Olivine and pyroxene from the mantle of asteroid 4 Vesta

- We identified Mg-rich olivine and pyroxenes fragments in howardite meteorites.
- They share parent body signatures with meteorites thought to come from 4 Vesta.
- The Mg-rich olivines are likely residues left by mantle partial melting.
- The Mg-rich pyroxenes crystallized from mantle partial melts.
- The vestan mantle was likely incompletely molten and chemically heterogeneous.

A number of meteorites contain evidence that rocky bodies formed and differentiated early in our solar system's history, and similar bodies likely contributed material to form the planets. These differentiated rocky bodies are expected to have mantles dominated by Mg-rich olivine, but direct evidence for such mantles beyond our own planet has been elusive. Here, we identify olivine fragments (Mg# = 80-92) in howardite meteorites. These Mg-rich olivine fragments do not correspond to an established lithology in the howardite-eucrite-diogenite (HED) meteorites, which are thought to be from the asteroid 4 Vesta; their occurrence in howardite breccias, combined with diagnostic oxygen three-isotope signatures and minor element chemistry, indicates they are vestan. The major element chemistry of these Mg-rich olivines suggests that they formed as mantle residues, in crustal layered intrusions, or in Mg-rich basalts. The trace element chemistry of these Mg-rich olivines supports an origin as mantle samples, but other formation scenarios could be possible. Interpreted as mantle samples, the range of Mg-rich olivine compositions indicates that Vesta's structure differs from that predicted by conventional models: Vesta has a chemically heterogeneous mantle that feeds serial magmatism. The range of olivine major element chemistries is consistent with models of an incompletely melted mantle such as in the model proposed by Wilson and Keil (2013) rather than a whole-mantle magma ocean for Vesta. Trace element chemistries of Mg-rich pyroxenes (Mg# = 85-92) provide support that some of these pyroxenes may represent initial fractional crystallization of mantle partial melts.