Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.514g&link_type=abstract
Meteoritics, vol. 30, no. 5, page 514
Other
Breccias, Polymict, Meteorites, Elephant Moraine 83309, Partial Melting, Plagioclase, Rare Earth Elements, Ureilites
Scientific paper
The absence of a related basaltic component has been considered as one of the major problems in understanding the origin of ureilites as products of internal differentiation processes. If such a loss occurred and was caused by impact stripping [1] or explosive volcanism [2], the best chance to find and study the basaltic material should be in polymict ureilites. EET 83309 was recognized as a micro-brecciated ureilite composed mainly of ureilitic mineral clasts, olivine and pigeonite, but also containing plagioclase, enstatite, troilite, suessite, C-rich matrix and minor Cr-sulfide [3,4]. With the ion microprobe, we measured the concentrations of rare earth elements (REE) in various components of EET 83309. The olivine and clinopyroxene, while having much wider ranges of major elemental compositions, display REE patterns typical of those found in monomict ureilites [5]. The LREE-enrichments in C-rich matrix, though present, are not as large as those observed in Kenna, Novo Urei or Goalpara [5]. The REE abundances of three plagioclase grains (up to ~0.8 mm) vary by a factor of ~25, from 0.02 to 0.5 x CI. The plagioclase grain (An=47) with the highest REE abundances has a flat REE pattern with a typical positive Eu anomaly (Eu/Eu*=41). The other two grains (An=0-10) display decreasing chondrite-normalized abundances from La to Pr and much higher Eu anomalies (Eu/Eu*=90 and 106). Surprisingly, the more albitic the plagioclase, the lower the REE concentrations and the larger the Eu anomalies. We also found some black Fe-rich matrix clasts and two quenched melt clasts in which fine euhedral olivine crystals and clinopyroxene laths are set in plagioclase-normative glass. Unfractionated REE distribution patterns (~4 x CI) are observed in both a large melt clast (~1.7 mm) and an Fe-rich black matrix clast (~0.3 mm). REE modeling indicates that this kind of melt can be produced by ~25% partial melting of chondritic material, leaving behind a ureilitic residue which is plagioclase-free and has a superchondritic Ca/Al ratio [6]. The whole-rock REE budget (~0.5 x CI) [1] can reasonably be accounted for by recombining plagioclase, black Fe-rich matrix and melt clasts. These observations provide support to the idea that ureilites formed by large degree (~25%) partial melting of chondritic parent body(ies) [e.g., 2]. The melt clasts found in EET 83309 represent either trapped original partial melt or shock-induced melt of rocks other than ureilites. In either case, they may represent the "missing" basaltic components related to the ultramafic ureilites. The black Fe-rich matrix clasts are probably the metallic Fe component derived from chondritic material in addition to melt clasts and ureilites. Some of the plagioclase could have formed from a melt with 4 x CI REEs, but the variations in REE abundances and in size of the Eu anomaly and their relationship with An content are difficult to explain. These plagioclase characteristics indicate either multiple melt sources [7] or more complex processing. References: [1] Warren P. H. and Kallemeyn G. W. (1989) Meteoritics, 24, 233-246. [2] Scott E. R. D. et al. (1993) GRL, 20, 415-418. [3] Mason B. (1986) Antarc. Meteorite Newslett., 9 (2), 15. [4] Prinz M. et al. (1987) LPS XVIII, 802-803. [5] Guan Y. and Crozaz G. (1995) LPS XXVI, 527-528. [6] Goodrich C. A. (1992) Meteoritics, 27, 327-352. [7] Prinz M. et al. (1988) LPS XIX, 947-948.
Crozaz Ghislaine
Guan Yifeng
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