Rhenium-osmium isotope systematics in meteorites I: Magmatic iron meteorite groups IIAB and IIIAB

Earth and Planetary Science Letters, 108 (1992) 191-202

191

Elsevier Science Publishers B.V., Amsterdam [CL]

Rhenium-osmium isotope systematics in meteorites I" Magmatic iron meteorite groups IIAB and IIIAB J o h n W . M o r g a n , R i c h a r d J.

Walker * a n d J e f f e r y N. G r o s s m a n

u.s. Geological Surcey, Mail Stop 981, Reston VA 22092, USA

Received April 4, 1991; revision accepted December 3, 1991

ABSTRACT Using resonance ionization mass spectrometry (RIMS), Re and Os abundances were determined by isotope dilution (ID) and tSTOs/IS~Os ratios measured in nineteen iron meteorites: eight from group IIAB, ten from group IIIAB, and Treysa (IIIB anomalous). Abundances range from 1.4 to 4800 ppb Re, and from 13 to 65000 ppb Os, and generally agree well with previous ID and neutron activation (NAA) results. The Re and Os data suggest that abundance trends in these iron groups may be entirely explained by fractional crystallization. Addition of late-formed metal to produce Re-Os variation in the B subgroups is not essential but cannot be excluded. Whole-rock isochrons for the IIAB and II1AB groups are statistically indistinguishable, Pooled data yield an initial ~7Os/~'Os of 0.794 + 0.010, with a slope of (7.92 + 0.20) x 10 2 corresponding to a magmatic iron meteorite age of 4.65 + 0.11 Ga (using a decay constant of 1.64 x 10 Jl a t ). Given the errors in the slope and half life, this age does not differ significantly from the canonical chondrite age of 4.56 Ga, but could be as young as 4.46 Ga.

I. Introduction T h e R e - O s isotopic system is u n i q u e l y suited to the geochronology of iron meteorites. T h e highly siderophile character of these e l e m e n t s contrasts sharply with the largely lithophile nature of such o t h e r useful long-lived radioactive e l e m e n t s as K, Rb, Sm, Lu, T h and U. T h e dating of iron m e t e o r i t e s was the target of an early application of the R e - O s m e t h o d by H e r r et al. (1961) [1]. A b o u t 20 years later, Luck and coworkers [2,3] m a d e use of an ion probe to repeat the work at higher precision a n d also analyzed the metal phases of several o r d i n a r y chondrites. The range of 187Re/lS6Osin the iron meteorites p e r m i t t e d a w e l l - c o n s t r a i n e d iron m e t e o r i t e isochron. T h e c h o n d r i t e data, however, were too narrowly clustered to give separately a satisfactory isochron, a n d plotted consistently above, although within error of, the iron meteorite isochron. Luck et al. inferred that n o significant age difference existed b e t w e e n chondrites and

* Present address: Department of Geology, University of Maryland, College Park. MD 20742, USA Elsevier Science Publishers B.V.

irons. By a s s u m i n g a c o m m o n age of 4.55 Ga, they derived a lS7Re decay c o n s t a n t c o r r e s p o n d ing to a half-life of 45.6 + 1.2 Ga. L a b o r a t o r y m e a s u r e m e n t s by L i n d n e r et al. suggest a shorter JSTRe half-life: 43.5 + 1.3 G a [4] and, later, 42.3 _+ 1.3 G a [5]. If the more recent L i n d n e r half-life is applied to published iron meteorite data [3,6], the conclusion must be drawn that the iron m e t e o r i t e s as a group are several h u n d r e d million years y o u n g e r than the chondrites [5,6]. Cores of large asteroids may cool to the R e - O s closure t e m p e r a t u r e this slowly [7], but not all iron m e t e o r i t e groups have such long cooling rates nor did they all originate in asteroidal cores. (Most iron m e t e o r i t e s may be classified into o n e of t h i r t e e n groups. T a x o n o m y is based chiefly on structure a n d kamacite b a n d width, and on Ga, G e a n d Ni c o n t e n t [8,9]. T e n of these groups are t h o u g h t to be m a g m a t i c because trace e l e m e n t distributions seem to originate by fractional crystallization of metallic liquid [10]. T h e origin of groups IAB, I I E and I I C D is still an o p e n question.) R a d i o g e n i c e n r i c h m e n t of ~°TAg in some iron m e t e o r i t e s argues against yotmg ages, however. In I V A - I V B , IIIAB, IIB and some a n o m a l o u s

192

irons, mVAg enrichment correlates with Pd content, suggesting that formation and fractionation occurred soon enough after nucleosynthesis to include mvPd (half-life = 6.5 Ma) [11-14]. The root of the disagreement between Re-Os and Pd-Ag methods may be experimental rather than cosmological. The Luck et al. Re-Os isochron for iron meteorites contains a miscellany of groups for which there is little reason to infer genetic relationships [2,3]. Although the points form a good linear array, the slope is controlled by IIA irons at high ~7Os/~S~'Os ratios and by members of group I I I A at low values. Interpretation of an iron meteorite isochron would be less ambiguous if it were derived from a homogeneous set of genetically related meteorites. Recently, Re and Os abundances in a large number of iron meteorites of magmatic origin were reported [10]. The spread of the R e / O s ratio in each of the most populous groups (IIAB and IIIAB) appears large enough to permit a good isochron to be defined. Since the members of each group probably have a common origin, within-group isochrons may be reliably interpreted. For the present Re-Os isotopic study, a suite of eighteen IIAB and I I I A B iron meteorites was selected to cover as wide a range as possible of R e / O s ratios [1-3,10,15,16]. In addition, the anomalous I I I B iron Treysa was included in the suite because Re-Os isotope systematics in this meteorite had been studied previously [1]. The purpose of the experiments was to (a) examine Re and Os fractionation trends in these iron meteorite groups using precise isotope dilution data in order to guide future work, and (b) establish whole-rock isochrons for each group in order to better define the Re-Os closure ages.

2. Samples Sawn pieces of iron meteorites weighing between 0.7 and 4 g were provided by Dr. Roy Clarke of the Smithsonian Institution, Washington. The pieces were cut into smaller rectangular chunks using a Leco "Vari-cut" saw with a diamond wafering blade lubricated with cutting oil. The (new) blade was first cleaned by sawing pieces of Grant (low in Re and Os). To avoid cross contamination, samples were sawn in ascending

J.W. MORGAN ET AL.

order of reported Re abundance. The small chunks were washed several times with ethanol to remove cutting oil. The clean pieces were lightly etched in 2 M HC1 for 2 minutes using magnetic stirring, washed with water and ethanol, and dried at 105°C.

3. Analytical methods For isotopic analysis, we used a resonance ionization mass spectrometer developed at the National Institute of Standards and Technology, Gaitherburg, MD [17] which was based on an NBS 60 °, 0.152 m radius-of-curvature thermal ionization machine. Because of the selectivity of resonance ionization, both Re and Os were loaded on the same Ta filament. The two elements were evaporated from the heated (2000°C) filament to produce a gas-phase reservoir of largely atomic species. An ultraviolet laser b e a m was produced by a N d - Y A G pulsed laser pumping a tunable dye laser and by doubling the frequency of the output with an angle-tuned second harmonic-generating KDP crystal. Osmium and Re were selectively photoionized'at wavelengths of 297.17 nm and 297.69 nm respectively. Backgrounds were measured on-mass but 0.1 nm off-resonance. Ions were detected and the signal amplified by a 17stage ion multiplier operating at a gain of about 105. The output passed via a pre-amplifier to a transient digitizer which integrated the waveforms and quantitated the signal. The chemical separation procedure for iron meteorites involved an alkaline fusion, distillation of Os, and anion exchange separation of Re as described previously [18]. Acid dissolution of iron meteorites has been employed with apparent success in earlier Re-Os isotopic work where Cr 6+ was the oxidant [1-3]. In our procedure, substitution of H 2 0 2 for Cr 6+ yields clear solutions, allows cleaner chemistry and results in lower blanks. Hydrogen peroxide is a weaker oxidant than chromium trioxide, however, and we therefore employed alkaline fusion rather than acid dissolution to ensure isotopic equilibration before distillation. Rhenium metal and osmium sponges, enriched in lSSRe (99.55%) and 1 9 0 0 S (96.59%) respectively, were used to prepare two sets of spike solutions, which differed in concentration by ap-

193

R H E N I U M - O S M I U M I S O T O P E S Y S T E M A T I C S IN M E T E O R I T E S

proximately an order of magnitude. Meteorites believed to have lower Re and Os contents than Filomena (Table 1) were analyzed using the more dilute set of spike solutions. Duplicate Filomena samples were analyzed using both sets of spike solutions to verify consistency. After the analytical work for this paper was completed, papers were published describing the application of negative thermal ionization mass spectrometry (NTIMS) to the isotopic measurement of Re and Os [19,20]. The spike solutions used in the present work therefore were recalibrated using this much more precise technique. In addition, rigorous spike-standard equilibration was achieved by fusion techniques similar to those described below for analysis of samples. This treatment greatly decreases the likelihood of multiple valence states and species of Re and particularly of Os. The Os spike recalibration agreed well with our previous values, but our Re stock spike solution was found to be 6% less concentrated than the value used in our previous meteorite studies [6,21,22]. The results presented in the present paper are based on the new calibrations. Aliquots of Re and Os spike solutions were each weighed into a Zr crucible and dried at ~ 50°C. Before fusion, the dried spikes in the crucible were taken up in H 2 0 , two pellets of N a O H added, and the solutions dried again. The spikes were fused with an additional 1 g of N a O H at 325°C for 1 hour. The weighed sample was added to the crucible and fused at 350°C for 30 minutes. To the cooled mixture, 5 g of N a 2 0 2 was added and the sample was fused at 700 ° for 90 minutes. After cooling, the melt cake was dissolved in 20 ml H 2 0 and the solution acidified with 25 ml 1:1 H z S O 4. The solution was transferred to a distillation apparatus and 5 ml 30% H 2 0 2 added. OsO 4 was distilled at about 105°C for 75 minutes with very careful drop-wise addition of a further 15 ml 30% H 2 0 2. The OsO 4 was trapped in 10 ml 6 M N a O H ; Re remains in the distillation flask. The Os distillate was acidified with 15 ml 1 : 1 H 2 S O 4 and transferred to a clean distillation set. Ten milliliters 30% U 2 0 2 was added and Os distilled into a receiver containing 5 ml HBr. The HBr solution was taken to dryness in a teflon vial and Os converted to the hexachlorosmate by evaporation with HCI. To re-

cover Re, the cooled residual solution from the first distillation flask was passed through an anion exchange column of 1 cm in diameter containing 5.5 g (equivalent of 3.5 g dry weight) Bio-Rad AG-1 × 8 resin (200-400 mesh). The column was successively washed with 25 ml 2.5 M H 2 8 0 4 , 25 ml 1.0 M H2S04, 100 ml 1 M HCI, 25 ml 0.8 M H N O 3 and 25 ml 4.0 M H N O 3. Re was finally eluted with 60 ml 4.0 M H N O 3. The eluate was taken to dryness in a teflon vial and redissolved in 1 ml 0.8 M H N O 3. The solution was loaded on a small column made from a polyethylene 1 mI graduated transfer pipet containing 0.18 g (wet weight) of resin. The column was washed with 1 ml 0.8 M H N O 3 and Re eluted in 3 ml 4.0 M H N O 3. The solution was taken to dryness in a teflon vial. Total analytical blanks were found to be approximately 250 pg Re and 160 pg Os. The blanks for iron meteorites were higher than those for our work on chondrites and terrestrial samples (typically about 70 pg Re and 10-20 pg Os [18]). The high blanks, particularly for Re, were due in part to attack on the Zr crucible during the extended fusion necessary to dissolve the largest iron pieces. Slight cross contamination from high abundances in such iron meteorites as Negrillos and Bennett County may also have contributed. 4. Results

