MINERALOGY AND COOLING HISTORY OF THE CALCIUM-ALUMINUM-CHROMIUM ENRICHED UREILITE, LEWIS-CLIFF-88774

The LEW 88774 ureilite is extraordinarily rich in Ca, Al, and Cr, and mineralogically quite different from other ureilites in that it consis ts mainly of exsolved pyroxene, olivine, Cr-rich spinel, and C. The pr esence of coarse exsolved pyroxene in LEW 88774 is unique because pyro xene in most other ureilites is not exsolved. The pyroxene has bulk Wo contents of 15-20 mol% and has coarse exsolution lamellae of augite a nd low-Ca pyroxene, similar to 50 mu m in width. The compositions of t he exsolved augite (Ca33.7Mg52.8Fe13.5) and host low-Ca pyroxene (Ca4. 4Mg75Fe20.6) show that these exsolution lamellae were equilibrated at similar to 1280 degrees C. A computer simulation of the cooling rate, obtained by solving the diffusion equation for reproducing the diffusi on profile of CaO across the lamellae, suggests that the pyroxene was cooled at 0.01 degrees C/year until the temperature reached 1160 degre es C. This cooling rate corresponds to a depth of at least 1 km in the parent body, assuming it was covered by a rock-like material. Therefo re, LEW 88774 was held at this high temperature for similar to 1.2 x 1 0(4) years. The proposed cooling history is consistent with that of ot her ureilites with coarse-grained unexsolved pigeonites. Lewis Cliff 8 8774 includes abundant Cr-rich spinel in comparison with other ureilit es. The range of FeO content of spinels in LEW 88774 is from 1.3 wt% t o 21 wt% [Fe/(Fe + Mg) = 0.04-0.6]. The Cr-rich and Fe-poor spinel in LEW 88774 has less Fe (FeO, 1.3 wt%) than spinels in other achondrites . We classify this spinel as an Fe, Al-bearing picrochromite. Most ure ilites are depleted in Ca and Al, but this meteorite has high-Ca and A l concentrations. In this respect, as well as mineral assemblage and t he presence of coarse exsolution lamellae in pyroxene, LEW 88774 is a unique ureilite. Most differentiated meteorites are poor in volatile e lements such as Zn, but the LEW 88774 spinels contain abundant Zn (up to 0.6 wt%). We note that such a high Zn concentration in spinel has b een observed in the carbonaceous chondrites and recrystallized chondri tes. This unusual ureilite has more primitive characteristics than mos t other ureilites.