Asteroid 4 Vesta is large enough to have a core-mantle-crust structure similar to Earth. However, this protoplanet did not experience plate tectonics and recycling of its crustal material, which means that samples from Vesta are very old and preserve evidence of the earliest solar system processes. To better understand these processes, scientists study howardite-eucrite-diogenite (HED) meteorites, which are thought to originate from Vesta, that encompass a variety of minerals, compositions, and textures. The range in sample variability is valuable because different rock types formed in different regions and at different depths within Vesta and allow scientists to piece together a more complete picture of Vesta’s evolution.

A recent collaborative study, led by Ai-Cheng Zhang of Nanjing University and the CAS Center for Excellence in Comparative Planetology in Hefei, China, studied unique features in meteorite Northwest Africa (NWA) 8321. Based on mineral size and composition, it is classified as a diogenite, a rock that likely formed in the deep crust at the base of a magma chamber. Its unique features relate to internal changes that were occurring in Vesta as the asteroid cooled, very early in the history of the solar system.

Zhang and colleagues observed that, in NWA 8321, olivine, a magnesium-rich silicate mineral also found in cooled Hawaiian lava flows, was replaced with small amounts of metal, metal sulfides, and other silicate minerals. The presence of metal and metal sulfides suggests that a sulfur-rich fluid modified the sample in a process called sulfidation. The study hypothesizes that because sulfidation occurred in a rock that formed in the lower crust or upper mantle of Vesta, the sulfur had to come from the interior of Vesta. Additional observations suggest that the olivine grains in NWA 8321 interacted with small amounts of molten rock as well, which can be generated at depth. The possible presence of sulfur-rich fluids in ancient Vesta is a significant new observation because previous geochemical studies indicate that Vesta was (and is) very dry. Fluids in this asteroid may have been dominated by sulfur, rather than water, and thus, are very different from Earth. READ MORE