UPb systematics of phosphates from equilibrated ordinary chondrites

EPSL ELSEVIER

Earth and Planetary Science Letters 121 (1994) 153-171

U-Pb systematics of phosphates from equilibrated ordinary chondrites Christa G6pel, Gerard Manh~s, Claude J. All~gre Laboratoire G~ochimie et Cosmochimie, LP.G., 4 place Jussieu, 75252 Paris, Cedex 05, France

(Received July 30, 1993; revision accepted November 11, 1993)

Abstract U-Pb systematics were determined from fifteen phosphate separates from equilibrated ordinary chondrites and from small bulk fragments of the same meteorites. The high 238U/2°4pb ratios of thirteen of these phosphate separates lead to extremely radiogenic Pb whose 2°6pb/Z°4pb ratios range from 250 up to 3500. The P b / P b model ages for these phosphates range from 4.563 to 4.502 Ga, with an analytical precision of 106 y and the U-Pb system is apparently concordant. The time interval observed, 60 × 106 y, reflects the thermal processing of the equilibrated chondrites and is consistent with that previously derived from the R b / S t , K / A r and Pu chronologies. The P b / P b ages of the phosphates from the seven H chondrites show a negative correlation versus their metamorphic grade. This is the first clear relationship ever observed between a long-lived chronometer and the intensity of metamorphism as reflected by metamorphic grade. Assuming that the P b / P b age indicates the accurate U-Pb closure time in phosphates, the P b / P b chronology is compatible with the model of a layered H chondrite parent body. However, this interpretation of the U / P b systematics is not unique; it postulates a slow cooling of the equilibrated materials at high temperature, in apparent conflict with petrological observations. Except for the H chondrites, which agree rather well with Pu systematics, comparison of the P b / P b chronology with published radiochronometric data does not reveal simple correlations. In the present debate concerning the thermal history of chondrites, the chronometric information derived from each isotope system is interpreted as the time of its thermal closure. However, this basic assumption may not be correct for all the radiochronologies and must be evaluated before the radiochronometric data can be applied as compelling time constraints for the period of 4.56-4.4 Ga of proto-planetary history.

1. Introduction C h r o n o m e t r i c i n f o r m a t i o n d e r i v e d from longlived r a d i o n u c l i d e s , short-lived r a d i o n u c l i d e s a n d m e t a l l i c cooling r a t e s can b e u s e d to establish a globally c o h e r e n t c h r o n o l o g i c a l s c e n a r i o for

[MK]

c h o n d r i t e s . O u t o f this c o m e s the following o b s e r vations [1, a n d r e f e r e n c e s herein]: (1) T h e time interval for the f o r m a t i o n of the c h o n d r i t i c p a r e n t b o d i e s was short, less t h a n a few million years, o c c u r r i n g at 4.55-4.56 Ga. (2) T h e t h e r m a l evolution o f these small b o d ies was r a p i d a n d o c c u r r e d within t h e first hund r e d million y e a r s just after t h e i r accretion. An accurate and reliable high-resolution c h r o n o l o g y for this early p e r i o d o f solar system

0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD1 0012-821X(94)E0217-8

154

C. G6pel et aL / E a r t h and Planetary Science Letters 121 (1994) 153-171

evolution, however, is still not well established. The heat source and the physical conditions that characterise the diverse thermal processes shaping the evolution of ordinary chondrites have not been identified. Have they been heated to 1150 K and subsequently cooled to 200 K in unbroken parent bodies? Conflicting answers to this question have provided motivation for and stirred up interest in the current debate [2-4]. The U-Pb system can contribute to the understanding of the early history of the chondritic materials. The production of 2°7pb was much higher in the early solar system than it is today because of the relatively short half life of 235U. Thus the ratio of the radiogenic Pb components 2°7pb*/z°6pb* varied rapidly at that time. This feature, coupled with the present analytical capability to measure this ratio with a precision of better than 0.1%, allows age determinations with a resolution of about 1 m.y. for meteoritic material of high U / P b ratio. Coupling the two U-Pb systems (Z35U and 238U) allows us to evaluate the coherency of the p a r e n t - d a u g h t e r relationship in chondritic material and determine if the U-Pb evolution occurred in a closed system or if it was modified by processes or subsequent events that may have occurred since the last 4.5 Ga. Since Patterson's work 35 years ago [5], an abundance of P b / P b data with increasing precision have confirmed the 4.55-4.56 Ga 2°7pb/ 2°6pb age of ordinary chondrites [6-11]. This age is accepted as "the formation age of chondrites" and also as that of the solar system. The improvement in the analytical precision has provided the motivation and means for investigating this problem further, and for addressing the following concerns: No correlation between the P b / P b chronology and chemical or petrological characteristics in chondrites has been found and the validity of these model ages at the level of current analytical precision (better than 107 y) has not yet been established. Chondrites appear to contain more radiogenic Pb than can be derived from the in-situ decay of U and Th, assuming an initial Pb component with a primordial isotopic composition [12,13]. The reason for this open U-Th-Pb behaviour has still not been identified. The objective of this study is to explore the

U-Pb systematics of ordinary chondrites. New data for phosphates separated from equilibrated ordinary chondrites are presented and the behaviour of U / P b in these materials is carefully considered in order to evaluate the meaning of the precise chronometric information now available. The initial results of this investigation were recently published in abstract form [14,15]. Although calcium phosphates (merrilite and apatite) are minor accessory minerals in ordinary chondrites (less than 0.5 vol%), they are major hosts for U, enriched by more than an order of magnitude relative to the bulk rock (0.1-3 ppm versus 0.01 ppm, respectively). Because of this, phosphates contain highly radiogenic Pb and are therefore suitable minerals for precise U / P b dating. Phosphates are secondary mineral phases, produced during post-formational thermal processing of chondrites. Their close spatial association with the metal phase suggests that they formed by oxidation of P-rich metal [16,17]. The U-Pb evolution in this mineral and the precise P b / P b chronometry available from the highly radiogenic Pb can clearly delineate many fine details of the thermal history of ordinary chondrites. However, because of analytical difficulties the U / P b systematics of the phosphates from only one ordinary chondrite (LL6 Saint-S6verin) have been published [18,19]. This work reports positive results from an extensive effort to overcome these problems and new Pb-Pb data from a number of phosphate separates. Phosphates from fifteen equilibrated chondrites were separated and analysed. These include seven H chondrites (H4 Forest Vale, H4 Ste. Marguerite, H5 Nadiabondi, H5 Allegan, H5 Richardton, H6 Guarefia, H6 Kernouv6), five L chondrites (L5 Homestead, L5 Knyahinya, L5 Ausson, L 5 - 6 Barwell, L6 Marion (Iowa)) and three LL chondrites (LL5 Tuxtuac, LL5 Guidder, LL6 Saint-S6verin (light)). It was not possible to study phosphates from type 3 chondrites because of their rarity and small grain size. Except for L5 Knyahinya and L5 Homestead, only meteorites displaying no shock features and preferentially those which had already been studied with other long-lived radionuclides or the with the 24apu fission-track method were selected. Although low

C. GOpel et al. /Earth and Planetary Science Letters 121 (1994) 153-171

K / A r ages and the presence of melt pockets are clear signs of shock events [20-22], Knyahinya and H o m e s t e a d were analysed at the beginning of this study in order to check the experimental procedure. In addition, whole-rock U-Pb systematics were measured in small fragments from several meteorites for which no previous wholerock data were available.

