For more than half a century, scientists have estimated the bulk chemical composition of Earth by comparison with its potential cosmic building blocks, as sampled by meteorites. In a conceptual breakthrough, Dauphas uses the unique isotopic content of different types of meteorite to identify those that best represent these building blocks. The author also evaluates whether the material added to Earth during its formation changed over time. Fischer-Gödde and Kleine show that not even the most recently accreted 0.5% of such material consisted of the type of meteorite long thought to be a major contributor to our planet's composition. This realization challenges our understanding of how Earth obtained its inventory of volatile elements and water.
In the 1970s, Earth was shown to have a different oxygen-isotope composition from most meteorites. The only meteorites that have a similar oxygen isotopic abundance are called enstatite chondrites, which are silicon-rich and highly reduced (most iron is in the form of metal or sulfide, rather than oxide). This similarity drove several models that based Earth's composition on enstatite chondrites. However, the mismatch in the elemental composition between such meteorites and Earth's rocks led most researchers to continue using models based on more-oxidized and volatile-rich meteorites known as carbonaceous chondrites.
Improvements in the ability to determine precise isotopic abundances led to the discovery that many elements can be used to distinguish between Earth and meteorites. In 2011, a study of these isotopic differences suggested that Earth was made from a mixture of meteorite types, not just the carbonaceous chondrites that had been the main component of most models. Dauphas takes this approach further by developing a methodology in which the isotopic disparity between different groups of meteorites and Earth can be used to track the composition of the materials that accreted to our planet throughout its formation.
The most important chemical differentiation event in Earth's history was the separation of its iron-metal core from its silicate mantle. When the core formed, elements that are more soluble in metal than in silicate were selectively removed from the mantle. Some elements (such as iridium, platinum, palladium and ruthenium) are so soluble in metal that the mantle should have been effectively stripped of them during core formation. However, the observed abundances of these elements in the mantle are in the same relative proportion as those seen in primitive meteorites. Furthermore, they are depleted by a factor of only about 350 with respect to their abundance in meteorites, compared with the million-fold depletion that would be expected were the mantle in chemical equilibrium with the core.”