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NUMBER 19 93 the original presence of superheavy elements in the Allende meteorite, is that the Tm excess may be a previously unrecognized fission product. Marvin, Wood, and Dickey (1970), who first described the Ca/Al-rich inclusions in the Allende meteorite, noted the general similarity between their chemistry and mineralogy and the sequence of compounds calculated to be among the highest temperature condensates to form in a solar nebula. Grossman (1972, 1973) has greatly extended this work; he concluded (1973:1137) that the Ca/Alrich inclusions in the Allende meteorite are enriched in Sc, Ir, and the lanthanides by a factor of 22.8 ± 2.2 relative to Type I carbonaceous chondrites, and that the large enrichments of these refractory trace elements provided strong evidence that the inclusions are high-temperature condensates from the solar nebula. All but two of the samples he analyzed belong to Group I, and his conclusion appears well-founded for this group. Can this conclusion be extended to Groups II and III, or do they have a different origin? The trace element distribution in the Group III samples is not greatly different from Group I, except for the negative Eu and Yb anomalies in the lanthanide distribution, and these two groups may well be genetically related. However, the strongly fractionated lanthanide distribution in Group II samples, the Eu, Tm, and Yb anomalies, and their relative depletion in other refractory elements such as Zr, Mo, and the platinum metals, strongly suggest extensive chemical differentiation of their parent material. One aspect still insufficiently explored is the possible correlations between mineralogy, texture, and chemistry of the Allende inclusions. Group I so far comprises only melilite-rich chondrules; Group II comprises four aggregates and one chondrule; the two representatives of Group III are both aggregates; Group IV comprises the common olivinerich chondrules, but also includes some aggregates. The term aggregate covers a variety of fine-grained white, pink, or pale gray inclusions with irregular form; if an aggregate approaches spherical form, the distinction between aggregate and chondrule may be arbitrary. The form of the aggregates and their contacts with the enclosing matrix vary greatly; some are angular fragments with sharp contacts, many are irregular in form, with intricate mossy-like margins against the matrix, others are elongated shards or stringers. Subparallel elongation of the latter type in a hand specimen often indicates a vaguely oriented fabric, suggesting an original flattening during deposition (as plastic glass fragments?) or a secondary flattening produced by later deformation. The overall impression is that some aggregates may be devitrified glass fragments, whereas others represent agglutinated dust particles. The formation of chondrules, specifically the melilite-rich chondrules of Group I, presents an additional problem. Grossman's condensation curves (1972) are for gas-solid equilibria, and if the Group I material originated in this way, it should have separated as a dust; the formation of chondrules involved segregation of this dust and subsequent melting, by such processes as lightning or shock effects. Alternatively, these chondrules may have originated in some nonequilibrium process, such as that suggested by Blander and Katz (1967): metastable nucleation of liquid droplets, formed by supersaturation of the nebular gas, component by component, as subcooling took place below the temperatures where equilibrium solids should have crystallized out. TABLE 3.—Abundances (ppm) of Eu and Yb in Group H and Group m samples, and the Eu/Yb ratios Elements Eu . . . . •&,.... Eu/Yb • . 37 0.31 0.51 0.62 3598 0.18 0.84 0.21 Group II 3803 0.17 0.73 0.23 4691 0.62 1.6 0.39 4692 0.13 0.37 0.35 Group 3593 0.36 1.9 0.17 III 4698 0.25 0.74 0.34
94 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES The mineralogy and chemistry of the Group I materials certainly support the concept that they represent an early high-temperature condensate. The condensation sequence established by Grossman shows that this would be followed by forsteritic olivine and enstatite, essentially the material of the Group IV chondrules and aggregates. The position of the Group II and Group III materials in this sequence is more enigmatic. In minor and traceelement concentrations, Group III materials are not very different from Group I, the most marked feature being the negative Eu and Yb anomalies. In view of the small number of Group III specimens, further speculation on their origin may be deferred. Group II materials are in marked contrast to Groups I and III in minor and trace elements, with relatively high alkalies, low refractories (Zr, Hf, Mo, and platinum metals), and especially the strongly fractionated lanthanide pattern. Evidently the medium from which this material condensed had undergone or was undergoing considerable fractionation of these elements. So far a complementary condensate, enriched in the heavier lanthanides, has not been identified; however, since Group II material forms only a small