ASTEROID DIFFERENTIATION - PYROCLASTIC VOLCANISM TO MAGMA OCEANS

Asteroid differentiation was driven by a complex array of magmatic processes. This paper summarizes theoretical and somewhat speculative research on the physics of these processes. Partial melts in asteroids migrate rapidly, taking < 10(6) years to reach surface regions. On relatively small (< 100 km) asteroids with sufficient volatiles in partial melts (<3000 ppm), explosive volcanism accelerated melts to greater than escape velocity, explaining the apparent lack of basaltic components on the parent asteroids of some differentiated meteorites. Partial melting products include the melts (some eucrites, angrites), residues (lodranites, ureilites), and unfractionated residues (acapulcoites). The high liquidus temperatures of magmatic iron meteorites, the existence of pallasites with only olivine, and the fact that enstatite achondrites formed from ultramatic magmas argue for the existence of magma oceans on some asteroids. Asteroidal magma oceans would have been turbulently 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 crystals. After turbulent convection slowed, crystals and magma would have segregated, leaving a body stratified from center to surface as follows: a metallic core, a small pallasite zone, a dunite region, a feldspathic pyroxenite, and basaltic intrusions and lava flows (if the basaltic components had not been lost by explosive volcanism). The pallasite and dunite zones probably formed from coarse (0.5-1 cm) residual olivine left after formation of the magma ocean at >50% partial melting of the silicate assemblage. Iron cores crystallized dendritically from the outside to the inside. The rapid melt migration rate of silicate melts suggests that Al-26 could not be responsible for forming asteroidal magma oceans because it would leave the interior before a sufficient amount of melting occurred. Other heat sources are more likely candidates. Our analysis suggests that if Earth-forming planetesimals had differentiated 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 were not met, Earth would not have a basaltic component.