Vesta (Greenwood et al., 2005), or giant impacts such as the Moon-forming event in the case of the Earth and Moon (Tonks and Melosh, 1993). As such, finding robust geochemical fingerprints of these ancient magma oceans is important for understanding the earliest stages of planetary evolution.A range of newly developed stable isotope systems are yielding novel insights into planetary accretion, differentiation and evolution (e.g., Greenwood et al., 2005;Georg et al., 2007). However, many of these systems are multiply affected by a range of processes, potentially including core formation and crystallisation of accessory minerals, which makes interpretation of such data challenging. The lithophile, major element magnesium (Mg) is almost unique in this regard, as stable isotope fractionation in magmatic systems will be almost entirely controlled by crystallisation of high-Mg, mafic minerals, assuming isotopic fractionations exist between such minerals and magma.The large degree of planetary melting required to generate a magma ocean would produce highly magnesian magmas, which would then subsequently crystallise large amounts of mafic, Mg-rich minerals such as olivine. Small Mg stable isotope fractionations exist between co-existing terrestrial mantle olivine, orthopyroxene and clinopyroxene, with olivine being the isotopically lightest phase (Handler et al., 2009;Young et al., 2009;Pogge von Strandmann et al., 2011;Xiao et al., 2013). Consequently, it should also be possible to detect progressive Mg isotope changes in the products of a crystallising magma ocean.Vesta is the second largest asteroid in the Solar System and comprises a metal core, silicate mantle and crust (Russell et al., 2012). Based on spectral observations and the Dawn Mission, the howardite-eucrite-diogenite (HED) meteorite suite is inferred to come from Vesta (McCord et al., 1970;Binzel and Xu, 1993; Russell et al., 2012). Most HED meteorites also share common nucleosynthetic isotope signatures for some elements that demonstrate their genetic affinity (Greenwood et al., 2005). These lithologically diverse meteorites provide a unique archive of the timing and processes of protoplanet formation and differentiation. Diogenites are mostly orthopyroxenites, and are conventionally viewed as cumulate igneous rocks formed in a magma ocean or bodies on Vesta, whereas eucrites are basaltic and gabbroic rocks predominantly composed of pigeonite and plagioclase (Mittlefehldt, 2014).Despite many decades of petrological, geochemical and chronological study, the petrogenesis of eucrites and diogenites and the relationship between them remain enigmatic. End-member models include limited partial melting or extensive (i.e. magma ocean) melting of Vesta (Stolper, 1977; Mandler and ElkinsTanton, 2013;Mittlefehldt, 2014). Nearly all models are difficult to reconcile with the large range of incompatible trace element concentrations in diogenites and eucrites. However, a recent study by Mandler and Elkins-Tanton (2013) has used both chemical and physical arguments to ...

<p>Meteorites provide the only direct record of the chronology and nature of the processes that occurred in the early solar system. In this study, meteorites were examined in order to gain insight into the timing and nature of magmatism and silicate differentiation on asteroidal bodies in the first few million years of the solar system. These bodies are considered the precursors to terrestrial planets, and as such they provide information about conditions in the solar system at the time of planet formation. This study focuses on eucrites, which are basaltic meteorites that are believed to represent the crust of the Howardite-Eucrite-Diogenite (HED) parent body. The processes of silicate differentiation and the relationship between eucrites and the diogenitic mafic cumulate of the HED parent body are poorly understood. The major and trace element chemistry of the minerals in the eucrite suite was measured. There is little variability in mineral major element concentrations in eucrites, however considerable variability was observed in mineral trace element concentrations, particularly with respect to incompatible elements in the mineral phases. Magnesium was separated from digested eucrite samples, and the Mg isotope composition of the eucrites was measured to high precision in order to date the samples using the short-lived ²⁶Al–²⁶Mg chronometer and examine magmatic evolution on the HED parent body. Correlations between incompatible elements in pyroxene and ²⁶Mg anomalies, produced by the decay of ²⁶Al, indicate that the eucrite suite was formed from a single, evolving magma body. Large trace element and Mg isotopic differences between eucrites and diogenites indicate that the two meteorite groups did not, as previously suggested, originate from the same magma body. Instead they may have formed from two large magma bodies, which were spatially or temporally separated on the HED parent body. The application of the short-lived ²⁶Al–²⁶Mg chronometer to this suite of eucrites constrains the onset of eucrite formation to ~3 Myr after the formation of the solar system’s first solids, as a result of rapid accretion and melting of planetesimals due to heating from the decay of ²⁶Al.</p>