I-Xe and 40Ar-39Ar analyses of silicate from the Eagle Station pallasite and the anomalous iron meteorite Enon

0016-7037/83/06100746$03.00/0

Gewhtmrca er Cosmochrmrca ACM Vol. 41. up. 1007-1012 8 Pergamon Press Ltd 1983.Printedin U.S.A.

I-Xe and 40Ar-3gAranalyses of silicate from the Eagle Station pallasite and the anomalous iron meteorite Enon S. NIEMEYER* Physics Dept., University of California, Berkeley, CA 94720 (Received November 4, 1982; accepted in revisedform February 21, 1983) Abstract-Silicate from two unusual iron-rich meteorites were analyzed by the I-Xe and 40Ar-‘9Ar techniques. Enon, an anomalous iron meteorite with chondritic silicate, shows no loss of radiogenic QAr at low temperature, and gives a plateau age of 4.59 + 0.03 Ga. Although the Xe data fail to define an I-Xe correlation (possibly due to a very low iodine content), the inferred Pu/U ratio is more than 20 above the chondritic value, and the Pu abundance derived from the concentration of Pu-fission Xe is 4 times greater than the abundance inferred for Cl meteorites. These findings for Enon, coupled with data for IAB iron meteorites, suggest that presence of chondritic silicate in an iron-rich meteorite is diagnostic of an old radiometric age with little subsequent thermal disturbance. The JZagleStation pallasite, the most W-rich meteorite known, gives a complex aAr-39Ar age pattern which suggests a recent (So.85 Ga) severe thermal disturbance. The absence of excess ‘29Xe, and the low trapped Ar and Xe contents, p consistent with this interpretation. The similarity between 40Ar-39Ardata for Eagle Station and for the ohvine-rich meteorite Chassigny lends credence to the previous suggestion of a connection between Chassigny and pahasites, in the sense that similar processes operating at similar times on di5ercnt parent bodies may have been involved in the formation of ohvine in both types of meteorites. INTRODUCI’ION

CHONDRITIC SILICATEinclusions in IAB iron meteorites have I-Xe ages comparable to carbonaceous and ordinary chondrites (NIEMEYER, 1979a). The POAr-39Ardata for these IAB inclusions also provided well-defined age plateaus which are as old and older than unshocked chondrites (NIEMEYER, 1979b). In contrast: BE iron meteorites contain silicate inclusions which are more differentiated than IAB silicates, and a presumed more complex history for this iron group was confirmed by the I-Xe and WAr-39Ar chronologies of silicate inclusions from two IIE irons (NIEMEYER. 1980). As described here, the work is extended to include an anomalous iron meteorite with chondritic silicate, Enon, and an anomalous pallasite, Eagle Station. Enon has an unique structure of mm-sized silicate grains uniformly distributed throughout a matrix composed of metal in about half the meteorite and troilite in the other half (BILD, 1976). The mineralogical data of BUNCH ef al. ( 1970) are consistent with chondritic silicates, and BILD’S( 1976) chemical data indicate a composition similar to CI chondrites, except that siderophiles are enriched by about a factor of 10. BILD suggested that Enon formed by the invasion of a metal/troilite mixture into porous chondritic material. But oxygen isotopic analyses place Enon in the cluster defined by the mesosiderites, which in turn fal! within the main population of

achondrites (CLAYTON and MAYEDA, 1978). The primary purpose of the present study was to deter-

* Present Address: Lawrence Livermore National Laboratory. P.O. Box 808, L-232, Livermore, CA 94550.

mine whether the chondritic silicate in such an anomalous meteorite is as ancient as IAB silicates. The Eagle Station pa&site belongs to a trio ofpallasites which are chemically distinct from the main group of pallasites (Sco-rr,1977a). Eagle Station also has more of the %-rich component (1.3%) than any other meteorite (CLAYTON and MAYEDA, 1978) and lies near the 160-mixing line defined by high-temperature minerals from C3 meteorites. The possibility, then, that this meteorite might bear other isotopic anomalies prompted this noble-gas chronological study. EXPERIMENTAL

