LETTER. Age of the eucrite "Caldera" from convergence of long-lived and short-lived chronometers

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1 Pergamon PPI S (96) Geochimica et Cosmochimica Acta, Vol. 60, No. 23, pp , 1996 Copyright 1996 Elsevier Science Ltd Printed in the USA. All rights reserved /96 $ LETTER Age of the eucrite "Caldera" from convergence of long-lived and short-lived chronometers MEENAKSHI WADHWA* and GONTER W. LUGMAIR Scripps Institution of Oceanography, University of Califomia, San Diego, La Jolla, CA 92093, USA (Received June 4, 1996; accepted in revised form September 17, 1996) Abstract--The long-lived 1475m-143Nd chronometer has been used in conjunction with the short-lived 146Sm-142Nd and 53Mn-53Cr chronometers to constrain the time of formation for the eucrite Caldera. The combination of these three chronometers has produced results that are consistent with one another and converge on a precise, high resolution age of Ma (within 95% confidence limits). So far this is the youngest crystallization age that has been confirmed for a noncumulate eucrite. Thus, this basaltic meteorite formed relatively late as compared to other noncumulate eucrites on a differentiated asteroidal parent body, most likely between 17 and 41 million years after the formation of the first condensates in the early Solar System. From these age constraints it is evident that Caldera either formed within a magma body on a large (>500 km) asteroid or in an extensive impact melt on the crust of its parent body. 1. INTRODUCTION High precision, high resolution formation ages of igneous meteorites are of considerable interest since they provide insight into timescales involved in the accretion and subsequent differentiation of meteorite parent bodies in the early Solar System. High resolution ages of eucritic meteorites are particularly interesting since these can place important constraints on the chronology and evolution of the basaltic crust on their asteroidal parent body. It has been recognized that relative chronometry from the distribution of extinct radionuclides (Podosek and Swindle, 1988, and references therein), such as 26A1, 1465m, 53Mn, and 6 Fe, could potentially be used in combination with absolute chronometers, such as 147Sm-143Nd, 87Rb-S7Sr, and 2 7pb-z 6pb, to obtain precise absolute age dates that could allow a resolution of a few million years for early Solar System events. In practice, however, this has been impeded largely due to the analytical difficulties of accurately measuring the decay products of many of these short-lived radionuclides and, therefore, the lack of an extensive database of correlated precise absolute and relative ages for various primitive meteorite types. Moreover, an important prerequisite for the use of extinct radionuclides as reliable relative chronometers is that they be homogeneously distributed at least in the region of the Solar System where these meteorites formed. Only recently tighter constraints regarding the distributions of at least some of these nuclides have begun to emerge (Lugmair and Galer, 1992; Lugmair and Maclsaac, 1995; MacPherson et al., 1995; Lugmair et al., 1996). * Present address: Department of Geology, The Field Museum, Roosevelt Road at Lake Shore Drive, Chicago, IL 60605, USA. We report here the results of a study of 1475m-143Nd, 1465m-142Nd (146Sm-decays to laznd with a half-life of -103 Ma), and 53Mn-53Cr (S3Mn decays by electron capture to S3Cr with a half-life of ~3.7 Ma) systematics in the eucrite Caldera, that has yielded a precise absolute age for this meteorite. Caldera, a find from Chile, is an unusual basaltic meteorite in that it is one of only two known non-antarctic unbrecciated noncumulate eucrites, the other being Ibitira. Previous