing to the small amount of meteorite available. The values for Srs’/Sr 86 in Table 5 have been corrected for contamination of Sr on the assumption that Srs7/Srs* ratio of the latter is always 0,705. They have also been normalized to the value g-340 for SrS8/Sr 88, but the measured values for Srs8/ Sr86 associated with the individual runs are also given to demonstrate the size of this correction. GAST’Svalues for Sr*‘/Sr 86 for the Moore County eucrite are also

33.9 -

125 ooi&arnination.

-

(526)

not measured

-

-

-

not measured

-

75

347

not measured 3400 (3490)

-

-

-

approx. 6400 not measured not measured

; 74 1 75

681 58) 163

(1320) not measured 1129 1132 1132

(pglg)

Concentration K in sample

-

76

330 -

(Pg)

(yg)

Round bracketed figures high-probable

Total-meteorite 2-B Oligoolase 50% concentrate I-A Oligoolaee 60% concentrate I-B oligoclase I, corrected for enstatite Enstatita I-A Enstatite I-B Enstatite 98% concentrate II-A Enstatite II, corm&d for plagioolase Moore County achondrite Total-meteorite 1-A Total-meteorite 1-B GAST ( 1962)

Bishopville aubrite: Total-meteorite 1-A Total-meteorite 1-B Total-meteorite 2-A

Tracer K handled

Sample K handled

0.05

0.03 -

2.0 -

0.4

0.35 0.69 -

0.8

(0.48) 0.13 916

0.62,

0.79,

5.2 18.6 -

-

1.6

(4050) -

-

-

-

-

-

-

(530)

680

600

(770) 565

K/Rb

9.3, -L9 18.9

1.0

2.3

1.2

1.4 1.4 1.3

(%)

(1.01) 0.73,

(6.78) 1

4.98 (6.44)

1.83 1.71 1 1.65 1.971 293) 1‘88

@g/g)

Reagent Concentration blank Rb in sample correction

0.06 0.04

0.3 0.06

0.6

0.6

1.0 1.0 0.5 0.5 1 0.5

(Pg)

Tracer Rb handled

-

0.1

0.1

0.4 0.4 0.1 0.1 0.2

(tlg)

Sample Rb handled

-

0.3

I.0

1.0

0.8

(%)

Reagent oorreotion

Table 6. Rb concentrations of the Bishopville and Moore County meteorites (total Rb as pg/g taken as 308 x Rbs’ as p moles/g)

The rubidium-strontium age of the Bishopville aubrite

1095

normalized to 8.340. The good agreement with our own is taken as further justification of the normalization procedure. RESULTS : GEOCHEMICALASPECTS The average reproducibility of the Sr determinations in Table 4 is less than 2 per cent as defined by the five duplicate analyses. It is thus evident that the difference in Sr concentration between the two total-meteorite samples, some 40 per cent, is real, which indicates that the aubrite is not homogeneous in l-2 g portions in its distribution of Sr. MORGAN and LOVERINCJ(1964) have found similar inhomogeneity in the distribution of U and Th in this meteorite. A significant difference is also present in Sr*‘/Sr 86 between TR-1 and TR-2 which correlates with the measured difference in Rb*‘/SP (Table 7). This suggests that the inhomogeneity is an original property of the aubrite rather than the result of present-day weathering. Table 7. Data used for the isochron diagram (Fig. 1) Rbe7/Sree

