Origin of volatile elements in terrestrial planets

Sulfur depletion in the Earth’s mantle: a consequence of both volatilization and core segregation.

Sulfur (S) is both volatile (easily vaporized) and siderophile (has affinity with metal phases). The origin of sulfur on Earth is a source of controversy, as it could have been volatilized during the Earth’s accretion and trapped in the core during its segregation. S is a major element in chondritic meteorites (2 to 6 wt% S), but relatively depleted in the Earth’s mantle (200 ppm). Several previous studies have argued that S was not accreted to the Earth in its early stages of formation, and instead was added to the mantle at the last meteoritic bombardments, following the end of core segregation. I conducted experiments at high pressure and temperature in the Laboratoire Magmas et Volcans (Clermont-Ferrand, France) to precisely constrain the distribution of S between metal and silicate. The results showed that sulfur terrestrial abundance can result from an equilibrium between core and mantle and does not formally require a sulfur addition in the late meteoritic bombardments (Boujibar et al. 2014, EPSL). These findings suggest that S was likely present in non-gaseous phases all along the Earth’s accretion in the inner regions of the Solar System.

Segregation of alkali elements (Na, K, Cs & Rb) in planetary cores.

The Earth’s and Mars' mantles are depleted in several other volatile elements. Some of these elements are thought to remain exclusively in the mantles (lithophile elements). They are therefore commonly used as direct tracers of volatile depletion. This paradigm was tested at the Geophysical Laboratory of the Carnegie Institution for Science. I investigated how alkali elements (Na, Rb and Cs), which are volatile and usually considered as lithophile, distribute between sulfide and silicate liquids. Sulfide liquids could have been segregated into planetary cores if the building blocks of terrestrial planets contained significant amounts of sulfur. Our experiments indicate that alkalis partition efficiently into sulfides that contain high levels of O and at high temperature (see Figures on the right). This work shows that the cores of Earth, Mars and the asteroid Vesta can be important reservoirs for alkali elements, if sulfides were involved during core-mantle differentiation (Boujibar et al., 2020, GCA).