The oxidation state of Hadean magmas and implications for early Earth’s atmosphere
Magmatic outgassing of volatiles from Earths interior probably played a critical part in determining the composition of the earliest atmosphere, more than 4,000 million years (Myr) ago. Given an elemental inventory of hydrogen, carbon, nitrogen, oxygen and sulphur, the identity of molecular species in gaseous volcanic emanations depends critically on the pressure (fugacity) of oxygen. Reduced melts having oxygen fugacities close to that defined by the ironwüstite buffer would yield volatile species such as CH4, H2, H2S, NH3 and CO, whereas melts close to the fayalitemagnetitequartz buffer would be similar to present-day conditions and would be dominated by H2O, CO2, SO2 and N2 (refs 14). Direct constraints on the oxidation state of terrestrial magmas before 3,850 Myr before present (that is, the Hadean eon) are tenuous because the rock record is sparse or absent. Samples from this earliest period of Earths history are limited to igneous detrital zircons that pre-date the known rock record, with ages approaching <4,400 Myr (refs 58). Here we report a redox-sensitive calibration to determine the oxidation state of Hadean magmatic melts that is based on the incorporation of cerium into zircon crystals. We find that the melts have average oxygen fugacities that are consistent with an oxidation state defined by the fayalitemagnetitequartz buffer, similar to present-day conditions. Moreover, selected Hadean zircons (having chemical characteristics consistent with crystallization specifically from mantle-derived melts) suggest oxygen fugacities similar to those of Archaean and present-day mantle-derived lavas as early as <4,350 Myr before present. These results suggest that outgassing of Earths interior later than <200 Myr into the history of Solar System formation would not have resulted in a reducing atmosphere.