HR: 08:30h
AN: V41A-01    [Abstracts]
TI: Extinct Nuclides and the Origin and Earliest History of the Earth
AU: * Jacobsen, S B
EM: jacobsen@neodymium.harvard.edu
AF: Department of Earth and Planetary Sciences, 20 Oxford Street, Cambridge, MA 02138 United States
AB: 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.
DE: 1015 Composition of the core
DE: 1020 Composition of the crust
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly