I-Xe and 40Ar-39Ar dating of silicate from Weekeroo Station and Netschaëvo IIE iron meteorites

I-Xe and 40Ar-3gAr dating of silicate from Weekeroo Station and NetschaGvo IIE iron meteorites SIDNEYNIEMEYER* Department of Physics, University of California, Berkeley, CA 94720, U.S.A. (Receioed 7 May 1979; accepted in revisedform 29 August 1979) Abstract-Silicate inclusions from two IIE iron meteorites were dated by the I-Xe and 4”Ar-39Ar techniques. Weekeroo Station, a ‘normal’ IIE iron, shows no loss of radiogenic 40Ar at low temperature, and the well-defined 40Ar-39Ar plateau yields an age of 4.54 f 0.03 Byr. The xenon data define a good I-Xe correlation with an age of + 10.9 k 0.5 Myr relative to BjurbGle [the monitor error (52.5 Myr) is not included]. Despite its relatively young age, Weekeroo Station’s (1zgXe/‘32Xe),,,,,,, ratio (= 0.84 + 0.05) lies significantly below the solar value. Netschaivo silicate has a chondritic composition, unlike ‘normal’ IIE silicate which is more differentiated. Nevertheless Netschagvo gives a 4”Ar-39Ar plateau-age of only 3.79 k 0.03 Byr, with the xenon data failing to define an I-Xe isochron. Only irons from the IAB and IIE groups contain silicate inclusions, but these two groups differ in many other respects, mostly suggesting that IAB meteorites are more primitive. The I-Xe chronology supports this suggestion inasmuch as Weekeroo Station formed well after (8-15 Myr) IAB silicates. In terms of SCOTT and WASSON’S (1976) model, ages for Weekeroo Station date the shock event which formed ‘normal’ IIE irons by mixing the low-melting fraction of the parent silicate with shock-liquefied metal. Scott and Wasson’s suggestion that Netscha&vo represents IIE parent material, however, is contradicted by Netscha&vo’s 3.8 Byr age. The four silicate-bearing IIE irons which have now been dated can be subdivided into distinct pairs: Weekeroo Station and Colomera formed near the beginning of the solar system, while NetschaZvo and Kodaikanal both formed only 3.8 Byr ago. A review of other properties of these meteorites generally support this subdivision. This work underscores the complexity of the history of IIE meteorites; in particular, an adequate model must account for the formation of two IIE irons at 3.8 Byr without disturbing rare gases in Weekeroo Station. All formation models are quite speculative, but the one which seems best to fit the available evidence postulates two parent bodies: the 3.8 Byr old silicate formed on one parent body, all other IIE material resided in a separate body, and subsequent collision(s) mixed the young silicate with IIE metal.

INTRODUCTION

ONLY iron meteorites from IAB and IIE groups contain silicate inclusions, but these two groups differ in many other respects. IAB iron meteorites apparently contain primitive material: siderophile elements in the metal matrix have chondritic abundances (relative to Ni), and the silicate inclusions are chondritic in mineralogy and chemical composition with unfractionated rare earth element patterns (BILD, 1977). The IAB group is also distinctive in that Ga, Ge, and Ni fractionations are much greater than in most iron groups (SCOTT and WASSON, 1975), and the angularity of the silicate inclusions suggests that these irons have not been molten since incorporation of silicate into the metal matrix. In contrast, IIE irons show: (a) much smaller variations in Ga, Ge, and Ni abundances, (b) weaker Ga-Ni’and Ge-Ni correlations, (c) bulk silicate compositions which are not chondritic (OLSEN and JAROSEWICH,1970), and (d) rounded silicate inclusions which indicate silicate-metal immiscibility.

* Present address: Department of Chemistry, University of California, San Diego, La Jolla, CA 92093, U.S.A.

The primitiveness of the IAB meteorites was confirmed by I-Xe and 40Ar-39Ar analyses (NIEMEYER, 1979a, b). The Ar study established that IAB silicates typically give very well-defined apparent-age plateaus, with little or no loss of radiogenic 40Ar at low temperature. Plateau ages ranged from 4.48 to 4.57 Byr, while unshocked chondrites give a mean age of only 4.47 Byr. Most IAB silicates also yielded well-defined I-Xe correlations with ages similar to chondrites. Surprisingly, I found that (1) I-Xe ages of silicates were correlated with Ni contents of the metal, and (2) troilite in Mundrabilla apparently formed _ 10 Myr before silicate. I concluded that, taken at face value, these and other results favor a nebular formation model. This paper reports the results of I&Xe and 40Ar-39Ar analyses of silicate from two IIE iron meteorites. One objective was to determine whether the many differences between the IAB and IIE groups would also be reflected in the ages. Although RbSr analyses have previously been reported for both these groups, a clear comparison is not possible since IAB silicates yielded rather imprecise ages (BURNETT and WASSERRURG,1967a) and IIE silicates differed among themselves. Ages for Weekeroo Station (BURNETTand

S. NIEMEYER

34

WASSERBURC,1967a) and Colomera (SANZ et al., 1970), both IIE meteorites, just barely overlap, but both are nominally consistent with formation at the beginning of the solar system. Kodaikanal, however, clearly formed much later, i.e. only 3.8 Byr ago (BURNETT and WASSERBURG, 1967b). Presence of excess IZ9Xe in Weekeroo Station (BOGARDet al., 1971) indicates the feasibility of dating IIE meteorites by the I-Xe technique. SCOTT and WAS~ON(1976) recently proposed a specific scenario for the formation of IIE meteorites: the parent material formed in a non-igneous process, analogous to that proposed for IAB irons, and then a shock event mixed the low-melting fraction of the silicate with shock-liquefied metal. They suggested that NetschaEvo (classified as IIE-anomalous) may represent parent material since its silicate inclusions are more chondritic, angular in shape, and even contain relict chondrules (OLSEN and JAROSEWICH,1971). Analysis of silicate from NetschaZvo and a ‘normal’ IIE iron, Weekeroo Station, provide a test of this hypothesis.

Table

TlZElp. (“C)

1. Argon

4oAr

data for irradiated

EXPERIMENTAL PROCEDURE Small aliquots ( - 10 mg) of each sample were reserved for characterization by X-ray diffraction and a scanning electron microscope equipped for energy-dispersive X-ray analysis. Netscha&o was received m the form of one small, predominately silicate fragment, and two larger fragments composed mostly of metal. The small silicate fragment plus silicate chipped from a large fragment weighed 262 mg. Silicate-enriched portions of the remaining fragments were heated for 20 min in hot 4 N HN03 under reflux conditions. The residue was washed several times in water and then methanol. Aliquots of both this sample (87 mg) and the previous 262 mg silicate sample were reserved, and the two samples were combined. This sample was then given a ‘final washing’ which consisted of ultrasonification in hot ( -60°C) distilled water, followed by centrifuging and decanting of the water. The same procedure was repeated three times using methanol, and then the sample was finally taken to dryness. The X-ray diffraction and SEM analysis confirmed the presence of the four major silicate phases previously reported for Netschasvo (BUNCH et al.. 1970). Metal accounted for less than 5”:; of the irradiated sample. A chisel and dental picks were used to mine three silicate inclusions from a metal block of Weekeroo Station. The