Abundances of Re and Os and isotope ratios measured in this work are summarized in Table 1. Agreement with previous ID [3] and N A A [1,10,15,16] results is generally good (Table 2), particularly with values reported by Pernicka and Wasson [10], and by other authors for meteorites where abundances are reasonably high. Our results are much lower than those of H e r r et al. [1] and other early N A A values for irons of low Os content (Central Missouri, Mount Joy, Grant [1]; Campbellsville, Breece {= Grant} [16]). Our results for Charcas are lower by about an order of magnitude than those of Luck et al. [3], who apparently analyzed a sample from the main mass in Paris. Our sample (USNM 1355) may have been artificially heat-treated [8], and thus might seem suspect. A sample of the Paris specimen (kindly provided by B. Zanda) was recently analyzed by N I T M S [23], and our values in Table 1

194

J.W. MORGAN ET AL.

agree reasonably with the new results: 149.5 ppb Re and 1117 ppb Os. There is a significant difference between our results and those of Luck et al. for Casas Grandes. Our sample is from the main mass in Washington ( U S N M 369) and results agree well with those of Pernicka and Wasson for a sample of similar

provenance [10,24] and also with those of Herr et al. [11. Results for Henbury are markedly different from literature values except for analyses reported by Herr et al. for a sample of unknown origin. The same authors analyzed a second sample (studied by Brown and Goldberg [25]) that

TABLE 1 Re and Os abundances and Os isotopic data in magmatic iron meteorite groups IIAB and IIIAB (20- errors in italics) Sample Wt. (rag) I1A Negrillos USNM 1222 IIA Bennett County USNM 1199 IIA Coahuila AMNH 3298 IIA Filomena USNM 1334 I1A Filomena USNM 1334 IIA Lombard USNM 1684 I1B Mount Joy USNM 160 l i b Sandia Mountains USNM 855 liB Central Missouri USNM 1377 IliA Costilla Peak USNM 7112 IliA Casas Grandes USNM 369 IliA Loreto USNM 1507 IIIA Henbury USNM 882 IliA Trenton USNM 2173 IliA Charcas USNM 1355 IIlB (Anom) Treysa USNM 1881 IliA Tamaragul USNM 2294 IIIB Campbellsville USNM 2672 IIIB Tieraco Creek USNM 927 II1B Grant USNM 830

136.3 155.3 275.5 592.4 545.9 488.0 373.7 558.9 719.4

214.9 134.7 262.9 303.1 248.6 178.8 312.2 554.2 635.3 431.4 6311.9

Rhenium

Osmium

(ppb)

(ppb)

ISVOs/IS¢' Os

tS7Re/lS¢' Os

4799.0 53.0 4767.// 48.0 122//.0 11.0 204.6 2.0 204.6 2.1 156.7 1.4 22.72 0.21 8.518 0.091 1.386 0.040

65270.0 590. 0 57360.11 570.0 9813.() 127.0 1041.0 12.0 11/30./t 8.2 789.7 7.1 134.1 1.2 55.76 0.45 12.74 0.11

1.044 0. Ol 0 1.048 0.010 1.200 0.016 1.426 0.014 1.439 0.013 1.450 0.015 1.311 0.015 1.272 0.018 1.165 0.016

2.953 O.042 3.338 0.042 5.006 0.08.5 7.942 0.117 8.028 0.104 8./)21 0.104 6.834 0.088 6.158 0.083 4.378 0.135

1444.0 12.2 394.6 4.3 380.3 3.8 257.5 3.4 180.7 1.6 132.0 2.1 78.32 0.78 35.80 0.41 3.464 O.056 3.186 0.071 2.831 0.075

18430.0 180.0 3630.0 35.0 3828.0 40.0 2765.0 30. 0 1537.0 9.0 1234.0 18.0 572.8 4.6 237.4 2.2 17.08 0.17 28.02 0.36 22.88 0.25

1.031 0.009 1.131 0.011 1.111 0.011 1.113 0. 015 1.177 0.012 1.160 0.017 1.245 0.019 1.256 0.012 1.437 O.021 1.167 0.021 1.161 0.021

3.146 0.042 4.372 0.006 3.995 0.058 3.745 O.064 4.732 0.052 4.304 0.129 5.509 0.069 6.077 0.092 8.196 O.160 4.576 0.117 4.979 0.142

R H E N I U M - O S M I U M IS OTOPE SYSTEMATICS IN M E T E O R I T E S

195

w a s in c l o s e a g r e e m e n t w i t h o t h e r l i t e r a t u r e values. O u r s a m p l e ( U S N M

c o m m u n . ) . In a n y e v e n t , it is c l e a r l y a n a n o m a -

882) c o u l d h a v e b e e n

l o u s s a m p l e as f a r as a b s o l u t e a b u n d a n c e is c o n -

t a k e n f r o m p i e c e s o f " s h r a p n e l " [8] a n d m a y h a v e

c e r n e d , a l t h o u g h i s o t o p i c a l l y it a p p e a r s c o n c o r d a n t with o t h e r IIIA irons.

b e e n u s e d f o r m e c h a n i c a l t e s t i n g (R. C l a r k e , p e r s .

TABLE 2 Comparison of rhenium and osmium analyses of magmatic iron meteorite groups IIAB and IIIAB Type

Locality

IIA IIA IIA IIA I IA IIA I1A 11A I1A IIA I IA IIA

Negrillos Negrillos Bennett County Bennett County Coahuila Coahuila Coahuila Coahuila Filomena Filomena

II A I 1A

IIA I IA 11A lib IIB IIB lib

Rhenium (ppb)

IliA lIIA Ilia IlIA I11A II1A IliA IIIA IIIA IlIA IIIA IIIA lIIA I I IA IIIA IIIB-An IIIB IIIB

Costilla Peak Henbury Henbury Henbury Henbury Henbury Casas Grandes Casas Grandes Casas Grandes Loreto Trenton Trenton Charcas

Others

This work

Others

4799

4800 4801/ 4200 4350 1400 14211 1430 1280

65270

50400 86000 59000 58000 105011 10700 11600

4767 122/)

204.6 204.6

II IB

Breece

IIIB IIIB IIIB

Tieraco Creek Campbellsville Campbellsville

156.7 22.7 8.52 1.39 1444 257.5

394.6

380.3 180.7 132.0

Descubridora

Tamaragul Treysa Grant Grant

Ref.

