It has long been recognized that Earth and other differentiated planetary bodies are chemically fractionated compared to primitive, chondritic meteorites and, by inference, the primordial disk from which they formed. However, it is not known whether the notable volatile depletions of planetary bodies are a consequence of accretion1 or inherited from prior nebular fractionation2. The isotopic compositions of the main constituents of planetary bodies can contribute to this debate3,4,5,6. Here we develop an analytical approach that corrects a major cause of measurement inaccuracy inherent in conventional methods, and show that all differentiated bodies have isotopically heavier magnesium compositions than chondritic meteorites. We argue that possible magnesium isotope fractionation during condensation of the solar nebula, core formation and silicate differentiation cannot explain these observations. However, isotopic fractionation between liquid and vapour, followed by vapour escape during accretionary growth of planetesimals, generates appropriate residual compositions. Our modelling implies that the isotopic compositions of magnesium, silicon and iron, and the relative abundances of the major elements of Earth and other planetary bodies, are a natural consequence of substantial (about 40 per cent by mass) vapour loss from growing planetesimals by this mechanism.
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We thank the Natural History Museum in London, NASA (National Aeronautics and Space Administration), O. Nebel, D. Ionov, S. Nielsen, E. Takazawa, K. Sims, Y. Niu, R. Brooker and C. Robinson for supplying us with various samples. We acknowledge C. Bierson for his help with direct outflow vapour loss modelling. This study was funded by NERC grant NE/L007428/1 to T.E., C.D.C. and M.J.W., which was motivated by NE/C0983/1. ERC Adv Grant 321209 ISONEB further supported the work of T.E. and C.D.C. NERC grant NE/K004778/1 to Z.M.L. funded P.J.C.
The authors declare no competing financial interests.
Reviewer Information Nature thanks F. Moynier and E. Young for their contribution to the peer review of this work.
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Extended data figures and tables
The compositions are plotted against their average literature oxygen isotope compositions53. These mass-dependent oxygen isotope measurements reflect parent body hydrothermal alteration20, so the correlation (R2 = 0.78) between Mg and O isotopes (as well as with the petrographic group54, indicated in brackets under sample names) implies that the Mg isotope compositions of some carbonaceous chondrites have been altered by hydrothermal processes. The most altered samples, to the upper right of this diagram, are excluded from our chondrite Mg isotope means.
The compositions are plotted against whole rock MgO (a) and Al2O3 (b) contents. The absence of correlations of Mg isotope compositions with MgO or Al2O3 indicates the absence of discernible Mg isotope fractionation during partial melting.
Extended Data Figure 3 Comparison between modelled compositions of a vapour-depleted liquid and observed planetary compositions.
As Fig. 3, showing a, Modelled changes in isotope compositions (‰ per amu) against total relative vapour loss (Ftotal, in mole fractions). b, Observed isotope compositions relative to enstatite chondrites (errors are 2 s.e.m.). c, Loss (mole fraction) of a given element (X), fX, versus Ftotal. d, Molar element/Ca ratio of the terrestrial mantle7, normalized to that of enstatite chondrites31 ((X/Ca)EH). Compared to Fig. 3, observed isotope compositions for Mars, and eucrite and angrite parents bodies as well as elemental and isotopic Fe observations are additionally included (b and d; Fe isotope data from ref. 55 and references therein, all other references as in Fig. 3). Comparison of observed Fe contents and isotope ratios are complicated by core formation because most Fe enters the core. In our model we assume that the iron in the core has not been affected by vaporization, inferred to occur later. For instance, the effect of ~48% Fe loss (c) on the current bulk silicate Earth Fe content is dependent on the fraction of Fe that entered the core before collisional vaporization and the oxygen fugacity evolution of the growing Earth. For reference, the datum labelled Fe** in d is therefore the Fe/Ca of the bulk Earth (calculated from ref. 56) instead of the Fe/Ca of the bulk silicate Earth. Similarly, Si can also enter the core, although its quantity is likely to be <3 wt% (ref. 57). Right-pointing arrow in a indicates the effect of 3 wt% Si in the core (3,000 K assumed for metal–silicate Si isotope fractionation factor58).
Extended Data Figure 4 Comparison between modelled compositions of a vapour depleted liquid and observed planetary compositions.
Similar to Extended Data Fig. 3, but for model runs with a CI chondrite initial composition. a, Modelled changes in isotope compositions (‰ per amu) against total relative vapour loss (Ftotal, in mole fractions). b, Observed isotope compositions relative to CI chondrites (errors are 2 s.e.m.). Note that observed Mg and Fe isotope compositions are presented relative to their chondritic mean, whereas Si isotope observations are relative to a mean of carbonaceous and ordinary chondrites59, because those chondrites have indistinguishable Si isotope compositions, yet are distinctly different from enstatite chondrites (see ref. 4 and references therein). c, Loss (mole fraction) of a given element (X), fX, versus Ftotal. d, Molar element/Ca ratio of the terrestrial mantle7, normalized to that of CI chondrites7 ((X/Ca)CI). As in Extended Data Fig. 3, Fe** in d is the Fe/Ca of the bulk Earth.
Extended Data Figure 5 Magnesium isotope compositions of reference samples analysed in multiple studies.
The shaded areas show the mean and 2 s.e.m. of the isotope compositions observed in this study. Data are from this study and refs 5, 10, 11, 12, 13, 14, 16, 17, 60, 62, 62. Note that the plotted composition of Murchison from ref. 10 is a mean of the two replicates presented in table 1 of ref. 10. The value for BHVO from ref. 16 is BHVO-1, all others are BHVO-2.
Extended Data Figure 6 Variation in velocity of individual impacts (normalized by target-body escape velocity) as a function of target body radius.
Central line denotes median value, shaded box encompasses the region spanning the 25th to 75th percentiles, upper lines denote the 90th percentile. Bulk density is assumed to be 3,000 kg m−3.
Extended Data Figure 7 Fractional mass loss in the Grand Tack simulation as a function of final body radius for the direct vapour outflow model.
This illustrates results both with (white boxes, as Fig. 2b) and without (shaded boxes) the inclusion of inheritance effects (see Methods). Boxes denote the median value, bars denote the 25th and 75th percentiles.
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Hin, R., Coath, C., Carter, P. et al. Magnesium isotope evidence that accretional vapour loss shapes planetary compositions. Nature 549, 511–515 (2017). https://doi.org/10.1038/nature23899
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