A short timescale for changing oxygen fugacity in the solar nebula revealed by high-resolution 26Al-26Mg dating of CAI rims

Most rocky objects in the solar system, including the primitive chondrites and the terrestrial planets themselves, formed at oxygen fugacities (fO2) near that of the Iron-Wüstite (IW) fO2 buffer. Conversely, the most ancient rocky objects of the solar system, the calcium aluminum-rich inclusions (CAIs), formed at fO2 values 5 orders of magnitude lower than the IW buffer in an environment more closely resembling a solar gas. High-resolution Mg isotope data and estimates for fO2 for rims on CAIs show that this shift from ∼solar to protoplanetary (chondritic) fO2 occurred in 100,000 to 300,000 yr for these objects. Magnesium isotopes show further that the rise in fO2 was accompanied by a rise in the partial pressure of Mg. These results establish that CAIs entered a region resembling where planet progenitors formed within 3 × 105 yr of their formation in the solar nebula.