87Rb–87Sr history of the Norton County enstatite achondrite

Earth and Planetary Science Letters, 32 (1976) 191-198

19)

© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [21

87Rb-STSr HISTORY OF THE NORTON COUNTY ENSTATITE ACHONDRITE J.F. MINSTER and C.J. ALLEGRE Laboratoire de G~ochimie et Cosmochimie, Institut de Physique du Globe, Universit~ de Paris VI, et D~partement des Sciences de la Terre, Universit~ de Paris VI1, Paris {France)

Revised version received July 24, 1976

87Rb-87Sr analysis of the Norton County achondrite has been achieved with special attention to the rubidium analysis. Enstatite crystals and polycrystalline material give an "age" of 4.48 +-0.04 X 109 years and an initial ratio 87Sr/86Srl = 0.7005 -+0.0004 (x = 1.39 x 10-1 t yr-1, maximum errors). The feldspar component of the meteorite contains about 70% of the strontium and 30% of the rubidium of the whole sample, and does not lie on the isochron. Its model age relative to the strontium initial ratio of Allende is 4.6 X 109 years. The data are consistent with a complex history dealing with an incomplete isotopic reequilibration of the meteorite, 120 m.y. after its formation at 4.6 X 109 years, with an initial ratio similar to that of Allende.

1. Introduction The Norton County achondrite fell in 1948 in Norton County (Kansas). Beck and La Paz [1] described the circumstances o f its fall and its mineralogy and bulk chemical composition. This meteorite consists essentially o f large enstatite crystals in a mass o f fine-grained polycrystalline enstatite and olivine with small flecks o f metallic phases (kamacite, schreibersite, troilite and graphite). In addition, Keil and Fredriksson [2] described metallic copper, ferromagnesian alabandite, daubreelite and titanoan troilite, Using the chemical analysis o f Beck and La Paz [1 ] and Wiik [3] Norton County is classified as a Ca-poor achondrite (aubrite). Some a'nalyses o f trace elements in enstatite [4] and schreibersite [5] have been performed. Rare earth elements have been studied b y Schmitt et al. [6] and Masuda [7]. 4 o K_4 o Ar ages have been measured several times: whereas Vinogradov et al. [8] and Kirsten et al. [9] reported variable ages between 2.3 and 5.03 X 10 9 years, Geiss and Hess [10] and Bogard et al. [11] obtained 4.2 to 4.5 × 10 9 years. At the same time Bogard et al. [11] have obtained a 8 7 R b - S V S r internal isochron age of 4.7 -+ 0.13 X 10 9 years with an initial ratio 87 Sr/86 Sr I = 0.700 -+ 0.002. Huey and Kohman

[12] obtained a P b - P b age o f 4.55 × 10 9 years for the bulk sample, which is older than their P b - P b age o f 4.50 X 10 9 years for the chondrites; they argued that this was consistent with the old 87 Rb_87 Sr age o f Bogard et al. [ 11 ]. Such an old age o f Norton County seems to be accepted by many people (see, e.g., Wasson [13]). Eight samples (total weight o f about 2 g) o f grossly separated enstatite crystals and matrix were provided to us by Dr. K. Keil from the University o f New Mexico. In addition, through our cooperation with Dr. M. Tatsumoto of the U.S.G.S. (Denver) we obtained about 2 g o f sample N 523.3 X, one of the two samples studied by Bogard et al. [11]. 2. Experimental technique Our experimental technique is similar to the one already described by Birck et al. [ 14,15 ]. S a m p l e preparation. M1 samples were washed with

distilled acetone. Since there is a great variation o f the Rb/Sr ratio between the "single" crystals and the polycrystaUine matrix, we carefully hand-picked under a microscope most o f the crystals in the matrix, keeping the larger ones for separate measurements.

