Mineralogy and cooling history of the calcium‐aluminum‐chromium enriched ureilite, Lewis Cliff 88774

Abstract— The LEW 88774 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 C. The presence of coarse exsolved pyroxene in LEW 88774 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, 50 μm in width. The compositions of the exsolved augite (Ca 33.7 Mg 52.8 Fe 13.5 ) and host low‐Ca pyroxene (Ca 4.4 Mg 75 Fe 20.6 ) show that these exsolution lamellae were equilibrated at 1280 °C. A computer simulation of the cooling rate, obtained by solving the diffusion equation for reproducing the diffusion profile of CaO across the lamellae, suggests that the pyroxene was cooled at 0.01 °C/year until the temperature reached 1160 °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. Therefore, LEW 88774 was held at this high temperature for 1.2 × 10 4 years. The proposed cooling history is consistent with that of other ureilites with coarsegrained unexsolved pigeonites. Lewis Cliff 88774 includes abundant Cr‐rich spinel in comparison with other ureilites. The range of FeO content of spinels in LEW 88774 is from 1.3 wt% to 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 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, LEW 88774 is a unique ureilite. Most differentiated meteorites are poor in volatile elements 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 been observed in the carbonaceous chondrites and recrystallized chondrites. This unusual ureilite has more primitive characteristics than most other ureilites.