Tracking the formation of magma oceans in the Solar System using stable magnesium isotopes

Abstract

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 ...

Lead and Mg isotopic age constraints on the evolution of the HED parent body

“…The mass‐dependent Mg isotope composition of NWA 4215 and Dhofar 700 has already been reported in Schiller et al. () and is comparable to that of other diogenites reported by Schiller et al. () when accounting for the isotopic difference between the DTS‐2b standard used in this work and the more commonly used DSM‐3 standard (Table ).…”

Multi-element ion-exchange chromatography and high-precision MC-ICP-MS isotope analysis of Mg and Ti from sub-mm-sized meteorite inclusions

“…The extremely sluggish kinetics of Cr speciation in acidic media at room temperature and the large differences in the selectivity of a cation exchanger for various Cr(III) species are known to be problematic for high Cr recovery through chromatographic Cr purication. 22 Thus, to speciate >97% of the Cr into the Cr(H 2 O) 6 3+ form characterized by a high affinity for the cation resin, we used a sample pre-treatment procedure involving dissolution in 2 M HNO 3 followed by dilution and equilibration in 0. Fig.…”

“…2 Studying their isotope composition potentially provides constraints on, for example, the timing and efficiency of planetary differentiation processes, [3][4][5][6] mixing and transport of primitive components in the solar protoplanetary disk, 7-9 thermal processing of presolar dust, 10 nucleosynthetic sources that contributed to the astrophysical birth environment of our young Sun, 11 biogeochemical processes on Earth 12 and evaporation and condensation processes. 13,14 Given that Mg is a moderately volatile element and Ti a highly refractory one, potential differences in the degree of stable isotope fractionation may discern the condensation and/or evaporation history of CAIs.…”

Magnesium stable isotopes support the lunar magma ocean cumulate remelting model for mare basalts

“…Many experimental and theoretical models along with elemental and isotopic data of lunar samples, including both stable and radiogenic isotopes, were used to determine the Moon's origin and magmatic evolution as well as the origin of lunar basalts, which are presumably partial melts of the cumulates produced during the LMO crystallization (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). Over the last 2 decades, the nontraditional stable isotopes have also provided new insights into the accretion and magmatic evolution of planetary bodies including the Moon (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35). Most of the data (except for volatile elements such as K and Zn) show that among bodies in the solar system the bulk silicate Earth (BSE) and bulk silicate Moon (BSM) are uniquely similar, despite some variations among the lunar rocks.…”

“…Most of the data (except for volatile elements such as K and Zn) show that among bodies in the solar system the bulk silicate Earth (BSE) and bulk silicate Moon (BSM) are uniquely similar, despite some variations among the lunar rocks. Magnesium, a major element with three stable isotopes, is potentially an important tool to study the Moon's early magmatic differentiation because its isotopic fractionation is only influenced by mineral crystallization and is not affected by core formation processes (23,31,(35)(36)(37)(38)(39). Most studies indicate that Mg isotopic compositions in the inner solar system are homogeneous and vary perhaps as a result of igneous differentiation processes (22,23,36,(38)(39)(40)(41)(42).…”