192 For the sample N 523.3 X a procedure similar to the one used b y Masuda [7] was used. The sample was gently crushed in an agate mortar. 200 mg o f the sample, grossly representative of the whole-rock sample, were designated as sample N 523 U. A second fraction was used for mineral separation; a light component, N 523 F, of density less than 2.9 was obtained. For the first separate, N 523 F1, we used a mixture o f methylene-iodide washed with dilute bromhydric acid, and acetone. For the second one, N 523 F2, we used filtered bromoform. This material represents about 1% by weight o f the sample. Starting from a part of N 523 P(see below) we obtained about 4% by weight o f this light component. The remaining fraction o f the sample N 523.3 X was sieved through a 100-mesh nylon siever. The finer fraction is called N 523 P; the largest crystals have been taken for N 523 S. Throughout this paper we use Masuda's notation of P for polycrystalline materials, S for "single" crystals, and U for whole-rock samples.

Chemistry and blanks. After addition of tracers (41 K, 87 Rb, 84 Sr) the samples were dissolved in teflon beakers by a mixture o f hydrofluoric, perchloric and nitric acids, and eluted on columns of cationic exchange resin (1 cm 3 AG 50W X12 200-400 mesh) using the elution procedure described by Birck and All~gre [16] (namely, separation of Ca from Sr b y neutralized citric acid). Great care was taken to achieve a complete destruction of the magnesian fluoride precipitate. Considering the low Rb and Sr concentrations of our samples, we applied blank corrections to each analysis. These corrections do not change significantly the slope o f the isochron since (1) the slope of the isochron is controlled b y the higher points, and (2) the direction o f our blank correction for these points is almost parallel to the isochron in the (87 Rb/8 6 St, 8 7 Sr/8 6 St) diagram. However, the initial ratio depends rather strongly on the position o f the lower points. Seven blanks have been measured from 0.02 to 0.05 ng for Rb and from 0.07 to 0.12 ng for Sr. The 87Sr/86Sr ratio of the blank is 0.707 + 0.003. The mean values have been used for the corrections.

Strontium. We checked the accuracy of our Sr mea-

TABLE 1 NBS standard BS 987 0.71017 0.71011 0.71017 0.71016 0.71014

+ 0.00005 -+0.00015 ± 0.00009 -+0.00010 + 0.00008

0.71014 -+ 0.00016 0.71039 +-0.00045 Spiked and unspiked runs normalized to 86Sr/88Sr = 0.1194. The two last values correspond to low signal runs (less than 3 X 10-12A). The errors are 20 mean.

surements by running the Sr standard NBS 987. The .results are shown in Table 1. For usual runs, we obtained a reproducibility of -+ 5 × 10 -s around a mean value o f 87 Sr/86 Sr = 0.71015 (given NBS value: SVSr/86Sr = 0.71014). Due to the low Sr content of our samples, we often obtained low signal runs (less than 3 X 10-12 A o f 8SSr). We checked the possibility o f an instrumental bias at that signal level b y running the standard under similar conditions. In both cases, our mean value is within the error limits (2o mean). Thus, since our samples are very radiogenic, the precision o f the isotopic ratio 87/86 Sr is not critical. The precision of the age is controlled by the s ~ Rb/ 86 Sr ratio, and thus the Rb determination. This requires a good control o f the mass discrimination during the Rb run (since it cannot be corrected) and an accurate determination o f the concentrations o f the tracer solutions.

Rubidium. The Rb was loaded on tungsten filaments (0.025 mm X 0.75 m m × 13 ram) in hydrofluoric acid solution, together with tantalum oxide and phosphoric acid, and run at a low and constant temperature (filament current: 1.6 A). At least four sets o f ten ratios were measured. Under these conditions, the mass discrimination was found to be constant, showing less than 0.2% variation, except for very small samples (say tess than 0.5 ng Rb). The 8 s Rb/87 Rb ratio o f normal Rb was measured several times at 2.6078 with an absolute spread o f +-0.11%. This value is independent o f the quantity loaded on the filament, and o f the Mg or K added on the load. These results

193 TABLE 2

TABLE 3

85 Rb/87 Rb value of standard solutions measured as described in the text

Computed and measured 85Rb/87 Rb ratios of various mixtures of normal Rb and spike