This work

North Chile * San Martin San Martin Tocopilla Tocopilla Tocopilla

Lombard Mount Joy Sandia Mountains Central Missouri Central Missouri

Osmium (ppb)

35.83 78.32 2.83 3.19 3.46

57360 9813

(a) (c) (c) (d,e) (a) (b) (c) (d)

1030 1040 221 21)5 19(1 252 222 180 160 26 111.2 2.4 1.9 1250 215 1311/ 1530 1260 1250 450 731 382 371 200 185 1150 130 31 93 4.8 3.2 4.1 4.1 3.5 5.1

I 1411 123/) 12911 11160 789.7 134.1 55.76 12.74

I 140 5111 86 25 12

18430 2765

19600 2250 13251) 159011 166110

3630

3220 9300 3890 3910 1600 1710 16100 1100 264 580 100 27 44 411 19 41

3828 1537 1234 237.4 572.8 22.08 28.02 17.08

* A meteorite in italics is paired with the one immediately above. (a) Herr et al. [1]; (b) Luck and All~gre [3]; (c) Pernicka and Wasson [10]; (d) Fouch6 et al. [15]; (el Crocket [16].

(c) (a) (d) (a) (b) (d) (a) (a) (c) (a) (c) (c) (a) (a) (b) (c) (d) (a) (b) (c) (a) (a) (c) (b) (d,e) (c) (a) (a) (c) (d,e) (c) (el (d,e)

J.w. MORGAN ET AL

196

105

,

,

f

I

k

IIIAI~ Irons

(a)

103

\

104

i

IIIAB Irons

~,10 2

.•

103

(b)

~/Treysa

#/Treyse (9

PC 10~

O 102

i as''' i

10'7

I~

i

m

/ i

9

Ni (wt. %)

m

10

i

100

11

i

i

8 9 Ni (wt. %)

I

10

11

Fig. 1. (a) Log Os vs. log Ni in IIIAB irons. (b) Log Re vs. log Ni in IIIAB irons. Triangles = IliA; squares = IIIB.

105

10 4

i

i

IIAB Irons

(a) 104

(b)

103

~ 103 n"

0

"-....

101

102 i

1015.2

6.4

516 6.0 Ni (wt. %)

10c 2

516 6.0 Ni (wt. %)

6.4

Fig. 2 (a). Log Os vs. log Ni in IIAB irons. (b). Log Re vs. log Ni in IIAB irons. Inverted triangles = IIA; dots = liB.

,

,,,

10~

,

,

,,

,

..... IIIAB' ......Irons' ............. .,

103

, /

&o,

r

,,

i

o

,,,,,,,i

,

. . . . . .IIAB . . . . .Irons .

102

,,,i,,

/ "

1t

/ "

1o,

10'

101 ===

10 0

,,

104

i

illl

(a) '''i

10 2

........

i

.

,,.,,,,I

olO~ppb)_

(b)

......

10 4

0 i

10 5

10 101

'

''''"'

,

10 2

,,h,.,l

........

q

ols0~ppb ) 10 4

......

10 5

Fig. 3 (a). Log Re vs. log Os in IIIAB irons (symbols as for Fig. 1. (b) Log Re vs. log Os in IIAB irons (symbols as for Fig. 2.)

RHENIUM-OSMIUM ISOTOPE SYSTEMATICS IN METEORITES

5. R h e n i u m a n d o s m i u m d i s t r i b u t i o n and IIIAB iron meteorites

in I I A B

In the IIAB and IIIAB groups, such refractory siderophile elements as Os and Re correlate inversely with Ni content over most of the compositional range [10]. On log-log plots against Ni, the linear portion of the inverse slope [as in fig. 2 of ref 10] suggests that the variations in this region result from fractional crystallization with essentially constant distribution coefficients. Pernicka and Wasson [10] show that in group IlIAB this relationship breaks down for irons containing more than 9.0% Ni, where abundances of Re, Os, Ir become essentially invariant with increasing Ni content. Using Ni data from [10], we see that the more precise ID data for Re and Os confirm these observations (Fig. 1). Further, the new data for the IIIB irons show an apparent positive correlation of Os with Ni. In the IIAB group the variation of Re and Os with Ni appears continuous and could be fitted with a quadratic curve. If a distinction is made between IIA and liB irons, however, the data are described more precisely on a log-log plot by separate straight lines through each group (Fig. 2). In the IIAB and IIIAB magmatic groups, as in all iron meteorites, abundances of refractory siderophile elements are highly correlated. The log-log variation of Re and Os in IIIAB irons appears linear (Fig. 3a). In detail, Re abundances within the IIIB subgroup seem slightly negatively

197

correlated with Os variation, as becomes more apparent when we discuss the variation of 187Re/ 186Os ratio with Os abundance. In IIAB irons, log-log Re-Os variations are not linear overall, but seem continuous (Fig. 3b). As in the relationship with Ni, the IIA and IIB subgroups fit separate straight lines on a log-log Re-Os abundance plot. The variation of R e / O s is conveniently illustrated using the 187Re/l~6Os ratio, which is then directly relatable to the isotope systematics discussed in the next section. Variation of l~TRe/ 186Os with log Os abundance in group IIIAB is linear overall (Fig. 4a) except for the two IIIB meteorites lowest in Re and Os: Grant (9.24% Ni) and Tieraco Creek (10.4% Ni) [10]. The lowest Ni II1B iron analyzed by us--Campbellsville (8.65% Ni)--plots within both the IliA and IIlB trends, both in Fig. 1 (Os and Re vs. Ni) and Fig. 3a (Re vs. Os). Campbellsville falls near the intersection of the trends defined by the A and B subgroups in many chemical variation diagrams, including the taxonomically important plots of Ga and Ge vs. Ni. Apparently this meteorite (and perhaps other IIIB irons with similar Ni content) represents a pivotal change in conditions during the formation of IIIAB irons. In the IIAB group, also, there is a difference between A and B subgroups in the variation of 187Re/18~Os with log Os abundance (Fig. 4b). As with the IliA irons, group IIA shows a negative correlation of 187Re/186Os ratio with Os content. Unlike the

9

10 IIIAB Irons

', 8,

, IIAB I'rons

(a) 8

ffl

~°6

3

~~

vw

w

•

'

Os (ppb)

•

,9 ~6 /

°

2101

(b)

~

~

,

O,s (ppb)

Fig. 4 (a) lS7Re/186Os vs. log Os in I]IAB irons (symbols as for Fig. 1. (b) IS7Re/186Os vs. log Os in IIAB irons (symbols as for Fig. 2).