PROCEDURES

Enon-A 1.62g piece was heated in 4 N HNOr for - 1H hours, after which little metal remained undissolved. A curious feature of this sample was that after etching away the metal/sulfide matrix, the silicate grains remained stuck together. even during uhrasonic agitation. The silicate residue (350 mg) was handpicked further to remove non-silicate phases, and then the sample was successively washed in CSz, water, and methanol. Eagle Station-Metal and weathered ohvihe were handpicked from the sample, leaving large oiivine fragments. The sample was then uhrasonically washed in HrO, followed by methanol. These two samples were irradiated together with the IAB and IIE meteorite samples previously reported (NIEMEYER, 1979a,b, 1980). These earlier papers describe the irradiation and detail the procedures for mass spectromeuy and data analysis which were also used in this study. The Ar data,

corrected for mass discrimination, radioactive decay, and interferences produced by reactions on K during the neutron irradiation, are listed in Table 1. Interferences from reactions on Ca could not be directly inferred since “Ar had decayed to undetectable levels. Xenon data, corrected for mass fractionation and blanks, are shown in Table 2. Abundances of trace elements and some noble-gas components which are derived from these data are listed in Table 3.

1007

S.

1008

Niemeyer

Eagle Statlo" ri.3371 3) 500 BOO 1000 1200 1300 14M) 1500 TOTAL

600

a00 1000 900

1100 1200 1250 1300 1350 1400 1500 TOTAL

' ' 2:; :f:", 3.6 '.? C.7 40.3

357 4075 2384

632 149 71 36 7 4 1722

0.2334(53! 0.060(27) O.l63(3i) 0.1758(60) 1.024(40) 0.986(20) !2.1(3.5) :.21(84)

'I.;53(!6j X107(15) 0.343(34) 0.3398(73) 1.71'10) !.665(50) :8.7(5.6) ?.O(l.?)

J.462[33) j.iC(42) 6.?5(67' 2.427(52) 1.389(86) ').3996(34) 0.159(42) 2.C88f72)

0.0145(15) 0.01148(60) o.ooss3(4a) 0.0l102(96) 0.0744(33) 0.5383(52) 4.89(37)

0.02430(85) 0.01745(81) 0.01299(81) 0.01593(73) 0.1121(19) 0.807(14) ?.27(591

11.3108(32) 0.3223(33) 0.3178(35) 3.3196(33) 0.3680(49) 0.3841(71) O.SSlf46)

2.61(X3) 2.16(57) O.C456(38)

3.94(66) 3.11(98) O.O682(57l

0.438(69) 0.272(78) C.3229(21)

-__

.-__

Oata we corrected according to the procedure described in the text. The lo errors 3n ratios we shown in parentheses for the iast tua signrficant P,dCeS.

s@'"t"

gas

W""tS

have

WTOPS

Of - 10%.

Ar in blanks for all temperilturesteps 1s the blank canposition is 36/38/39/400.42/0.15/.051 100. Temperature steps above 1500°C had errors greater than MI and dre not sham brt ~TP Included I" the totals.

RESULTS AND DISCUSSION Enon

The abundance of J8Ar-spallation translates into an exposure age of 80 Ma using BILD's(1976) chemical data and production rates previously employed by NIEMEYER (1979b). The amount of ‘26Xe-spallation gives a less precise estimate of 150 Ma if Ba is assumed to be at the chondritic level. The difference between these two values probably reflects errors

‘;&q:e

.00366(11) .00283(21) .003a%(O9) .00321(19)

600

800 IO00 1200 1300 1400

1500 0.4 ‘OTAL12.9 500

80@ 900 1000 ,100 1200 1250 1300 1350 1400 1500 1600 TOTA,

152.0 5.0 2.3 4.1 17.2 13.2 6.6 8.2 3.4 I.0

.00034(67) .00325(071

.003X(261 .@0394 39) .a0390 27) .CO304(25) .00202(42) .00085(31) .00056(65) .00328(:51

.00341(07! .00334(29) .00437(41) .00568(40) .00718(32) .01,81(42l .0303(15, .0304(13) .0176(11) .0121(20) .0101(15 .0084(21 .00645(13!