silicate from Netschaevo

36Ar

37Ar “OArlOO

(x10-%m3STP/g) NETSCeVO

400 600 800 900 1000 1100 1200 1300 1350 1400 1450 1500 1700 TOTAL

36 58 424 383 443 540 349 174 94 99 9 3 2 2613

400 600 800 900 1000 1100 1200 1250 1300 1400 TOTAL

184 1492 4156 1431 855 841 406 73 18 2 9459

(305.2

0.3599(51) 0.1592(25) 0.00452 (57) 0.00782(54) 0.00953(94) 0.00633(34) 0.00906(52) 0.01806(74) 0.0353(21) 0.0578(24) 0.062(16) 0.116(41) 0.217(95) 0.01965(42) WEEKEROO STATION 0.0946(12) 0.0594(12) 0.04623(74) 0.06470(84) 0.1362(15) 0.3352(37) 1.707(17) 6.978(94) 21.31(85) 10.7(3.5) 0.254(16)

and Weekeroo

Station

38Ar

3qAr

179.1(2.4) 183.7(2.4) 2.723(35) 2.647(36) 5.397(85) 2.364(25) 2.391(32) 1.661(21) 1.572(20) 1.730(23) 2.441(92) 3.83(39) 4.8(2.0) 9.32(21)

0.6064(84) 0.898(12) 1.244(16) 0.6872(8$ 0.5184(68) 0.5118(52) 0.5210(79) 0.5465(73) 0.5480(79) 0.5399 (88) 0.557(23) 0.527(55) 0.39(17) 0.6732(57)

mg)

1.58(91) 4.94(28) 2.24(12) Z-288(89) 4.28(14) 2.382(80) 3.78(21) 3.70(U) 6.57 (56) 8.8(1.1) ___ --__3.368(70) (281.3 ------------_ 26.6(1.9) 130(10) 337(39) _-2.80(26)

mg) 0.5385 (55) 0.1084(20) 0.1013(14) 0.2245(24) 0.5272 (53) 0.7825(80) 3.032(31) 10.73(15) 32.1(1.3) 16.1(5.3) 0.502 (30)

O-3340(38) 0.3206(40) 0.3250(39) O-3329(36) 0.3337 (35) 0.3378(38) 0.3311(40) O-3201(96) 0.334(36) 0.41(14) 0.3278(20)

Data are corrected according to the procedure described in the text. The In errors on ratios are shown in parentheses for the last two significant places. Absolute gas amounts have errors of _ lo”,,. Amounts of 40Ar (in 10e9 cm3 STP) in the 1500°C blanks are 4.0 for NetschaEvo and 5.2 for Weekeroo Station; cold blanks are about a factor of 2 lower. Blanks contain no detectable “Ar or “‘Ar. and “Ar/*“Ar and 38Ar/40Ar are _ 10% and -4Oo/, respectively above atmospheric ratios. The time between irradiation and analysis of the first temperature step is 331 days for Netschaevo and 501 days for Weekeroo Station

I-Xe and 40Ar-39Ar dating of silicate

35

resulting sample had an average grain size of _ 10~. This Exposure ages are also calculated from total amounts of sample was then given the same ‘final washing’ described 38Ar“P and average production rates. The Ca and Fe profor Netschaevo. The mineralogical characterization of duction rates estimated for IAB silicates are used here also, Weekeroo Station revealed major amounts of silica, orthoCa abundances are taken from Table 2, and Fe abundances pyroxene, diopside, and plagioclase, as well as lesser are from BILDand W~sso~ (1977) for Netscha&vo and from amounts of chromite and K-feldspar. These results are conOLSENand JARO~EWICH (1970) for Weekeroo Station. sistent with previous mineralogical studies (BUNCH and Minimum uncertainties for both types of Ar exposure OLSEN,1968; BUNCHet al., 1970; OLSENand JAROSEWICH, ages are ~30%. The good agreement for both Weekeroo 1970). Station and NetschaG;A then, is rather surprising. Even These two samples were irradiated together with the IAB the highly uncertain Xe,, age for Netschaevo agrees samples which have already been reported (NIEMEYER, with the Ar ages, suggesting that 20Myr is a fairly good 1979a, b). Details of the irradiation and mass spectroscopy estimate of its exposure age. The ‘26Xe,, age for Weekeroo are given in the IAB papers. Station is much greater than its Ar ages, but the ‘*‘Xe age is quite unreliable since trace-element abundances vary by more than an order of magnitude in Weekeroo Station DATA ANALYSIS (EVENSENet al., 1979). None of these exposure ages for Weekeroo Station falls very close to the -600 Myr age Data are handled according to the same procedures dedetermined from 38Ar in FeNi (BOGARDet al., 1968). scribed in the IAB papers. Rather than recount all the Since these samples were irradiated along with the IAB details listed therein, only brief summaries are given here, samples, the St. Severin age monitor reported in NIEMEYER along with specific notes concerning Weekeroo Station and NetschaZvo. (1979b) also applies here (J = 0.03754 k 0.00051). After subtracting the trapped + spallation component from Argon 40Ar, a small correction in each case, an apparent age is Argon data are corrected for (1) mass discrimination, (2) calculated for each temperature step using decay of 3’Ar and 39Ar, (3) procedural blanks, (4) neutron interactions on Ca (except for 37Ar), and (5) neutron intert = (l/n) In [l + J(40Ar*/39Ar*)] actions on K (except for 39Ar). Data as they stand after these corrections are listed in Table 1, and errors associated with each correction are included in the listed 10 where 1 = 5.543 x lo- ” yr-’ (STEIGERand JKGER,1977). uncertainties. Ages quoted here from previous studies are also revised ‘Fast’-neutron fluences relative to the St. Severin age using the new decay constants estiblished by Steiger and monitor are shown in Table 2. Fluence corrections to 37Ar Jlger. and 39Ar have not been made in Table 1, but are included Plateau ages and total K-Ar ages are given in Table 3. in ages and elemental abundances shown in Tables 2 and 3. The lo errors include uncertainties in J, relative neutron Total amounts of 37Ar and 39Ar* give the Ca and K fluences, and 40Ar*/39Ar*. abundances listed in Table 2. Values for Netschatvo are in Xenon good agreement with BILD and WAS~~N(1977), but lower than OLSEN and JARO~EWICH(1971). The K content of Xenon data, corrected only for mass discrimination, are shown in Table 4. Blank, spallation, and fission comooWeekeroo Station is consistent with analysis of a 12-inclunents are subtracted following the procedures described in sion sample (OLSENand JAROSEWICH,1970), but my Ca NIEMEYER (1979a). The amount of ‘26Xe._ is small in Netsabundance is lower by about l/3. Quite possibly this differ.r cha&o (1.i x 10-‘4cm3 STP/g) but is much larger in ence is due to inability to detect 37Ar in all but 3 temperaWeekeroo Station (623 x 10-‘4cm3 STP/g). The (lz4Xe/ ture steps. lz6Xe) ratio for Weekeroo Station, as well as the amount, Three components contribute to “Ar: trapped, spallaagree Ai the la level with B~GARDet al. (1971). tion, and 38Ar,, from neutron capture on 37Cl. A unique Fission data are presented in Table 5. Main sources for partitioning is not possible, and the following conventional this component are neutron-induced fission of 235U and procedure is adopted (see e.g. JESSBERGER et al., 1974): spontaneous fission of *44Pu. The (‘34Xe/‘32Xe)f ratio for (1) If 36Ar/38Ar > 0.60, Cl-derived Ar is assumed to be Weekeroo Station is in excellent agreement with the 1.84 negligible, and amounts of trapped and spallation Ar are ratio expected for 235U The case for NetschaZvo is condetermined from the measured 36Ar/38Ar ratio. siderably more complicated (Fin. _ 1). Despite large errors (2) If 36Ar/38Ar < 0.60, trapped Ar is assumed to be on the %O-1200°C points, these points define a good line negligible; thus only spallation contributes to 36Ar, and which gives (‘34Xe/‘3ZXe), = 1.35. The 130&1400”C 38Arc, is estimated assuming (36Ar/37Ar),, = 0.60. points, however, lie well off this line; if the trapped compoThe resulting concentrations of trapped-Ar and Cl sition is assumed to be AVCC, these points give (determined from 38Ar,-, and irradiation parameters) are (‘34Xe/‘32Xe), = 1.81. Assignment of an error is subjective in both cases: the 90&12oo”C line has an extremely low shown in Table 2. NetschaEvo’s extremely high Cl content is most remarkable; some terrestrial contamination is reduced variance (5 x 10e4), indicating that the la error (1968) fit is much likely since 70% of the 38Arc, is released below 600°C. Yet, (kO.50) calculated by the WILLIAM~CJN too great. Assumption of a trapped composition for the over 20% of the 38Arc, is released above 9oo”C, the point 130&14OO”Cpoints essentially determines the second fisat which the 40Arp39Ar age-plateau begins (see discussion sion composition. I conclude that, despite these uncertainbelow). Thus the ‘true’ Cl content of NetschaZvo may well ties, a reasonable interpretation is that the higher-temperabe greater than Weekeroo Station and all IAB silicates. ture points are consistent with U fission alone, while the Following the approach of NIEMEYER (1979b), cosmic-ray exposure ages are kstimated by three methods (see Table 2j. 90&12OO”C points strongly suggest both 244Pu and 235U The “Ar-Ca age is calculated from the (38Ar.../37Ar),, fission. Two different Pu/U ratios for Netscha&vo are not imratio determined by the ‘plateau’ defined by higher-temperplausible. Analysis of Xe from a bulk sample of St. Severin ature steps, where the bulk of the “Ar is usually released. yielded Pu/U = 0.0154 (PODOSEK,1972), whereas a whitBoth Netschaevo and Weekeroo Station give 3 consecutive temperature steps with 38Ar,,/37Ar ratios within _ 15% of lockite separate from St. Severin gave Pu/U = 0.033 (LEWIS 1975). It is interesting to note that Netscha&vo also each other. Netschagvo’s exposure age, however, depends contains whitlockite, and release of fission Xe from St. heavily upon the partitioning procedure since the Cl-comSeverin whitlockite peaked at 1200°C. Implications of the ponent dominates 38Ar.