Sm-Nd work on Ibitira (Prinzhofer et al., 1992) has shown that although the 147Sm-143Nd systematics yielded a precise age of Ga, the 146Snl]ln4sm ratio was determined to be This was taken to indicate that Ibitira was indeed an ancient object that formed close to ~4.56 Ga ago, but that its Sm-Nd systematics had been subsequently disturbed by partial isotopic equilibration. Similarly, the formation ages of other noncumulate (but brecciated) eucrites, such as Juvinas and Chervony Kut, have also been estimated to lie close to ~4.56 Ga (Lugmair et al., 1975, 1994a,b; Wadhwa and Lugmair, 1995). Texturally, Caldera differs from Ibitira, and from other (brecciated) noncumulate eucrites, in having a relatively coarse grain size (typically a few millimeters) and a hypidiomorphic granular texture, indicative of a slower cooling rate (Boctor et al., 1994; N. Z. Boctor, pers. commun.). 2. EXPERIMENTAL A 1.5 g bulk sample of Caldera was gently crushed with a boron carbide mortar and then sieved to obtain various grain-size fractions. There is extensive yellow-brown staining on almost all grain surfaces in Caldera. Because of the unknown terrestrial residence time of this meteorite, and the possible terrestrial origin of this staining, it was clear that it had to be removed before reliable isotopic measurements on mineral separates could be attempted. To remove the staining the 4889

2 4890 M. Wadhwa and G. W. Lugmair Table 1, Sm-Nd isotopic results for Caldera. Nd isotopic data are normalized to 148Nd/144Nd = (equivalent to H6Nd/I44Nd = ). Uncertainties (20) on ratios, given in parentheses, are for the least significant figures. Sample Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd 144Sm/144Nd 142" CAL TRI (4) (14).04034(8) (0.37) CALTR (4) (14) (9) (0.37) CAL PI (18) (14) (4) (0.37) CALPx (6) (14) (11) (0.36) * 142 = {[(142Nd/144Nd)measured/(142Nd/144Nd)staadard]- I } x 104. The standard 142Nd/144Nd value of used here is the mean of tenestrial standards given in Lugmair and Galer (1992) /zm fraction was washed in an ultrasonic bath for up to 2 h in 1.8 N HC1. Thereafter, plagioclase (P1) and pyroxene (Px) mineral separates were prepared using a Frantz Isodynamic magnetic separator followed by handpicking. In order to optimize the purity of the mineral fractions, the P1 and Px separates were again cleaned in 1.8 N HC1 for 15 rain and a second round of handpicking followed. Two bulk samples (TR1 and TR2), weighing 161 and 127 mg, respectively, were used for Sm-Nd analyses. TR1 was an unetched chip while TR2 was crushed and then ultrasonically cleaned for a half hour in 1.8 N HC1 before dissolution. Mineral separates (P1 and Px), mostly clean of the surface coating, and TR1 and aliquots of the acid-cleaned bulk sample TR2 were dissolved in HF/HNO3. For the Mn-Cr analyses the rest of TR2 (TR in Table 2) was dissolved in HF/HNO3 to obtain the silicate "Sil" (solution) and "Sp" (residue) fractions. The silicate portion, Sil, was decanted, and the chromite residue, Sp, was then washed in H20 and dissolved in a Teflon bomb in HF/HNO3. All samples were eventually brought into solution in HC1 for aliquoting. The parent solutions for mineral separates and bulk samples were allquoted into three portions: ~90% for isotopic measurements; ~5% for samarium and neodymium concentrations; ~5% reserved as a "safety fraction." Samarium, neodymium, and chromium separations were made using cation exchange chromatography. Manganese and chromium concentrations were determined in aliquots of the dissolved samples of bulk rock TR, Px, silicate fraction (Sil), and chromite (Sp) with graphite furnace atomic absorption spectrometry. The accuracy of the measured concentrations was typically better than _+5%, and a 2a uncertainty of _+ 