2.6080 2.6104 2.6078 2.6050 2.6076

85 Rb/87Rb measured

8SRb/87Rb calculated

Absolute difference

~*

1.5195 2.0417 2.2145 2.2488 2.2807 2.4141 2.5420

1.5176 2.0378 2.2142 2.2499 2.2831 2.4123 2.5443

0.0019 0.0039 0.0003 0.0011 0.0024 0.0018 0.0023

1.2 1.9 0.14 -0.49 -1.0 0.75 -0.90

Mean 2.6078 -+ 0.11%

Each value, corresponds to at least 50 measured ratios. For these runs, the variation of the mass discrimination is less than 0.2%. The error on the mean is the absolute spread.

are given in Table 2. Fig. 1 shows the evolution of the ratio during two different runs of the same sample and the reproducibility between these two runs. Table 3 and Fig. 2 show the comparison between computed and measured 8 s Rb/S 7 Rb ratios of various mixtures of normal Rb and spike. For these mixtures we used two different standard solutions, and two different spike solutions. For the calculated values we used o u r m e a n m e a s u r e d c o m p o s i t i o n o f t h e n o r m a l (8 s Rb/8 7 Rb = 2 . 6 0 7 8 ) a n d o u r m e a s u r e d value for t h e spike i s o t o p i c c o m p o s i t i o n (8 s R b / 8 7 Rb = 0 . 0 0 8 1 ) . This last value was less precise b u t never critical. In all cases, m e a s u r e d a n d c a l c u l a t e d values agree w i t h i n 0.2%. Since R b has n e v e r b e e n f o u n d y e t to b e f r a c t i o n a t e d at t h a t level (see, e.g., G a r n e r et

For the "calculated" value, we used 85Rb/87Rbnormal = 2.6078 and 8SRb/87Rbspike = 0.0081. [85 Rb/87 Rbm_87 Rb/8 s Rb c *~ = t ~ ) X 103 where subscripts m and c refer to measured and calculated, respectively.

RUBIDIUM MIXINGLINE

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0,437

0.4345

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3SRb/I7gb © e l c w l a t ~ l Fig. 2. Rb mixing line between normal and spike. Abscissa are calculated (c) 85 Rb/87 Rb ratios; ordinates are the measured (m) values. Insert shows: Fig. 1. Evolution with time of the 85 Rb/87 Rb ratio during two different runs of the same sample. The change in the mass discrimination is negligible, the reproducibility between the two runs better than 0.2%. Each point is a set o f t e n ratios with its 2o error. Time is approximate.

CSRb/87Rbm-SSRb/87Rbc~

~= \ "

85Rb/87Rb~-~

j X 10 3

assuming 0.2% error on 8 5 Rb/8 7 Rbm.

194 al. I17]), this precision was considered to be attained for our samples.

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o o.~ Tracers. We used a unique mixture o f tracers solution which was repetitively calibrated with different solutions of standards. Both 87 Rb and 84 Sr concentrations agreed within 0.1% between the different calibrations. Tracer solutions calibrated with these standards have been used to measure the concentrations of the NBS wafers: the obtained values agreed with the published NBS data within 0.1%. However, in view of the data given in Table 3 the error of the 8 ? Rb concentration of our tracer may be about 0.2%.

COUNTY

4.48 ± 0 . 0 4

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In summary. We consider that the error of our 87 Rb/ 86 Sr ratio is about -+0.5% for well-spiked samples. Each time that the evolution of the internal mass discrimination during the Rb run was higher than 0.5% or that the sample-to-spike ratio was too high, we attributed a -+1.0% error to this ratio. Through the reproducibility of our runs we estimate an error of -+2% for the K concentrations.

Fig. 3.87Sr/a6Sr versus 87Rb/86Sr. The errors are +20 mean and -+0.5% respectively. The lower insert shows the lower points. The upper insert shows: _ (87 Sr/86Srm--87Sr/86Sra.l × -\

87Sr/86Sr m

]

103

where subscript m stands for the measured value and a the value adjusted through the least-square program. The dotted lines correspond to the +20 error envelope.