2. Analytical procedure Phosphate separates were made by a combination of magnetic susceptibility and density separation techniques. 25-50 g of chondrite starting material was crushed in an aluminium oxide mortar and ground in a boron carbide mortar until the grain size of the main fraction was between 37 and 74 p,m. Two grain size fractions, 74-149 txm and 37-74 jxm, were selected and independently processed through the following steps: The metal phase was eliminated by a hand magnet before passing the sieved fractions through a Frantz magnetic separator. Phosphates, together with feldspar, pass through undeflected and are found in the > 1.6 A non-magnetic fraction. Two separation steps using heavy liquids follow, the first with bromoform eliminates feldspar (and boron carbide chips from the second mortar) and the second step with methylen iodide eliminates traces of aluminium oxide from the first mortar. Depending on the purity of the phosphate fraction, observed under a binocular microscope, the two grain-size fractions were combined, ground to a finer size and a new separation step with bromoform performed. This procedure minimises the number of mixed grains. After the final control under the binocular microscope, the phosphate separates were transferred into a Teflon beaker and weighed, with 0.8-5 mg often representing the complete yield from 10 g of starting material. The purpose of the washing sequence described below is to eliminate any contamination introduced during the mineral separation procedure. The problem here is that surface contamination cannot be removed from phosphates by simple acid washing because of their solubility in

155

acids. Therefore, each separate was washed for about 5 min with 0.3 ml of acetone under ultrasonic agitation at room temperature. The wash solution was isolated from the sample and spiked with a mixed 2°sPb-Z33u tracer. The amount of common Pb found in these acetone rinses generally ranged between 10 and 300 × 10-12 g, those of U at the level of chemistry blank (1 x 10 12 g). The procedure was repeated until the Pb content analysed in the leachates was equivalent to the chemistry blank (6 x 10 ~2 g). Complete dissolution of phosphates was accomplished by exposure to 0.4 ml of 0.5 M HBr for 15 min at room temperature with intermittent ultrasonic agitation. The solution was decanted by centrifuging and transferred into a beaker containing a 2°spb-233U spike. The very small residue which sometimes remained was exposed again to a 0.4 ml fraction of 0.5 M HBr, this time for 3 h in order to ensure complete dissolution of the phosphates. The beakers containing the dissolutions were subsequently warmed in order to equilibrate the samples with the 2°sPb-233U spike, and the U and Pb were finally separated following the procedures described in [13,23]. Total blanks for the chemistry including the dissolution procedures were ~ 6 × 1 0 12 g Pb and ~ 1 × 10-12 g U. Because of the small amounts available, the U and Pb isotopic compositions were measured using the electron multiplier detector. Single Re filaments, using the phosphoric acid-silica gel technique, were employed for the Pb and U isotopic measurements. The raw data were corrected for instrumental mass fractionations of 1.0_+ 0.3%o per mass unit (2o-) for Pb and 1.5_+0.7%o per mass unit (2o-) for U, as determined by regular measurements of the standards SRM 981 and SRM 960, respectively. In the present study the U and Pb concentrations of the phosphates are measured with an uncertainty of 10-30%. The main reason for the low accuracy comes from the uncertainty in the small sample weight, which was obtained using a regular analytical balance. The phosphate separates may also contain other mineral phases as impurities and these stay as small residues after dissolution with 0.5 M HBr. The U / P b abundance ratios, however, are not affected by either

H6 H6 H6 1-15 HS H5 HS H4 H4

L6 L6 L5-6 L5-6 L5 L5 L5 LS LS

LL5 LL5

Kernouv# Kernouv# Guare~a Nadiabondi Allegan Allegan Allegan Forest Vale Ste.Marguerite

Marion (Iowa) Marion (Iowa) Barwell Barwell Knyahinya Homstead Homstead Homstead Ausson

Tuxtuac Guidder

4"

+

204pb

U

+

0,8 0,5

2,3

185,4

58,3

0,2 0,1

27,6 19,9

57,6

0,1 0,1

0,7 0,2

61,9 30,5

0,1

1,3

118,5

25,6 18,6

7,2 4,8

548,6 373,6

25,8

1,8 0,3

153,9 36,3

0,5 0,6 0,1

0,2

36,5

62,4 63,4 27,0

0,2

32,3

7,7

4,6

6,0

6,1 5,6

7,8 5,7

7,9

10,0 9,9 7,6

7,4 10.6

6,8

9,5 11,4

10,0 5,3

9,6

9,1

(ppb) (ppb) (ppb)

Pb

.t.

12,47 + 0,21

4,69 4- 0,05

2.19 + 0,24

27.28 4- 0,95 51 + S

53 _+ 2 92 + 13

92 + 5

16,03 4- 0,15 14,49 +_ 0,39 49 + 2

9,56 4- 0,12 58 + 5

4,30 + 0,08

1,12 4- 0,01 2,04 4- 0,03

4,60 + 0,10 15,99 + 0,29

38,00 _.+ 0,91

45,4 4- 2,0

236U/204pb

+ + 4+

0,20 0,23 0,11 0,018

15,865 + 0,022 31,80 +_ 0,10

102,33 43,38 61,41 20,109

65,64 _+ 0,15

114,22 + 0,28

36,177 + 0,037 32,754 _+ 0,050 65,73 4- 0,13

62,01 __+. 0,20

25,56 + 0,12

23,934 ::I: 0,018

19.151 + 0,014 19,894 _+ 0,015

40,178 + 0,071

21,999 4- 0,019

65,853 _+ 0,065 55,836 + 0,066

§ 2o6pb/2 o4pb

2

3,7

3,6

4,5172 4- 0,0018 4,5593 + 0,0012 4,5477 __+ 0,0019 4,5359 + 0,0019 4,5385 + 0,0015

1,3796 + 0,0033 2,0312 + 0,0013 1,9872 -+ 0,0013 1.7773 4- 0,0013

3,7 3,3

4,6142 __+ 0,0042 4,5667 +__ 0,0016

1.7349 + 0,0032 1,1311 ± 0,0188

0,6901

32.19 _+ 0,23

15,851 _+ 0,024

0,90714 +__ 0,00049 0,7537 __+ 0,0011

0.83184 + 0.00032

2,3052 4- 0,0021 1,6080 +__ 0,0039

1,4027 4- 0,0098 1,2971 __+ 0,0211 1,9958 -+ 0,0013

4,4 3,9

3,8 4,1 4,0

4,5606 + 0,0027 4,5590 + 0,0013 4,4978 _+ 0,0020 45,16 + 0,97 7O+_ 5 20,118 + 0,019

4,5330 __+ 0,0026

3,8

4,5536 __+ 0,0010 1,105 + 0,020 0,6551 + 0,0046 0,7219 + 0,0024 0,6855 + 0,0052

126 + 15

4,5654 + 0,0038

3,9

0,0012

4,5503 + 1,2687 _+ 0,0071 0,6838 4- 0,0017

69_+ 2

1,1291 _+ 0.0071

3,5 3,9

4,5577 _+ 0,0018 4~5522 +_ 0,0010 0,6552 + 0,0016

3,6

4,4189 ± 0,0017 1,5343 ± 0,0013 1.1751 +_ 0,0094

0,71807 + 0,00040 0,6859 +__ 0,0020

69+ 2 124 + 5

3,4

4,4346 _+ 0,0013

1,4389 + 0,0025

0,70727 __+ 0,00067

+ 0,0034

0,8124 __+ 0,0011

3,9 3,8

3,4

3,3

4,5334 + 0,0019

2,8

Th/U

4,5324 +± 0,0008

(~E)

1,1878 + 0,0065 1,8659 + 0,0013

1,0320 __+ 0,0085

#

PblPb age

36,44 +_ 0,14

4

0,80426 + 0,00032

0,85443 + 0,00028 0.84274 __+ 0,00034

0,81617 __+ 0,00035 0,73252 __+ 0,00075

0,6907 + 0,0015

0,6773 __+ 0,0016

./.

2OBpb/2O6p b

+

207pb/2O6pb

32,802 _+ 0,056

69+

25,69 + 0,14

23,972 + 0,026

19,155 4- 0,014 19,912 + 0,018

22,012 + 0,020 40,65 + 0,25

57,59 _+ 0,92

69+

2O6pb/2O4 pb

+ All values are corrected for blank chemistry, mass fractionation and spike contribution; ~ isotope ratio is corrected for mass discrimination and spike; # P b / P b age is calculated using the decay constants AU238 = 1.55125 × 10 m y 1 and AU235 = 9.8485 x 10 10 y 1 [73] and the primordial Pb isotopic composition from T a t s u m o t o et al. [12] for correction of the 2°4pb; he atomic ratio is calculated assuming an evolution in a closed system since 4.55AE with the decay constant hTh 232 = 0.49475 x 10-11 y - 1 [73].

type

sample

Table 1 U-Pb systematics of fragments from ordinary chondrites

/

,<~

~-

~.

,~

.""

