Earth’s volatile contents established by melting and vaporisation

摘要

The silicate Earth is strongly depleted in moderately volatile elements (e.g. Pb, Zn, In, alkalis) relative to CI chondrites, the meteorites which compositionally most closely resemble the Sun. Qualitatively the depletion “trend” may be explained by accretion of 10-20% of a volatile-rich body to a reduced volatile-free protoEarth^, followed by partial extraction of some elements to the core. Several issues remain, however, notably the overabundance of In in silicate Earth which leads to questions about the sources of Earth’s volatiles^,. Here we have examined the melting processes which attended accretion on Earth and precursor bodies and performed vaporisation experiments under conditions of fixed temperature and oxygen partial pressure. We find that the pattern of volatile element depletion in silicate Earth is consistent with partial melting and vaporisation rather than with simple accretion of a volatile-rich chondrite-like body. We argue that melting and vaporisation on precursor bodies and possibly during the giant moon-forming impact^– was responsible for establishing the observed abundances of moderately volatile elements in the Earth.