.00329(10) .00311(21) .00470(35) .00855(48) .01070(38) .0161(11) .0490(16) .0507(171 .0283(15) .0188(32) .0133(20) .CllS(?C) .00853(71~

I

m the assumed chemical compositlon for the .Ye exposure age. The 80 Ma age is similar to both the IAB silicates (NIEMEYER. 1979b) and mesosiderites (BEGEMANN e't af..1976). The concentration oftrapped Xe m Enon is slmrlar to the silicates in IAB irons (NIEMEYER, 1979a),as isthe Te concentration. But Enon has substantially lower abundances of the two halogens, I and Cl. To ascribe these depletions to loss of volatiles does not tit with the chondritic level of trapped Xe. and may instead reflect a depletion of halogens. This depletion may be either indigenous or caused by the acid treatment. The tiAr-3’Ar apparent-age plot m Fig. 1 shows J. good plateau for the 800-1000 C steps, which accounts for 9 I % of the released “A?. This three-step plateau gives an age of 4.59 2 .03 Ga. The absence of any evidence for loss of @A? (the * denotes Kderived) at low temperature indicates that Enon has not been subjected to significant disturbances since its formation. The drop-off in apparent age at higher temperatures is probably due to the inability to correct for interferences from neutron reactions with Ca. but for Enon, these interferences have a small effect on the age. The total K-Ar age 1s 4.58 Ga. and the c$orrection for Ca interferences estimated from BILD'S ( 1976) Ca/K ratio raises the total age by only 0.006 Ga. The Ar compositions show no evidence for atmospheric Ar, but even if all the trapped Ar in the plateau steps IS atmospheric Ar, the plateau age would be reduced by only -0.003 Ga. Thus, the 4.59 s .03 Ga plateau age is quite likely a reliable age, and within the resolution of this chronometer, Enon is at least as old as the oldest IAB silicate inclusion 14.57 Ga). An I-Xe plot for Enon is shown in Fig. 2. The

,‘dI’O”

10.1482,54i .i482(OTi 0.888(16) .l?e2(56) il.F544(:-0) .1493(26) '1.9209(861 .1438(25)

I

11.68111?) 0.:33(::) 3.0,(11! l.d607!‘:l

i,.lSS(Ol) 3.334(0@) 0.414(23) X413(14) 3.221(02) 3.215(03) 0.276(06) '3.299(06) 0.214(06) O.l58(15l 0.112(18 ?.1?0(24 1.1851(141

i

.1031(321 .0529(54,

1.9724(4bj ~.1103(9') 1.113(19~ 0.986(11) 0.9822!74) o.994(74, 3.5953(73) 0.903(11) 0.896(17) 0.869(32) KE I:",i O.STZ?i34,

.14$18) .1560(39) .,530(B) .,598(28) .1649(201 .,759(251 .170,(24) .1513(X) .i20?(691 .1506(49, .,54(14) .:538(06,

Z*iSli,’ 2.5?5!59) 1.343r103 4.313(54: !7.9(1.3) 11.:3(54! 2.03113) 4.3,,3i:

!.i.OZI’L '.283(22' '.608fb7 ?.a,7:;9\ 2.725(32; 4.15714:: '1.82(241 ,>.6,3i;lri J.6,!)0> i.50(411 ?.43(25, 2.43(52' '.970!3h

-.11;,(5J, '9.5109(53)

3.4251(D) ).39C3(28~ I.:S'?(?Ol '.304(46l

~.llOliZd! -.4029(60, 1.3987(511 1.3952(28) X4074(29) 2.4894(43) 1.5189(6C) 1.551(111 ,!.618(40) 1.513(23) '1.449(26) ".W74~141

1.3647:43 ,1.495(11) 0.3RlO(d6\ 3.4741(61‘ 0.882(27) '.%4(65) :.,7(2n) ".556(661

,.3372(33 ,1.3595(491 0.3299(51: '1.3537(631 11.3421(381 11.3579(37! 0.4697(691 .,.5094(81) %546(12, .X631(51) 3.458(23] a.42q37, 1.3557(25'

Xe and Ar Table

3.

Abundances components

K Sample

Eagle Enon

sta.

Cl

U

I

Te

'6Artr

(ppb)(ppnl

34 108?

