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NUMBER 19 123 that El Taco silicate inclusions were derived by transformation of chondrite material, cannot be answered with certainty. Furthermore, there are many ways in which a chondrite could be transformed to give the El Taco silicates. Three features have to be explained: the lower FeO content of the mafic minerals, the apparent differences in bulk composition, and the disequilibrium between different inclusions. One simple model that would explain most of the El Taco silicate inclusion features would be the incorporation of chondritic matter into a hot iron metal, at or near to the nickel-iron melting point of about 1500° C. Heating of a chondrite with Htype olivine and pyroxene in an atmosphere of CO and CO2 in equilibrium with one another will lead to a reduction of iron silicates above temperatures of about 1200° C. It has been shown by Mueller (1963) and Speidel and Nafziger (1968) that an olivine-silica-metal assemblage will change towards lower fayalite contents in the olivine when heated from 1100° to 1300° C. A reduction of the silicates to about 6 mole percent FeO, which is found in most of the silicates of iron meteorites with silicate inclusions (Bunch, Neil, and Olsen, 1970), may then reflect the equilibrium at the temperature of molten nickel-iron. At this temperature albite and diopside will melt also, but not olivine and enstatite, a condition which could explain the depletion of the former minerals discussed above. The apparent disequilibrium between different inclusions was probably not established at these high temperatures, but later during cooling, and may be the last change produced by a continuing reduction of iron oxide from the silicates. The lower FeO mole percent of olivine may then simply reflect the fact that olivine is easier to reduce than pyroxene. That this is the case was shown in the ureilite Havero, where olivine next to carbon-rich veins is reduced to lower FeO contents, whereas pyroxene is not (Wlotzka, 1972). The question remains, why did the hightemperature reduced assemblage not change back to its original, more oxidized form upon slow cooling? This difficulty can be overcome by changing from a closed to an open system, which would be the result in a change from rapid heating to slow cooling. A rapid heating process, as by an introduction of silicates into a molten metal pool or invasion of a chondritic rock by a metal melt, would essentially result in a closed system with respect to C/CO/CO2 because the time for escape of the gases was too short. Thus the equilibrium between CO and CO2 would establish the low Fa and Fs content of the mafic minerals at this high temperature. Slow cooling, which took place during the y-a transformation between 850° and 600° C over some 100 million years (Goldstein and Short, 1967), will result in an open system, where oxygen removed by the reduction of Fe-silicates by carbon may escape and provide for the continuing reduction. The differences from inclusion to inclusion can then be explained by local differences in carbon available per inclusion or weight-unit of silicates. These differences are mainly due to a different FeO content of the olivines, since the pyroxene compositions are more uniform. As olivine is more easily reduced than pyroxene, its composition will follow these changes better than that of pyroxene. Thus the low FeO content of olivines from the small, veinlike inclusion 6 may be caused by the higher amounts of carbon available from the metal in this area. The proportionality of Fe and Ca in orthopyroxene (Figure 10) is not in contradiction to a loss of FeO by reduction. Both FeO and CaO will be leaving the pyroxene grains by diffusion—FeO because it is reduced to Fe metal, and CaO because at lower temperatures the solubility of CaSiO3 in the enstatite is lowered (Davis and Boyd, 1966). Another possibility is a genetic relationship of the material of the El Taco inclusions to chondrites like Kakangari (Graham and Hutchison, 1974), which already contain olivines with a low fayalite and pyroxene with a low ferrosilite content, so that a reduction is not necessary. Only the late reduction leading to the disequilibrium between olivine and pyroxene and between different inclusions has to be operative here too. Summary and Conclusions It has been shown that (1) the silicate minerals of the El Taco inclusions have a composition close to the minerals of chondrites with the exception of a lower iron content of olivine and pyroxene; (2) the iron content of olivine and pyroxene shows small but distinct variations from inclusion to inclusion; and (3) the bulk chemical composition is
