Extinct Nuclides and the Origin and Earliest History of the Earth

There are well established and measurable 129Xe/130Xe, 136Xe/130Xe, 182W/183W and 142Nd/144Nd isotopic variations in the Earth due to decay of the now extinct nuclides 129I, 244Pu, 182Hf and 146Sm. These nuclides record primarily differentiation processes which acted during the first ~300 million years of Earth history (4.57-4.25 Ga). Tracing isotopic variations of these systems through the geologic history may be used to study re-mixing of early formed reservoirs. This may, for example, provide a simple way of determining the rate of crustal recycling into the mantle. Among these systems the 182Hf-182W system is the only one directly related to the core formation process (and, by inference, accretion) of Earth. The 182Hf-182W system provides strong constraints which yield a mean time of accretion and core formation of ~10 Ma. The coupled 146Sm - 147Sm chronometer makes it possible to constrain the age of the initial silicate differentiation in the mantle source region of some of the Earth's oldest surviving crustal rocks to ~4.47 Ga. This also puts an upper limit on the total accretion interval of ~100 Ma which is consistent with the extraction of a primordial crust enriched in LREE from the mantle after the last giant impact on Earth. The I-Pu-Xe system is fundamentally unreliable as a chronometer of accretion. The presence of a large 129Xe excess in the deep Earth is consistent with a very early formation and a short time interval for the accretion of the Earth, but may primarily date a combination of gas/dust fractionation in the solar nebula and the closure of the atmosphere, rather than accretion itself.