3. Rosults, interpretation and discussion The analytical results are given in Table 4. As already quoted by Bogard et at. [ l 1 ], the concentrations of K, Rb and Sr are low and strongly variable between enstatite crystals and matrix. We attribute these differences to the light minerals rich in Rb and Sr, mostly present in the matrix, and which we consider to be oligoclase. We were also intrigued by the concentrations in the crystals (N 102 S); a microprobe analysis identified this sample as diopside, containing enstatite as exsolution lamellae. Large, 5 m m size forsterite crystals were also found, showing that these minerals can occur as fairly large crystals. The results are plotted in a conventional (8 7 Rb/8 6 SF, 8 ? St/8 6 S r ) diagram (Fig. 3). The points do not fall on a single line within their experimental error: if we calculate a least-square best-fit line for all the samples, the parameter ( S / N - 2 ) 1/2 equals 3.6 and definitively attributes this scatter to "geological" errors in the sense of Brooks et al. [18]. However, as shown in Fig. 3, if the two feldspars points are discarded, the other points fall on a well-defined line. Since blank positions are generally on the lower right-hand side

of the line, and since the feldspars are Rb- and Srrich minerals, an explanation of the data by contamination effects is not satisfactory. Papanastassiou and Wasserburg [19] argued that in Apollo 16 breccias, feldspar was originally in equilibrium with the other phases in the rock, and was only slightly affected by impact-induced crushing and subsequent metamorphism. We assume here that feldspar in Norton County behaved in the same manner as that in Apollo 16 breccias. In such a model, the "whole-rock" sample and the oligoclase define a line related to tl{e primary age, whereas the whole-rock and the other minerals define a secondary isochron. A least-square treatment of feldspars and whole-rock data gives an age of 4.68 -+ 0.12 X 109 years and initial ratio 87Sr/86Sri = 0.6985 -+ 0.0004 (20 errors ( S / N - 2 ) 1/2 = 0.7). This initial ratio is close to 87 Sr/86 SrAllend e = 0.69877 -+ 0.00005 [20,21] by not significantly different from BABI [22]. If we take the Allende value as the initial ratio of Norton County, we obtain an age of 4.63 × 109 years which is close to the age of the most primitive chondrites and achondrites. The secondary isochron gives an age of 4.48 -+ 0.04 × 109 years

diopside + enstatite matrix enstatite crystals entatite crystals matrix enstatite crystals feldspar feldspar whole rock

N 102 N 120 N 133 N 104 N 523 N 523 N 523 N 523 N 523

19.3 45.0 57.6 76.0 56.9 37.3 4.1 2.9 210.0

Weight (rag) 0.71418 (10) 0.72511 (10) 0.8494 (2) 0.8970 (7) 0.72926 (8) 0".9414 (5) 0.72165 (14) 0.71182 (8) 0.7509 (4)

87Sr/86Sr

0.213 (l.0) 0.3816 (0.5) 2.35 (1.0) 3.05 (1.0) 0.455 (1.0) 3.741 (0.5) 0.3458 (0.5) 0.1979 (0.5) 0.7757 (0.5)

87Rb/86Sr

0.0414 0.348 0.242 0.130 0.281 0.184 8.44 6.77 0.299

Rb (ppm) 3.7 0.2 0.2 0.3 0.2 0.4 0.1 0.1 0.1

Rbblan k

0.539 2.630 0.297 0.124 1.748 0.145 69.24 97.1 1.10

Sr (ppm)

1.0 0.1 0.6 1.0 0.1 2.0 0.03 0.02 0.1

Srblank

192. 153. 338. 130. 524. 640. 230.

95.0 24.0 4420.0 4330.0 68.8

K/Rb

67.0 37.0

K (ppm)

The uncertainties of the 87 Sr/86 Sr ratios are 20 mean and concern the last digit. The uncertainties of the 87 Rb/86 Sr ratios are in percent. The blanks are in percent.

