DISTINGUISHING NEBULAR AND IGNEOUS HISTORIES AMONG OLIVINE-DOMINATED ASTEROIDS

Melting models indicate that the composition and abundance of olivine systematically co-vary and are therefore excellent petrologic indicators. However, heliocentric distance, and thus surface temperature, has a significant effect on the spectra of olivine-rich asteroids. We show that composition and temperature complexly interact spectrally, and must be simultaneously taken into account in order to infer olivine composition accurately.

Using near-infrared data obtained at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea with SpeX, a low- to medium-resolution near-infrared spectrograph (0.8 to 2.5 µm) and pre-existing visbile spectra we examine the spectra of olivine dominated asteroids. We find that most (7/9) of the olivine-dominated asteroids are magnesian and thus likely sampled mantles differentiated from ordinary chondrite sources (e.g., pallasites). However, two other olivine-rich asteroids (289 Nenetta and 246 Asporina) are found to be more ferroan. Melting models show that partial melting cannot produce olivine-rich residues that are more ferroan than the chondrite precursor from which they formed. Thus, even moderately ferroan olivine must have non-ordinary chondrite origins, and therefore likely originate from oxidized R chondrite or melts thereof, which reflect variations in nebular composition within the asteroid belt. This is consistent with the meteoritic record in which R chondrites and brachinites are rare relative to pallasites.