0.46 0.61

38Arsp Sample 1x10-6cm3 STP/g)

Eagle

of trace elements and rare-gas in Eagle Station ano Enon

(ppn)(ppn)(ppb)

.35 .Ol

3.7 3.5

'32x+

lr10-'0c.3STP/g)

0.12 0.64

0.6 0.8

0.113 2.07

'26xesp (x10-'4cm3 STP/g)

Sta. 0.76

(0.5

1.6

1.72~.07

5.11

112

6.0

1.22+.14

En@*

1009

in Enon

Procedures for calculating these abundances are the sane as those described I" Ni~myer (1979a.b).

lOete"nined

by the 1000-1500 C steps for Eagle Statlon and the 1200-1600 C steps fpr Enon.

spallation component has been subtracted from the data, so presumably the plotted points constitute a mixture of trapped Xe and iodinederived Xe. The line in Fig. 2 is a least-squares fit to all points except the 600 C point. This first step is excluded because it probably contains a significant contribution from atmospheric Xe, and including it changes the best-fit line appreciably due to its very small-error. Although the slope corresponds to a (‘291/‘271)~ratio of 1.12 X 10W4.indistinguishable from the chondrite Bjurbole, the scatter about this line indicates that most likely this iodine ratio is not valid. Two possible sources of the scatter are: 1) changing ( ‘29Xe/‘MXe)trapped ratios, a possibility which is supported by the heavy-isotope plot shown in Fig. 3 and discussed below, and 2) terrestrial iodine contamination to which this sample is especially vulnerable due to its very low 1 content. Thus, although Enon contains at least two components which contribute to ‘29Xe, it is not possible to determine whether any of the ‘29Xe variations are due to in situ decay of ‘291. The antiquity of Enon can also be addressed by estimating the abundance of 2”Pu at the time of Xe retention: spallation-corrected data are plotted in Fig. 3, from which a (‘34Xe/‘32Xe)-fission ratio of 1.22

FIG. 2. I-Xe correlation diagram for Enon. Data are corrected for blanks and spallation. Points are identified by release temperatures in degrees C. The point labelled Novo is for Novo Urei (PHINNEY, 197 1). The line is a least-squares fit to the 800-16OO“C points. The large lo error on the slope, coupled with the high reduced variance (i.e. chisquare per degree of freedom) of 2.5, indicates that the data do not constitute a two-component mixture, as required to obtain a valid ‘291/‘271 ratio.

-t 0.14 is determined by the intercept of the line defined by the 1200- 1600 points. The x2 value indicates the error may be overestimated by 20%. The lowertemperature points are excluded because of the influence of an air-Xe component, and the 1200°C point is included since it brings the line through the Novo Urei point, which represents the most “fissionfree” Xe composition directly measured in a meteorite. This particular assignment of trapped components according to release temperature is sup ported by the systematics observed on a ‘24Xe/‘30Xe VS. ‘26Xe/‘WXe diagram. (If the 1200 C point is excluded from the Fig. 3 fit, the line then passes subo'70~l

I

i

1

0.65 1

0.45

u lntaapt

1500

- 1.22 *0.14

1200

5w

0.40

A,.

0.35 0.08

ml

1000

0.10

0.12

1100

0.14

0.16

0.18

130x*113z)(,

FIG. I. Apparent age plot for Enon. Numbers adjacent to the data points are release temperatures in degrees C. Some higher-temperature steps with little 39A? are shown but not identified. Apparent ages are plotted with 2~ errors

where “Af. using define

the error includes only the uncertainty on 4oAr+/ Trapped and spallation contributions are subtracted (“‘Ar/36Ar),,+,p = 1.0 k 0.5. The 800-1000°C steps a plateau with an age of 4.59 + .03 Ga.