198

IlIB subgroup, however, lIB meteorites show ~STRe/l~'Os and Os abundance to be positively correlated. Pernicka and Wasson [10] considered the variation of Re and P G E in IIAB and I I I A B iron meteorites in the light of extensive I N A A analyses. Their discussion was limited chiefly to the I I I A B irons since they proposed that the course of elemental fractionation in the IIAB and I I I A B groups was effected by the same mechanisms, although operating under dissimilar conditions. The linear log-log variation with Ni of Re and Os in the I I I A (and IIIB irons with < 9.0% Ni) is readily explained. Such variation results from fractional crystallization when partition coefficients (k L- for element E ) are invariant, or more generally, when (kL.-- 1 ) / ( k N i - 1) remains constant even though individual partition coefficients may vary with composition and temperature [26]. A similar explanation may apply to the trace element systematics of the I I A irons. For IIIB iron meteorites with Ni > 9.0%, Pernicka and Wasson explain the drop in R e / O s , and the small variability in Re and Os abundances, by late addition of primary melt with approximately chondritic R e / O s . To arrive at ~ 9.0% Ni, the core at this point was probably 85% crystallized [10]. The remaining liquid may have had a high R e / O s ratio, but would have very low absolute Re and Os abundances, because these elements are selectively taken up by solid metal. Given the I I I A trends for log Os and log Re vs. log Ni and lSVRe/lS('Os vs. log Os (Figs. 1 and 4a), an iron meteorite with Ni content similar to Tieraco Creek (10.4% Ni) would be expected to contain about 0.1 ppb Os and have lS7Re/lS~Os ratios of about 15-20. In reality, Tieraco Creek contains 28 ppb Os and has IS7Re/~S6Os ratios close to 5. Given the suggestion by Pernicka and Wasson of late primary metal addition, > 99% of the Os and Re now observed in this meteorite would have been contributed by added metal. A very small amount of late-arriving primary metal would both raise abundances and lower the R e / O s ratio. Pernicka and Wasson suggest that as little as 0.1% addition of metal compositionally similar to Costilla Peak would suffice. Costilla Peak may not be typical of unfractionated late forming metal liquid however because the high Re and Os abundances and low l~7Re/lS~'Os suggest that this

J.W. M O R G A N

E T AI..

meteorite represents a fractionated metal that crystallized at an early stage from a parental core. The observed la7Re/~S6Os ratio in Tieraco Creek more closely resembles that found in the metal phase of ordinary chondrites [3]; thus a contribution of about 1.0% of chondritic metal may be a more realistic estimate. Addition of new metal towards the end of core freezing may plausibly explain Re and Os trends in IIIB irons, but does not specifically predict almost constant Re and Os abundances as observed in the present work. But for just three meteorites coincidence cannot be ruled out. Nevertheless, late metal addition is not the only, and perhaps not the most generally applicable, explanation of the trends observed in I1B and IlIB subgroups. In late stages of fractional crystallization, such minor elements as S, P and C may have a marked influence on the partitioning of trace elements and Ni [26]. The most drastic changes in behavior are likely to be produced by S, because distribution coefficients vary as the square of the S concentration, but linearly for P and C. In addition, S is very insoluble in solid Fe-Ni metal and becomes concentrated in residual liquid. The formation of an abundant additional S-rich phase can drastically change trace element trends, even though changes in partition coefficients may not be large. For the I I I A B iron trend of log Ir vs. log Ni (fig. 2, ref. 27), Kracher and Wasson [27] consider the effects of an increase of S concentration in the liquid during crystallization of I I I A B irons. When the S concentration is sufficient, the liquid metal precipitates an additional p h a s e - either solid FeS or an immiscible sulfide melt. Partitioning behavior when two phases separate simultaneously from a melt is described by Greenland [28]. For the present case, the treatment is greatly simplified because partitioning of siderophile elements into sulfide is relatively insignificant in the presence of liquid and solid metal, and therefore may be ignored. The slope of log E vs. log Ni then depends on ( a . k L 1 ) / ( a ' k r ~ i - 1), where E represents Ir, Os and Re, and the weight fractions in the combined phases now forming from the melt are a and ( 1 - a) respectively for solid metal and the new phase (solid FeS or immiscible S-rich melt). Because a may be small (Kracher and Wasson [27] suggest a value of 0.17), the effect of the second