C~

.~

c~

157

c. G6pel et al. /Earth and Planetary Science Letters 121 (1994) 153-171

of these e x p e r i m e n t a l u n c e r t a i n t i e s and are defined with a precision of 1 - 2 % . T h e chemical p r o c e d u r e for analysis of the whole-rock f r a g m e n t s was similar to that described for the phosphates, with exception of the dissolution step, which was carried out with a mixture of c o n c e n t r a t e d H F - H B r . M e a s u r e d blanks for the whole-rock chemistry were slightly higher, ~ 20 × 10 -12 g Pb a n d ~ 1 x 10 12 g U. L o a d i n g onto the r h e n i u m filaments a n d the therm o i o n i s a t i o n p r o c e d u r e s were identical with those for the p h o s p h a t e separates. T h e 2°7pb/Z°6pb m o d e l age is calculated by assuming that the m e a s u r e d Pb isotopic composition is p r o d u c e d by closed-system U - P b evolution, starting from an initial Pb c o m p o n e n t with a p r i m o r d i a l Pb isotopic c o m p o s i t i o n (2°6Pb/Z°4pb

5I

I

lO

I

~

I

5Ga

0.64 -

4

I

= 9.307, 2°7pb/2°4pb = 10.294 [12,13]). T h e precision of the raw data, the correction of the e x p e r i m e n t a l mass d i s c r i m i n a t i o n a n d the chemical c o n t a m i n a t i o n i n t r o d u c e d d u r i n g processing a n d analysis all c o n t r i b u t e to the analytical uncertainty of the 2°7pb/2°6pb m o d e l age. T h e correction for analytical c o n t a m i n a t i o n i n t r o d u c e s an u n c e r t a i n t y of less t h a n 0.6 × 106 y for most samples, b e c a u s e of the low Pb b l a n k levels a n d the near-4.5 G a P b / P b model age for this mode r n terrestrial c o n t a m i n a t i o n . T h e c o n t r i b u t i o n of i n s t r u m e n t a l mass d i s c r i m i n a t i o n to the uncertainty in the 2°7pb/Z°6Pb model age decreases with the r a d i o g e n i c character of the Pb sample, so that an u n c e r t a i n t y of 0.03% per mass unit in the d i s c r i m i n a t i o n i n t r o d u c e s an u n c e r t a i n t y in the P b / P b ages of 6.5 × 10 ~' y w h e n 2 ° 6 p b /

I

62

).

.60

82

.60

L, ~

,'

L5 Kn

V o

0,.

I

H4 FV

Concordia

3

I

H4 SM

~.56

",,, --~

LL5 LI Tx

H

+

",

u

I,.5~

0.62

a,.

L5-6 Ba

-

'...~,, ... |

0

H6 Ke ~::.. H5 Na 0.60 -

H6 Gu

---

4"52

----

4.50

I Ii

4,-,

4 .5

Concordia ~ I

0

I

0.5

I

Concordia

"1

I [~

1.0

4.5'

I

I

0.5

I

~.I

1.0

238 U / 206 P b *

Fig. 1. Modified U-Pb Concordia diagram, after Tera and Wasserburg [74], showing whole-rock fragments from ordinary chondrites. 2°6pb* and 2°7pb* are the calculated radiogenic components assuming a primordial Pb isotopic composition for the measured 2°4pb. In (a) fragments from H chondrites are shown, in (b) data from L and LL chondrites are indicated as • and • respectively. All bulk fragments from ordinary chondrites plot to the left of the Concordia curve: they show an apparent excess of radiogenic Pb relative to in-situ decay of U. Different samples of the same meteorite show different Pb/Pb model ages. In cases when a more radiogenic initial lead isotopic composition (from troilites of equilibrated chondrites [7]) is used for the measured 2°4pb, the samples are indicated as small dots. H6 Guarefia = H6 Gu; H6 Kernouv~ = H6 Ke; H5 Nadiabondi = H5 Na; H5 Allegan = H5 Al; H4 Ste. Marguerite = H4 SM; H4 Forest Vale = H4 FV; L5-6 Barwell = L5 6 Ba; L5 Homestead = L5 Ho; L5 Knyahinya = L5 Kn; LL5 Guidder = LL5 Gd; LL5 Tuxtuac = LL5 Tx.

H6 H6 H6 H6 145 /45 H5 H4 H4 144

L6 L5..6 L5 L5 L5 L5

LL6 LL5 LL5

Guare~a Guare~a Kernouwl Kernouvd Allegan Richardton Nadiabondi Ste. Marguerite Ste. Marguerite Forest Vale

Marion (Iowa) Bat'weft Ausson Knyahinya Knyahinya Homestead

St.S~verin Guidder Tuxtuac

÷

1990,1

1130,0

878,8 129,1

552,1

1341,9 nd

3,9

2,0

1,0 1,0

0,8

2,0 2,1

825,2

805,6

3,1

1489,1

2,5

1396,7

1610,2 6102,0 478,4

2,8 1,0 2,5

1,8

1230,0

4,1

3031,8

3,4

2,3

5,0

]6897,0 5283,4 2588,9

Pb (ppb)

1,0

4.0

weight (rag)

4-

0,7

0,1

1,5

0,5

1,5 0,6 0,4

0,1

0,6

0,2 nd

0,3

415,6

227,0

444,7

903,6

908,7 162,0 nd

467,0

1471,2

790,1 nd

nd

466,5 37,7

348.0

2,1 0,9 0,4

960,1

3432,6 nd nd

O (ppb)

4-

1,5

7,4 3,2 0,7

204pb (ppb)

.c-

497 ± 38

1410 ± 180

251 ± 5

1491 ± 114

534 ± 10 243 ± 60

3389 ± 740

951 ± 24

2879 ± 226

425 ± 20 91 4- 29

139 ± 5

562 ± 8

397 ± 8

238U/204pb

§ 4-

448 ± 1

1156 ± 6

261,4 ± 0,6

1318 ± 5

248,6 ± 0,5

184,0 ± 0,5

2420 ± 14 538,5 ± 0,9

1

514 ± 38

270 ± 5 1445 ± 182

1517 ± 114

291 ± 25

557 ± 10 266 ± 61

3468 ± 753

977 ± 24

364 ± 1

364 ± 1 931 ±

2960 ± 228

924 ± 170

443 ± 18 109 ± 29

160 ± 5

1779 ± 32 586 ± 6

746 ± 11

410 ± 2

206pb/204pb

2564 ± 16

677 ± 4

409 ± 2 73,2 ± 0,2

574 ± 1 150,8 ± 0,2

725 ± 1 1718 ± 3

407,6 ± 0,4

206pb/204pb

4-

+

± 0,0006

0,6238 ± 0,0011

0,61456 ± 0,00047

0,63590 + 0,00041

0,60564 + 0,00034

0,62629 ± 0,00150

0,61614 ± 0,00039 0,6304 ± 0,0042

0,61392 ± 0,00044

0,60593 ± 0,00025

0,63464 ± 0,00027

0,62457 ± 0,00027

__+ 0,0008

0,5503 ± 0,0042

2,4872 ± 0,0017

1,7328 ± 0,0013

0,2736 ± 0,0016

2,3638 ± 0,0020

0,3545 4- 0,0010 2,3543 ± 0,0040

1,3562 ± 0,0012

0,7184

0,2689 ± 0,00O2

0,2919 __+ 0,0008

0,2972 ± 0,0066

1,6497 ± 0,0239

0,62792 ± 0,00097

0,4635 + 0,0028

0,6617 ± 0,0123

1,5990 ± 0,0023

0,5519 + 0,0004 0,6882 ± 0,0006

0,6151

0,6383 ± 0,0005

208pb/206pb

0,62844 + 0,00050

0,64604 ± 0,00098

0,60485 ± 0,00022 0,60805 ± 0,00020 0,61452 ± 0,00023

0,61002 __+ 0,00020

207pb/206pb

± 0,0005

4,5436 ± 0,0021

4,5353 ± 0,0006

4,5536 ± 0,0007

4,5142 ± 0,0006

4,5330 ± 0,0008

4,5268 _+ 0,0009 4,5395 ± 0,0010

4,5382 + 0,0007

4,5107 + 0,0005

4,5609 ± 0,0007

4,5627 ± 0,0006

4,5627 ± 0,0007

4,5556 ± 0,0034

4,5514 ± 0,0006

4,5239 ± 0,0005 4,5502 ± 0,0007

4,5211

4,5044 ± 0,0005

4,5044 __+ 0,0005

± 0,0015 4,5452 ± 0,0029

4,5358 + 0,0014

4,5571

4,5145 ± 0,0013

4,5280 + 0,0017 4,5426 ± 0,0024

4,5384 ± 0,0008

4,5113 ± 0,0005

4,5630 4 0,0006

4,5534 _+ 0,0006 4,5657 ± 0,0068

4,5563 ± 0.0008

4,5214 ± 0,0005

4,5056 ± 0,0005

(A~)

(~E)

