A magma ocean on Vesta: Core formation and petrogenesis of eucrites and diogenites

Abstract

Abstract-Available evidence strongly suggests that the HED (howardite, eucrite, diogenite) meteorites are samples of asteroid 4 Vesta. Abundances of the moderately siderophile elements (Ni, Co, Mo, W and P) in the HED mantle indicate that the parent body may have been completely molten during its early history. Duriig cooling of a chondritic composition magma ocean, equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0.80, when the convective system freezes and melts segregate from crystals by gravitational forces. The extruded liquids are similar in composition to Main Group and Stannern trend eucrites, and the last pyroxenes to precipitate out of this ocean (before convective lockup) span the compositional range of the diogenites. Subsequent fractional crystallization of a Main Group eucrite liquid, which has been isolated as a body of magma, produces the Nuevo Laredo trend and the cumulate eucrites. The predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites. Thus, eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with Vesta.

Lobate and flow-like features on asteroid Vesta

“…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.…”

“…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.…”

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

“…In this model, total melting would not be expected. Over the last 30 years a number of additional models have been proposed that are basically variants of these (e.g., Warren and Jerde, 1987;Hewins and Newsom, 1988;Righter and Drake, 1997;Ruzicka et al, 1997;Warren, 1997;Mittlefehldt and Lindstrom, 2003;Greenwood et al, 2005), however, consensus on a single model of HED petrogenesis has been elusive.…”

Dawn: A journey in space and time

“…By modeling siderophile element abundances, modest core sizes (percentages of the asteroid by mass) of 21.7% [Dreibus et al, 1997], 0-25% [Ruzicka et al, 1997], and 5-25% [Righter and Drake, 1997] have been estimated. Natural remanent magnetization in HEDs suggest that this core generated an ancient global magnetic field, with a surface field similar to that of the Earth (5-50 microTeslas) [Morden, 1992;Collinson and Morden, 1994].…”

“…Diogenites are cumulate orthopyroxenites, sometimes with minor olivine, and howardites are regolith breccias composed of fragments of eucrites and diogenites. Attempts to relate eucrites and diogenites require complex igneous models involving melting scenarios that generated diverse liquids which produced a variety of cumulate rocks [Stolper, 1977;Longhi and Pan, 1988;Grove and Bartels, 1992] or fractional crystallization of an extensive magma ocean [Righter and Drake, 1997;Ruzicka et al, 1997;Warren, 1997]. Assuming serial magmatism, Wilson and Keil [1996] modeled the sizes and flow rates of conduits that carried magmas from source regions to Vesta's surface.…”