S. NIEMEYER

36

Table 2. Ca. K, Cl, and trapped-Ar abundances and cosmic-ray exposure ages FlUelICe correction1 (X)

Sample

Weekeroo

Station

Ca2 (X)

(38Ar) 4 cl,p;:, @lo-'0 cm3STP/g)

38Ar-L2 exposure age5 CL4 (MY~) (ppm)

36Ar SP exposure age5 (Myr)

126x, SP exposure age5 (MYr)

-0.2 to.7

0.83

777

-_

1700

18

20

16

-0.6 10.7

2.52

1360

17

20

410

350

930

______---

’ Fluences relative to the St. Severin age monitor: calculated from counting 5”Co in Ni flux wires. * Calculated from 37Ar abundances, estimated 15%uncertainties. ACalculated from j9Ar* abundances, estimated 15% uncertainties. 4 These abundances are calculated by partitioning 38Ar among the trapped, spallogenic. and Cl-derived components (see text). Since the partitioning is not unique, these are only rough approximations. .’ Procedures for estimating these exposure ages are described in the text; minimum errors for the “Ar-Ca and total ““Ar,, ages are _ 305’01and “(‘Xe,, ages are even more uncertain. apparent non-zero Pu/U ratio are discussed later along with the “‘Arpa9Ar and I-Xe results. Abundances of trapped-Xe, U, I, and Te are also listed in Table 5. Procedures for determining these abundances are given in NIEMEYER (1979a). Trapped-Xe concentrations are comparable to previous results for IIE silicates (ALEXANDER and MANUEL. 1968: BOGARDet ul., 1971), which are generally lower than Xe contents of IAB silicates. The U, Te, and I abundances of these IIE silicates are similar to IAB silicates, with the possible exception of the low iodine content in Weekeroo Station. I noted previously that although iodine and tellurium are positively correlated in chondrites, IAB silicates show no definite correlation, with perhaps a hint of the opposite trend. In Netscha&vo and Weekeroo Station I and Te are apparently anti-correlated. The I-Xe dating method consists in demonstrating and evaluating a correlation between “*Xe* (produced by neutron capture on l”I) and lZ9Xe* (from decay of lz91). A standard I-Xe correlation diagram for Weekeroo Station, shown in Fig. 2, shows such a correlation for the high-temperature points. The slope of the line gives the ‘29Xe*/‘28Xe* ratio. from which the I-Xe age is derived. Details of the procedure are given in NIEMEYER (1979a); in particular, difficulties in determining the absolute neutron flux are discussed there. The ‘29Xe/‘3zXe ratio in the trapped component can also be inferred from the I-Xe correlation by assuming a value for (‘2sXe/‘32Xe),,. RESULTS Weekeroo Station

The 100&16OO”C points for Weekeroo Station define the good I-Xe correlation shown in Fig. 2. The slope of this line corresponds to formation 10.9 _t 0.5 Myr after the ordinary chondrite BjurbBle.

The large uncertainty associated with the neutron-flux calibration (+2.5 Myr) is not included in the error. Figure 3 shows the 40Ar-39Ar apparent-age pattern for Weekeroo Station. No significant loss of 4oAr* is observed at low temperature, and the 60&1300°C steps define a good plateau with an age of 4.54 k 0.03 Myr. The (40Ar/36Ar),r+s,, ratio of 0.2 + 0.4 is determined by a 40Ar/36Ar vs 39Ar*/36Ar plot. Weekeroo Station’s (‘Z9Xe/‘32Xe),, ratio of 0.84 + 0.05 lies significantly below the primordial solar value of z 1.0, a result which does not depend critically upon the choice of low-temperature cutoff for the I-Xe correlation. Previous I-Xe analyses of chondrites show a weak correlation between (“‘Xe/ ‘32Xe),r ratios and I-Xe ages (PODOSEK, 1970), but Weekeroo Station conflicts with the trend; its late formation is coupled with a relatively low trapped ratio. Arapahoe’s trapped ratio of 0.56 is the lowest yet measured (DROZD and PODOSEK, 1976), but this value was determined by a long extrapolation. In addition, Arapahoe’s I-Xe is surprisingly old, suggesting to some critics that an artifact is responsible for both results: an erroneously steep line will have an erroneously low intercept. In the case of Weekeroo Station, a shock event probably reset the I-Xe clock (see discussion below), and the degree to which the silicate was closed for Xe determined the change in the trapped ratio. For example, if parent material with the 13’Xe/ “‘1 ratio measured in Weekeroo

Table 3. Plateau ages and total K-~Ar ages

Plateau age1 (Byr)

Sample Net sch&~vo

3.79*.03

Weekeroo

4.54A.03

Station

Total

TelUp.