## Pb/Pb age

# Pb/Pb age

~- All values are corrected for blank chemistry, mass fractionation and spike contribution; ~ ISOtope ratio is corrected for mass discrimination and spike contribution; P b / P b age is calculated using the decay constants AU238 = 1.55125 × 10- my i and AU23~ = 9.8485 × 10 -1° y - 1 and applying the primordial Pb isotopic composition from Tatsumoto et al. [12]; ## P b / P b age is calculated using the isotopic composition of troilites (2°6pb/2°4pb = 18.40, 2°Tpb/2°4pb = 15.54) from ordinary chondrites determined by Unruh [7] for correction of the 2°4pb; he atomic T h / U ratio is calculated assuming an evolution in a closed system since 4.55AE with the decay constant ATh 232 = 0.49475 :x: 10- i t y - 1.

type

sample

Table 2 U-Pb systematics of phosphates from ordinary chondrites J

2,0

6,8 10,0

1,0

94

1,2 9,4

2,8 5,5

0,8

1.1

1,1

6,1

2,6 6,1 1,6

2,2

2,4

2,3

Th/U

I

,.....,

.q c~

pc

C. G6pel et al. /Earth and Planetary Science Letters 121 (1994) 153-171 2 ° 4 p b = 20 a n d 0.6 × 10 6 y w h e n

2°6pb/Z°4pb =

3. Results

100, decreasing to 0.43 × 106 y for pure radiogenic Pb. Combining these sources of error, the 2°7pb/ 2°6pb model ages can be obtained with an overall analytical precision of 1 to 4 × 10 6 y for the fragments and 0.5 to 1 × 106 y for most phosphate separates. Only the Pb-Pb model ages for the phosphates from the H5 Nadiabondi and LL5 Tuxtuac meteorites have an uncertainty larger than 1 m.y. For Nadiabondi the major contribution to the 3.4 m.y. uncertainty is the analytical blank correction: only 0.15 × 10 -9 g of Pb was available for the analysis because of the small sample size (1 mg) and the low U-Pb concentration (38 and 129 ppb respectively). In the case of the Tuxtuac phosphate, the uncertainty of 2.1 m.y. is chiefly caused by a low beam intensity during the mass spectrometric measurement. I

I

159

3.1. U-Pb systematics of chondrite fragments U-Pb results for whole-rock data are summarised in Table 1 and graphically presented in Fig. 1. U concentrations of the whole-rock samples vary from 5 to 11 ppb. Although Th concentrations were not measured, T h / U ratios can be recalculated assuming closed system behaviour since 4.55 Ga and an initial primordial Pb isotopic composition. The T h / U ratios thus calculated range from 2.8 to 4.4. Typical chondritic U concentrations are ~ 10 ppb U and T h / U ratios cluster around 3.8 [24-26]. The spread in Th and U concentrations in these samples is probably due to heterogeneous sampling because of the small sample size (20-50 mg). Total Pb concentrations show a large range, from 0.02 p p m (L5 I

H4 Ste.Marguerite - - ~ \

~,.51

.51

\ H5 Nadiabondi~

LL6 St.Severin

~

0.62 H5 Allegan

I~

H5 Richardton

el

/

~.

b. 6

LL6 Tuxtuac - - ~ " ~

/'

m-~-_l --e

L5 Knyahinya ~ . ~

A.~

¢,O

~4

L5-6 Barwell

O4

F / /

"" 0.61 LL6 Guidder - ~ J

a.

H6 Kernouv~

L5 Ausson

0

__//

L5 Homestead L6 Marion (Iowa) 0.60

H6 Guarefia

0

Concordia ]

0.8

H

I 0.9

-~1 tl 1.0

4.50

~ I

I

/,

0.8

0.9

1.0

Concordia

238U / 206 P b * Fig. 2. U-Pb Concordia diagram for phosphates from ordinary chondrites. U-Pb systematics of phosphates from (a) H chondrites and (b) L and LL chondrites. Symbols as in Fig. 1. As in Fig. 1, 2°6pb* and 2°7pb* are the calculated radiogenic components assuming that a primordial Pb isotopic composition is associated to the measured 2°4pb. More radiogenic phosphates plot on the Concordia curve. Less radiogenic phosphates are exceptions: H5 Nadiabondi and H5 Allegan (a), and LL6 St. Severin and L5 Knyahinya (b). These samples show a small apparent excess of radiogenic Pb. With a more radiogenic isotopic composition [7], they show a small apparent deficit of radiogenic Pb relative to the U in-situ decay (cirf). The slightly discordant character of these four phosphates depends critically on the Pb isotopic composition chosen for correction of the measured 2°4pb.

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Homestead) up to 0.35 ppm (H5 Allegan). The concentration of non-radiogenic 2°4pb ranges from 0.1 to 7.2 ppb and is not correlated with the metamorphic grade. The Pb isotopic compositions vary from 2°6pb/ 2°4pb = 16 (LL5 Tuxtuac) to 2°6pb/z°apb = 126 (L6 Homestead). The radiogenic character of the Pb is clearly linked to the content of non-radiogenic 2°4pb of the sample. In the P b / P b isotope diagram the fragments define a linear array which passes close to the primordial Pb isotopic composition. The best-fit line defines an age of 4.5456 _+ 0.0005 Ga although the data scatter significantly about this line (xrZed.= 16). This scatter is also apparent in the Pb-Pb model ages for the individual fragments, which range from 4.41 to 4.61 Ga. No correlation seems to exist between the metamorphic grade and the P b / P b model age if we consider all meteorite types. However, if we consider only the H chondrites, the P b / P b model ages range from 4.52 Ga (H6 Guarefia) to 4.61 Ga (H4 Forest Vale) and correlate nicely with the metamorphic grade: the most equilibrated samples exhibit the youngest ages. All fragments display an excess of radiogenic Pb relative to that produced by the in-situ decay of U, assuming primordial Pb isotopic composition for the measured 2°4pb. This excess of 2°6pb* and 2°7pb* ranges from 0% (Ste. Marguerite) up to 754% for the most discordant sample (Allegan). This U-Pb feature indicates a recent U-Pb perturbation (ca. 0-0.5 Ga) which is similar to that observed in all bulk samples of equilibrated chondrites [6-10].

3.2. U-Pb systematics of the phosphates U-Pb concentrations and Pb isotopic compositions measured in the phosphate separates are summarised in Table 2 and illustrated in Fig. 2. U concentrations vary by two orders of magnitude, from 38 ppb (H5 Nadiabondi) to 3.4 ppm (H6 Guarefia). With the exception of Nadiabondi this represents an enrichment of 20-150, relative to the bulk rock. It has been noted by several authors that the U concentration of phosphates is related to the metamorphic g r a d e - - t h e more equilibrated the chondrite, the higher the U con-

tent of the phosphate [27-30]. While the U concentrations in apatites from H6 chondrites typically cluster around 3200 ppb, those from H 4 - 5 chondrites lie around 2000 ppb and phosphates from H3 chondrites show U concentrations of < 200 ppb. The phosphate separates we analysed represent mixtures with variable proportions of apatite and merrilite and the separation technique does not discriminate between these two phosphate minerals. The U / P b systematics of the phosphate separates of type 5 - 6 chondrites is dominated by the apatite: it is more abundant and its U concentration is an order of magnitude higher than merrilite. The proportion of U contained in phosphates, relative to the bulk rock, can be calculated using the modal abundances of phosphates determined by Hagee et al. [31]. The percentage of U from the total rock which is contained in phosphates is quite variable: 7% in L 5 - 6 Barwell, 100% in H6 Guarefia, 14% in L6 Marion (Iowa), 16% in LL6 Saint-S6verin (dark) and 24% in LL6 SaintS6verin (total). Two detailed fission-track mapping studies from Saint-S6verin and Nadiabondi provide similar results [27,32]. In the case of H6 Kernouv6 and H6 Guarefia, 88 and 83% of the U measured in the phosphates resides in apatite, for the L 5 - 6 Barwell and LL6 Saint-S6verin, this value is 58% and 50% respectively. For H5 Nadiabondi it is variable, ranging from 30 to 80%, depending on the U concentration of the merrilite (49 or 87 ppb). The aliquot used for determining the U concentration was lost by accident during the analysis of the Forest Vale phosphate and in the following we assume that the U / P b system is concordant for that sample. The 2°4pb concentrations range from 0.1 ppb up to 7 ppb and in most cases they are higher (up to a factor of 5) than the 2°4pb concentrations in the whole-rock fragments. The 238U/2°4pb ratios lie between 91 (H5 Nadiabondi) and 3400 (H6 Guarefia, L 5 - 6 Barwell). These values are 7-300 × higher than those measured in the fragments, which clearly reflects the U enrichment in the phosphates. The high values of 23SU/x°4pb result in very radiogenic Pb isotopic compositions: 2°6pb/2°4pb

