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Science 21 August 1992:
Vol. 257. no. 5073, pp. 1096 - 1099
DOI: 10.1126/science.257.5073.1096
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Articles

Mesosiderite Clasts with the Most Extreme Positive Europium Anomalies Among Solar System Rocks

David W. Mittlefehldt 1, Alan E. Rubin 2, and Andrew M. Davis 3

1 C23, Lockheed Engineering and Sciences Company, 2400 Nasa Road 1, Houston, TX 77058
2 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024
3 Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637

Pigeonite-plagioclase gabbros that occur as clasts in mesosiderites (brecciated stony-iron meteorites) show extreme fractionations of the rare-earth elements (REEs) with larger positive europium anomalies than any previously known for igneous rocks from the Earth, moon, or meteorite parent bodies and greater depletions of light REEs relative to heavy REEs than known for comparable cumulate gabbros. The REE pattern for merrillite in one of these clasts is depleted in light REEs and has a large positive europium anomaly as a result of metamorphic equilibration with the silicates. The extreme REE ratios exhibited by the mesosiderite clasts demonstrate that multistage igneous processes must have occurred on some asteroids in the early solar system. Melting of the crust by large-scale impacts or electrical induction from an early T-Tauri-phase sun may be responsible for these processes.


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