Partial melting of the acapulcoite-lodranite meteorite parent body

McCoy, Timothy J.
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Many asteroids experienced partial melting and incomplete differentiation. Our knowledge of the detailed of these processes are incomplete, owing to the paucity of partially melted meteorites. I studied two groups of meteorites which originated on a common parent body and are residues of a wide range of partial melting - acapulcoites and lodranites. These meteorites formed from a chemically and isotopically heterogeneous precursor chondrite. Heating and cooling occurred early in the history of the solar system, as evidenced by the ~4.51 Ga39Ar-40Ar age of acapulcoites and the ~4.48 Ga39Ar-40Ar age of the lodranite Gibson. The heating was probably caused by non-collisional heat sources. Acapulcoites (Acapulco, Monument Draw, Yamato 74063, ALH A77081, ALH A81261, ALH A81315, ALH 78230, ALH A81187 and ALH 84190) formed by low degrees of partial melting (e.g., Fe,Ni-FeS eutectic melting, but not silicate partial melting). Fe,Ni-FeS partial melts concentrated into micron- to centimeter-sized veins, but migration distances were short. In contrast, lodranites (Lodran, Gibson, Yamato 791491, Yamato 791493, Yamato 74357, Yamato 8002, Yamato 75274, MAC 88177, LEW 88280, EET 84302 and FRO 90011) experienced higher degrees of partial melting, including silicate partial melting. The higher degree of partial melting allowed efficient melt migration, depleting the residues in plagioclase and troilite. Volatiles played a major role in melt migration, driving partial melts to the surface where they were erupted at greater than the escape velocity and lost into space. Thus, basaltic partial melts are not sampled as discrete meteorites. In one meteorite (LEW 86220), these basaltic, Fe,Ni, FeS-rich partial melts from a lodranite source region were injected into a cooler, acapulcoite region. The acapulcoite-lodranite parent body experienced a range of partial melting and melt migration. Cooling of this body may have been complex, with slow cooling at high temperatures, rapid cooling at intermediate temperatures, and slow cooling at low temperatures, indicating that the body may have broken up and gravitationally reassembled. One to three impact events liberated the acapulcoites and lodranites from their parent body. These meteorites represent our best opportunity to understand partial melting and incomplete differentiation of asteroids.
Thesis (Ph. D.)--University of Hawaii at Manoa, 1994.
Includes bibliographical references (leaves 136-146)
xiii, 146 leaves, bound illus. 29 cm
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