K-Ar age (Byr)

(1000-1400

C)

3.4ot.03

(600-1300

C)

4.54i.03

’ Plateaus are defined by the temperature fractions shown in parentheses. The trapped + spallation correction assumed (40Ar/36Ar),,+,, = 0.2 + 0.4 for Weekeroo Station and 0.4 k 0.4 for NetschaCvo.

I-Xe and 40Ar-39Ar dating of silicate Table 4. Xenon data for irradiated silicate from Netschdvo 132x, f" 10-12 cl&mf ef

g;*

37 and Weekeroo Station

~124X@

'26x,

128X,

129x,

130x,

132X,

132~~

132x,

132x,

132x,

131x, 132%

_'34x0

_'36x,

132Xe

132x,

NRTSCHAi&O (305.2mg) 400 GO0 800 900 1000 1105 1200

I.300 1350 1400 1450 1500 1700 TOTAL

23.0 2.7 3.2 3.6 3.1 3.0

2.9 3.8 4.0 8.0 1.0 0.7 0.7 57.3

.00345(26)

.00457(17) %0.12(87) .00345(26) .00661(3%) 96.1(2.%) .00315(30) .00458(27) 3%.94(99> .00356(25) .00436(32) 29.17(42) .00368(36) .00454(27) 37.80(89) .00395(17) .oa713(51) 37*7&O) .00452(41) .00712(701 39.9?(62) .00488(34) .00555(16) 23.36(34) .00452(30) .00456(68) 15.19(14) .00409(19) .00473(24) 8.192(82) .00571(50) .00465(75) 7.162(85) 5.703(%0) .0088(11) .0038(12) .0022(13) .00403(94) 4.031(69) .00387(13) .00.500(14) 50.3(1.1)

0.965(10) 1.009(11) 0.994(12) 0.988(12) 0.977(15) 0.992(11) 0.9?3(13) 1.064(16) 1.094(13) L.133(14) 1.141(20) 1.126(26) 1.286128) 1.019al)

1.248(013) .1534(09) 2.844(67) .1556(28) 2.275(51) .1519(M) 2.8?0(42) .1493(18) 6,3?(13) .1453(35f .1467(15) 11.22(32) .1434(27) 14.67(20) .1582(29) 6.99%(71) 4.%32(27) .1566(25) 2.085(16) .1585(21) 1.599(10) .1583(74) 1.209(20) .1580(4%) 1.014(17) .1451(47) 3.620(93) .1534(09)

.3907(32)

.4060(41) .4179(43) .4267(61) .4532(40) .4690(89) .4913(64) .4401(76) .4208(36) .4114(30) .406(U) .394(13) .400(16) .4144(27>

.3328(23) .3464(43) .3646(40) .3938(34) .4304(62) .4445(52) .4647(59) .3991(46) .3740(47) .3572(22) .367(11) .3546(78) .3481(%4) .3651f41)

WR~KEROO STATION (281.3mgf 58.1 400 16.5 600 38.4 800 24.0 900 20.0 1000 13.4 1100 11.1 1200 4.3 1250 1300 3.6 1400 1.4 1.0 1600 TOTAL 198.8

.00368(U) .00553(21) .00592(17) .00948(17) .02662(61) .0771(13) .1064(16) .0775(12) .0518(09) .0165(14) .0073(11) .01410(18)

.00362(11) .00651(26) .00761(15) .01401(39) .04542(76) .1382(11) .1933(26) .1412(25) .0903(26) .0242<12) .0088(U) .02253(25)

1.956(11) 0.994(06) 2.241(1%) 1.036(06) 1.038(0%) 1.103(06) 1.609(U) 1.290(09) 2.%41(28) 1.760(12) 5.083(46) 2.434(22) 8.02%(49) 3.378(24) 6.049(37) 2.7X5(17) 4.597(29) 2.13%(20) 1.043(09) 1.261(16) 0.333(04) 1.025(18) 1.944(11) 1.299(07)

.1538(12) 0.831105) 1.419(04) .1533(13) 1.033(04) .1549(14) 1.232(05) .1580(09) 3.070(21> .1885(19) %.685(84) .2697(23) .3144(21) 12.017655) 8.483(55) .2522(20) 5.516(29) .1921(36) 2.083(22) .1629(48) 1.169(28) .1495(1%) 1.911(06) .1651(10)

.3345(25) .3365(21) .3306(21) .3907(U) .3960(15) .3454(27) .4223(33) .3859(23) .5725(29) .5951(38) .7064(40f .7825(63) .7044(34) .7713(79) .7886(78) .%7%6(69) .4734(66) .492200) .4119(%6) .3462(53) .3865(21) .4298(U) .3936(16) .3979(28)

Data are corrected for mass discrimination only. The Iu errors are shown in parentheses for the last two significant

places. Absolute gas amounts have errors of + 10%. The amounts of i’*Xe (in lo-l3 cm3 STP) for 1500°C blanks are.0.8 for Netschaevo and 2.3 for Weekeroo Station; cold blanks are only 10% lower. The compositions are atmospheric except for excesses at izsXe (- 8 x lo-” cm3 STP) for both samples and at lz9Xe (-7 x lO_” cm3 SIP) for Netschaevo only.

Station formed at 0 Myr with trapped xenon of solar composition, a shock event 11 Myr later would lead

to a revised trapped ratio ranging from the ambient value then at the shock locality (completely open system) to a value of 1.5 for a system closed to xenon. Viewed in this way, the low trapped ratio for Weekeroo Station (where there is neither a long extrapolation nor a steep correlation line) supports the idea that low ratios occurred at some places and

times in the primitive solar system. DROZD and PO~ISEK (1976) have already discussed how complicated models, e.g. gas-dust fra~tionation~ can lead to such effects. The alternative possibility that both the age and trapped ratio are artifacts, however, cannot be fully excluded. For example, if a single phase carries most of the iodine, then the shock event may cause partial losses of Xe and I while still maintaining a correla-

Table 5. Fission data and abundances of trapped-Xe, U, I, and Te for IIE silicates __-('32Xe) ('32X& f2 f fX10-'2cm3STP/R) 1'3*Xef tr

Sample

u3 fppbf

5

('32Xe)tr

I4 (~10-10cm3STi'/%) (ppb)

&

Netscha?vo

1.35 (Y-12) 1.81 (13-14,AVCC)