amount of the total meteorite (probably about 5% or less), the complementary enriched material may be too dispersed to be recognizable. Possibly Group II materials represent a late condensate, subsequent to the segregation of Group I and Group IV. The Group IV material is almost entirely magnesiumiron silicates; separation of this material from the parent medium would enrich the latter in other major elements, among which would be Ca, Al, and the alkalies. Since olivine is a poor host for most trace elements, these too would remain in the parent medium and increase in concentration. Pyroxene is a mineral which preferentially concentrates the heavier lanthanides, and its separation along with olivine would lead to a depletion of these elements in the residual material. In this connection the composition of interstitial glass in the olivine-pyroxene chondrules reported by Clarke, et al. (1971) is revealing; this glass composition is (weight percent) SiO2, 47.8; A12O3, 24.7; FeO, 0.7; MgO, 6.3; CaO, 17.5; Na2O; 2.0; K2O, 0.2, a composition similar in many respects to the bulk composition of Group II aggregates. This discussion thus suggests at least two periods of segregation for the Ca/Al-rich inclusions, one before the separation of olivine and one following it. Much additional work needs to be done to verify this, and to fill out the details of the process. Clearly, however, the Allende meteorite preserves a remarkable record of the origin of chondrules and chondrites, but one which requires extensive and intensive research to decipher. Literature Cited Ahrens, L. H., and H. von Michaelis 1969. The Composition of Stony Meteorites, III: Some Inter-element Relationships. Earth and Planetary Science Letters, 5:395-400. Blander, M., and J. L. Katz 1967. Condensation of Primordial Dust. Geochimica et Cosmochimica Ada, 31:1025-1034. Boynton, W. V. 1975. Fractionation of the Solar Nebula: Condensation of Yttrium and the Rare Earth Elements. Geochimica et Cosmochimica Ada, 39:569-584. Clarke, R. S., Jr., E. Jarosewich, B. Mason, J. Nelen, M. Gomez, and J. R. Hyde 1971. The Allende, Mexico, Meteorite Shower. <strong>Smithsonian</strong> Contributions to the Earth Sciences, 5:1-53. Conrad, R. L., R. A. Schmitt, and W. V. Boynton 1975. Rare-Earth and Other Elemental Abundances in Allende Inclusions. Meteoritics, 10:384-387. Gast, P. W., N. J. Hubbard, and H. Wiesmann 1970. Chemical Composition and Petrogenesis of Basalts from Tranquillity Base. Proceedings of the Apollo 11 Lunar Science Conference, 2:1143-1163. Graham, A. L., and B. Mason 1972. Niobium in Meteorites. Geochimica et Cosmochimica Ada, 36:917-922. Gray, C. M., D. A. Papanastassiou, and C. J. Wasserburg 1973. Primitive "Sr/^Sr in the Allende Carbonaceous Chondrite. Icarus, 20:213-239. Grossman, L. 1972. Condensation in the Primitive Solar Nebula. Geochimica et Cosmochimica Ada, 36:597-619. 1973. Refractory Trace Elements in Ca-Al-rich Inclusions in the Allende Meteorite. Geochimica et Cosmochimica Acta, 37:1119-1140. Marvin, U. B., J. A. Wood, and J. S. Dickey, Jr. 1970. Ca-Al Rich Phases in the Allende Meteorite. Earth and Planetary Science Letters, 7:346-350. Mason, B. In press. Meteorites. Chapter B-l in The Data of Geochemistry, sixth edition. U.S. Geological Survey Professional Paper, 440-B, Part 1. Mason, B., and P. M. Martin 1974. Minor and Trace Element Distribution in Mellilite and Pyroxene from the Allende Meteorite. Earth and Planetary Science Letters, 22:141-144. Osborn, T. W., R. G. Warren, R. H. Smith, H. Wakita, D. L. Zellmer, and R. A. Schmitt 1974. Elemental Composition of Individual Chondrules
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SMITHSONIAN CONTRIBUTIONS TO THE EA
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Contents MINERALOGY COMPOSITION OF
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Constituent SiO TiO Al 0 fl 2 3 Cr
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PER CENT CBLCIUM FIGURE 3.—The ca
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Constituent TiO,, 2 2 3 FeO* MnO Mg
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20 Cfl 40 .60 SMITHSONIAN CONTRIBUT
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35'50' I L 6.. 3.. 2.. r FOKT DEFIA
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12 SMITHSONIAN CONTRIBUTIONS TO THE
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14 20 SMITHSONIAN CONTRIBUTIONS TO
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16 Constituent Si0 2 TiO2 A12O3 Cr
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18 20 40 £0 SMITHSONIAN CONTRIBUTI
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20 40 20 SMITHSONIAN CONTRIBUTIONS
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Abyssal Basaltic Glasses as Indicat
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24 Electron-M icroprobe Analysis An
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30 Hill in the NE Atlantic, and fro
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32 cores the arrangement is stratig
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36 ATLANTIC OCEAN LAT. LONG. DEPTH
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