Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.226g&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 226
Computer Science
5
Scientific paper
The Acapulco meteorite is currently studied by a consortium in order to elucidate the origin and history of its parent body. This meteorite is characterized by an approximately chondritic bulk chemistry, its major element composition is close to that of average H chondrites. Volatile elements and noble gases are, however, enriched nearly to the level of C1 chondrites and U and Th show an even higher enrichment (Palme et al., 1981). The texture of Acapulco is equigranular; chondrules are absent. Previous work with long-lived isotopes indicated some unexpected results considering the recrystallized texture and the estimated high equilibration temperature (1350 degrees K): a K/Ar age of 4.7 +- 0.3 aeon, an internal Sm-Nd isochrone indicating a recrystallization age of 4.60 + 0.03 aeon and recently an Ar/Ar age of 4.51 aeon (Palme et al., 1982; Prinzhofer et al., 1991). We present U and Pb concentrations and Pb isotopic compositions measured in bulk samples and mineral separates from Acapulco. Small bulk samples show slightly radiogenic Pb isotopic compositions (^206Pb/^204Pb= 22-36); similar to the majority of meteorites they appear to have more radiogenic Pb than could have been derived from U decay. The U concentrations range from 64 to 96 ppb. Compared to C1, Acapulco is enriched in U by a factor of 5-10. ^204Pb concentrations of the fragments (~6 ppb) are lower than C1 level (47 ppb); they lie in the range of H 4-6 chondrites (0.2 up to 7 ppb) (Tatsumoto et al., 1976). Three different mineral phases were separated and analyzed: plagioclase, troilite, and phosphate. The phosphate separate shows a high U concentration. Using the modal abundance of the minerals, phosphates account for half of the U in the bulk rock. The Acapulco phosphate contains more ^204Pb (17,1 ppb) than phosphates from L-, H-, and LL- chondrites, and its resulting Pb isotopic composition is less radiogenic. The Pb/Pb model age of the phosphate relative to primordial lead is 4.557 +- 0.002 aeon. Table 1, which in the hard copy appears here, shows U-Pb data of some Acapulco samples. The plagioclase is the main carrier of the ^204Pb in the bulk meteorite (80%). Different separates show low U concentrations, the Pb isotopic compositions are close to the primordial lead, but distinct. The Pb isotopic composition of the troilite is intermediate; U and Pb concentrations are similar to those measured by Unruh in troilites from L chondrites (Unruh, 1982). The U-Pb systematics clearly indicate that this meterotie has been affected by two distinct processes: the plagioclase data indicate an old perturbation or reequilibration process, whereas the isotopic composition of the troilite and the bulk fragments translates a recent addition of radiogenic lead. We will discuss these data and evaluate different models concerning the history of Acapulco. References Gopel, C., Manhes, G., and Allegre, C.J. (1991) Meteoritics (abstract), 26, 338. Palme et al. (1981) Geochimica Cosmochimica Acta, 45, 727-752. Prinzhofer, A., Papanastassiou, D., Wasserburg, G.J.(1992) Geochimica Cosmochimica Acta, 56, 797-815. Tatsumoto, M., Unruh, D.M., Desborough, G.A. (1976) Geochemica Cosmochimica Acta, 40, 617-634. Unruh, D.M.(1982) Earth Planet. Sci. Lett. 58, 75-94.
Allègre Claude J.
Gopel Ch.
Manhes Gerard
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