3.1

5.9%

0.8

0.53 2.02

81

0.4

Weekeroo Station

1.86 (10-16) t.21

8.8

5.3%

15.8

1.67 +.04

13

0.9

-m-

' Determined by 134Xe/‘32Xe vs 13’Xe/ i32Xe plots for the points specified in parentheses (numbers refer to temperature of fractions in hundreds of degrees C). The IO error for Weekeroo Station includes only the statistical error; see text for discussion of un~r~inties for Netschaevo. ’ Calculated using (‘36Xe/ ‘3zXe), ratios determined from 130Xe-‘32Xe-‘36Xe systematics. 3 Calculated by the procedure of PODOSEK (1972) and estimated neutron fluences of &,srma, = 2.1 x 10’9cm-z and d&i,h.rma, = 6.2 x IO” dE/E cm-‘. Netschafvo’s U abundance is only an approximate lower limit; a reasonable upper limit is given by assuming all ‘j2Xer is from 13’IJ induced fission, 4 Calculated from abundances of ‘*‘Xe* and measured izsXe*/ i2’I ratio, 20% uncertainties. SGiven by i3’Xe* abundances and neutron fluence given in footnote 3; Te concentrations are upper limits due to possible interference from i”Ba.

S. NIEMEYER

38

tion. For the slope artificially iodine is ably does

preferential loss of iodine relative to xenon, would be increased and the trapped ratio lowered. Since such a preferential loss of unlikely, this alternative explanation probnot apply.

NetschaZvo NetschaCvo did not give an I-Xe correlation (see Fig. 4); xenon released above 1000°C has an almost constant 129Xe/‘32Xe ratio. The 40Ar-39Ar apparentage plot in Fig. 5 shows a fairly good plateau beginning at the 1000°C step. The slight decrease at 1300°C may be due to recoil of 39Ar*. The age of the 100&14OO”C plateau is 3.79 f 0.03 Byr. Forge heating of NetschJvo at -7O&1ooo”C in the 19th Century (BUCHWALD, 1975) probably caused the loss of 40Ar* at low temperature. If the duration of the forge heating was of the same order as the laboratory heating, i.e. hours rather than days, the forge temperature can be estimated by matching the inferred 40Ar* loss (22%) with the laboratory temperature required to produce an equivalent 39Ar* loss. Inspection of Fig. 5 indicates a temperature of 60&8oo”C, consistent with previous estimates. It is almost certain, therefore, that an event 3.8 Byr ago reset the K-Ar clock in Netschalvo and homogenized the xenon isotopes. This case provides an especially good example of the benefits of simultaneously dating a sample by the I-Xe and 40Ar-39Ar techniques. The 3.8 Byr old event apparently homogenized I, Te, and U, as higher-temperature fractions (2 1200°C) show a fair degree of correlation among ‘**Xe* and lz9Xe*, 13’Xe*, and fission xenon. Assuming then that the measured Pu/U ratios were established -4.6 Byr ago, the 90&1200°C points indicate a 244Pu/238U ratio of 0.052. Chronological interpretation of this ratio is hindered by possible chemical fractionations between Pu and U. Nonetheless. I note that Netschagvo’s Pu/U ratio is intermediate to St. Severin whitlockite (LEWIS, 1975) and Allende inclusions (PODOSEKand LEWIS, 1972). Among all the iron meteorites with silicate inclusions, only Netschaevo contains chondrules. Yet only it and one other iron are clearly younger than the solar system. But it is not clear whether Netscha2vo’s chondrules originally formed -4.5 Byr ago and then survived the 3.8 Byr old event. or whether the chondrules actually formed only 3.8 Byr ago. Resolution of this question might provide important clues to the ongoing debate concerning the mechanism for chondrule formation. DISCUSSION Many of the chemical and mineralogical differences between IAB and IIE meteorites were noted in the introduction ; the present results establish a significant age difference as well. Weekeroo Station, a ‘normal’ IIE meteorite, formed 8 Myr after the youngest IAB silicate previously dated. SCOTT and WAS~N (1976) proposed that IIE parent material formed in a man-

ner analogous to IAB meteorites, from which normal IIE meteorites were derived by shock mixing. In terms of this model, Weekeroo Station’s I-Xe age indicates that IIE parent material and IAB meteorites formed at approximately the same time, with the IIE shock event occurring shortly thereafter. The 40Ar-39Ar age of Weekeroo Station falls within the range defined by IAB silicates, its high quality plateau also typical of IAB silicates. A genetic relation between these two groups, however, is ruled out by their different oxygen isotopic compositions (CLAYTON and MAYEDA, 1978). Scott and Wasson’s suggestion that Netschafvo is a sample of parent material, however, is contradicted by its 3.8 Byr age. Such a young age is quite surprising considering that Netschai+vo silicate is more chondritic than Weekeroo Station’s Apparently the IIE group has had a complex history, which could account also for its structural and mineralogical diversity. Before further discussion of the present results, I review previous TIE dating studies. Subdivision oj silicate-bearing

IIE irons

Silicate inclusions have been discovered in only 5 out of 12 HE irons; inclusions in Elga are virtually unstudied, but precise ages are now available for the other four. Kodaikanal gave a good RbSr isochron with an age of 3.7 + 0.1 Byr (BURNETT and WASSERBURG, 1967b), making it contemporaneous with NetschaEvo. Burnett and Wasserburg argue that the moderately low initial 87Sr/86Sr ratio for Kodaikanal is inconsistent with simple homogenization of a 4.6 Byr old inclusion at 3.7 Byr, but instead requires a ‘secondary’ body made from pre-existing bodies. Rb-Sr data for Weekeroo Station (BURNETT and WASSERBURG,1967a) yielded an age of 4.28’:::: Byr, while 40Ar~-40K data defined a 4.41 + 0.1 Byr age (BOGARD et a[., 1968). The more precise 40Ar-39Ar age agrees with the Rb-Sr result only at the extreme upper limit, and more recent Rb-Sr analyses (EVENsim et al., 1979) also give an age significantly younger than the 40Ar 39Ar age. Possibly the RbSr system was disturbed by an event which had little or no effect on the K-Ar system. Colomera defined a precise linear array corresponding to an age of 4.51 f 0.04 Byr, although a few discordant points were excluded in order to define the isochron (SANZ et nl., 1970). WASSERBURG et ul. (1968) also reported a 40Ar--K age of 4.2 Byr for Colomera K-feldspar. Table 6 shows exploratory argon and xenon compositions measured in unirradiated Colomera feldspar and a separate enriched in diopside. Since K and Ca abundances are not known, 40Ar--K and cosmic-ray exposure ages cannot be reliably determined from these data. (The Colomera sample intended for this study was, unfortunately, compromised before I received it by inadvertent exposure to iodine-bearing heavy liquids.) On the basis of ages. 1 propose subdividing these four silicate-bearing IIE meteorites into two distinct

39

I-Xe and 40Ar-3qAr dating of silicate

0.55

I

I

I

Netschcao

0.50

3z ::

-1 0.45

t2

$!

