Refractory inclusions from the ungrouped carbonaceous chondrites MacAlpineHills 87300 and 88107

MacAlpine Hills (MAC) 87300 and 88107 are two unusual carbonaceous chondrit es that are intermediate in chemical composition between the CO3 and CM2 me teorite groups. Calcium-aluminum-rich inclusions (CAIs) from these two mete orites are mostly spinel-pyroxene and melilite-rich (Type A) varieties. Spi nel-pyroxene inclusions have either a banded or nodular texture, with alumi nous diopside rimming Fe-poor spinel. Melilite-rich inclusions (Angstrom k( 4-42)) are irregular in shape and contain minor spinel (FeO <1 wt%), perovs kite and, more rarely, hibonite. The CAIs in MAC 88107 and 87300 are simila r in primary mineralogy to cars from low petrologic grade CO3 meteorites bu t differ in that they commonly contain phyllosilicates. The two meteorites also differ somewhat from each other: melilite is more abundant and slightl y more Al-rich in inclusions from MAC 88107 than in those from MAC 87300, a nd phyllosilicate is more abundant and Mg-poor in MAC 87300 CAIs relative t o that in MAC 88107. These differences suggest that the two meteorites are not paired. The CAI sizes and the abundance of melilite-rich CAIs in MAC 88 107 and 87300 suggests a genetic relationship to CO3 meteorites, but the CA Is in both have suffered a greater degree of aqueous alteration than is obs erved in CO meteorites. Aluminum-rich melilite in CAIs from both meteorites generally contains excess Mg-26, presumably from the in situ decay of Al-2 6. Although well-defined isochrons are not observed, the Mg-26 excesses are consistent with initial Al-26/Al-27 ratios of approximately 3-5 x 10(-5) A n unusual hibonite-bearing inclusion is isotopically heterogeneous, with tw o large and abutting hibonite crystals showing significant differences in t heir degrees of mass-dependent fractionation of Mg-25/Mg-24. Th, two crysta ls also show differences in their inferred initial Al-26/Al-27 ratios, 1 x 10(-5) vs. less than or equal to 3 x 10(-6).