RHENIUM-OSMIUM ISOTOPE SYSTEMATICS IN METEORITES

phase may be dramatic. The effect may be illustrated semiquantitatively by using Ir (for which there are good partitioning data) as a surrogate for Re and Os. The solid metal/liquid metal partition coefficient for Ni is slightly less than 1 over a wide range of compositions and temperature, but for Ir corresponding values are in the range 2 to 12 [26]. In the two phase system, ( k N i - 1) is small and negative (typically - 0 . 1 ) and ( k I r - 1) might be about 4 or 5. Thus ( a ' k l r - 1 ) / ( a ' k N i - - 1 ) is large and negative, in accordance with the trends in IIA and IIIA iron subgroups. When the sulfide phase begins to form ( a . k y i -- 1) would remain negative but become much larger (about -0.8). Although k~r increases with increasing S content of the melt and with failing temperature, ( a . k ~ r - 1) becomes small and may approach zero. Accordingly, we would expect ( a - k l r - l ) / ( a "kNi -- 1) to become very small and the slope to break sharply on the appearance of the sulfide phase. In IIIAB irons, detailed numerical modeling predicts a break in the steep inverse correlation of log Ir vs. log Ni at about 20-50 ppb Ir [27]. Pernicka and Wasson [10] observe a break at precisely this Ir value. The IIIAB results from this work and [10] show breaks in the slopes for log Re and log Os vs. log Ni at corresponding values. We suggest that the IIIB trend is satisfactorily explained by the effects of S in the late stages of crystallization of the parent body core. Pernicka arid Wasson prefer late addition of primary metal, and for the IIIB irons this may be an equally acceptable model if one assumes the almost constant abundances of Ir, Re and Os in IIIB irons to be coincidental. It is more difficult, however, to account for the trends in IIB irons by the late addition mechanism. To be sure, addition of a small amount of late-forming chondritic metal to a suitably fractionated liquid metal would satisfactorily reproduce the Os content and R e / O s ratio of a IIB iron like Central Missouri. The additions would need to be precisely titrated against Ni, however, to produce the linear log Os and log Re vs. log Ni relationships seen in Fig. 2. In fact, these relationships seem to be first-order evidence of fractional crystallization and suggest a continuation of trends seen in IIA irons. The original S content of the IIAB magma was proba-

199

b[y significantly higher than that of the IIIAB magma, perhaps by as much as a factor of 5-10 [26,27]. Thus the separation of a S-rich phase may have taken place at higher temperature and Scontent than in the IIIAB irons. Temperature and S content work in opposite senses on k E for Ir (and probably Os and Re) [26]. Nevertheless, the significantly negative slope for log Os and log Re vs. log Ni in IIAB irons (Fig. 2) strongly suggests that at the onset of FeS precipitation (or formation of an immiscible S-rich liquid) the values for k~: are sufficiently high for ( a • k E - 1) >> 0. With high S in the liquid, kNi may also increase and might become greater than unity at the onset of separation of a separate S-rich phase. In the presence of a second phase, the log E vs. log Ni will have a negative slope for small values of a even when kNi > 1. Thus in at least a qualitative sense, the break in slope between the A and B subgroups in the IIAB and IIIAB iron meteorites may be explained by separation of a S-rich phase. Future precise analysis of larger sample suites of magmatic irons (and particularly the B subgroups) may reveal further fine structure in the cosmochemistry of siderophile elements. 6. Rhenium-osmium isotope systematics in IIAB and IIIAB iron meteorites

Isotopic data for lS7Re/lS6Os a n d 1870S/1860S are given in Table 1. I I A B : The eight meteorite samples from the IIA subgroup have a spread in lSVRe/IS6Os of almost a factor of 3 (3.15-9.12). This range in a single subgroup almost exactly matches the variation in a suite of twelve iron meteorite samples from several groups (IA, IIA, IIIA, IVB and a mesosiderite) analyzed by Luck and Allbgre [3]. The data for the three IIB irons are too few and have an insufficient 1~7Re/lS~Os variation to constrain a useful isochron in their own right. They do appear to lie closely within the array of IIA irons on an isochron plot (Fig. 5). If we accept that the IIA and IIB subgroups are genetically related, a combined regression yields the following results: initial 187Os/~86Os = 0.798 + 0.013, slope = 0.07906 + 0.0024. The initial ratio agrees within error with previous values for iron meteorites [1-3] and with the initial ratio of 0.802

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1.4

o} 1.2 O

¢

O 1.0

Initial = 0.794+_0.010 0.8

Slope = 0.0792+_0.0020

0

'

2

'

a 187

'

d'

Re/18SOs

8

'10

Fig. 5. lsochron for IIAB and IIAB iron meteorites (symbols as for Figs. I and 2).

present measurements. There may be some justification, therefore, for pooling these data to obtain a magmatic iron meteorite "best estimate": initial ISYOs/lS6Os = 0.794 +_ 0.010, slope = 0.0792 _+ 0.0020, age = 4.65 +_ 0.11 Ga (Fig. 4). The best-estimate age, as quoted, is just within the experimental error limits of the canonical chondrite age of 4.56 Ga (or the "4.56 time bin" [29]). Currently, however, there is a 3% error in the lS7Re half-life determination [5] and if this is incorporated into the error on the iron meteorite age we find a lower limit of 4.47 Ga. Thus, at the latest, the formation of the IIAB and I I I A B iron meteorites was complete by about 100 Ma after the origin of the chondrites. 7. Comparison with other iron meteorite ages and with cooling rates

_+ 0.049 for carbonaceous chondrites [6]. When adopting the revised Lindner et al. decay constant of ( 1 . 6 4 + 0 . 0 5 ) × 10 -~l yr -~ [5], the I I A B slope corresponds to an age of 4.64 + 0.13 Ga (excluding the error in the decay constant). l I M B : In the following discussion, we deal with the I I I A B irons sensu strictu. We have omitted Treysa, which is classified as an anomalous IIIB iron that combines high Ni with IIIA-like Re and Os abundances [9]. The range of IS7Re/lS~'Os in the suite of I I I A irons analyzed in the present work is barely a factor of two. Literature results suggest that this range should be larger. A sample of Charcas analyzed previously had a usefully low ratio of 2.87 [3], but we could not reproduce this result. Accordingly, our isochron for I I I A irons alone has errors that are too large for the isochron to be informative. Unlike the case of the I I A B irons, combination of the I I I A and I I I B irons significantly improves the spread in 1 8 7 R e / 1 8 7 0 s due to the high ratio in Campbellsville (Fig. 5). The resulting isochron is nearly identical to that of the IIAB iron meteorites: initial ~SVOs/lS6Os = 0.798 +_ 0.018, slope = 0.0776 _+ 0.0041, age = 4.56 +_ 0.23 Ga. The I I A B and I I I A B iron meteorites do not seem to have a common origin. Nevertheless, Re-Os isotope systematics of the IIAB and I I I A B groups are identical within the resolution of our