0.40

0.35 I2

I

I

I

0.13

0.14

0.15

13’Xef 13’Xe

Fig. 1. Determination of fission composition for Netschdvo. Data are corrected for blanks and spallation. Points are identified by release. temperature in degrees C. The 900-1200°C points define the correlation line; the intercept of the line gives (‘“*Xe/ t”*Xe), = 1.35. Nominally, this corresponds to a 14QPu/z38U ratio of 0.052 (4.6 Byr ago), but the uncertainties do not unequivocally exclude a zero contribution from Z’L4Pu.

8

I

W eekeroo

1

I

Station

6-

Slope = 0.339 f 0.006 (t’PXe/‘3zXe),a

I 5

I 10

= 0.84 f 0.05

I 15

28

‘28Xe/‘32Xe Fig. 2. I-Xe correlation diagram for Weekeroo Station. Data are corrected for blanks, spallation, and fission. Points are identified by release temperature in degrees C. The line is a least-squares fit to the lOOO-1600°C points; points not included in the fit are shown as open circles. The error on the slope is adjusted to compensate for neglecting the non-zero correlation coefficients introduced by the blank, spahation, and fission corrections; the procedure is described in N~EMEVER (1979a). In this case the adjustment reduced the error by about a factor of 3. The slope corresponds to an age of + 10.9 f 0.5 Myr. The (12qXe/132Xe)T,,ratio of 0.84 k 0.05 is inferred by assuming that the trapped component on the correlation line has ‘2*Xe/‘3zXe = 0.082.

S. NIEMEYER

40

I

,

Weekeroo

I Station -I

I

I 0.4

0.2

Cumuhtive fraction Fig. 3. Apparent

age plot for Weekeroo

Station.

temperatures in degrees C. Some higher-tem~rature

I

I

I

0.6

0.8

1.0

19Ar* released

Numbers adjacent to the data points are release steps with little 39Ar* are shown, but not identi-

ages are fied; steps with greater than 5OU;,errors on the 4oAr*13’Ar* ratios are not shown. Apparent plotted with 2a errors where the error includes only the uncertainty on s”Ar*/39Arf. The 60--l 300-C steps define an excellent plateau with an age of 4.54 It 0.03 Byr.

pairs: Weekeroo Station and Colomera formed near the beginning of the solar system, whereas Netschatvo and Kodaikanal both formed only 3.8 Byr ago. Qther properties of these meteorites, summarized in Table 7, generally support the subdivision. Abundances of Ar and Xe are lower in NetschaZvo and Kodaikanal than in Weekeroo Station and Colomera (these meteorites are henceforth referred to as the ‘young pair’ and “old pair’ respectively). Total 129Xe/132Xeratios of the young pair are close to the solar value, whereas the old pair show substantial excess lz9Xe*. The rare-gas results thus suggest loss

of gases from the young pair during the 3.8 BIT event. while the old pair remained relatively undisturbed. Mineralogical evidence is not clear-cut, but some features (e.g., pyroxene compositions and trace elements in chromite) conform to the proposed subdivision. Abundances of Ni and Ga in metal also follow the expected grouping. The link between Netschagvo and Kodaikana~ is further strengthened by their similar. yet unusually low, exposure ages. Not all properties reflect this subdivision, but the overall evidence is convincing. A definitive test may be provided by oxygen isotopes, which have proved to be a valuable

Netschaibo

Silicate

* 800

1.0

#400

t 0.9 I 0

I 20

I 40

I 80

I 60 “axe/

I 100

1:

13’Xe

Fig. 4. I--Xe correlation diagram for NetschaEvo. Data are corrected for blanks, spallation, and fission. No correlation is observed, instead points 2 1000°C show an almost constant ‘z9Xe/‘32Xe ratio. This pattern is attributed to homogenization of xenon isotopes during the 3.8 BYF event which reset the K-Ar clock.

I-Xe and 40Ar-39Ar dating of silicate

H E

41

900 3.0.,-&J(J

?! x 2 2.5 -

2.0

800 I 0.8

I 0.6

I 0.4

I 0.2

0

1.0

Cumulative fraction “Ar* released Fig. 5. K/Ca and apparent-age plots for NetschaEvo. Apparent ages are plotted with 2a errors where the error includes only the uncertainty on 40Ar*/39Ar*. The 100&1400”C steps define a fairly good plateau corresponding to an age of 3.79 k 0.03 Byr. (The slight age decrease at 1300°C may be due to recoil of 39Ar*.) Loss of 40Ar* at lower temperatures is probably due to forge heating of this meteorite (BUCHWALD, 1975).

tool for confirming

and defining genetic relationships

(CLAYTONet al., 1976). CLAYTONand MAYEDA(1978) found that oxygen isotopic compositions of pyroxene from the old pair are identical within error, while a pyroxene plus olivine sample from Netschacvo is distinctly different. Kodaikanal has not yet been analyzed, but the proposed subdivision predicts that its oxygen composition will be similar to NetschaZvo. Implications regarding origin of IIE meteorites

Silicate inclusions with rounded shapes (Table 7) require melting the silicate and metal during or after

mixing of these two phases (see e.g., WASSERBURG et al., 1965). A shock event probably caused the melting since localized shock can be followed by rapid cooling, without gravitational separation of silicate and metal. Absence of silicate inclusions in many IIE irons may represent cases in which cooling was not sufficiently rapid to trap the silicate. The rounded shapes, differentiated bulk compositions, and unequilibrated nature of old IIE silicates are consistent with the shock-mixing origin proposed by SCOTTand WAXIN (1976). Evidence for immiscible liquids within silicate inclusions supports a molten silicate phase, but the

Table 6. Xenon and argondatafor unirradiated Colomera silicate (CS)*

l 32Xe

124Xe

126X,

12EXe

12gXe

Sample

10-12 cm3STP/g)

Colomera feldspar

49 25

3.69 t.15

6.56 k.23

25.39 21.11

130.6 to.9

Colomera diopside

33 +3

1.06 2.04

1.15 f.05

9.15 +.sa

140.8 +2.3

Sample

(x

4oAr (x 10-k cm3STP/g)

13*x,

40Ar

30Ar

36Ar

36Ar

Colomera feldspar

51.6 f5.2

14,932 2461

1.256 A.073

Colomera diopside

2.1 k.3

2,177 +ia

1.496

130Xe

l 31Xe

134Xe

l 36Xe

22.16 k.33

107.17 5.85

38.18 k.44

32.61 2.43

15.72 k.39

78.25 t1.33

39.60 t.61

33.16 2.53

= 100

+.022

* Data are corrected for mass discrimination and blanks. Listed errors are la, absolute gas amounts have an additional 10% error. Sample weights are 30.4 mg for feldspar and 29.4 mg for the diopside-enriched sample.

