HED Meteorites and Their Relationship to the Geology of Vesta and the Dawn Mission

Abstract

Howardite-e ucrite-diogenite (HED) meteorites, thought to be derived from 4 Vesta, provide the best sampling available for any differentiated asteroid. However, deviations in oxygen isotopic composition from a common mass-fractionation line suggest that a few eucrite-like meteorites are from other bodies, or that Vesta was not completely homogenized during differentiation. The petrology and geochemistry of HEDs provide insights into igneous processes that produced a crust composed of basalts, gabbros, and ultramafic cumulate rocks. Although most HED magmas were fractionated, it is unresolved whether some cucrites may have been primary melts. The geochemistry of HEDs indicates that bulk Vesta is depleted in volatile elements and is relatively reduced, but has chondritic refractory element abundances. The compositions of HEDs may favor a magma ocean model, but inconislencies remain. Geochronology indicates that Vesta accreted and differentiated within the first several million years of solar system history, that magmatism continued over a span of . . . . . \0 Myr, and that its thermal history extended for perhaps 100 Myr. The protracted cooling history is probabl y responsible for thermal metamorphism of most HEDs. Impact chronology indicates that Vesta ex perienced many significant collisions, including during the late heavy bombardment. The age of the huge south pole crater is controversial, but it probably ejected Vestoids and many HEDs. Continued impacts produced a regolith composed of eu-(me and diogenite fragments containing only minor exotic materials. HED meteorites serve ib ground truth for orbital spectroscopic and chemical analyses by the Dawn spacecraft, and H,( \1cSween Jr. (C8J). A

Spectral and mineralogical characterization of inner main-belt V-type asteroids

“…Diogenites material originated from a plutonic layer deep in Vesta's lower crust/upper mantle (McSween et al 2010) and it has been suggested that the lack of this mineralogy among the V-type asteroids could be due to the difficulty in excavating large fragments from depths (Moskovitz et al 2010;Burbine et al 2009). However, numerical simulations (Asphaug 1997) of the South Pole crater formation have shown that a large fraction of the excavated material would have come from Vesta's mantle and not its crust, because it has a diogenite-dominated composition.…”

“…A close connection between asteroid (4) Vesta and these basaltic achondrites was established during the earliest studies, including visible and infrared spectroscopic investigations (McCoord et al 1970;McFadden et al 1977;Xu et al 1995;Burbine et al 2001), numerical and dynamical simulations (Asphaug 1997), and spatial resolved observations ; Thomas et al 1997). However, recent studies in oxygen isotopic composition from a common mass-fractionation line suggest that a few eucrite-like A&A 533, A77 (2011) meteorites come from other bodies, or that (4) Vesta was not completely homogenized during differentiation (McSween et al 2010). It is noteworthy that a laboratory analysis of iron meteorites also indicates an origin from at least 70 distinct differentiated parent bodies (Mittlefehldt et al 1998).…”

Geologic map of the northern hemisphere of Vesta based on Dawn Framing Camera (FC) images

“…One of the efforts of the Dawn Science Team has been to investigate the geology of asteroid Vesta through geologic mapping. Geologic mapping attempts to decipher the evolution of a given body (e.g., Coradini et al, 2011;Greeley and Batson, 1990), to provide the context for the spectrally derived composition of the surface (e.g., De Sanctis et al, 2012a), and, in the case of Vesta, to understand the possible link with achondrite meteorites (e.g., McSween et al, 2011;Mayne et al, 2010;Sykes and Vilas, 2001). The first Dawn observations of Vesta's shape and morphology were reported by Jaumann et al (2012).…”

Zinc isotopes in HEDs: Clues to the formation of 4-Vesta, and the unique composition of Pecora Escarpment 82502

“…The source of this additional heat may have been from large impacts (Nyquist et al, 1986), heating at depth after burial of surface basalts (Yamaguchi et al, 1996(Yamaguchi et al, , 1997, or the differentiation into layers (Takeda, 1997). A large impact crater of 460 km diameter, 13 km depth with a large central peak, has been identified on Vesta (Thomas et al, 1997) which may have extended beyond the crust into the mantle layer and ejected many Vestoids and HEDs (McSween et al, 2010). The age of this impact crater is controversial, with some evidence suggesting it occurred during the Late Heavy Bombardment period of 3.5-4.0 billion years ago (Scott et al, 2009b), and other evidence supporting an age of only 1 billion years (Asphaug, 1997).…”