C. G6pel et al. /Earth and Planetary Science Letters 121 (1994) 153-171

ratios range from 108 to 3470. With the exception of H5 Allegan and H5 Nadiabondi, the other thirteen chondrites exhibit 2°6pb/2°4pb ratios higher than 250. Phosphates from five chondrites, H4 Ste. Marguerite, H6 Kernouv6, L 5 - 6 Barwell, L5 Homestead and LL5 Guidder show 2°6pb/ 2°4pb ratios higher than 1000. The reliability of the 2°7pb/Z°6Pb model ages with a 1 × 10 6 y precision can be tested by duplicate analyses, which were performed on H6 Guarefia, H4 Ste. Marguerite and H6 Kernouv6. Each duplicate involved a distinct mineral separation and chemical processing procedure. In the cases of H6 Guarefia and H4 Ste. Marguerite, the two analyses corresponded to the same grain-size fractions. Even so, the isotopic compositions were distinctly different: 2°6Pb/2°4pb = 410 and 745 for H6 Guarefia and 2°6pb/Z°4pb = 924 and 2960 for H4 Ste. Marguerite. However, because of their highly radiogenic character the Pb-Pb model ages are identical to within less than 0.1 m.y. For Ste. Marguerite the difference in the isotopic compositions probably results from variations in the U concentration in the phosphate separates and for Guarefia it is due to differences in the non-radiogenic 2°4pb concentrations. The duplicate samples exhibit similar T h / U ratios, as calculated from their Pb isotopic compositions (1.09 and 1.15 for Ste. Marguerite, 2.35 and 2.37 for Guarefia). This suggests that the phosphate separates are mixtures with similar proportions of apatite and merrilite, which are characterised by distinct T h / U ratios [28]. Two phosphate size fractions, 7 4 - 1 4 9 / x m and 37-74 /xm, were prepared from H6 Kernouv6. The two phosphate separates show distinct calculated T h / U ratios (2.63 and 2.19 respectively), suggesting that they contain variable proportions of apatite and merrilite. The Pb-Pb model ages (4.524 and 4.521 Ga) differ by 3 × 106 y. We consider this age difference as significant and related to the different grain sizes of the phosphates. Grain size may influence incorporation of U and Pb and the closure temperature of the U-Pb system (see Section 4.1). The study of phosphate separates from the Saint-Sdverin (LL6) reported by Manh~s et al. [19] indicated a concordant U-Pb system within

161

the available analytical precision, with a measured 2°6pb/Z°4pb ratio of 157.4 and a Pb-Pb model age of 4.55 _+ 0.01 Ga. Chen and Wasserburg [18] later published two new analyses with slightly more radiogenic Pb, 2 ° 6 p b / Z ° 4 p b = 159 and 191, and more precise P b / P b model ages of 4.552 _+ 0.004 Ga and 4.551 _+ 0.004 Ga. The Pb isotopic composition of the Saint-S6verin phosphate separate analysed in this study is even more radiogenic (2°6pb/Z°4pb = 270.3) and the resulting P b / P b model age is 4.5536_+ 0.0007 Ga. The Pb-Pb model ages from all three investigations are compatible within their analytical precision. However in the Pb-Pb diagram these isotopic compositions define a good linear array which corresponds to an age of 4.5575 _+ 0.0026 Ga with X~ed.= 0.43. This age is determined without making any assumptions about the isotopic composition of the Pb associated with the non-radiogenic 2°4pb in these separates. The Pb-Pb model ages of all phosphates are summarised in Table 2. These values, ranging from 4.504 Ga for H6 Guarefia up to 4.563 Ga for H4 Ste. Marguerite, were obtained by assuming a primordial isotopic composition for the measured 2°4pb. Considering H, L, and LL chondrites together, no correlation is found between metamorphic grade and age. In contrast, when the phosphates from the H chondrites are considered alone, the Pb-Pb model ages show distinct correlations both with metamorphic grade and U concentration (Fig. 3). The database for L and LL chondrites is quite small: no L4 or LL4 chondrites were analysed. No correlation can be discerned between the P b / P b age and the metamorphic grade for L or LL chondrites. The range of ages covered by both classes is however included in the age distribution defined by the H chondrites. It is also compatible with the correlation of the P b / P b age versus U concentration of the phosphates observed for the H chondrites (Fig. 3). The distribution of the available U-Pb data is shown in a Concordia diagram (Fig. 2), assuming a primordial Pb isotope composition is associated with the measured 2°4pb. Most phosphates seem to be concordant within their analytical uncertainty. Note that the apparently concordant char-

162

C. G6pel et al. /Earth and Planetary Science Letters 121 (1994) 153-171

the Pb-Pb age variation is 6 and 8 x 10 6 y for the less radiogenic samples (H5 Allegan, H5 Nadiabondi; Table 2). The correlation between the Pb-Pb ages and the metamorphism grades of the H chondrites is preserved when the Pb component associated with the measured 2°4pb is restricted to the isotope fields previously defined, regardless of its origin.

4.56

~,~,4.54

.~4.52 a.

4.50

4. Discussion 1000

2000

3000

U concentration of phosphates (ppb)

Fig. 3. U concentration measured in phosphates showing the negative correlation between the U concentrations of phosphates and their Pb/Pb model ages. H5 Richardton = H5 Ri; L6 Marion (Iowa) = L6 Ma; L5 Ausson = L5 Au; LL6 SaintS~verin = LL6 StS. Other abbreviations as in Fig. 1.

acter and the Pb-Pb model ages for the highly radiogenic phosphates ( 2 ° 6 p b / 2 ° 4 p b > 4 0 0 ) are nearly independent of the isotopic composition of non-radiogenic Pb associated with the observed 2°4pb. If this behaviour reflects the isotopic closure of the U / P b system in the phosphates, the Pb-Pb model ages can be considered to be absolute ages with an accuracy better than 3 m.y. For the less radiogenic phosphates (2°6pb/2°4pb < 400), the U-Pb concordant character and the Pb-Pb age are more dependent on the Pb isotope composition associated with the 2°4pb. While this composition cannot be accurately defined, it can be deduced from the U-Pb systematics of bulk ordinary chondrites in that it lies along a trend defined by primordial Pb and the Pb observed in troilites from ordinary chondrites [7]. The troilite is depleted in U but it shows Pb with a radiogenic composition (2°6pb/Z°4pb = 18), one which clearly has not been produced by in-situ decay of U. This radiogenic Pb component was interpreted as resulting from terrestrial Pb contamination prior to analysis [7]. When the troilite Pb composition is used instead of primordial Pb, the apparent Pb-Pb ages differ by only 0.3 x 10 6 y for the most radiogenic samples (H6 Kernouv6, L 5 - 6 Barwell) whereas

4.1. Interpretation o f the U-Pb systematics of chondritic phosphates A straightforward interpretation of the P b / P b model ages of the phosphates is based on the following considerations: (i) Phosphates are the product of thermal metamorphism; they formed during the oxidation and migration of phosphorus, which was originally contained in metal grains. (ii) The apparently concordant character of the U / P b system in the phosphates suggests that the P b / P b age accurately indicates the time of the U / P b closure in this phase. The U / P b concordance is good for phosphates exhibiting very radiogenic isotopic compositions and independent of any assumptions about the isotopic character of non-radiogenic Pb. The slightly discordant behaviour for the phosphates with less radiogenic isotopic compositions is probably not the result of U decay in an open system, but rather it is due to the non-radiogenic Pb associated with the measured 2°4pb. This Pb component may be characterised by a more radiogenic composition than the primordial Pb. Its nature will be discussed in another paper. This interpretation of the U-Pb systematics in the phosphates provides two important constraints: (i) The oldest age displayed by phosphates from H chondrites, 4.563 Ga, provides a lower limit for the accretion time of these meteorites. This value is close to the best P b / P b age determination for the refractory inclusions of the Allende meteorite (t = 4.566 _+