5% is estimated for the Mn/Cr ratios. Samarium and neodymium concentrations were obtained using the isotope dilution technique. Chromium and neodymium isotopic measurements were made on the La Jolla VG 54E single-collector mass spectrometer using the Faraday cup. The number of repeat measurements of the isotopic composition (each consisting of 30 blocks of 10 ratios) on each sample ranged from 2 to 6 for neodymium and 12 to 18 for chromium. Neodymium isotopic analyses were carded out as in Lugmair and Galer (1992). Details of chromium isotopic measurements will be described elsewhere. Finally, elemental X-ray mapping was performed on three thin sections of Caldera to identify any possible phases rich in rare earth elements (REEs). These analyses were made with the Cameca SX- 50 electron microprobe at the University of Chicago Sm-Nd Systematics 3. RESULTS AND DISCUSSION Results of Sm-Nd analyses are given in Table 1. Note that mass balance constraints from concentrations of samarium and neodymium require the presence of a trace phase rich in REEs. Previous petrographic studies (Boctor et al., 1994) had failed to identify any phosphates in Caldera. However, X-ray mapping of thin sections of Caldera showed that a calcium phosphate mineral (containing percent-level LREEs) was present in very low abundances (<0.01 modal %). There is a large spread in the Sm/Nd ratios between plagioctase and pyroxene in Caldera (Table 1 ). Good isochrons for both the ~47Sm-]43Nd and the 146Sm-~4ZNd systems were obtained (Figs. 1 and 2). Note that even the data from the unetched bulk sample, TR1, fall on the isochrons, which indicates that if the surficial staining on mineral grains is indeed of terrestrial origin, only insignificant amounts of REEs could have been introduced. The ~47Sm- ]4aNd system yields an age of 4544 _+ 19 Ma with an initial ~43Nd/]"Nd ratio of _+ 24. The short-lived 146Sm-~4ZNd system results in a ~46Sm/]~Sm ratio of _+ ll and an initial e~42 of -3.0 _ These Sm-Nd results, within errors, are indistinguishable from results previously obtained on other achondrites such as the angrites, Angra dos Reis, and LEW86010 (Lugmair and Marti, 1977; Lugmair and Galer, 1992; Nyquist et al., 1994), and the noncumulate eucrites, Juvinas, Ibitira, and Chervony Kut (Lugmair et al., 1975; Prinzhofer et al., 1992; Wadhwa and Lugmair, 1995). This indicates that both Sm-Nd systems in all these meteorites closed contemporaneously within the uncertainties afforded by the data, i.e., Ma for J47Sm-143Nd and >15 Ma "o ~: z ~ c CALDERA i / Age = Ga I,= 0. 74t II TRI~ / TR2.,, o.~,5o ~ o.2s--~- o.3so Sm/144Nd FIG Sm-143Nd evolution diagram for Caldera. The age is calculated using k = a -~. The deviations of the data points from the best-tit line in e-units (parts per 104 ) and the individual 2~7 errors are shown in the inset figure.

3 Age of the eucrite meteorite Caldera I,~ oi ~-1 CALDERA j 14eSm/1~Sm " " 11. 1(142) ~ I P x ~ ---_---_--_----,-~~----_--_- / PI i TR2 TR1 i i i i i ~,~Sm/-~Nd FIG. 2. '46Sm- 142Nd evolution diagram for Caldera, plotted as ~ 142 vs. 144Sm/~44Nd. The dotted lines show the range of uncertainty (2a) in the standard value of -0.1 e-units. for t46sm-i42nd (although in the case of Ibitira, the Sm-Nd systematics were subsequently disturbed by partial isotopic equilibration; Prinzhofer et al., 1992) Mn-Cr Systematics The considerably finer resolution of the 53Mn- 53Cr system, compared to both Sm-Nd systems, paints an interestingly different picture which is, however, consistent with the Sm- Nd systematics. Mn-Cr systematics are summarized in Table 2 and illustrated in Fig. 3. Minerals in Caldera have a wider range of 55Mn/52Cr ratios, from ~0 (chromite) to ~8 (pyroxene), as compared to other noncumulate eucrites (Lugmair