42

S. NIEMEYER Table 7. Properties of siIicate-daring

IIE meteorites

Young pair

Netschalvo Evidence ( 36Ar)

total (x 10-8cm3STP/g)

supporting

Kodaikanai proposed

0.5a

(132Xe) trapped (x 10-10cm3STP/g) (12gXe/132Xe) total Fe/(Fe+Mgf:e orthopyroxene clinopyroxene Chromite: Hn(O=32)e Znf0=32je

0.9d,1.7a

1.014=

0.98b

2.17d,1.46a

abundantf

minor

undetected

.19 undetected

.19

Metal phase Ni(%)l Ga(ppm) i Immiscible liquids within silicate inclusions

2.2,3.5

-_-

8.6 24.8

8.71 21.9

undetected

Cosmic-ray exposure ages @VT) ___--.~...--------c-

20a

angular

Chondrules

K-feldspar' Equilibrated silicate assemblagee Cooling rates (‘C/.Myr)

yesf Or

4 . PAb8PAnl 3. Be

undetected

o.3-o.5i 1.30-1.3!

1 grain' .24 .15,.23 .28 .09

2.6.3.8

2.5,3.7

7.51 28.2

7.86 28.4

observed'

600,4008 _---1 ---

15& ------_.-.------

Shape of silicate Lnclusfonsk

IO-3sa

.35,.46 .49,.41

observedj

undetected

Other

Feldspar

.2L .28

.I7 .13..27

undetected

Oxygen compositionsh 6170. 6180

15-4tiaPC

'(0.1-0.3)b

.14 .lO

Colomera

subdivision 0.8-l.0b’c

0.5a

Olivinee

Old pair Weekeroo Station

comparisons rounded

rounded

no OrPOAb

no n

80

present

orShb*l,%o

rounded no Or Ab 6 92.+

common

conrmon

yes

no

no

no

li

-400i

li

-1op

a This work. b ALEXANDER and MANUEL(1968). c BOGARDer nt. (1968). d BOGARDet at. (1971). ’ BUNCHet al, (1970). r OLSENand JAROSEWICH (1971). ?WA~~ERBURG et al. (1968). h CLAYTONand MAYEDA(1978). i SCOOTand WAS~N (1976). “OLSEN and J~~os~wtc~(1970). ' BU~HWALD(1975). ! BOGARDet (11.(1969). mCHANGand W;~NKE(1969). ” BENCEand BURNETT(1969). OBUNCH and OLSEN(1968). 0 From WASSON’S(1971) equation and BUCHWALD’S(1975) bandwidth.

I-Xe and 40Ar-3gAr dating of silicate

abundance of rare gases suggest that the molten period was brief. The I-Xe age of Weekeroo Station suggests that this event occurred at + 11 Myr. In contrast to old IIE meteorites, Netschaevo’s silicate inclusions are angular, equilibrated, and show no evidence for liquid immiscibility. Furthermore, its bulk mineralogy is much more chondritic, due essentially to the presence of olivine (OLSENand JAROSEWICH, 1971). These characteristics argue that Netschaevo formed under different conditions than old IIE meteorites. But Kodaikanal, the other young IIE iron, has more rounded inclusions, much less olivine, and a di~uilibrium assemblage. In these respects, Kodaikanaf more closely resembles the old IIE irons rather than Netschaevo. These differences may be due to differing degrees of heating during various shock events. The results of this dating study coupled with the proposed subdivision of silicate-bearing IIE meteorites provide new insights into the history of this group. Resolution of iron meteorites into distinct groups via Ni, Ga, and Ge systematics has been interpreted as evidence for a genetic relationship among members of a single group (SCOTTand WASSON,1975). Applying Occam’s razor, one postulates a common parent body for all IIE meteorites. An adequate formation model must also account for the disturbances at 3.8 Byr in the young IIE irons only. It is difficult to envision a planetary-wide process which meets this demand. A formation model involving only one parent body can be maintained by postulating that a shock event produced the young IIE silicates, since such an event allows localized heating. But such a model encounters at least two apparent difficulties: (1) the shock event must reset Netschaevo’s K-Ar and I-Xe clocks without melting the silicate or metal, and (2) an open-system resetting of Netschagvo’s I-Xe clock is implied by the measured upper limit of c 1.2 for the (‘29Xe/‘32Xe),, ratio, since a closed-system resetting would produce a trapped ratio of c 10. It is doubtful whether a shock event during which silicate and metal remain solid can provide such an open system, in which case recourse to a more complicated two-shock evolution may be needed e.g. one shock event at 3.8 Byr resetting the radiogenic clocks while metal and silicate are still separated, and a later shock injecting silicate into metal without melting the silicate. An alternative model involves two parent bodies: old IIE meteorites and metal of all IIE meteorites are part of one parent body, but young IIE silicates formed on a separate’ parent body. Subsequent colhsion(s), possibly involving other bodies or perhaps between these two postulated parent bodies, then mixed the 3.8 Byr old silicate with IIE metal. This model relaxes the constraints imposed by the undisturbed old IIE meteorites and also accounts for the different oxygen compositions. Given such a model, the oxygen composition and trace elements of Netschatvo (BILDand WASSON,1977) suggest that the par-

43

ent body for young IIE silicates is closely related to ordinary chondrites. Perhaps the postulated collision is connected with the collisional degassing of ordinary chondrites during recent (z 30-700 Myr) solar system history @OGARD et nl., 1976). All these models are speculative, but the last one seems best to fit all the facts. My results certainly underscore the complexity of the history of IIE meteorites, and suggest that shock processes played an important role in forming both young and old IIE irons. Systemtics

of I-Xe

dating

DROZD and POWSEK (1977) reviewed possible breakdowns in the assumptions of the I-Xe method; the I-Xe study of IAB iron meteorites (NIEMEYER, 1979a) provided support for many of these assumptions. Drozd and Podosek pointed out that isotopic redistribution of I and/or Xe, caused by either thermal or nonth~m~ events, might Iead to erroneous ages and trapped ratios. Absence of an I-Xe correlation for Netschatvo is interpreted to be the result of complete, or nearly complete, mobilization and redistribution of Xe isotopes 3.8 Byr ago. Of course this result does not rule out creation of a spurious I-Xe correlation by isotopic redistribution in another circumstance, but at least it demonstrates that in one instance such an event destroyed the I-Xe correlation rather than created an erroneous one. SUMMARY The primary findings of this dating study of silicate from the IIE irons, Weekeroo Station and Netschaevo, are : (1) Weekeroo Station shows no loss of 40Ar* at low temperature, and the well-defined 40Ar-39Ar plateau yields an age of 4.54 f 0.03 Byr. The good I-Xe correlation gives an age of + 10.9 Myr, indicating formation well after (8-l 5 Myr) IAB silicates. (2) Netschaevo gives a 40Ar-39Ar plateau with an age of 3.79 & 0.03 Byr. The absence of an I-Xe correlation is attributed to xenon redistribution 3.8 Byr ago. (3) These results contradict SCOTT and WASON’S (1976) suggestion that Netschdvo represents IIE parent material. (4) Weekeroo Station’s (129Xe/‘32Xe),, ratio of 0.84 f 0.05 supports DROZD and POWSEK’S (1976) claim for trapped ratios significantly below the primordial solar value. (5) The four silicate-bearing IIE meteorites which have now been dated can be subdivided into distinguishable pairs: Weekeroo Station and Colomera formed near the beginning of the solar system, while Netschaevo and Kodaikanal both formed only 3.8 Byr ago. This subdivision is also reflected in many other properties of these meteorites. (6) A complex history is indicated for the IIE group. Although all models are quite speculative, I lean

