Chondrites are thought to chemically represent the building blocks of the Earth. However, refractory element (e.g. Al) contents in the silicate Earth relative to many other elements are not the same as in any chondrite . Although broadly explained by core formation and nebular volatile depletion, in detail such processes cannot explain all deviations relative to chondrites.Several studies have investigated mass-dependent fractionation of Mg isotopes in Earth and chondrites to better constrain Earth's accretion history. Results have varied, most likely due to analytical issues. We have developed a critical mixture double spiking procedure to generate more accurate and precise isotope ratios in systems with three isotopes, such as Mg . Our Mg isotope analyses show that Earth has 25Mg/24Mg significantly heavier by 0.02‰ than chondrites, as are samples from Mars and the eucrite and angrite parent bodies . We propose that vapour-liquid fractionation during vapour loss from planetesimals is the most plausible explanation for the heavy 25Mg/24Mg of differentiated planetary bodies. We have simulated the chemical consequences of such vapour loss for Mg, Si and Fe isotope ratios and for concentrations of nine major and minor elements. Constrained by the observed Mg isotope difference between Earth and chondrites, the model implies that 40% vapour loss occurred. Such vapour loss transforms the initially chondritic element and isotope ratios of the residual silicate into approximately the bulk silicate Earth composition. Physical models suggest that impacts between planetesimals can cause substantial melting and that vapour associated with this melt escapes. Our models also imply that total vapour losses may accumulate to tens of percent, supporting the hypothesis that accretionary loss of 40% vapour from planetesimals early in Earth's accretion history may have shaped our planet's composition. A consequence of such large vapour loss is that the planetesimals from which the early Earth grew were void of volatiles and that these were delivered by volatile-rich material late in Earth's accretion.  Palme, H. and O'Neill, H.S.C., 2003. In: The Mantle and Core, Carlson R.W. (Ed).  Coath, C.D. et al., 2017. Chem. Geol. 451, 78-89.  Hin, R.C. et al., 2017, Nature 549, 511-515.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- 1009 Geochemical modeling;
- GEOCHEMISTRYDE: 1038 Mantle processes;
- GEOCHEMISTRYDE: 1041 Stable isotope geochemistry;
- GEOCHEMISTRYDE: 1060 Planetary geochemistry;