et al., 1994a, b). Inspite of this wide range in 55Mn/ 52Cr ratios, the 53Cr/52Cr ratios in all the samples measured are the same within uncertainties. Although typical errors are 8-12 ppm, the total range in 53Cr/S2Cr ratios is only about 4 ppm, i.e., e-units, with an average of 1.12 e. It should be noted that since Caldera has a short cosmic ray exposure age of ~ 14 Ma (Shukolyukov and Begemarm, 1996), spallogenic isotope effects in chromium are negligible. Since the S3Cr/52Cr ratios in all samples are the same, an upper limit for the 53Mn/55Mn ratio of 1.2 x 10-7 can be calculated from the uncertainties. This means that the short-lived radionuclide 53Mn, which was present in the incipient solar nebula, was no longer extant by the time the Mn-Cr system closed in Caldera. Table 2. Mn-Cr isotopic results for Caldera. Cr isotopic data are aocmalized to 50Cr#2Cr = m~rt in eomt, matiem are t,/i~:aly ~ ~lt. Staple Me (ppm) Cr (ppm) SSlVlaP~ ex531" (+ 5~t) CALTR ,09 CALSp x CAL Sil "0.11 CAl. Px i.15 4" 0.12 "t(53) = {[(53Cr#2Cr)measmed/(J3Cr/J2Cr)standardl-l} 53Cr/57-Cr is taken as x I04. The stlmdard vahm of The errors on f.(53) given here are not 2anmm, which would be smaller, but are bascd on estimates from the i~roducibility of the repe.~ eneasurements of the sampte and standards. indicates that formation of Caldera must have occurred ~ 19 Ma after Chervony Kut. Although a precise crystallization age for Chervony Kut is not known, it is obvious that the upper limit on the age of this differentiated meteorite cannot exceed that of the first condensates in the early Solar System, which is anchored at Ma (G6pel et al., Therefore, within 95% confidence limits, the upper limit on the age of Caldera cannot exceed 4549 Ma. It should be noted that if the upper limit of the ~3Mn/~Mn ratio in Caldera is compared with a 53Mn/55Mn ratio of (4.4 _ 1.0) x 10-2, as determined for calcium aluminum inclusions (CAIs) by Birck and All~gre (1985), an age difference of at least 30 Ma is obtained for these objects, thereby leading to an upper limit of 4536 Ma for the age of Caldera. However, there appear to be clear discrepancies in assuming that (1) the initial value of the 53Mn/55Mn ratio for the Solar System was (4.4 _+ 1.1) 10-5, as represented by the average value of this ratio in CAIs and/or (2) the 53Mn/ 55Mn ratio was homogeneous in the early Solar System. This can be seen by the fact that comparison of the 53Mn/55Mn ratio in CAIs (Birck and All~gre, 1985) with that determined in the LEW86010 angrite (Lugmair et al., 1992) indicates that CAIs are older than the angrites by -20 Ma, whereas Pb-Pb systematics (G6pel et al, 1991; Lugmair and Galer, _, t.~, ~ ~, [~Mn/UMn <1.2E-7 [ / ~F i H^vo(.co.l.12, [ ~o." 3.3. Age Constraints for Caldera From the long-lived ~47Sm-t43Nd chronometer, the age of Caidera is established as Ma, allowing an age range of Ma for its formation. However, further constraints for this age interval can be obtained from the short-lived 53Mn-53Cr and 146Sin-142Nd chronometers. It was shown for the eucrite Chervony Kut that S3Mn was still present at the time of its solidification and that the 53Mn/55Mn ratio at that time was ( ) x 10-6 (Lugmair et al., 1994a and unpubl, data). If both eucrites originate from the same parent body with a common 53Mn/55Mn reservoir (see below) the upper limit of the 53Mn/55Mn ratio in Caldera 6) 1.00 ~" R 0,501 0'00 1,,.... -r Earth & Moon SSMnl,~Cr 10.0 FIG Mn-53Cr evolution diagram for Caldera, plotted as (c 53) vs. 5SMn/52Cr. For reference, the solid horizontal line through (0,0) represents the terrestrial standard; dashed fines are 53Mn-53Cr isocitrons for the eucrites Chervony Kut (CK) (Lugmalr et al., 1994a, and unpubl, data) and Juvinas (JUV) (Lugmalr et al., 1994h).