S P S S P S F1 F= U

Mineralogy

Sample

Analytical results, Norton County achondrite

TABLE 4

196 and 87 Sr/86 Sr = 0.7005 + 0.0004 (20 errors ( S / N - 2 ) 1/2 = 1.08) Our interpretation of the results is that Norton County formed at 4.6 X 10 9 years with the initial 8 "7 St/86 Sr I similar to that o f Allende and about 120 m.y. later (i.e., at 4.48 X 109 years) impact-induced metamorphism occurred, which resulted in incomplete isotopic reequilibration. As we will discuss (Fig. 4) these ages are significantly different from the results o f B o g a r d et al. [11]. This two-stage evolution model o f Norton County is supported by several arguments: Reid and Cohen [4] have discussed the textural aspects of the enstatite in enstatite achondrites and concluded that all the aubrites, except Shallowater, have been shocked; this shock caused the disordered crystallization in the meteorites. For Norton County, this could have mildly reset the 87 Rb_87 Sr clock. Using a simple two-stage model calculation for the evolution o f the 8 v Sr/86 Sr initial ratio with time and using the value o f l m e t a m . = 0.7005 and 1 = Allende for initial value, we can calculate the s v Rb/86 Sr ratio o f the reservoir using 2xT= 120 m.y. We obtain a value

I

0.703

I

I

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I

I

87 Rb _ 0 8 0.702

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0.699

0.698

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r

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4.3

4.4

4..5

4..6

4.7

4.8

Age

(10 9 yrs)

Fig. 4. 8 7 Sr/ 8 6 SrI versus age diagram. The various internal isochrons plot along an evolution line corresponding to a cfiondritic Rb]Sr ratio. The errors are 20 propagated errors. NC1 = Norton County [ 11 ] ; NC2 = this work; NC3 = N 523.3 X data only; G = Guarena [31 ] ; C = Colomera [28]; SS = Saint Severin [32] ; BA = basaltic achondrites [16].

o f about 0.8 in agreement with our "whole-rock" value o f 0.76. The major objection to such an interpretation is the fact that small feldspar grains should be more resistant to resetting than large enstatite crystals. In fact the enstatite crystals do not consist o f a simple mineral assemblage: they are cracked and the cracks are filled with foreign minerals [4] which might contain most of the Rb and the Sr o f the "single" crystals samples. As discussed by Ashworth and Barber [23], mild shock conditions might have resulted in disorder effects and inversion to clinoenstatite; they could have induced isotopic reequilibration o f the foreign minerals. Moreover, the feldspar contains 70% of the Sr and 30% o f the Rb o f the whole sample, and can behave as a reservoir, since no other mineral is present to accept the Sr losses. On the other side, the concentrations o f the other minerals are so low, and the interval of time between the initial event and the secondary event is so short that eventual 87 Sr incomes in the feldspars cannot be seen. Finally, if the Rb and Sr diffusions are similar, they are not fractionated relative to each other, and the rich reservoir is good initial-event keeper. Riley and Compston [24] have shown that the (T,I) diagram is very useful for discussion o f R b - S r systematics. As quoted by Sanz and Wasserburg [25], a proper plotting in such a diagram should take into account the anticorrelation o f the errors o f the age and the initial ratio. This coefficient can be calculated, and it can be shown that the error distributions o f the parameters of a best-fit line are close to gaussian (Minster, in preparation). In Fig. 4, we have shown our results together with some o f the primitive internal isochrons in meteorites. It is clear that our results on Norton County (NC2) are in agreement with ,the chondritic and achondritic data available. This is in agreement with the old I - X e age o f the aubrites Shallowater and Pena Blanca Spring [26,27]. The previous determination (NC1) by Bogard et al. [11 ] is incompatible with this global picture. Even if we restrict our data to sample N 523.3 X, also used by Bogard et al. [11 ], tile determinations are far outside o f the error limits (NC1 and NC3). Note here that ( S / N - 2 ) ~/2 = 2.0 for NC3 and that our data on N 523.3 X do not define an isochron. This discrepancy is difficult to explain, the Caltech group attributes it either to a too high initial ratio [25] or to the use o f an electron multiplier [28] for most o f the radiogenic points.