Ages of silicate inclusions in lAB and I I E groups of iron meteorites and in mesosiderites have been determined using long-lived radionuclides. Both 4°Ar-3°Ar and SVRb-SVSr suggest ages similar to those of chondrites for IAB meteorites and for two I I E irons, Weekeroo Station and Colomera [30-33]. Two other I I E iron meteorites, however (Kodaikanal and Netschaevo) yield significantly younger 4°Ar->~Ar and 87RbS7Sr ages of 3.8 Ga, apparently the result of severe shock metamorphism [31,34,35]. I-Xe data on the lAB and liE irons are generally compatible with the 4°Ar-3~Ar and S7Rb-S7Sr results [31,36]. Due to the absence of silicate inclusions, this approach has not been applied to IIAB and I I I A B irons. Of more direct interest to the age question of IIAB and I I I A B magmatic irons is the in-situ decay of mvpd to l°7Ag ( t l / 2 = 6.5 × l0 s' a) [1114]. An internal isochron in the IIB and I I I A B irons between metal, sulfide and schreibersite suggests an initial mVAg/mgAg close to the terrestrial value of 1.08 and a slope corresponding to l°7Ag*/l°Spd = 1.5 x 10 5 (107Ag, is radiogenic Ag produced by in-situ decay of l(}7pd) [14]. This is a value very similar to those found in I V A and IVB iron meteorites. Because the initial l°7pd/ msPd at 4.56 Ga is not known (radiogenic 1°7As has yet to be detected in chondrites), the l°7Ag*/ msPd values cannot be strictly interpreted as ab-

RHENIUM-OSMIUM ISOTOPE SYSTEMATICSIN METFORITES

solute ages. Nevertheless, if the premise behind the Ag-Pd method is accepted, ten half-lives of l°7pd (65 Ma) would be a firm upper limit on how much younger the iron meteorites can be relative to the 4.56 Ga old chondrites. The ~°TAg* data for iron meteorites imply fast cooling rates of greater than 150 K / M a [13-14]. For IIB and IIIAB irons, metallographic cooling rates based on kamacite growth given by many authors are typically very much lower: 1-10 K / M a [37]. Rates for taenite-free IIA hexahedrites based on the growth of plate rhabdites fall in a similar range [38]. Cooling rates estimated for iron meteorites by Narayan and Goldstein [39] are faster than other estimates by two orders of magnitude. The Narayan-Goldstein estimates for IIIAB irons of 150-1400 K / M a (and IIB irons seem to have similar values) are in good agreement with those required by the 1°TAg data [12-14], and suggest small parent bodies of the order of < 20 km in diameter. Cooling rates have recently been re-examined [40], however, with the result that the IIB and IIIB values of Narayan and Goldstein have been reduced by a factor of 50, and IIIA results by a factor of 100. These rates imply parent bodies of about 150-200 km in diameter in which the cores would freeze in roughly 100 m.y. [7,38]. Because the iron meteorites are unlikely to be older than chondrites, we can place limits of between 4.47 and 4.56 Ga on the Re-Os iron meteorite age. This time range is compatible with both l°7pd-1°VAg and cooling rate data. In particular, the apparent discrepancy between 187RelSTOs and l°7pd-l°7Ag implicit in the combination of earlier Re-Os isotopic data [2,3,6] with the revised 187Re half-life [5] seems to have been removed, at least for the present. The current experimental uncertainty in the Re-Os age does not allow new constraints to be placed on the cooling history of the IIAB and IIAB iron meteorites. Nevertheless, two different isotopic systems now can be applied directly to the non-silicate phases of iron meteorites. With improved techniques for isotopic measurement [19,20] we may confidently expect the chronology of the iron meteorites soon to become much more Precise. The cosmochemical data presented here and in [10] may provide a ready guide for sample choice in future high-precision Re-Os isotope studies.

201

Acknowledgements We thank Roy S. Clarke of the Smithsonian Institution for the iron meteorites used in this work, and for much sound advice about iron meteorites in general. Perceptive comments by P.A. Baedecker, J.A. Philpotts and three anonymous reviewers were most beneficial. This work was supported by NASA contract 91-143.

References 1 W. Herr, W. Hoffmeister, B. Hirt, J. Geiss and F.G. Houtermans, Versuch zur Oatierung von Eisenmeteoriten nach der Rhenium-Osmium-Methode, Z. Naturforsch. 16a, 1053-1058, 1961. 2 J.M. Luck, J.L. Birck and C.J. All~gre, 187Re-1~VOs systematics in meteorites: early chronology of the Solar System and age of the Galaxy, Nature 283, 256-259, 1980. 3 J.M. Luck and C.J. All~gre, lSTRe-187Os chronology in meteorites and cosmochemical consequences, Nature 302, 130-132, 1983. 4 M. Lindner, D.A. Leich, R.J. Borg, G.P. Russ, J.M. Bazan, D.S. Simons and A.R. Date, Direct laboratory determination of the Re 187 half-life, Nature 320, 246-247, 1986. 5 M. Lindner, D.A. Leich, G.P. Russ, J.M. Bazan and R.J. Borg, Direct determination of the half-life of lSTRe, Geochim. Cosmochim. Acta 53, 1597-1606, 1989. 6 R.J. Walker and J.W. Morgan, Rhenium-osmium isotope systematics of carbonaceous chondrites, Science 243, 519521, 1989. 7 J.M. Herndon and M.W. Rowe, Thermal models of inhomogeneously accreted meteorite parent bodies, Nat. Phys. Sci, 244, 40-41, 1973. 8 V.F. Buchwald, Handbook of Iron Meteorites, Vol. 2, pp. 442-447, University of California Press, 1975, 9 D.J. Malvin, D. Wang and J.T. Wasson, Chemical classification of iron meteorites--X. Multielement studies of 43 irons, resolution of group IIIE from IIIAB and evaluation of Cu as a taxonomic parameter, Geochim. Cosmochim. Acta 48, 785-804, 1984. 10 E. Pernicka and J.T. Wasson. Ru, Re, Os, Pt and Au in iron meteorites, Geochim. Cosmochim. Acta 51, 17171726, 1987. 11 W.R. Kelly and G.J. Wasserburg, Evidence for the existence of l°7pd in the early solar system, Geophys. Res. Letts. 5, 1079-1082, 1978. 12 T. Kaiser and G.J. Wasserburg, The isotopic composition and concentration of Ag in iron meteorites and the origin of exotic silver, Geochim. Cosmochim. Acta 47, 43-58, 1983~ 13 J.H. Chert and G.J. Wasserburg, The isotopic composition of silver and lead in two iron meteorites: Cape York and Grant, Geochim. Cosmochim. Acta 47, 1725-1737, 1983. 14 J.H. Chen and G.J. Wasserburg, The isotopic composition of Ag in meteorites and the presence of l°7pd in protoplanets, Geochim. Cosmochim. Acta 54, 1729-1743, 1990.