FK. 3. Determination of fission composition for Enon. Data are corrected for blanks and spallation. The point labelled Novo is for Novo Urei (PHINNEY, 1971). The line is a least-squares fit to the 1200- 1600°C points; the reduced variance is 0.63. The intercept gives (‘“Xe/‘32Xe)r = 1.22, which nominally corresponds to a 2uPu/238U ratio of 0.12 (4.56 Ga ago).

stantially above the Novo Urei pomt and the mtercept value is reduced to 1.05. which corresponds to a much higher Pu/U ratio.) Using the parameters detailed in NIEMEYER (1979a), the (‘“Xe/‘32Xe)f ratio of 1.22 corresponds to ?‘w”~%I = 0. I? at 4.56 Ga ago. An alternative normalization using Nd, where the Nd concentration is estimated from BILD’S ( 1976) La and Sm determinations and cosmic ratios, gives ‘“Pu/Nd = 14 X 1O-4 weight ratio. Both normalizations indicate an order~f-ma~itude enrichment of za4Pu above the cosmic abundances suggested by HUDSON et al. (1983) [Pu/~~*U = DO7 and Pu/Nd - 2 X 10e4], and the 2a limit from the error on the (1”Xe/‘32Xe)-fission ratio gives Pu concentrations which are 2-4 times above the cosmic ratio at 4.56 Ga ago. An alternative approach to evaluating Enon’s Pu enhancement is to directly compare the abundance of Xe atoms from decay of ‘“Pu to the abundance inferred for Cl meteorites. In the calculation of the abundance of (‘34Xe)h, rather than using the Fig. 3 correlation line to partition between U and Pu fission-Xe, I instead separately sum the 600- 1100 C and 1200- 1600 C steps after subtracting the spailation component, and then subtract air Xe from the low-temperature composition and Nova Urei Xe from the high-temperature composition. The resulting (‘“Xe/‘32Xe)-fission ratios are 1.88 for the lowtemperature steps, which within error is consistent with either no Pu-fission Xe or a chondritic Pu/U ratio, and 1.21 for the high-temperature steps. in excellent agreement with the value deduced from the Fig. 3 correlation line. This agreement is not a trivial resuit, since, in contrast to the correlation approach. here the cont~bution of each temperature step is weighted by the amount of Xe. The inferred abundance of ‘34Xe from Pu fission for the high-temperature steps is 4.14 X lo-‘* cm’ STP/g., This calculation is not overwhelmingly dependent upon the assignment of trapped composition, as an AVCC composition gives only a 14% lower (‘34Xe), abundance. Comparing Enon’s calculated Pu abundanee to the Cl concentration derived from HUDSON ~4[a/.‘~ ( 1983) 2?u/23*U ratio and ANDERS and EBIHARA‘S ! 1982) Cl U concentration indicates that Enon is enriched in Pu by about a factor of 6. Thus the high Pu/U and Pu/Nd ratios apparently stem from an enhancement of Pu rather than depietions of U and Nd. This conclusion is supported by BILD’s i 19761 measurements of La and Sm at about 2X the Cl levels (normalized to Mg); and it also represents the strongest argument against an artifact produced by the nitric acid treatment. This apparent enhancement of Pu identifies Enon as a potentially important meteorite for gaining a better understanding of the behavior of Pu in the early solar system; in addition, the high Pu level is further evidence for the antiquity of Enon, as already deduced from the 40Ar-39Ardata. The noble-gas chronologies of silicates tn iron

meteorites

(NIEMEY~K.

i Y79a.b.

1380) suggested that and mineralogy for silicate inclusions may be diagnostic of radiometric ages as old and older than unshocked chondrites. with reiattvely simple age spectra with little evidence for subsequent thermal disturbance. The prevalence of simple age spectra for the silicate inclusions is especially noteworthy since chondrites rarely give such simple patterns. The present finding that the chondritic siiicate in Enon, despite Its anomalous structure and its oxygen isotopic affiliation with achondrites, is also very old and undisturbed supports the validity of this diagnostic. The events which led to the formation ot Enon must have taken place very early in the solar system. for its “‘Ar-j9.4r age is 120 Ma older than the mean age of unshocked chondrites ENRIGHT and TURNER, 1977). This age difference is significantly; greater than Enon’s ! o error of 30 Ma, which includes uncertainties in J, relative neutron fluences. and 4of4r*/39.4r*. Thus, most likely to within -30 Ma. Enon is contemporaneous with the oldest solar system objects. The unusual nature of Enon coupled with its antiquity merits further geochemical and chronological investigations. 3 ohondritic composition

