PLANETARY BASALT EVIDENCE FOR SULFIDE CONTROL OF FE FRACTIONATION IN THE INNER SOLAR SYSTEM

The compositions of basalts of the terrestrial planets document a radial decrease in the Fe contents of their mantles that likely reflects the fractionation of Fe during the condensation of the Solar nebula. Orbital dynamic arguments for wide mixing zones during the accretion of the terrestrial planets have overshadowed the classic demonstration by Lewis (1972) that the metal/silicate ratios of the planets are broadly consistent with that predicted by the solar condensation sequence for a radial decrease in temperature in an adiabatic cloud. The large increase in planetary basalt data in the decades since provides an opportunity to revisit this possibility. As predicted by Lewis, planetary basalt data indicates that the Fe content of the Mercurian mantle appears to be the lowest in the solar system, in contrast the mantles of Mars and 4 Vesta (Eucrite meteorites) have the highest Fe contents, while the Venusian and modern Terrestrial mantles have intermediate Fe values. This radial decrease in Fe content is not, however, continuous, as basalts of Venus and the Earth have similar Fe contents, whereas there is a discontinuous increase to the Fe contents of Mars and Eucrites. A similar discontinuous increase exists between Archean komatiites and ferropicrites on the Earth and lunar mare basalts, with the latter two having the Fe contents of Martian and Eucrite basalts. Broadly there are 2 distinct basalt populations, one poor in Fe, but high in Al, characteristic of the inner terrestrial planets, and the other rich in Fe, but poor in Al, characteristic of the Moon, the early Earth, Mars, and the asteroid belt. This dichotomy may be produced by a “sulfide line” in the solar nebula at approximately 700K near the vicinity of the Earth. Sunward of the sulfide line, the Fe content of silicate condensates increases radially with decreasing temperature through equilibration with metallic Fe. Outside the sulfide line, however, the Fe content of silicate condensates will be buffered at a relatively high value, determined by the relative solar abundance of Fe and S. The presence of minor ferropicrites in the Archean on the Earth may indicate the proximity of the Earth to the sulfide line, resulting in the co-accretion of these 2 distinct silicate condensates.