c. G6pel et al. / Earth and Planetary Science Letters 121 (1994) 153-171

0.002 Ga [18,33]), so a time interval of AT = 3 + 2.6 × 106 y is suggested. (ii) The time interval of 6 x 107 y defined by the range of P b / P b ages of phosphates (4.5634.502 Ga) constrains the duration of the early thermal evolution of the equilibrated chondrites. However, such a straightforward interpretation of these precise P b / P b ages of the phosphates is not unique. A more detailed understanding of the U-Pb system in the chondritic material is required before this P b / P b chronology can be validated as a real time constraint for the early period (4.4-4.56 Ga) of the proto-planetary history. Two points need to be considered: The Concordia curve is nearly linear for times older than 4 Ga. Therefore, it is ambiguous if the U-Pb system is strictly concordant or if it was disturbed during the first 0.2 Ga of the solar system. The U-Pb ratio needs to be defined with a relative precision of better than 10 - 4 in order to make this distinction. Such a precision is two orders of magnitude greater than that realised during this study and cannot be attained even with the most precise procedures available today [34]. As a consequence, the P b / P b ages of the phosphates may translate either (i) the thermal closure during an early stow cooling process or complete U / P b resetting in early metamorphic events, or (ii) a partial resetting during an early metamorphic event. In the first case the P b / P b age must be considered as close to an absolute age, but whether the process was continuous or episodic in character cannot be determined. In the second case, the P b / P b age indicates the early nature of the resetting event, but it does not accurately define the time it occurred. The structure of the U-Pb systematics observed does not allow us to distinguish between these two alternatives. The meaning of the U-Pb record in the phosphates differs depending on the metamorphic grade and the peak temperature of the parent rock. First, merrilite and apatite are enriched in U relative to the bulk material and they can act as radiogenic Pb donors to adjacent minerals. Both types of phosphate can also incorporate Pb. As in the case of Sr and Mn (ionic radius, r =

163

1.12A), Pb (r = 1.20 ,~,) may substitute for Ca (r = 1.18A) [35], which is a major structural element in phosphate. Secondly, the closure temperature of the U / P b system in meteoritic Cl-apatite can be estimated from experimental studies of Pb diffusion in terrestrial F-apatite [36,37]. This temperature is calculated according to Dodson's formalism [38], using the diffusion parameters defined by Cherniak et al. [37] (activation energy E = 54.6 _+ 1.7 k c a l / m o l - 1 and a pre-exponential factor D o = 1.27 x 10 4 cm 2 s-~). For phosphates with radii between 25 ~ m and 65/xm, the corresponding closure temperature is 687 + 15 K for a cooling rate of 1°C/106 y, 727_+ 16K for 10°C/106 y, and 771 _+ 19 K for 100°C/106 y. Type 5-6 chondrites were affected by temperatures of the order of 950-1200 K [39], higher than the estimated closure temperature (690-750 K) for the U-Pb system in the phosphates. The phosphates of these equilibrated chondrites have obtained a significant part (type 5) or most (type 6, i.e., Guarefia 100%) of the total U contained in the bulk rock. Their P b / P b ages may reflect the U / P b 'closure of this mineral phase during the cooling of the parent body, corresponding to the termination of Pb diffusion from this phase. Type 4 chondrites were subjected to intermediate temperatures of between 650 and 950 K. This may be lower than the U / P b closure temperature of the phosphates, which means that U / P b closure in the phosphates may not be related to the cooling period for these objects. The P b / P b age of the phosphates is affected by the diffusion of both U and Pb in the bulk material into the new forming phosphate mineral. The transfer of U from the bulk rock towards the minerals is incomplete, as is evident from the lower fraction of the total U carried by phosphate in the type 4 chondrites. Depending on the relative mobility of U and Pb, the P b / P b age establishes an upper or a lower limit for the time of the last U / P b closure of the bulk material. The smaller the difference between the mobility of U and Pb, the more closely the P b / P b age of the phosphates will record the U / P b closure of the bulk material. The whole-rock U / P b closure is related to the condensation a n d / o r accretion processes of chondrites.

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The correlation between the P b / P b age and the U concentration of the phosphates (Fig. 3) could be interpreted as indicating the time scale for the migration of U out of the chondrule glass into the phosphates. This conclusion would be valid if the type 5 - 6 chondrites had been affected by temperatures lower than those of the Pb closure in the phosphates. However, this is not the case. The early heating of the unequilibrated material governs the migration of U into the growing phosphates [27]. It also affects Pb diffusion into or out of the phosphates, but there is no unique time relationship between the increase in U concentration and Pb closure in the phosphates since so many complex and ill-constrained factors are involved. In conclusion, the 6 × 10 7 y Pb-Pb time interval observed for chondritic phosphates reflects the early thermal processing of equilibrated chondrites. However, the U-Pb systematics of these phases do not allow us to delineate thermal events of an episodic character (metamorphic events) from those of a more continuous nature (internal heating followed by slow cooling). The correlation between the P b / P b ages of the phosphates and the metamorphic grade of the seven H chondrites is the first clear relationship observed between long-lived radionuclear chronology and the intensity of metamorphism. 4.2. Accretion and thermal h&tory o f the ordinary chondrite parent bodies: The debate

The formation, the structure and the thermal history of chondritic parent bodies has long been the subject of an ongoing debate. There is no agreement about even the existence of correlations between long-lived and short-lived time scales, metallographic cooling rates and petrological observations. Three contradictory models have been developed for the accretion and thermal evolution of these objects. One model for chondritic parent bodies proposes a layered structure which was preserved during the early thermal evolution. In this model the most metamorphosed material (type 6) is located in the interior of the parent body and is successively surrounded by less and less meta-

morphosed material. The 108 y time span derived from the R b / S r , K / A t and Pu chronometers and the metallographic cooling rates in equilibrated chondrites is interpreted as a record of monotonic cooling from the peak temperature of 1150 K [39-41]. This time scale constrains the size of the parent bodies to objects with radii between 100 and 200 km. The onion skin model predicts inverse relationships between both the metailographic cooling rates and the radiometric ages versus metamorphic grade. The thermal evolution of such objects seems to be compatible with low-temperature accretion and an internal heat source such as 26A1. Such models also account for the relative proportions of the different metamorphic grades of H and L chondrites observed in terrestrial collections [42]. The other two types of models point out the difficulty in interpreting the 108 y time span as the duration of monotonic cooling of the parent bodies in the light of a variety of petrographic observations which have been made on equilibrated chondrites. Many petrographic features appear to require rapid cooling from a high temperature. One example is the hot accretion model discussed by Hutchison et al. [4] and Christophe Michel Levy [43]. Accretion at different temperatures followed by rapid cooling induces different degrees of autometamorphism in this model. However, when the apparent lack of correlation between metallographic cooling rates and the metamorphic grades of chondrites is considered, a third type of model appears to be required. This model postulates that the metamorphism recorded by the radiometric ages and the final stage of the cooling (T < 500°C) registered by the metallographic cooling rates occurred in distinct planetary environments [2]. The initial metamorphism could have been driven by 26A1 decay [4446], external heating of small objects during a very luminous early phase of the sun [47] or by magnetic induction [48]. The final episode of cooling of these materials depends on their depth in the parent body and therefore no correlation with the metamorphic grade is expected. The most recent publications concerning the observation of a relationship between cooling

c. G6pel et al. /Earth and Planetary Science Letters 121 (1994) 153-171

rates, age determinations and mineralogy are still contradictory: (i) Taylor et al. [2] emphasise the absence of any correlation between metallic cooling rates and metamorphic grade on the basis of 27 H chondrites, 10 L and 4 LL chondrites and consequently reject any model with a preserved onion skin structure. Other authors favour the onion skin [49,3,50] except in the case of the L chondrites whose parent body may have undergone a complex sequence of events which compromised the chronological information. They consider that the metallographic and fission-track cooling rates, as well as the 39Ar/4°gr systematics measured on unshocked meteorites, do support the onion skin model for the H and LL parent bodies. 4.3. H o w compatible are U / Pb systematics with a layered H chondrite parent body?