The abundance of ‘b,4r-spalIation agrees to withrn ‘0% with MEGRUE’S ( I968) dete~ination for an unirradiated Eagle Station olivine sample. The corresponding exposure age is 58 Ma, assuming an Fe abundance of 14.6% (from MECRUE) and tit = 9 *. 1Vi0 cm3 STP/g Fe/Ma. This exposure age is comparable to Enon and other stony-irons. The K abundance shown in Table 3 is much higher than MEGRUE’S( i 933) value of 0.54 ppm, which may be due to an inhomogenous disttibution of pyroxene symplectites (BUSECK, 1977). MEGRUE was unable to obtain a reliable K-Ar age from his data because of the low K concentration. The apparent-age plot shown in Fig. 4 for the present data reveals a complex pattern. The 800 and 1000 C steps, accounting for

FIG J. ,4pparent age piot for Eagk Station. Apparent ages are plotted with 20 errors where the error includes only the uncertainty on *O.4P/‘*.4r? Trapped and spallation contributions are subtracted using (40Ar/36Ar),,+ip= 1.0 -C 0.5. The data do not define a plateau. but instead indicate that Eagle Station experienceda relativelyrecent thermal event (so.85 Ga ago) which degassed 290% of the radiogenic “Ar.

Xe and

Ar in Enon

-40% of the 39Ar+, give fairly similar ages with an average of 0.86 Ga. The greater apparent age for the first step at 600 C can be reasonably attributed to atmospheric Ar. The rise in apparent ages for the > 1000 C steps indicates an incomplete outgassing at a relatively recent time. The decomposition between spallogenic and trapped components is impeded by substantial interferences from chlorine-derived 38Ar. The conventional. but non-unique. decomposition indicates that the spallogenic and trapped components are predominately released above 1300 C. whereas 99% of the 39Ar is extracted in the 5 1300 C steps. Consequently. we calculate a “total” K-Ar age of 1.6 Ga using the 5 1300 C fractions and ignoring the contributions to 4”Ar from the spallogenic and trapped components. If the recent outgassing indeed occurred at -0.85 Ga, the total K-Ar age indicates that for an initial retention of Ar beginning 4.5 Ga ago. the 0.85 Ga event degassed more than 90% of the radiogenic Ar. The small amounts of trapped Ar and Xe are consistent with such a degree of outgassing. The Xe data give no indication of excess “‘Xe produced by decay of “91. The ‘24Xe-‘26Xe-‘3”Xe systematics for blank-corrected data indicate that the composition of trapped Xe is close to air. with essentially no spaliation Xe present. Despite an order of magnitude variation in ‘2sXe/‘30Xe ratios. the ‘z9Xe/‘30Xe ratios are constant within 2a errors. The average (weighted by error) ‘29Xe/130Xe ratio for all steps except the last is 6.41 -t .03. which is intermediate to air and Novo Urei. The absence of excess lz9Xe is consistent with the recent extensive outgassing indicated by the 4oAr-39Ardata. especially in light of the very low iodine conrent. Fission Xe in Eagle Station is highly enriched relative to the trapped component. especially at higher temperatures. Most notably, the ‘30Xe/‘32.Xeratio for the 1500°C step is zero within error. The (‘34Xe/ ‘3’Xe)-fission ratio listed in Table 3 hints at a small 244Pu contribution, but at the 20 level all the fission Xe can be attributed to induced fission of %. Because nobie-gas chronologies for other paliasites are not known, it is an open question whether the recent thermal disturbance inferred for Eagle Station is a common phenomena for the pallasites. The nature of the events which formed the pallasites. and their relative ordering, is still an open question (e.g.. SCOTT. 1977b; BUSECK. 1977). but the thermal event which last reset the K-Ar clock may be a combination of the deformation which fractured the macroscopic olivine grains and a later slow annealing which rounded microscopic olivine shards. In any case, the recent severe degassing of Eagle Station prevents an evaluation of whether the unusual oxygen isotopic composition is accompanied by unusual “‘1 and ?“Pu abundances. The origin of pallasitic olivine is commonly attributed to the differentiation of chondritic material.