2(/2 15 K.F. Fouch6 and A.A. Smales, The distribution of gold and rhenium in iron meteorites, Chem. Geol. 1, 329-339, 1966. 16 J.H. Crocket, Some aspects of the geochemistry of Ru, Os, lr and Pt in iron meteorites, Geochim. Cosmochim. Acta 36, 517-535, 1972. 17 J.D. Fassett, L.J. Moore, J.C. Travis and J.R. DeVoe, Laser resonance ionization mass spectrometry, Science 230, 262-267, 1985. 18 J.W. Morgan and R.J. Walker, Isotopic determinations of rhenium and osmium in meteorites by using fusion, distillation and ion-exchange separations, Anal. Chim. Acta 222, 291-300, 1989. 19 R.A. Creaser, D.A. Papanastassiou and G.J. Wasserburg, Negative thermal ion mass spectrometry of osmium, rhenium and iridium, Geochim. Cosmochim. Acta 55, 397401, 1991. 20 J. Volkening, T. Walczyk and K.G. H e u m a n n , O s m i u m isotope ratio determinations by negative thermal ionization mass spectrometry, Int. J. Mass Spectrosc. lon Processes, in press, 1991. 21 J.W. Morgan, R.J. Walker and J.N. Grossman, Rheniumosmium isotope systematics in enstatite chondrites, Lunar Planet. Sci. XXI, 809-810, 1990. 22 J.W. Morgan, R.J. Walker and J.N. Grossman, R h e n i u m and osmium abundances and Os-187/Os-186 ratios in IIAB and IIIAB iron meteorites, Lunar Planet. Sci. XXII, 919-920, 1991. 23 M.F. Horan, J.W. Morgan, R.J. Walker and J.N. Grossman, Re-Os isotope constraints on the age of iron meteorites, Science, submitted. 24 E.R.D. Scott, J.T. Wasson and V.F. Buchwald, The chemical classification of iron m e t e o r i t e s - - V I I . A reinvestigation of irons with Ge concentrations between 25 and 80 ppm, Geochim. Cosmochim. Acta 37, 1957-1983. 25 H. Brown and E. Goldberg, A new determination of the relative abundance of rhenium in nature, Phys. Rev. 76, 1260 1261, 1949. 26 J. Willis and J.l. Goldstein, The effects of C, P, and S on trace element partitioning during solidification in Fe-Ni alloys, in: Proc. Lunar Planet. Sci. Conf 13th, J. Geophys. Res. Suppl. 87, A435-A445, 1982, 27 A. Kracher and J.T. Wasson, The role of S in the evolution of the parental cores of the iron meteorites, Geochim. Cosmochim. Acta 46, 2419-2426, 1982.

J.W. M O R G A N

E T AL.

28 L.P. Greenland, An equation for trace element distribution during magmatic crystallization, Am. Mineral. 55, 455-465, 1970. 29 G.R. Tilton, Age of the solar system, in: Meteorites and the Early Solar System, J.F. Kerridge and M.S. Matthews, eds., pp. 259-275, Univ. Arizona Press, 1988. 30 S. Niemeyer, 4°Ar->Ar dating of inclusions from lAB iron meteorites, Geochim. Cosmochim. Acta 43, 1829 1840, 1979. 31 S. Niemeyer, I-Xe and a{~Ar->%r dating of silicates from Weekeroo Station and Netschaevo l i e iron meteorites, Geochim. Cosmochim. Acta 44, 33-44, 1980. 32 D.S. Burnett and G.J. Wasserburg, S7Rb-SVSr ages of silicate inclusions in iron meteorites, Earth Planet. Sci. Lett. 2, 397-400, 1967. 33 H.G. Sanz, D.S. Burnett and G.J. Wasserburg, A precise S7Rb-STSr age and initial ~VSr/Sf'Sr for the Colomera iron meteorite, Geochim. Cosmochim. Acta 34, 1227-1239, 1970. 34 D.S. Burnett and G.J. Wasserburg, Evidence for the formation of an iron meteorite at 3.8×109 years, Earth Planet. Sci. Lett. 2, 137 147, 1967. 35 N.H. Evenson, P.J. Hamilton, G.E. Harlow, R, Klimentidis, R.K. O'Nions and M. Prinz, Silicate inclusions in Weekeroo Station: Planetary differentiates in an iron meteorite, Lunar Planet. Sci. X, 376-378, 1979. 36 S. Niemeyer, l-Xe dating of silicate and troilite from IAB iron meteorites, Geochim. Cosmochim. Acta 43, 843-860, 1979. 37 J.A. Wood, Review of the metallographic cooling rates of meteorites and a new model for the planetesimals in which they formed, in: Asteroids, T. Gehrels, ed., pp. 849-891, University of Arizona Press, 1979. 38 E. Randich and J.I. Goldstein, Cooling rates of seven hexahedrites, Geochim. Cosmochim. Acta 42, 221-233, 1978. 39 C. Narayan and J.l. Goldstein, A major revision of iron meteorite cooling r a t e s - - a n experimental study of the growth of the Widmanstatten pattern, Geochim. Cosmochim. Acta 49, 397-410, 1985. 40 V. Saikumar and J.I. Goldstein, An evaluation of the methods to determine the cooling rates of iron meteorites, Geochim. Cosmochim. Acta 52, 715-726, 1990.