Magnesium oxide‐iron oxide mass balance constraints and a more detailed model for the relationship between eucrites and diogenites

Abstract

Abstract-According to a currently popular model for petrogenesis on the howardite, eucrite, and diogenite (HED) parent asteroid, the diogenites are not comagmatic with most eucrites but instead formed in separate orthopyroxenite-dominated plutons. This model can be tested for consistency with mass balance for MgO and FeO, assuming the overall diogenite/(diogenite + eucrite) ratio, d, of the parent asteroid is at least comparable to the average d for the eucrite + diogenite dominated howardite regolith breccias. Average mg# (=MgO/ [MgO + FeO]) is much lower for eucrites, especially noncumulate eucrites, than for diogenites. Unless the diogenite parent magmas eventually produced a large proportion of low-mg# residual basalt and gabbro (RBG), the implied initial magma's mg# is vastly higher than that of any noncumulate eucrite. Starting from a source previously depleted by putative primary eucrite genesis, melt mg# can be estimated as a finction of the exchange reaction KD and degree of melting. Using several very conservative assumptions (e.g., assuming that the total [MgO + FeO] concentration is nearly the same in the nascent melt as in the residual solids), the degree of melting required to yield a melt with mg# high enough to satisfy mass balance, without implying an RBG component that accounts for >50% of all eucrites, is an implausibly high 60-80 wt?!.The separate orthopyroxenitic plutons (SOP) model also seems inconsistent with the uniform density of melts across the diogenite-eucrite compositional spectrum (2.77-2.82 g/cm3), which implies that diogenitic magmas should have been as capable as eucrites of extruding to form lavas. This difficulty cannot be reduced by simply assuming that later-formed magmas were systematically both more plutonic and more MgOrich than earlier ones, because the plutonic cumulate eucrites equilibrated with melts systematically lower in mg# than noncumulate eucrites. Conceivably, the bulk mg# of the asteroid's silicate system was increased between primary-melt eucrite genesis and SOP diogenite genesis by graphite-fieled reduction of FeO. However, the graphite oxidation process generates a huge proportion of gas, which would have enhanced the buoyancy of the nascent diogenite-parent magmas, thus exacerbating the difficulty of achieving the implied high degrees of partial melting.To avoid these difficulties but still form most eucrites as rapidly cooled extrusives, I propose the NERD (noncumulate eucrites as extruded residua of diogenites) model. In this model, the diogenites form as early cumulates from a large magma system (probably a global "magma ocean") that yields a large proportion of eucritic melt as residuum. This residual melt zone undergoes relatively little crystallization during a period when it is episodically tapped to produce extrusions, dikes and sills of rapidly cooled noncumulate eucrites. Slight (-5-10%) porosity in the nascent eucritic crust keeps it marginally buoyant over the residual melt zone. The common thermal metamorphism of noncumulate eucrites results f...

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.…”

“…A fractional crystallization sequence then proceeded to form orthopyroxenites (diogenites), followed by increasing plagioclase content to form basalts (eucrites). Since the eucrites formed last in this model from the remaining melt, it is sometimes called the "residual liquid" model of eucrite petrogenesis (e.g., Warren, 1997). Alternatively, the original parent body formed and cooled, then underwent heating events that caused primary partial melts (Stolper, 1977).…”

Dawn: A journey in space and time

“…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.…”