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

Lobate and flow-like features on asteroid Vesta

“…Alternatively, if the volcanism on Vesta manifested itself only as a global magma ocean, which cooled and crystallized to form basaltic minerals in widespread flow fields as opposed to discrete individual flows (similar to the larger basinfilling lunar maria), then this might also explain the lack of observed volcanic flow features. In either case, the lack of discrete volcanic features on Vesta is consistent with the hypothesis, based on analysis of basaltic material in the HED suite, that volcanism on Vesta occurred only during the first $ 10-100 Ma of Vesta's history (e.g., Schiller et al, 2010;McSween et al, 2011), and that since then impact processes have degraded any ancient volcanic features to the point where they cannot be distinguished. It is possible that ongoing detailed study of Vesta's surface as part of the quadrangle mapping process may yet identify some small battered fragments of ancient volcanic features; perhaps the best place to investigate is Vestalia Terra, which appears to be a continent-sized fragment of Vesta's original crust (Buczkowski et al, 2013), albeit disrupted and blanketed by superposed impact ejecta.…”

“…Earth-based telescopic observations showed spectral evidence of basaltic mineralogies, and the inferred relationship of the HED meteorites to Vesta is well established (e.g., McCord et al, 1970;Feierberg et al, 1980;Gaffey, 1997;Binzel et al, 1997;Cochran and Villas, 1998) and has been confirmed by the Dawn observations De Sanctis et al, 2012). Petrologic study of the Vesta-derived HED meteorites demonstrates the presence of basaltic minerals and glassy textures that are diagnostic of basaltic lava flows on Earth (McSween et al, 2011(McSween et al, , 2013, or alternatively that these basaltic minerals were derived from crystallization of a global magma ocean that may have once covered Vesta (e.g., Righter and Drake, 1997;Ruzicka et al, 1997;Warren, 1997). Given the strong likelihood of basaltic volcanism on Vesta inferred from telescopic data and the HED connection, Keil (1996, 1997) used mathematic and petrologic models to predict the types of volcanic deposits that should occur on Vesta, including: (a) surface basaltic lava flows with widths of a few hundred meters to a few kilometers, lengths between a few kilometers to several tens of kilometers, thicknesses between 5 and 20 m, and erupted volumes o3 km 3 ; (b) formation of channelized flows on steeper slopes; (c) a lack of uniform sheet flows; d) a lack of shield volcanoes; (e) shallow intrusions (dikes) with widths of $ 1 m and vertical extents of o10 km, (f) deep intrusions (dikes) with thicknesses ≤3 m and lateral extents ≤30 km, and volumes between 3 and 10,000 km 3 ; and (g) explosive eruptions only occurring in the form of optically-dense lava fountains that form lava ponds feeding lava flows, such that no broad pyroclastic fall deposits should occur on Vesta.…”

“…There is no unequivocal morphologic evidence of ancient volcanic features preserved on Vesta, likely because of the early cessation of volcanism within $ 10-100 Ma after formation (e.g., Schiller et al, 2010;McSween et al, 2011) and the subsequent bombardment by impactors at all size ranges. We find that all lobate, flowlike materials are located close to impact craters or on steep slopes, indicating a likely formation as a result of impact and erosional processes.…”

“…Thus, given these studies, we expected prior to Vesta arrival that some volcanic features, particularly lobate lava flows, would be present on the surface, although they would probably have been heavily disrupted by impact craters and thus not easily recognized. Indeed, volcanism on Vesta is thought to have ceased early in Vesta's history, perhaps within $ 10-100 Ma after formation (e.g., Schiller et al, 2010;McSween et al, 2011).…”

“…The presence of basaltic compositions and textures in the HEDs (e.g., McSween et al, 2011;McSween et al, 2013 and references therein) suggest that basaltic lava flows once extruded onto Vesta's surface (e.g., Wilson and Keil, 1996) or alternatively were derived from the crystallized remnants of a global magma ocean (Righter and Drake, 1997;Ruzicka et al, 1997;Warren, 1997), and that Vesta must have differentiated into a crust, mantle, and core (e.g., Keil, 2002;McSween et al, 2011). In addition, Vesta's location in the main asteroid belt, prior observations by the Hubble Space Telescope , the nature of howardites (i.e., impact breccias: see McSween et al, 2011 and references therein), and spacecraft observations of other asteroids (e.g., Sullivan et al, 2002) all suggest that Vesta should have a heavily cratered surface, and contain resulting impact-related flow features.…”