Sti7/Sree

Total-meteorite 1-A Total-meteorite 1-B

0.482

0.7260

0.469

0.7257

Total-meteorite 2-A Total-meteorite 2-B

0.393 0.379

0.7217 0.7213

Oligoclase

0.3602

0.7210

Enstatite I Enstatite II

1.14, 1.10,

0.7605 0.7606

The analytical data also show an order-of-magnitude difference in the concentrations of Rb and Sr between the oligoclase and the enstatite. The presence of a small variation in the abundance of oligoolase from place to place within the aubrite would therefore explain its observed inhomogeneity in these elements. If this were the case, the inhomogeneity would date from the time at which these minerals were formed. In this connection, the calculation of material balance shows that all the Rb and Sr measured in the average total-rock can be simultaneously accounted for by their measured concentrations in the oligoclase and enstatite fractions. The enstatite data show a pronounced increase in the ratio Rb/Sr relative to that of the oligoclase. This is an unexpected geochemical effect, which has been observed since in this laboratory also in garnet and enstatite from terrestrial rocks. Using the calculated concentrations of Rb and Sr for the pure oligoclase and the pure enstatite (discussed below), about 27 per cent of the Rb in the Bishopville aubrite is situated in the enstatite, as compared with only about 10 per cent of its Sr. The large difference in the Sr content of the two enstatite fractions apparently confirms the visual estimate that enstatite II contained about 2 per cent of oligoclase. This amount would increase the Sr concentration from 1.6 ,ug/g as measured in enstatite I to the 3.1 ,ug/g in enstatite II. In addition, the corrected value for Sra7/Sr s6 for enstatite II now becomes identical with the measured value in

W. COIXPSTON, J. F. LOVERING and M. J. VERNON

1096

enstatite I-A (Table 4). The data for enstatite I-B are particularly sensitive to error in the blank correction, and must be considered unreliable for this reason. However it is noteworthy that if the blank correction on this dissolution has been too great by about 30 per cent, so that the corrected Sr*7fSr86value can be made equal to those of enstatite I-A and the pure enstatite II, the norresponding value for the ratio Rbs7/Sr86 of enstatite I-B is now within 3 per cent of the value for enstatite II. We consider that this integration of the enstatite data is sufficiently convincing to give the co-ordinate point for the mean enstatite equal weight with the others in Fig. 1. 076-

074-

0

/ 0.2

I

04

I

0.6 Rb%r*6

!

5

0.6

I.0

Fig. 1. S$7/SfsS versus Rbs’/Sfs6 fop the Bishopville caubriteand its mineral constituents. The 4.67 x lo@ years isochron (CAST, 1962)through initial Srs7/S@sof 0.697 is also shown.

THE APPARENTAQE OF THE BISHOPVILLE AUBRITE

The meaning of age as inferred from Rb-Sr analyses of meteorites has been discussed by GAST (1962). His “model I” age refers to an assumed common point of time at which the ratio Rbs’/Sr 86 in four chondrites and four aohondrites acquired fixed values. It assumes that the Sr*‘/Srs6 ratios at this time were the same for all samples of the meteorites, and that the latter thereafter remained closed chemical systems. By the use of precisely analogous assumptions, an age may be oaloula~d for the Bishopville aubrite and its oonstituel~t minerals. These assumptions are: (i) that the Sr*7/SrS6ratios were the same at one point of time in each mineral phase of the aubrite, and (ii) that the ratio Rb87/Srs6 in each mineral phase became fixed at this time, and remained fixed up to the present time (except for the radioactive decay of Rbs7). Table 7 shows the values for Rbs7fSr86 and Srs’~Sr86for the Bishopville samples and Fig. 3.displays these data as Sr87/Sr~*-Rb87/Sr86co-ordinate points. All points fit a single straight line to within experimental error. This is consistent with the assumptions given above, so that the age of the samples can be calculated from the