197 However, a 4% discrepancy is difficult to attribute to mass discrimination. Since their initial ratio is rather similar to ours, we tentatively a t t r i b u t e the age difference to calibration o f tracers. In any case, such a bias might also affect the old R b - S r age determinations o f this group on iron m e t e o r i t e s [29,30] and these data should be revised b e f o r e use in the present picture o f the early chronology of the solar system.

4. Conclusion We do not consider that the present data have solved the p r o b l e m o f the primary age o f f o r m a t i o n o f the N o r t o n C o u n t y aubrite. It is shown that N o r t o n C o u n t y has had a c o m p l e x geologic history, perhaps related to its brecciated character. If so, this brecciation probably t o o k place about 4.48 X 109 years ago and the primary age is about 120 m.y. older. Such a result indicates that one should be very careful about the primary age d e t e r m i n a t i o n s o f brecciated objects. At the m o m e n t we have no evidence that N o r t o n C o u n t y is one o f the " o l d e s t " objects o f the solar system.

Acknowledgements We thank Dr. K. Keil and M. Tatsunroto for providing us tire samples, J.L. Birck, N. Shimizu, M. Semet and all our colleagues o f the laboratory for their constant help and criticism, and one o f the reviewers for his criticisms on the manuscript.

References 1 C.W. Beck and L. La Paz, The Nortonite fall and its mineralogy, Am. Mineral. 36 (1951) 45-59. 2 K. Keil and K. Fredriksson, Electron microprobe analysis of some rare minerals in the Norton County achondrite, Geochim. Cosmochim. Acta 27 (1963) 939-947. 3 H.B. Wiik, The chemical composition of some stony meteorites, Geochim. Cosmochim. Acta 9 (1956) 2 7 9 289. 4 A.M. Reid and A.J. Cohen, Some characteristics of enstatire achondrites, Geochim. Cosmochim. Acta 31 (1965) 661-672. 5 J.T. Wasson and C.M. Wai, Composition of the metal, schreibersite and perryite of enstatite achondrites and origin of enstatite condrites and achondrites, Geochim. Cosmochim. Aeta 34 (1970) 169-184.