4 4892 M. Wadhwa and G. W. Lugmair 1992) indicate an age difference of only ~8 Ma. This raises the possibility that the inferred 53Mn/55Mn ratio in CAIs could be "artificially" elevated as a result of the presence of either nucleosynthetic anomalies (Papanastassiou, 1986) or 53Cr excesses from the decay of 53Mn in the interstellar medium. Additionally, it has recently been shown (Lugmair and Maclsaac, 1995; Lugmair et al., 1996) that the initial 53Mn/55Mn ratio in the parent bodies of different meteorite groups may have been different. Therefore, we are certain that the direct comparison of 53Mn/55Mn ratios between meteorites may give meaningful age differences only when the meteorites are believed to have either originated on the same parent body or if it can be shown by Mn-Cr systematics that meteorite families are closely related to one another (see Lugmair et al., 1996). In the case of the noncumulate eucrites Caldera, Chervony Kut, and also Juvinas, it is evident from Fig. 2 and from Lugmair et al. (1996) that their Mn- Cr isochrons all intercept, within error, very close to their respective bulk rock Mn/Cr ratios of ~ 1.7 to 2 at a common 53Cr excess of ~1.1 e-units. This suggests that all three meteorites originated from a reservoir with the same initial 53Mn/55Mn ratio (which is much higher than measured so far for any other meteorite type). Thus, Mn-Cr relative chronometry appears feasible between these objects. A lower bound on the age of Caldera is estimated from Sm-Nd systematics. Lugmair and Galer (1992) evaluated all existing ~46Sm/144Sm data for consistency and interlaboratory bias. Their best estimate for the ~46Sm/l~Sm ratio at 4558 Ma is _+ 10. Subsequent determinations of the 1465n~ ~44Sm ratio for angrites as well as other meteorites that formed close to ~4.56 Ga (Prinzhofer et al., 1992; Nyquist et al., 1994; Stewart et al., 1994; Sharma et al., 1995) confirm that the above value for this ratio at 4558 Ma is indeed reasonable. The 1465m/~44Sm ratio obtained by us for Caldera is _+ 11, which is in agreement, within error, with the above value. To determine a lower limit for the age of Caldera, we compare the maximum value for the best estimate of the J46Srn]l~Sm ratio at 4558 Ma (i.e., ) with the minimum allowable value for Caldera (i.e., ). This indicates that the maximum time difference between the age of the angrite LEW86010 (4558 Ma) and that of Caldera cannot be more than ~38 Ma, suggesting that the minimum age of Caldera cannot be less than Ma. (Note: If we were to use the upper limit of the measured ~46Sm/144Sm value for LEW86010 of , as determined by Lugmair and Galer (1992), then the minimum age of Caldera would be 4506 Ma.) However, this age is younger than the minimum age (from 2a uncertainties) obtained from the J47Sm-14aNd chronometer. Therefore, the minimum age allowed by this long-lived chronometer (i.e., 4525 Ma) is the strict lower bound on the formation age of Caldera. Finally, it should be noted that if the nominal best estimate value of is taken for the ~46Sm/1445m ratio in the Solar System at 4558 Ma before present (Lugmair and Galer, 1992), the lower limit on the age of Caldera (for a minimum ~46Sm/ ~44Sm ratio of , within 2a errors) becomes 4540 Ma. From the above, it is clear that combining the constraints from the ~47Sm-143Nd and the Mn-Cr systematics restricts the age range for the formation of Caldera between 4525 and 4549 Ma. Additionally, if one uses the nominal best estimate value of for the ~46Sm/144Sm ratio in the Solar System at 4558 Ma (Lugmair and Galer, 1992), the age range for Caldera becomes Ma. Conservatively, however, the maximum allowable age range for