Redox state of early magmas

Abstract

A study of cerium in zircon minerals has allowed an assessment of the redox conditions that prevailed when Earth's earliest magmas formed. The results suggest that the mantle became oxidized sooner than had been thought. See Letter p.79 The composition of Earth's earliest atmosphere, which accumulated more than four billion years ago during the Hadean eon, may have been influenced by magmatic outgassing of volatiles from Earth's interior. This paper reports a redox-sensitive calibration to determine the oxidation state of Hadean magmatic melts based on the incorporation of cerium into zircon crystals. The authors find that the melts have oxygen fugacities that are consistent with the idea that Earth's mantle reached its present-day oxidation state as early as 4.35 billion years ago. The findings suggest that outgassing of Earth's interior about 200 million years into the history of Solar System formation would not have resulted in a reducing atmosphere.