If the P b / P b age accurately registers thermal closure of the U-Pb system in phosphates during the cooling phase, the observed correlation between P b / P b systematics and the metamorphic grade of the host H chondrites appear to be compatible with a layered H parent body and with the in-situ decay of the short-lived nuclides 26A1 and eventually 6°Fe [51] as internal heat sources. We consider a classical model for the thermal evolution of such a parent body in order to evaluate if the 26A1 systematics are compatible with the P b / P b time scale. In this model the accretion of the primitive material is assumed to have occurred at low temperature and in a time interval that is short relative to the mean half-life of 26A1 [52,53,41,42]. Because of the low thermal diffusivity of chondritic materials and the short half-life of 26A1, the peak temperature of most equilibrated chondrites constrains the amount of 26A1 present at the time of accretion. The inferred value of 1150 K [3] agrees with the equilibration temperatures derived by the Ca pyroxene thermometer of ~< 1190 K [54] and by the oxygen isotopic data (1200 + 100 K [55]) and suggests that 27AI/26AI = 5 × 10 -6 at the time of accretion of the parent body. If the value 27Al/26A1 ~ 5 × 10 - 5 determined in some CAIs of carbonaceous chondrites [56,1] is repre-

165

sentative for all primitive solar matter at the time of CAI formation, the difference between the estimated 27A1/Z6A1 ratio of ordinary chondrite parent bodies and the observed CAI value of 5 × 10 5 gives the time delay between the formation of CAIs and the accretion of the chondritic parent bodies. The calculated time difference is 2.3 _ 0.2 x 106 y. The time interval between the P b / P b age of the Allende inclusions and the P b / P b age of the accretion of the parent body (AT = 3.0 _+ 2.6 X 10 6 y) is therefore compatible with the :*A1 time constraint. If the P b / P b age of phosphates in H4 chondrites dates the early heating of the H parent body, evidence for the early occurrence of 26A1 in ordinary chondritic material should be sought in type 4 chondrites, as proposed by Pellas and St6rzer [41] and Minster and Allbgre [40]. The subsequent investigation in H chondrites, specifically those meteorites showing the oldest P b / P b ages, H4 Ste. Marguerite and H4 Forest Vale, has indeed yielded evidence of 26A1 [57], but more work is needed to understand the A I / M g systematics in this material. A first attempt by Shukolyukov and Lugmair to detect the presence of 6°Fe in H4 Ste. Marguerite was not conclusive

[581. If the complete accretion of the H chondrite parent body occurred in a short time interval, the P b / P b ages of the unequilibrated material should be similar to or slightly older than those of the phosphates from H4 chondrites. Internal U / P b systematics were determined in two unequilibrated ordinary chondrites (L3 Mez6-Madaras and H3 Sharps) [59]. The Pb isotopic compositions range from 2°6pb/2°4pb = 9.1 to 16 and they define a linear array in the 2°7'2°6'2°4pb diagram. The corresponding P b / P b ages, 4.48 _+ 0.011 Ga for Mez6-Madaras and 4.472 _+ 0.005 Ga for H3 Sharps, are significantly younger than the P b / P b age of phosphates in H6 chondrites. If this low age is a common feature of unequilibrated ordinary chondrites, it does not easily fit into the model of a layered H chondrite parent body. And if the accretion of the superficial material occurred at the same time as that sited at greater depth, an ad-hoc continuous re-equilibration of the U-Pb system at low temperature ( < 300 K)

c. Ggpelet al. /Earth and Planetary Science Letters 121 (1994) 153-171

166

m u s t b e i n v o k e d . O n t h e o t h e r h a n d , if t h e a c c r e t i o n o f t y p e 3 m a t e r i a l o c c u r r e d later, 26A1 in-situ decay cannot be the source of the U-Pb re-equilibration, and another source must be considered. If the Pb/Pb age of the phosphates records the time of the U-Pb closure during the cooling of a layered body, the Pb/Pb c h r o n o l o g y indicates that the type 6 material must have spent a b o u t 6 X 107 y at a t e m p e r a t u r e a b o v e 700 K a n d a b o u t 3 x 107 y at t e m p e r a t u r e s h i g h e r t h a n 1100 K. S u c h a l o n g d u r a t i o n a n d slow c o o l i n g from high temperatures are incompatible with the petrographic observations made on equilibrated H chondrites. The occurrence of Fe-Mg silicates o f a s m a l l g r a i n size [60], t h e p r e s e r v a t i o n o f c l i n o b r o n z i t e [43] a n d s t r i a t e d p y r o x e n e

A T Pb (myr) 50

25

[61], a n d t h e survival o f p o l y c r y s t a l l i n e t a e n i t e [4] r e q u i r e a v e r y s h o r t t i m e (days o r w e e k s ) at t e m p e r a t u r e s a b o v e 1000 K a n d fast c o o l i n g .

4.4. Comparison o f the U-Pb systematics in chondritic phosphates with other chronometers A comparison between the U/Pb systematics o f p h o s p h a t e s a n d o t h e r c h r o n o m e t e r s is m a d e in o r d e r to e v a l u a t e t h e c o n s i s t e n c y w i t h p u b l i s h e d c h r o n o m e t r i c d a t a , s p e c i f i c a l l y w i t h r e s p e c t to t h e o n i o n skin m o d e l for H c h o n d r i t e s . A m o r e q u a n t i t a t i v e t r e a t m e n t will b e p r e s e n t e d e l s e w h e r e . T h e c o m p a r i s o n is b a s e d o n u n s h o c k e d c h o n drites and assumes that the age derived from each chronometer defines the time passed after

A T Pb (myr)

Allende H5 Ha

50

0

A T Pb (myr)

Allende

25

0

5O

Allende

25

0

4.50

g 4.40 I-

<3 4.30

4.50

4.52

4.54

Pb / Pb age (IE)

4.56

4.50

4.52

4.54

Pb / Pb age (/:E)

4.56

4.50

4.52

4.54

4.56

Pb / Pb age (/E)

Fig. 4. (a) 39Ar/4°Ar measurements on small whole-rock fragments and on mineral separates [63,66,75,76] are shown as * for H chondrites, as o for L chondrites and as [] for LL chondrites. Measurements of feldspar and pyroxene separates are indicated as • for H chondrites and o for the LL class. The P b / P b ages are indicated as absolute ages (lower abscissa) or as time differences (AT) (upper abscissa) relative to 4.566 AE, which corresponds to the P b / P b age measured for Allende refractory inclusions [33]. Concordant A r / A r and P b / P b ages, using the accepted decay constants, should plot on the solid line. This figure shows the different resolution (+3 x 107 y and -+ 1 x 106 y) for the 39Ar/4°Ar and P b / P b chronometers. The A r / A r ages of the chondrites range between 4.52 and 4.43AE. This 9 -+ 4 x 107 y time interval can be compared to the 6 x 107 y interval established by the P b / P b systematics of phosphates. However, no significant correlation is seen between the two chronologies. (b) Comparison of the chronologies based on the Sr initial ratio and the Pb/Pb system of the phosphates. Time intervals based on the initial Sr ratio of phosphates [19,68-70] are calculated assuming a primitive value similar to that in CAIs from Allende [77] and a Sr isotopic re-equilibration at the whole-rock scale. Concordant chronologies and Pb/Pb ages should plot on the solid line. The time intervals for the three H chondrites based on the initial Sr method range between 6 and 9 x 107 y (shaded area). This time interval is similar to that defined by the P b / P b systematics of phosphates, but no correlation is seen between the two chronologies. (c) I / X e chronology and P b / P b chronology of phosphates are compared. The I / X e ages (see compilation in [78]) are plotted as time difference relative to L4 Bjurb61e. The I / X e ages of H chondrites define a time interval of 16 -+ 5 x 10 6 y (indicated as the shaded area) which is 3-5 x smaller than the time interval defined by the Pb/Pb systematics in phosphates. No correlation is observed between these chronologies.

C. G6pel et al. /Earth and Planetary Science Letters 121 (1994) 153-171

the material had reached the closure temperature, cooling down from a higher temperature. This closure temperature corresponds to the temperature of isotopic closure below which the parent and daughter elements are no longer exchanged [38]. It is assumed that the coherence of the radioactive decay constants recommended by Steiger and Jiiger [62] is sufficient to prevent the introduction of systematic age biases larger than 3 x 10 7 y for ages around 4.5 Ga. Turner et al. [63] showed that the A r / A r ages of most individual samples are indistinguishable from the mean age (4.48 _+ 0.03 Ga). Later compilations of H chondrite A r / A r data by Pellas and Fieni [49], including the data from Turner et al. [63], Bogard et al. [64] and Kaneoka [65], showed that of twelve chondrites eleven overlap at 4.48 +_ 0.04 Ga. Considering that the accretion of the H chondrites occurred around 4.56 Ga, the clustering of the A r / A r ages defines the time interval of 8 _+ 4 × 10 7 y for their thermal evolution. This value is similar to that derived from the P b / P b systematics in phosphates. A comparison between the P b / P b age of the phosphates and the A r / A r ages measured on small bulk fragments [63] and separated minerals [66] has been made for five H chondrites (H4 Forest Vale, H5 Richardton, H5 Nadiabondi, H6 Kernouv6, H6 Guarefia; Fig. 4a). The 30 × 106 y precision of the A r / A r ages does not allow us to evaluate how well they correlate with the P b / P b ages of the phosphates, but such a correlation cannot be excluded. The ages defined by R b / S r whole-rock isochrons for H and LL chondrites are interpreted as the formation ages of the chondrites, assuming that the Rb fractionation is linked to condensation-accretion or to an early event preceding metamorphism. The absence of a wholerock isochron for the L chondrites is interpreted as the result of Rb loss by volatilisation from the meteorites during the shock events which have affected most of them. Using the accepted decay constant, ASVRb = 1.42 × 10 -11 y - i [62], based on the H, EL and E classifications the 87Rb/sVSr age for chondrites is 4.498 _+ 0.015 Ga [67]. The P b / P b ages of most chondritic phosphates are significantly higher than the R b / S r age of the whole-rock chondrites. This age differ-