124 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES close to chondrites with deviations mainly in the alkalies, Ca, and Al. We suggest an explanation of these features by an in-situ reduction of chondritic material at the temperatures of molten nickel-iron together with partial melting and redistribution of the more fusible minerals, feldspar and diopside. The varying Fe/Mg ratios of olivine and pyroxene in different inclusions are explained as a minor disequilibrium due to continuing reduction at lower temperatures. The following observations appear to support this explanation: Partial melting is supported by the rare-earth data on Woodbine by Masuda (1969) and Hintenberger and Berghof on El Taco (1970). The rare earth pattern shows a depletion in Eu and other deviations from the chondritic pattern, which can be explained by loss of a feldspar component. Evidence for reduction of the mafic minerals is the FeO-depletion of the marginal zones of olivines and pyroxenes. This is supported by the observed reduction of Cr, which moves from chromite into troilite. Another hint in this direction is the lower Ni-content of the metal particles in the silicate inclusions compared to the metal host, which has been reported by Bunch, Keil, and Olsen (1970). This could be a dilution effect caused by the newly formed metallic iron. Not much evidence is available for an in-situ process. A simple relation between size or location of the inclusions and their composition has not yet been found, except for the fact that the small veinlike inclusion 6 has one of the lowest and most variable FeO-contents in its mafic minerals. More inclusions of this type should be studied to answer this question. On the other hand, the variations in composition and texture between large inclusions, together with a homogeneous composition of individual inclusions, are easier to understand if the differentiations took place before incorporation into the metal host. In any case, however, these differentiations are small compared to the differentiations which separate achondrites or mesosiderites from chondritic material. These inclusions are still close to primitive material and not what one would expect at the boundary between iron core and mantle of a planetary body. The "raisin-bread" theory, which sees the iron meteorites formed in small pools dispersed through silicates (Urey, 1959), fits the observations on El Taco better. It does not seem possible, however, to go one step further, as Wasson (1970) does, and assume that group I irons and their silicate inclusions are primitive condensates and were never molten. These inclusions vary in bulk and mineral composition, which means they were either formed under different conditions in different regions (if one wants to retain their primitive nature) or they were differentiated by melting processes. The latter seems much more probable. Literature Cited Ahrens, L. H., and H. von Michaelis 1968. Fractionation of Some Abundant Lithophile Element Ratios in Chondrites. Pages 257-272 in L. H. Ahrens, Origin and Distribution of the Elements. Oxford: Pergamon Press. 1969. The Composition of Stony Meteorites, III: Some Inter-Element Relationships. Earth and Planetary Science Letters, 5:395-403. Bence, A. E., and A. L. Albee 1968. Empirical Correction Factors for the Electron Microanalysis of Silicates and Oxides. Journal of Geology, 76:382-403. Bunch, T. E., and W. Cassidy 1968. Impact-induced Deformation in the Campo del Cielo Meteorite. Pages 601-612 in B. French and N. Short, Shock Metamorphism of Natural Materials. Baltimore: Mono Book Corp. Bunch, T. E., K. Keil, and E. Olsen 1970. Mineralogy and Petrology of Silicate Inclusions in Iron Meteorites. Contributions to Mineralogy and Petrology, 25:297-340. Cassidy, W. A., L. M. Villar, T. E. Bunch, T. P. Kohman, and D. J. Milton 1965. Meteorites and Craters of Campo del Cielo, Argentina. Science, 149:1055-1064. Davis, B. T. C, and F. R. Boyd 1966. The Join Mg2Si6O-CaMgSi2O6 at 30 kb Pressure and Its Application to Pyroxenes from Kimberlites. Journal of Geophysical Research, 71:3567-3576. El Goresy, A. 1967. Quantitative Electron Microprobe Analyses of K- Feldspar Grains from the Odessa Iron Meteorite [Abstract]. 30th Meeting of the Meteoritical Society, October 25-27, 1967. Moffett Field, California: Ames Research Center. Goldsmith, J. R., and R. C. Newton 1974. An Experimental Determination of the Alkali Feldspar Solvus. Pages 337-359 in W. S. Mackenzie and J. Zussman, The Feldspars. Manchester: University Press. Goldstein, J. I., and J. M. Short 1967. The Iron Meteorites: Their Thermal History and
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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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14 20 SMITHSONIAN CONTRIBUTIONS TO
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16 Constituent Si0 2 TiO2 A12O3 Cr
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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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