HR: 09:00h
AN: V21F-05    [Abstracts]
TI: Extinct Plutonium Geochemistry of Ancient Hadean Zircons
AU: Turner, G
EM: grenville.turner@manchester.ac.uk
AF: SEAES, University of Manchester, Manchester, M13 9PL United Kingdom
AU: * Gilmour, J
EM: jamie.gilmour@manchester.ac.uk
AF: SEAES, University of Manchester, Manchester, M13 9PL United Kingdom
AU: Crowther, S
EM: sarah.crowther@manchester.ac.uk
AF: SEAES, University of Manchester, Manchester, M13 9PL United Kingdom
AU: Busfield, A
EM: a.busfield@manchester.ac.uk
AF: SEAES, University of Manchester, Manchester, M13 9PL United Kingdom
AU: Mojzsis, S
EM: Stephen.Mojzsis@colorado.edu
AF: Department of Geological Sciences, University of Colorado, Boulder, CO 80309-0399 United States
AU: Harrison, M
EM: mark.harrison@anu.edu.au
AF: RSES, Australian National University, Canberra, ACT 0200 Australia
AB: The abundance of 244Pu in the early solar system has important implications for r-process nucleosynthesis and models of noble gas transport within the Earth's mantle. Our recent discovery(1) of xenon isotopes from the in-situ decay of 244Pu in ancient Jack Hills zircons promises to provide a new time-sensitive window on the first 500 Ma of Earth history. We have extended this initial work by the use of resonance ioniisation mass spectrometry to analyse xenon released by stepped heating from 17 individual zircons with Pb-Pb ages in the range 3.95 to 4.18 Ga. Our immediate objectives are to determine the causes of variations in the inferred Pu/U ratios and in the longer term to determine the initial Pu/U ratio of the Earth. The Pu/U ratios calculated for individual zircons may be expected to vary as a result of igneous fractionation and also from differential loss of Pu and U fission xenon in the last 4 Ga. We have studied the effects of xenon loss by irradiating the zircons with thermal neutrons to generate xenon from 235U neutron fission in order to determine U/Xe ratios and apparent ages. 131Xe/134Xe and 132Xe/134Xe ratios can be used to calculate the relative contributions from 244Pu and 238U spontaneous fission and 235U neutron fission. The measured Pu/U ratios (back calculated to 4.56 Ga on the basis of the individual Pb-Pb ages) range from zero to 0.012. The highest ratio in our initial study was 0.008 (note that the published ratio has been revised upwards on the basis of improved decay parameters for 238U spontaneous fission). Comparison of Pb-Pb and U-Xe ages indicate varying amounts of xenon loss, over 50% in some cases. While this accounts for some of the variability in the inferred Pu/U, igneous fractionation may also play a part, and we are currently attempting to investigate this by a comparison with REE abundances. Reference: (1) Turner et al. (2004) Science, 306, 89-91.
DE: 8439 Physics and chemistry of magma bodies
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005