S. NIEMEYER

44

towards a model involving two parent bodies, one parent body for just the 3.8 Byr old silicate and a separate parent body for all other IIE material. Ackrzowledgements-I am especially pleased to acknowledge the advice and guidance of Professor JOHN H. REYNOLDS during this work. A sample of NetschSvo was kindly provided by Dr G. KURAT of the Naturhistorisches Museum Wien, and the Weekeroo Station material was obtained through the courtesy of Dr C. B. M~~RE of Arizona State University. Discussions with Dr JOHN T. WASin the initial stage of this work were very helpful. I have benefited greatly from the advice and encouragement of Dr DOUGLAS LEIGH, Dr JIOUGLAS PHINNEY, Dr URS FRICK, and Dr ANTHONY ZAIKOWSKI. Reviews by Dr C. M. HOHENRERG and Dr G. TURNER provided valuable comments. I also thank Mr G. A. MCCRORY for technical assistance. Dr ROBERT MONIO~ for assistance with data analysis, and MS JOAN AMOROW for typing the manuscript. This work (paper number 120) was supported by NASA under grant NGL 05-003-409. SON

REFERENCES ALEXANDERE. C. JR and MANUEL 0. K. (196X) Noble gases in silicate inclusions of Kodaikanal. Earth Planet. Sci. Lett. 4, 363-367. BENCE A. E. and BURNETT D. S. (1969) Chemistry and mineralogy of the silicates and metal of the Kodaikanal meteorite. Geochim. Cosmochim. Acta 33, 387407. BILD R. W. (1977) Silicate inclusions in group IAB irons and a relation to the anomalous stones Winona and Mt. Morris (Wis.). Geochim. Cosmochim. Actu 41, 1439--1456. BILV R. W. and WAS~ON J. T. (1977) NetschaEvo: A new class of chondritic meteorite. Science 197, 5X-62. BCK;ARD D. D., BURNETT D. S. and WASSERBURG G. J. (1969) Cosmogenic rare gases and the 40K-40Ar age of the Kodaikanal iron meteorite. Earth Planet. Sci. Lett. 5, 273-281. B(K;ARD D. D., BURNETT D., EBERHARDT P. and WASSERRURG G. J. (196X) ““Arp4’K ages of silicate inclusions in iron meteorites. Earth Planet. Sci. Lett. 3, 275-283. B~GARD D. D.. HUSAIN L. and WRIGHT R. J. (1976) 40Ar~m39Ar dating of collisional events in chondrite parent bodies. J. Geoph_w Res. 81, 56645678. BO(;ARD D. D., HUNEKE J. C., BURNETT D. S. and WASSERHI!RC; G. J. (1971) Xe and Kr analyses of silicate inclusions from iron meteorites. Geochim. Cosmochim. Acta 35, 1231-1254. BLIC%WAL~ V. F. (1975) Handhook of Iron Meteorite.\. liniv. of California Press. BUNCH T. E. and OLSEN E. (196X) Potassium feldspar in Weekeroo Station, Kodaikanal and Colomera iron meteorites. Science 160. 1223-1225: Science 162. 1507 I 508. BUNCH T. E., KEEL K. and OLSEN E. (1970) Mineralogy and petrology of silicate inclusions in iron meteorites. Contrih. Minerul.

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BCRNETT D. S. and WASSERBURG G. J. (1967a) *‘Rb~“Sr ages of silicate inclusions in iron meteorites. Earth Plunet. Sci. Left. 2, 397-408. BCRNET~ D. S. and WASSERBURGG. J. (1967b) Evidence for the formation of an iron meteorite at 3.X x IO9 years. Earth Plunet. Sci. Lett. 2, l37- 147. CHANG C. T. and WKNKE H. (1969) Beryllium-IO in iron meteorites, their cosmic ray exposure and terrestrial ages. In Meteorite Reseurch (ed. P. M. Millman), 397-406. Reidel.

CLAYTON R. N. and MAYEDA T. K. (197X) Genetic relations between iron and stony meteorites. Earth Planet. Sci. Left. 40, 16X-174. CLAYTON R. N.. ONUMA N. and MAYEDA T. K. (1976) A classification of meteorites based on oxygen isotopes. Eurth Planet. Sci. Lett. 30, 10-18. DROZD R. J. and PODCISEKF. A. (1976) Primordial ‘*‘Xe in meteorites. Earrh Planet. Sci. Left. 31, 15-30. DROZD R. J. and POWSEK F. A. (1977) Systematics of iodine-xenon dating. Geochem. J. 11, 231-237. EVENSENN. M., HAMILTON P. J., HARLOW G. E., KLIMENTIDIS R., O’NIONS R. K. and PRINZ M. (1979) Silicate inclusions in Weekeroo Station: planetary differentiates in an iron meteorite (abstract). In Lunar and Planetary Science X, pp. 37637X. The Lunar and Planetary Institute, Houston. JESSBERGERE. K., HUNEKE J. C.. POWSEK F. A. and WASSERBURG G. J. (1974) High resolution argon analysis of neutron-irradiated Apollo I6 rocks and separated minerals. Proc. Lunar Sci. Con$ 5th. 1419-1449. LEWIS R. S. (1975) Rare gases in separated whitlockite from the St. Severin chondrite: xenon and krypton from fission of extinct 244Pu. Geochim. Cosmochim. Actu 39, 417-m432.

NIEMEYER S. (1979a) I~-Xe dating of silicate and troilite from IAB iron meteorites. Geochim. Cosmochim. Acta 43. 843m~X60. N~EMEYERS. (1979b) “‘Ar m3”Ar dating of inclusions from IAB iron meteorites. Grochim. Cosmochim. Actu 43, in press. OLSEN E. and JAROSEWlCtl E. (IY70) The chemical composition of the silicate inclusions in the Weekeroo Station iron meteorite. Earth Plunet. Sci. Lett. 8, 261-266. OLSEN E. and JAROSEWICH E. (1971) Chondrules: first occurrence in an iron meteorite. Science 174, 5X3--585. P~D~SEK F. A. (1970) Dating of meteorites by the hightemperature release of iodine-correlated XelZ9. Geochim. Cosnwchim.

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POWSEK F. A. (1972) Gas retention chronology of Petersburg and other meteorites. Geochim. Cosmochim. Acta 36, 755~. 772.

PODOSEK F. A. and LEWIS R. S. (1072) lz91 and 244Pu abundances in white inclusions of the Allende meteorite. Eurrh Planet. Sci. Let?. 15, I01 109. SANZ H. G., BURNETT D. S. and WASSERBURC~G. J. (1970) age and initial s’Sr/*%r for the A precise “Rbis’Sr Colomera iron meteorite. Geo&im. Cosmochim. .4ctu 34. 1127 1239. SCOTT E. R. D. and WASS~N .I. T. (1975) Classification and properties of iron meteorites. Rec. Geophys. Space Phq’s. 13, 527.~546. SCOTT E. R. D. and WASSON J. 7‘. (lY76) Chemical classification of iron meteorites VIII. Groups IC, IIE. IIIF and 97 other irons. Geochim. C’osmochim. Actu 40, IO3 115.

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