101 I

either as a cumulate from fractional crystallization or as a residue from partial melting. It is natural, then. to consider the olivine-rich meteorite. Chassigny. as a possible metal-poor relative to the pallasites (BUSECK. 1977). although SCOTT (1977) argues against a direct relation because of Chassigny’s high fayalite content and plagioclase content. Moreover. the recently reported oxygen isotopic composition for Chassigny (CLAYTON and MAYEDA, 1982) is significantly different from the main group of pallasites. Nevertheless. it is interesting to note that the *Ar‘“Ar data for Chassigny (BOGARD and NYQUIST. 1979) and Eagle Station both fail to define an age plateau. although the apparent-age variations are much greater for Eagle Station. and the total K-Ar ages are fairly similar (1.3 Ga for Chassigny and 1.6 Ga for Eagle Station). Of course. the great disparity in oxygen compositions for Chassigny and Eagle Station renders a common parent body very unlikely; yet the “chronological similarity” lends credence to the suggestion of a connection between Chassigny and the pallasites. in the sense that similar processes operating at similar times may have been involved in the formation of olivine in both types of meteorites. Acknowledgemenrs-I thank Professor John H. Reynolds for his support and advice during this work. The Enon sample was kindly provided by Dr. J. T. Wasson and Dr. R. W. Bild of UCLA. The Eagle Station sample was obtained through the courtesy of E. Olsen of the Field Museum of Natural History, Chicago. Dr. Bryant Hudson’s generous sharing of his computer programs and insights into the interpretation of Pu-fission Xe data contributed significantly to this paper. Reviews by Dr. D. D. Bogard. two anonymous reviewers. and Dr. L. E. Nyquist as associate editor. provided valuable critical input. I alsothank Mr. G. A. McCrory for technical assistance and Ms. Pam Beringer for typing the manuscript. This work was partially supported by NASA under grant NGL 05-003-409. REFERENCES

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BILDR. W. (1976) A study of primitive and unusual meteorites. Ph.D. Thesis, University of California. Los Angeles. B~GARDD. D. and NYQUIST L. E. (1979) Ar-39/Ar-40 chronology of related achondrites (abstract). Mefeorrrics 14, 356. BUNCH T. E.. KEI~ K. and OLSEN E. (1970) Mineralogy and petrology of silicate inclusions in iron meteorites. Contrih. Mineral. Petrol. 25, 297-340. BUSECK P. R. (1977) Pallasite meteorites-mineralogy, petrology and geochemistry. Geochim Cosmochim. Acto 41, 71 l-740. CLAYTONR. N. and MAYEDAT. K. (1978) Genetrc relations between iron and stony meteorites. Earth Planet. Sci. Lett. 40. 168-174.

R. N. dnd MAYEDAT. K. i 1982)Oxygen isotopes in carbonaceous chondrites and in achondrites (abstract). in Lunar and P/aneraryScienceXII. The Lunar and Planetary Institute, Houston. p. 117-I 18. ENRIGHT M. C. and TURNER G. (1977) History and we ofchondrite parent bodies from aAr/‘yAr ages (abstract). ,Meteorir lcs 12, 2 I 7. HUDSONG. B.. KENNEDY B. M.. POWSEK F. .,\. .md HOHENBERGC. M. (1983) The early solar system abundance of ‘“P~ as Inferred from the St. Sevenn chondrite. ./ Geophys. Res. (submitted). MEGRUEG. H. (1968) Rare gas chronology of hypersthene achondrites and pallasites. J. Geophys. Res 73. 20272033. NIEMEYERS. (1979a). I-Xe dating of silicate and trolhte from IAB iron meteorites. Geochrm. Cosmochim. .4cra CLAY’TOh

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.~IEMEYER 5. I :979bj *‘Ar-“‘Ar datmg of inclwons IAB meteorites. Geoc,hlm Cosmochim. .Icro 43. 1840.

from 1829-

\IIEVEYER S. i 1980)

l-Xe and ““Ar-“‘ATdating of silicate from Weekeroo Station and Netschaevo IIE iron meteorites. Geochlm. Cosmochim. rlcta 44, 33-44.

PHlNNE\rD. L. (!971;

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SWTT

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