Mathematics – Logic
Scientific paper
May 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997m%26ps...32..343c&link_type=abstract
Meteoritics, vol. 32, pages 343-348
Mathematics
Logic
16
Scientific paper
The LEW88774 ureilite is extraordinarily rich in Ca, Al, and Cr, and mineralogically quite different from other ureilites, in that it consists mainly of exsolved pyroxene, olivine, Cr-rich spinel and carbon. The presence of coarse exsolved pyroxene in LEW88774 is unique because pyroxene in most other ureilites is not exsolved. The pyroxene has bulk Wo contents of 15-20 mol % and has coarse exsolution lamellae of augite and low-Ca pyroxene, ca. 50(m in width. The compositions of the exsolved augite (Ca33.7Mg52.8Fe13.5) and host low-Ca pyroxene (Ca4.4Mg75Fe20.6) show that these exsolution lamellae were equilibrated at about 1280oC. A computer simulation of the cooling rate, by solving the diffusion equation for reproducing the diffusion profile of CaO across the lamellae, suggests that the pyroxene was cooled at 0.01oC/year until the temperature reached 1160oC. This cooling rate corresponds to a depth of at least 1 km in the parent body, assuming a rock-like material. Therefore, LEW88774 was held at this high temperature for ~1.2(104 years. The proposed cooling history is consistent with that of other ureilites with coarse-grained unexsolved pigeonites. LEW88774 includes abundant Cr-rich spinel in comparison with other ureilites. The range of FeO content of spinels in LEW88774 is from 1.3 wt% to 21 wt% [Fe/(Fe+Mg)=0.04-0.6]. The Cr-rich, and Fe-poor spinel in LEW88774 has less Fe (FeO 1.3 wt%) than spinels in other achondrites. We classify this spinel as an Fe, Al- bearing picrochromite. Most ureilites are depleted in Ca and Al, but this meteorite has high Ca and Al concentrations. In this respect as well as mineral assemblage and the presence of coarse exsolution lamellae in pyroxene, LEW88774 is a unique ureilite. Most differentiated meteorites are poor in volatile elements such as Zn, but the LEW88774 spinels contain abundant Zn (up to 0.6 wt%). We note that such a high Zn concentration in spinel has been observed in the carbonaceous chondrites and recrystallized chondrites. This unusual ureilite has more primitive characteristics than most other ureilites.
Chikami Jun
Mikouchi Takashi
Miyamoto Masamichi
Takeda Hiroshi
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