ASTEROID DIFFERENTIATION - PYROCLASTIC VOLCANISM TO MAGMA OCEANS

Asteroid differentiation was driven by a complex array of magmatic pro cesses. This paper summarizes theoretical and somewhat speculative res earch on the physics of these processes. Partial melts in asteroids mi grate rapidly, taking < 10(6) years to reach surface regions. On relat ively small (< 100 km) asteroids with sufficient volatiles in partial melts (<3000 ppm), explosive volcanism accelerated melts to greater th an escape velocity, explaining the apparent lack of basaltic component s on the parent asteroids of some differentiated meteorites. Partial m elting products include the melts (some eucrites, angrites), residues (lodranites, ureilites), and unfractionated residues (acapulcoites). T he high liquidus temperatures of magmatic iron meteorites, the existen ce of pallasites with only olivine, and the fact that enstatite achond rites formed from ultramatic magmas argue for the existence of magma o ceans on some asteroids. Asteroidal magma oceans would have been turbu lently convective. This would have prevented crystals nucleated at the upper cooling surface (the only place for crystal nucleation in a low -pressure body) from settling until the magma became choked with cryst als. After turbulent convection slowed, crystals and magma would have segregated, leaving a body stratified from center to surface as follow s: a metallic core, a small pallasite zone, a dunite region, a feldspa thic pyroxenite, and basaltic intrusions and lava flows (if the basalt ic components had not been lost by explosive volcanism). The pallasite and dunite zones probably formed from coarse (0.5-1 cm) residual oliv ine left after formation of the magma ocean at >50% partial melting of the silicate assemblage. Iron cores crystallized dendritically from t he outside to the inside. The rapid melt migration rate of silicate me lts suggests that Al-26 could not be responsible for forming asteroida l magma oceans because it would leave the interior before a sufficient amount of melting occurred. Other heat sources are more likely candid ates. Our analysis suggests that if Earth-forming planetesimals had di fferentiated they were either small (< 100 km) and poor in volatiles ( < 1000 ppm) or they were rich in volatiles and large enough (>300 km) to retain the products of pyroclastic eruptions; if these conditions w ere not met, Earth would not have a basaltic component.