Caldera, within 95% confidence limits, is the former, which can be translated to an absolute age of formation of 4537 _+ 12 Ma. It should be noted that the Pb-Pb age for Caldera was recently reported as 4516 _+ 3 Ma (Galer and Lugmair, 1996). This is lower than our estimated age range for Caldera and, as noted by these authors, could be a result of the protracted cooling history of this meteorite, such that the times and temperatures of isotopic closure for the Sm-Nd and Mn-Cr systems were different from those of the Pb-Pb system Implications for Basalt Formation on the Eucrite Parent Body Compared to the age of formation of the refractory CAIs in chondrites (GOpel et al., 1991), it is evident that the eucrite Caldera formed between 17 and 41 million years after the condensation of the first solid objects in the early Solar System. In contrast, Sm-Nd systematics in another unbrecciated noncumulate eucrite, Ibitira, are consistent with an old age close to ~4.56 Ga (Prinzhofer et al., 1992). Two of the brecciated noncumulate eucrites, Chervony Kut and Juvinas, show clear evidence of the presence of live 53Mn and are believed to have formed within a few (i.e., <10) million years of the formation of CAIs (Lugmair et al., 1994a,b). Therefore, the formation age of 4537 _+ 12 Ma for Caldera is thus far the youngest age that has been precisely established for any noncumulate eucrite. At that time, the short-lived radionuclide, 26A1, very likely a major heat source for differentiation in the early Solar System, was extinct (Lee et al., 1977; MacPherson et al., 1995, and references therein). Thus, the source magma for Caldera was either generated at depth in a relatively large asteroidal body ( >500 km in diameter, which could perhaps retain heat for millions of years after the complete decay of a6ai), or was formed in an impact melt on the surface of the eucrite parent body (EPB), when the chromium isotopes were completely reequilibrated. As stated above, 53Mn-53Cr data for some noncumulate eucrites indicate that these objects have crystallized, most likely as basaltic flows on the EPB, < 10 million years after the formation of CAIs. If Caldera indeed crystallized in a magma body, it implies that endogenous basalt formation on the EPB may have lasted millions of years longer than has previously been assumed. However, certain petrographic characteristics of Caldera, such as the presence of minute "relict" pyroxene blebs within plagioclase grains, appear consistent with its formation in an impact melt (N. Z. Boctor, pers. commun.). In this case, the coarse-grained crystalline texture of Caldera would require that the impact produced basaltic melt pool be extensive enough to allow slow cooling. Therefore, an impact origin for Caldera would imply that the crust of the EPB was extensively remelted by energetic impacts that produced relatively deep pools of basaltic melt

5 Age of the eucrite meteorite Caldera 4893 on its surface. More detailed petrographic and geochemical studies on this rock are needed to evaluate this possibility further. Finally, the combination of long-lived and short-lived chronometers, such as in the case of Caldera, will yield high resolution absolute age dates for many other meteorite types in the near future as increasing amounts of self-consistent datasets for long-lived and short-lived radionuclides become available. This will eventually contribute towards a much higher resolution picture of the sequence of condensation, accretion and differentiation events in the early Solar System. Acknowledgments--We would like to thank Chris Maclsaac for his invaluable assistance in the laboratory and Deniz Oezen for diligently handpicking the plagioclase and pyroxene mineral separates used in this study. We are grateful to Ian Steele for helping with the x-ray mapping of sections of Caldera. We thank Dimitri Papanastassiou, Doug Macdougall, and an anonymous reviewer for very helpful comments. Support for this work was provided by NASA grant NAGW Editorial handling: J. D. Macdougall REFERENCES Birck J.