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 (f(O2)) near that of the Iron-Wustite (IW) f(O2) buffer. Conversely, the most ancient rocky objects of the solar system, the calcium aluminum-rich inclusions (CAIs), formed at f(O2) 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 f(O2) for rims on CAIs show that this shift from similar to solar to protoplanetary (chondritic) f(O2) occurred in 100,000 to 300,000 yr for these objects. Magnesium isotopes show further that the rise in f(O2) 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 x 10(5) yr of their formation in the solar nebula. (c) 2005 Elsevier B.V. All rights reserved.