A Plethora of Partially Melted Asteroids?

A number of recent quantitative mineralogical interpretations of S-type asteroids have identified compositions consistent with partially differentiated assemblages [e.g., 1-5]. However, partially differentiated meteorites (often designated as " primitive achon-drites ") constitute only a tiny fraction of the meteorite falls represented in terrestrial collections (~0.2% [6]). These two results would seem to be contradictory and would appear to raise serious questions concerning the validity of the asteroid spectral interpretations. Several factors must be considered when assessing the significance of this apparent discrepancy. The first is the general disconnect between meteorite fall frequency and the relative abundances of corresponding assemblages among main belt asteroids. If one considers the diversity of meteorite types (as opposed to the fall frequency for particular types) it is evident that most (~80%) of the meteorite parent bodies represented in our meteorite collection underwent strong heating and at least partial melting and differentiation [7]. Parent body location with respect to main belt " escape hatches " (the 3:1, 5:2 and ν 6 resonances in particular) appears to be the primary factor controlling the fall frequency of meteorites. Thus the fall abundance of meteorites cannot provide a robust check on the plausibility of asteroid spectral interpretations. Actually this apparently contradictory relationship is expected due to the processes that occur during the heating of small planetary bodies. The early transient asteroid and meteorite parent body heat sources were – by definition-finite since the bodies were not vaporized. This may seem obvious, but it has significant implications for the relative abundances of differentiated , partially differentiated, and undifferentiated assemblages among the asteroids. With a finite heat source, there is a limit to the peak temperature that can be attained within the parent body. If this peak temperature is elevated but below the point where the first melt appears (eutectic melt ~970°C for FeNi-FeS and ~1075°C for chondritic silicates [e.g., 8-10]), the parent body assemblage will only be metamorphosed and not melted. If the peak temperature is above the melting point of all the constituent mineral phases, the body will experience complete melting, magmatic differentiation and fractional crystallization. The temperature of complete melting will vary (~1500-1800°C) depending on the phase composition of the initial assemblage. The final melting temperature will also depend upon whether the melt phase is retained in or removed from the melt region. Silicate partial melt assemblages are formed in the temperature interval between the onset of silicate …