SPECTRAL DETECTION OF PARTIAL MELTING IN ASTEROIDS

Eighty percent of the parent bodies represented in our meteorite collections underwent heating sufficient to produce partial or total melting (Keil, 2000, Planet. Space Sci. 48, 887-903). Moreover, the enthalpy of fusion (heat required to convert a solid phase to a liquid phase without an attendant temperature rise) slows the temperature increase through the several hundred degree interval over which melting occurs. Thus parent bodies with limited heat sources can initiate partial melting but not attain complete melting. HEDs and angrites crystallized from extracted partial melts. Ureilites are partial melt residues. Lodranites and acapulcoites underwent limited partial melting and little or no melt extraction. Because of the short lifetimes of stony meteoroids compared to iron meteoroids, partial melt derived assemblages are underrepresented in our meteorite collections and probably constitute a significant subset of the asteroids.

Reflectance spectroscopy can readily distinguish those assemblage formed by crystallization of extracted partial melts (e.g., HEDs, angrites) due to their non-chondritic mineral compositions and combinations. However recognition of bodies composed of partial melt residues is more difficult. Assemblages which are strongly depleted in pyroxene can be readily recognized, but ambiguities exist with olivine dominated cumulate assemblages. Assemblages such as lodranites and acapulcoites which underwent low degrees of partial melting but little or no partial melt extraction are difficult to distinguish from chondrites. Although a calcium- and iron-rich eutectic silicate melt formed and subsequently crystallized, the bulk composition has not changed. However, the abundance and composition of pyroxene phases was changed. Whereas in the initial chondritic assemblage, pyroxenes consisted of low-Ca Opx and accessory low iron augite, the solidified partial melt introduces a third pyroxene, typically a eucrite-like high-Fe calcic pigeonite. The presence of a small component of strongly absorbing eucrite-like pyroxene produces subtle shifts in the absorption band centers. These shifts can be detected in high precision NIR spectra and provide a good indication of asteroids whose parent bodies attained temperatures sufficient to produce partial melting of silicates.