Geochemistry: Evaporating planetesimals.

Abstract

In comparison to primitive, chondritic meteorites, which are widely thought to be the building blocks of Earth, Earth and other differentiated planetary bodies are chemically fractionated, with Earth's crust and mantlethe 'silicate Earth'being strongly depleted in moderately volatile elements (such as lead, zinc, indium and the alkali metals). Two papers in this week's issue suggest that this difference in composition between chondritic meteorites and Earth could be a natural consequence of vapour loss from magma on the surface of growing planetesimals. Ashley Norris and Bernard Wood examined the melting processes that would have occurred during accretion on Earth and its precursor bodies and performed vaporization experiments under conditions of fixed temperature and oxygen partial pressure. They found that the pattern of volatile-element depletion in the silicate Earth is consistent with partial melting and vaporization rather than with simple accretion of a volatile-rich chondrite-like body. Remco Hin and co-authors show that differentiated planetary bodies have isotopically heavier magnesium compositions compared to chondritic meteorites, and conclude that this could be due to the isotopic fractionation between liquid and vapour, followed by vapour escape during accretionary growth of planetesimals.