RHEASILVIA BASIN ON ASTEROID VESTA - A WINDOW ON THE MANTLE?

The calculated excavation depth of the huge (500-km diameter) Rheasilvia basin is twice the likely thickness of the crust, so Vesta’s mantle should be exposed. A chondritic precursor for Vesta would require a high abundance of olivine in the mantle, and some Vestan meteorites (diogenites) are composed of orthopyroxene plus olivine, confirming its presence. Spectrometers on the Dawn orbiting spacecraft have identified and mapped diogenites in the most deeply excavated portions of the floor of Rheasilvia, as uplifted materials in locations on the crater wall, and as a component of ejecta outside the crater. Orbital geochemical analyses indicate a lower abundance of iron within Rheasilvia, relative to the rest of Vesta, supporting the diogenite identification. However, only orthopyroxene spectral features have been observed; so far, olivine has eluded detection by Dawn instruments. Experiments indicate that the olivine 1-micron feature can be masked by orthopyroxene in harzburgite diogenites, which typically contain only 10-35% olivine, or it may have been diluted during impact mixing. If olivine is really absent, this may suggest emplacement of diogenite plutons near the crust-mantle boundary, or the formation of a depleted upper mantle. Geochemical properties of diogenites, however, are inconsistent with their formation as residues from partial melting. The occurrence of diogenite at ~20 km depth in areas of the crater 200 km apart may suggest a widespread, uniform layer of cumulate diogenite, supporting the hypothesis of an early magma ocean. Although this model is popular, geochemical evidence in diogenites indicates a more complex petrogenesis and may require crystallization of multiple magmas. The geologic context for diogenites on Vesta provided by Dawn is consistent with the properties of the meteorites themselves, but does not distinguish unambiguously between models for their formation.