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The Acapulco meteorite: Chemistry, mineralogy and irradiation effects

1981; Elsevier BV; Volume: 45; Issue: 5 Linguagem: Inglês

10.1016/0016-7037(81)90045-4

1872-9533

H. Palme, L. Schultz, B. Spettel, H. W. Weber, H. Wänke, M. Christophe Michel‐Lévy, J. C. Lorin,

Planetary Science and Exploration

The Acapulco meteorite fell in August, 1976, at El Quemado, near Acapulco, Mexico. It is a unique object with chondritic composition but achondritic texture. High degree of recrystallisation and mineral chemical data indicate formation of the meteorite under redox conditions intermediate between those of H- and E-chondrites at ~ 1100°C, from which it cooled at a rate > 10°C/Myr. The major element composition is within the range of H-chondrites. Troilite and metal, and associated trace elements, are inhomogeneously distributed. Chromium is a factor of two higher than in H-chondrites. Enrichments of P and U indicate high phosphate content. Limited extent of partial melting may explain the light REE enrichment. However other incompatible elements have normal H-chondritic abundances or are even depleted like K and Rb. Moderately volatile or volatile elements (e.g. Mn, Ga, Ge, Zn) are enriched nearly to the level of C1-chondrites. Planetary noble gases are also significantly higher than in equilibrated ordinary chondrites. High temperature recrystallisation has not affected volatile element abundances. Compared to H-chondrites Acapulco is enriched in refractory siderophile elements. The distribution of W and other siderophile elements between metal and silicate phases are indicative of the higher temperature and lower oxygen fugacity of the assemblage. However, contrary to previous claims, the distribution of W cannot be used to calculate the equilibration temperature. Low K and high U contents are also reflected in the anomalous amounts of 40Ar and 4He. The old K-Ar age (4.7 ± 0.3 Gyr) and high 244Pu track densities indicate mobilisation of U and Pu-rich phases shortly after formation of the parent material. This and other evidence suggests that Acapulco may represent a rock formed in the early stages of incipient melting of a chondritic parent body. However, since compositional differences between Acapulco and H-chondrites cannot be explained by fractionation processes on the Acapulco parent body. Acapulco must have originated from a different parent body. Lack of depletion of volatile elements, absence of chondrules and reduced mineral composition indicate some relationship of Acapulco to silicate inclusions in iron meteorites and to other unusual meteorites. Oxygen isotopes and chemical data suggest that there are at least three different groups of reduced chondritic meteorites: (a) Acapulco, Lodran, and probably Allan Hills A 77081; (b) Pontlyfni, Mount Morris, Winona and silicate inclusions in IAB iron meteorites, and (c) Kakangari. An exposure age of 5 × 106 yr is deduced from spallogenic rare gas data.