The rubidium-strontium

age of the Bishopville aubrite

1097

slope of the line (which becomes an isochron), and the value for their joint initial ratio of Srs7/Srss can be found as an intercept of the extrapolated isochron on the Sr8’/Sras axis. The apparent age is 3.7 x lo9 years, taking Rbs’ as 1.39 x lO-ll/ year, with a 96 per cent confidence interval of 3.5 to 3.9 x lo9 years. Very similar values for these parameters are obtained if Rb87/Sr8s is regressed against Sr87/Sr86. This value for age differs very significantly from GAST’S Model I age, 4.67 x lo9 yesrs, and his Model II age, 4.7 x lo9 years, which measures the age of derivation of the achondrites from their chondritic source, and also from the results of PINSON et al. (1965) which independently confirm GAST’S values. The joint initial value for Sr87JSr86is O-7015 with 95 per cent confidence interval of O-6995 to 0.7035. GAST (1962) obtains 0.700 from Model I assumptions, and 0.701 from the Model II (neglecting the data for the exceptional Beardsley chondrite), which when normalized to Sre8/fWs equal to 8.340, become O-696 and O-697. Similarily the value found by PINSON et’al. (1965) becomes O-697. These results are in excellent agreement with our own measurements on the Moore County achondrite (Table 6), which provides a firm basis for the intercomparison of data. It therefore appears that the value for initial Sr87/S@ inferred above for the Bishopville aubrite is significantly greater than the “primordial” values inferred from measurements on other meteorites. INTERPRETATIONSOF THE APPARENT AGE I. These observations are mostly simply interpreted at their face-value, namely that the aubrite was generated during a high-temperature event some 3.7 x lo9 years ago. On this hypothesis, the constituent enstatite and oligoclase were first crystallized at this time, and have suffered no significant exchange of Rb and Sr since. On the other hand, alternative interpretations of the data which question the “closed system” assumption for the mineral phases are formally possible. These are set out below. II. Very little is known of the diffusion of trace amounts of Rb and Sr at elevated temperatures from plagioclase snd enstatite. Nor has the post-crystallization thermal history of the Bishopville aubrite been quantitatively defined. Since knowledge of these parameters is absent, it becomes possible to suggest that diffusive mixing of radiogenic Sr87 with common Sr from one mineral phase to another may have proceeded for a long interval of time after the formation of the minerals themselves, thereby keeping the Sr isotopic composition homogeneous over large volumes of the aubrite. The apparent age of 3.7 x lo9 years may be significantly less on this idea than the age of crystallization of the oligoclese and enstatite. The latter may be as great as GAST’S Model II age, but if so the aubrite data would testify either to continuously elevated temperatures for tclmost one billion years later, or else to the occurrence of a severe metamorphic event at 3.7 x lo9 years. The latter may possibly correspond to the breakup of a parent meteorite body. III. Rather than a continuous exchange of radiogenic with common Sr between mineral phases, the effect of metamorphism may be better described as the nett loss of rrtdiogenic Sr 87 from Rb-rich phases, accompanied by its absorption in Sr-rich phases. Referring now to Fig. 1, it is possible that the low gradient of the isochron is due to leakage of radiogenic Sr 87 from the enstatite. This process may have

1098

W. COUPSTON, J. F. LOVERINGand M. J. VERNON

occurred at any time, or over any period of time, subsequent to crystallization. Thus, the apparent age of the aubrite would again represent a minimum value for its crystallization, but the value 3.7 x lo9 years need not denote any specific event in time. This particular interpretation is capable of semi-quantitative development. It may be assumed that the total-meteorite samples would more nearly represent closed chemical systems than any mineral phase, since their volumes are so much greater. (This is the common experience in dating terrestrial rocks.) Thus, an isochron defined from total-rook data alone may register a higher value for age than the 3.7 x log years value if the enstatite has “leaked”. The gradient of a least-squares line fitted to the four data points corresponding to the duplicate analyses of the two total-rook samples is almost the same as that of Fig. 1, but its 95 per cent confidence interval is considerably wider owing to the reduced range in values for Sre7/Sra6. In terms of apparent age, the 95 per cent interval is 4.3 x log-2.7 x lo9 years, so that the age is still significantly less than GAST’S model ages at 4.7 x log years. Thus, the total-meteorite samples provide positive data against the hypothesis of substantial leakage of radiogenic Srs7 from the enstatite, as long as we can assume that the total-meteorite samples themselves have remained chemically closed. It is possible to calculate the approximate amount of radiogenic Srs7 supposedly lost per gram of enstatite if the true age and initial Srs7/Sras ratio for the enstatite’s common Sr could be specified. GAST’S MODEL II values, an age of 4.7 x log years and Sr*7/Sr*6 of O-697, would seem reasonable limits for the latter; their adoption implies that the aubrite has formed at effectively the same time as the Ca-rich achondrites. The ratio radiogenic SrE7/Rbs7 equivalent to this age is O-067. Thus the Srs7 produced per gram of enstatite can be calculated from its measured Rbs’ content assuming that the amount of the latter has not altered since crystallization except for radioactive decay. The radiogenic Srs7 actually present in the enstatite can be calculated from the primary data as radiogenic