167

ence confirms the apparent discrepancy between the R b / S r and P b / P b chronometers in chondrites [67]. If the R b / S r age of the whole-rock dates the formation of the parent body and if the P b / P b age of phosphates registers the closure of the U-Pb system during the cooling in the parent body, the R b / S r age should be similar to or older than the P b / P b age of the phosphate from H4 Ste. Marguerite. The S7Rb decay constant would have to be decreased by 1.4% +_ 0.3% with respect to the presently accepted value in order to bring these two chronometers into concordance. This somewhat model-dependant correction is similar to the 1% correction proposed by Minster et al. [67]. The internal R b / S r system of most chondrites has been disturbed by shock events. In the case of the few equilibrated chondrites where an internal isochron is obtained, the age is younger and the initial 87Sr/~6Sr ratio is higher than that of the whole-rock isochron. These features are compatible with re-equilibration in a closed system; however the uncertainty associated with the R b / S r age, which is in most cases greater than 30 m.y., does not allow us to evaluate whether a correlation exists with the metamorphic grade, nor even with the P b / P b age of phosphates from the same object. An approach using the evolution of initial strontium (SVSr/86Sr)o [68] has been applied to the R b / S r system of the phosphates from ordinary chondrites in order to compensate for the lack of internal R b / S r isochrons and insufficient precision associated with the internal R b / S r systematics [69,70]. Assuming an internal re-equilibration on the whole-rock scale, the interval of metamorphism for equilibrated chondrites ranges from a few to 15 x 10 7 y and no relationship between the radiogenic character of the initial Sr value and the metamorphic grade is discerned. Initial Sr and the P b / P b age of phosphates can be compared for six samples (H4 Forest Vale, H6 Kernouv~, H6 Guarefia, L5 Ausson, L5-6 Barwell, LL5 Tuxtuac; Fig. 4b). The error bars for LL6 Saint-S6verin were too large to include in the interpretation of the Sr data. For the three H chondrites, the time interval based on initial Sr ranges between 6 and 9 x 10 7 y, similar to the

168

C. GSpel et al. / Earth and Planetary' Science Letters 121 (1994) 153-171

P b / P b time interval of 6 × 107 y, but it is not correlated with metamorphic grade. I-Xe chronology is based on the presence early in the solar system's history of the short-lived 129I [71] and on the assumption that the iodine isotopic composition 1291/127l w a s homogeneous in the early solar nebula. A comparison between the I / X e chronology and the P b / P b chronology is interesting because the two chronometers are capable of similar time resolution and they both involve volatile elements. The comparison (Fig. 4c) includes ten samples (H5 Allegan, H5 Nadiabondi, H5 R i c h a r d t o n , L6 Guarefia, H6 Kernouvfi, L5 Ausson, L 5 - 6 Barwell, LL5 Guidder, LL5 Tuxtuac, LL6 Saint-Sdverin). The I / X e time interval, 2 × 10 7 y, is three times smaller than the time interval defined by P b / P b systematics. No correlation is apparent between the two chronologies when H, L and LL chondrites are considered. Within the H class, samples of metamorphic grade 5 or 6 do not show differences and the time span of 15 m.y. defined by the I-Xe chronology of three H 5 chondrites is identical to the range defined by all chondrites. The apparent

A T Pb (myr) 50

A T Pb (myr)

Allende

25

0

.,;

1000

lack of correlation between the two methods suggests that the chronometric interpretation of the experimental results is incorrect for one of the two chronometers, or that these chronometers date different events. The second case supports the idea that the I / X e clock registered early events and was not strongly affected by metamorphism [59,70]. Metallographic cooling rates, which are conventionally reported for a temperature of 773 K, are deduced from the Ni diffusion profiles in Ni-Fe grains of chondrites [72]. Comparison with P b / P b isotope systematics (Fig. 5a) includes seven H chondrites. A positive correlation is found, which is in agreement with that between metallographic cooling rates and metamorphic grade of apparently unshocked H chondrites [3]. The 244pu chronothermometry is based on the fission tracks recorded in Pu-rich phosphates and adjacent mineral phases which are depleted in actinides [41]. The comparison of fission-track cooling rates in merrilite with P b / P b systematics includes five H chondrites and is shown in Fig. 5b. A positive correlation is seen, which suggests

e" , u

100

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H5 Ri H5 AI o

0

~ii

r-

o u

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-

H4 SM

0

50

100

10

o o

N;

o e-

5

o . m

E

~Z~!~,~ ,,,~...... .....;~

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l~'~

r~ L L 6

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L5 Au

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I 4.54

.a_

Pb / Pb a o e (/E~

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St.S 1 4.50

4.52

4.54

4.56

Pb / Pb a qe (/E)

Fig. 5. Metallographic and 244pu fission-track-derived cooling rates plotted against P b / P b ages of phosphates. The X-axis is identical to that in Fig. 4. Data are taken from Taylor et al. [2], Lipschutz et al. [3] and Pellas and St6rzer [41]. The shaded areas emphasize the positive correlation between the P b / P b ages and the cooling rates determined by the two methods. Symbols as previously.

c. Giipel et al. / Earth and Planetary Science Letters 121 (1994) 153-171

agreement with that between fission track cooling rates and metamorphic grade of the H chondrites.

5. Conclusions The U / P b systematics of phosphates define a time interval of 6 × 10 7 y for the thermal processing of equilibrated chondrites. This time interval is well within the broad band of 10 8 y derived from the K / A t , R b / S r and Pu chronologies. The P b / P b ages of phosphates from equilibrated chondrites, interpreted as thermal closure times for the U / P b system, are correlated with the metamorphic grade of their host meteorites, supporting a model for a large parent body (100 km in radius) which has preserved its layered structure and which was internally heated up by the in-situ decay of 26A1. Correlation of the P b / P b systematics with fission tracks and metallographic cooling rates lends support to this model. In contrast, no clear correlation is observed with the A r / A r , R b / S r and I / X e chronologies. The above interpretation of the P b / P b systematics in equilibrated chondrites conflicts with many petrographic observations made in equilibrated H chondrites and it does not appear to agree with the few P b / P b ages from unequilibrated chondrites. A number of interpretations for the failure of concordancy with different radiochronometers and for the conflicting petrological features in ordinary chondrites have been presented: (i) Complex a n d / o r different individual histories for ordinary chondrites, (ii) an incorrect interpretation of the radiochronometric implications in the different material, and (iii) some combination of the above. The coherence of the U / P b systematics of phosphates confirms that the record of the thermal history of at least some chondrites has been preserved, but the open question remains: What is controlling their P b / P b age? The interpretation of this P b / P b chronology as the U / P b thermal closure of the phosphates during slow cooling is not unique. There is now a need to find a coherent relationship between the shorter lived nuclides and the apparently systematic P b / P b ages.

169

6. Acknowledgements We thank P. Pellas of the Musde Histoire Naturelle in Paris (Knyahinya, Homestead, Kernouv6, Guarefia, Ste. Marguerite, Allegan, Guidder, Nadiabondi, Ausson, Tuxtuac, Forest Vale, SaintS6verin), H. W~inke of the Max-Planck Institut in Mainz (Marion (Iowa) and Richardton) and R. Hutchison of the Natural History Museum in London (Barwell) for kindly supplying these samples. We further thank P. Pellas for his stimulating and enthusiastic discussion concerning the thermal history of chondrites and acknowledge that this study is based upon his earlier work. We are also indebted to Mme. Christophe and R. Hutchison for helpful advice concerning the mineralogy of ordinary chondrites. We thank M. Ghelis for introducing us to the technique of phosphate separation. We are also deeply indebted to C.M. Hohenberg for correcting and refining the English during the course of a long weekend. G. Tilton and two anonymous reviewers made valuable comments on the manuscript. This work was supported by A T P Plan#tologie 913725 and DO1961 and is IPG contribution 1278.

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