-L. and All~gre C. J. (1985) Evidence for the presence of 53Mn in the early Solar System. Geophys. Res. Lett. 12, Boctor N. Z., Palme H., Spettel B., E1 Goresy A., and MacPherson G. J. (1994) Caldera: A second unbrecciated noncumulate eucrite. Meteoritics 29, 445. Galer S. J. G. and Lugmair G. W. (1996) Lead isotope systematics of non-cumulate eucrites. Meteoritics and Planetary Sciences 31 (Suppl.), A47-A48. G6pel C., Manh~s G., and All~gre C. J. (1991) Constraints on the time of accretion and thermal evolution of chondrite parent bodies by precise U-Pb dating of phosphates. Meteoritics 26, 73. Lee T., Papanastassiou D. A., and Wasserburg G. J. (1977) 26A1 in the early solar system: fossil or fuel? Astrophys. J. 211, L107- Lll0. Lugmair G. W. and Galer S. J. (1992) Age and isotopic relationships among the angfites Lewis Cliff and Angra dos Reis. Geochim. Cosmochim. Acta 56, Lugmalr G.W. and MacIsaac C. (1995) Radial heterogeneity of 53Mn in the early Solar System? Lunar Planet. Sci. XXVI, Lugmair G. W. and Marti K. (1977) Sm-Nd-Pu timepieces in the Angra dos Reis meteorite. Earth Planet. Sci. Lett. 35, Lugmair G.W., Scheinin N.B., and Marti K. (1975) Search for extinct 146Sm, I. The isotopic abundance of 142Nd in the Juvinas meteorite. Earth Planet. Sci. Lett. 27, Lugmair G.W., Maclsaac C., and Shukolyukov A. (1992) The 53Mn-SaCr isotope system and early planetary evolution. Lunar Planet. Sc.i XXIII, Lugmair G. W., Maclsaac C., and Shukolyukov A. (1994a) Small time differences in differentiated meteorites recorded by the 53Mn- 53Cr chronometer. Lunar Planet. Sci. XXV, Lugmair G. W., Maclsaac C., and Shukolyukov A. (1994b) Small time differences recorded in differentiated meteorites. Meteoritics 29, Lugmair G. W., Shukolyukov A., and Maclsaac C. (1996) Radial heterogeneity of ~3Mn in the early Solar System and the place of origin of ordinary chondrites. Lunar Planet. Sci. XXVII, MacPherson G. J., Davis A. M., and Zinner E. K. (1995) The distribution of aluminum-26 in the early Solar System--A reappraisal. Meteoritics 30, Nyquist L. E., Bansal B., Wiesmann H., and Shih C.-Y. (1994) Neodymium, strontium and chromium isotopic studies of the LEW86010 and Angra dos Reis meteorites and the chronology of the angrite parent body. Meteoritics 29, Papanastassiou D. A. (1986) Cr isotopic anomalies in the Allende meteorite. Astrophys. J. 308, L27-L30. Prinzhofer A., Papanastassiou D. A., and Wasserburg G. J. (1992) Samarium-neodymium evolution of meteorites. Geochim. Cosmochim. Acta 56, Podosek F. A. and Swindle T. D. (1988) Extinct radionuclides. In Meteorites and the Early Solar System (ed. J.F. Kerridge and M. S. Matthews), pp Univ. Arizona Press. Sharma M., Papanastassiou D. A., and Wasserburg G. J. (1995) Sm- Nd systematics of a large eucrite clast in the Vaca Muerta mesosiderite and initial solar system 146Sm abundance. Lunar Planet. Sci. XXVI, Stewart B. W., Papanastassiou D. A., and Wasserburg G. J. (1994) Sm-Nd chronology and petrogenesis of mesosiderites. Geochim. Cosmochim. Acta 58, Shukolyukov A. and Begemann F. (t996) Cosmogenic and fissiogenic noble gases and 81Kr-Kr exposure age clusters of eucrites. Meteoritics and Planetary Science 31, Wadhwa M. and Lugmair G. W. (1995) Sm-Nd systematics of the eucrite Chervony Kut. Lunar Planet. Sci.XXVI,