Sra7 = Sr86((Sr87/Sr*6)present-

0*697},

assuming also that the common Sr content has not changed since crystallization. Thus on this hypothesis, the difference between these amounts, 0.0014 ,ug/g, is the amount lost by diffusion from the enstatite. It constitutes about 12 per cent of its total radiogenic Srs7. The amount of displaced radiogenic Sre7 per gram of the average total rock would be 0.0012 ,ug, since its enstatite content is about 86 per cent. The problem is to locate this displaced Sr *‘. In principle it may have been absorbed by the oligoclase or some other mineral phase, or totally lost from the totalmeteorite. It is relevant to calculatewhat enrichment in the oligoclase Srs’/Sr%atio would result if absorption did occur in this instance, and to compare the result with its actual Sra7/Srs6 ratio. The primary data give the present-day Srs’ concentration of the (pure) oligoclase as 5.4 ,ug/g, which is equivalent to about 0.65 ,ug/g of total the fraction rock. The displaced radiogenio Srs7 would therefore constitute 0*0012/0.65 of this, i.e. about 0.2 per cent, so that the Sr87/Sr8a ratio of the oligoclase would have increased by closely this fraction-for example from 0.7200 to 0.7215. The co-ordinate point for the oligoclase should lie above the Model II isochron by

The rubidium-strontium

age of the Bishopville

aubrite

1099

this amount. Figure 1 graphically displays this interpretation, with the Model II isochron passing through the initial value for Srs7/Srse of O-697, with gradient equal to O-067. The oligoclase point is very little displaced above the isochron, although it is possible that experimental errors in measuring the isotopic ratios have obscured the effect. However, accepting the measured data as correct, it appears from this analysis that either the enstatite has not lost any more than a few per aent of its radiogenic SP, or else, if it has lost as much as 12 per cent, nearly all the released radiogenic Sr87 has escaped from the total rock. Summarizing the interpretations of the apparent age of the aubrite, interpretstion I is the face-value conclusion that the aubrite and its constituent minerals were formed 3.7 x lo9 years ago. Interpretation II makes this value a minimum age for the aubrite but with consequences for the aubrite’s thermal history and Interpretation III considers the possibility of postcrysttallization loss of radiogenio Sr*’ which would make the apparent &ge an indefinite minimum value. The Bishopville data suggest that important chemical fractionation in at least certain meteorites has occurred as late as 3.7 x lo9 years, or hes extended for about 1 x lo9 years from the 4.7 x lo9 years datum. To return to the meteorite-Earth analogy, we may raise the question of what is the Sr isotopic evidence for the formation of the Earth, or at least its upper mantle during this time-interval. Is it possible that the ratio Srs7~Sr86 in the upper mantle may have developed from an initial value as high as O-7015 (the Bishopville initial ratio) rather than O-6971 This question appears to be conclusively answered by the measurements on Archaean greenstone and anorthosite by HEDGE and WALTHALL (1963). These authors obtained values of about 0+6985 (normalized to 8.340 for Sr8s/Sr86) for the initial Srs?fSrs@ of these rocks, which is consistent, as they show, with the development of the Sr in their source regions from O-697. Acknowledgenzenta-Weare indebted to A. J. EASTONfor the mineral separations and sodium analysis, and to V. M. BOFINUER and Mrs. A. &OWN for assistancewith the mass-spectrometry. REFERENCES CLAIMR. D. (1959) Surface ionization source for mass spectrometry. J. f&i. ~~~~~. 36,38-g. FAURE G. and HURLEY P. M. (1863) The isotopic composition of strontium. in oceanic and continental bssalts: Application to the origin of igneous roohs. J. Petrol. 4, 31-50. GASTP. W. (1962) The isotopic composition of strontium and the age of stone meteorites--I. Beochim. et Comnochim. Acta 26, 927-43. HEDCE C. E. and WALTHAXLF. G. (1963) R,adiogenic strontium-87 as an index of Geologic processss. S&lace IQO, 12 14-7. LOVERINUJ. F. (1964) Electron probe study of the Bishopville enstatite achondrite (in prepara-. tion). MORGANJ. W. snd LOVERING J. F. (1964) Uranium and thorium abundancesin stony meteorites 2. The Achondritic meteorites, J. Geophys. Res. 69, 1989-1994. PINSON W. W. JR., SCHNETZLER C. C., BEISERE., FAIRBAIRNH. W. and HUIRLEY P. M. (1965) Rb-Sr age of stony meteorites. cieochirn. et Comochim. Acta &l%,455-466. WASSERBVR~G. J., WENT. and Anonsox J. (1964) Strontium oontamin~tioninmineralanalysis. Gemhim. et Coe~ch~~. Acta 23, 407-10. WHK~ES A. B. L. (1960) Voltage coefficientofVictoreen High-Meg resistors. Rev. Sci. I~~~TUWL 31, 208-o.