R.A. Schmitt, R.H. Smith, J.E. Lasch, A.W. Mosen, D.A. Olehy and J. Vasilevskis, Abundances of the fourteen rare earth elements, scandium and yttrium in meteoritic and terrestrial matter, Geochim. Cosmochim. Aeta 27 (1963) 577-622. A. Masuda, Lanthanides in the Norton County achondrite, Geochem. J. 2 (1968) 111-135. A.P. Vinogradov, 1.K. Sadoroschnji and K.G. Knorre, Argon in meteorites, Meteoritika 18 (1960) 92. T. Kirsten, D. Krankowsky and J. Zahringer, Edelgas und Kalium Bestimarungen an einer gr6sseren Zahl yon Steinen Meteoriten, Geochim. Cosmochim. Acta 27 (1963) 13-42. 10 J. Geiss and D.C. Hess, Argon-potassium ages and the isotopic composition of argon from meteorites, Astrophys. J. 127 (1958) 224-236. 11 D.D. Bogard, D.S. Burnett, P. Eberhardt and G.J. Wasserburg, 87Rb-S7Sr isochron and 4 ° K - 4 ° A r ages of the Norton County achondrite, Earth Planet. Sci. Lett. 3(1967) 179 189. 12 S.M. Huey and T.P. Kohman, 2°Tpb-2°6pb isochron and ages of chondrites, J. Geophys. Res. 78 (1973) 32273244. 13 J.T. Wasson, Meteorites (Springer Verlag, New York, N.Y., 1974). 14 J.L. Bkck and C.J. All~gre, 87Rb-87Sr systematics of Muntsche Tundra mafic pluton, Earth Planet. Sci. Lett. 20 (1973) 266-274. 15 J.L. Birck, S. t:ourcade and C.J. All~gre, 87Rb_87Sr age of rocks from the Apollo 15 landing site and significance of internal isochrons, Earth Planet. Sci. Lett. 26 (1975) 29-35. 16 J.L. Birck and C.J. AU~gre, Chronology and chemical history of basaltic achondrites parent body, studied by the 87 Rb-87 Sr method, in preparation. 17 •E.L. Garner, L.A. Machlan and I.L. Barnes, Tile isotopic composition of lithium, potassiunr, and rubidium in some Apollo 11, 12, 14, 15 and 16 samples, Proc. 6th Lunar Sci. Conf. (1975) 1845-1855. 18 C. Brooks, S.R. Hart and I. Wendt, Realistic use of two errors regression treatments as applied to rubidiumstrontium data, Rev. Geophys. Space Phys. 10 (1972) 551-577. 19 D.A. Papanastassiou and G.J. Wasserburg, R b - S r systematics in Luna 20 and Apollo 16 samples, Earth Planet. Sci. Lett. 17 (1972) 52-63. 20 C.M. Gray., D.A. Papanastassiou and G.J. Wasserburg, The identification of early condensates from the solar nebula Icarus 20 (1973) 213 239. 21 G.W. Wetherill, R. Mark and Lee-Hu, Chondrites: initial 87Sr/86Sr ratios and the early history of the solar system, Science 182 (1973) 2781-2783. 22 D.A. Papanastassiou and G.J. Wasserburg, Initial strontium isotopic abundances and the resolution of small time differences in the formation of planetary objects, Earth Planet. Sci. Lett. 5 (1969) 361-376. 23 J.R. Ashworth and D.J. Barber, Electron petrograpby of

198

24

25

26 27

28

shocks effects in a gas-rich enstatite achondrite, Contrib. Mineral, Petrol. 49 (1975) 1 4 9 - 1 6 2 . G.H. Riley and W. Compston, Theoretical and technical aspects of R b - S r geochronology, Geochim. Cosmochim. Acta 26 (1962) 1255-1281. H.G. Sanz and G.J. Wasserburg, Determination of an internal 87 Rb_87 Sr isochron for the Olivenza chondrite, Earth Planet. Sci. Lett. 3 (1969) 3 3 5 - 3 4 5 . C.M. Hohenberg, l - X e dating of the Shallowater achondrite, Earth Planet Sci. Lett. 3 (1967) 3 5 7 - 3 6 2 . F.A. Podosek, Dating of meteorites by the high temperature release of iodine correlated 129 Xe, Geo chim. Cosmochim. Acta 34 (1970) 3 4 1 - 3 6 5 . H.G. Sanz, D.S. Burnett and G.J. Wasserburg, A precise 87Rb/87Sr age and initial 87Sr/86Sr for the Colomera iron meteorite, Geochim. Cosmochim. Acta 34 (1969) 1227-1239.

29 D.S. Burnett and G.J. Wasserburg, Evidence for the formation of an iron meteorite at 3.8 X 109 years, Earth Planet. Sci. Lett. 2 (1967) 1 3 7 - 1 4 7 . 30 D.S. Burnett and G.J. Wasserburg, 8 7 R b - 8 7 S r ages of silicate inclusions in iron meteorites, Earth Planet. Sci. Lett. 2 (1967) 3 9 7 - 4 0 8 . 31 G.J. Wasserburg, D.A. Papanastassiou and H.G. Sanz, Initial strontium for a chondrite and the determination of a metamorphism or formation interval, Earth Planet. Sci. Lett. 7 (1969) 3 3 - 4 3 . 32 G. Manhes, J.F. Minster and C.J. All~gre, Comparative Pb-Pb and R b - S r ages of Saint Severin meteorite and some constraints about speculative chronology of the early solar system, Meteoritics 10 (1975) 541.