HR: 1330h
AN: V13B-06    [Abstracts]
TI: Experiments of U Solubility in Earth's Core
AU: * Bao, X
EM: xbao@uwo.ca
AF: Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7 Canada
AU: Secco, R A
EM: secco@uwo.ca
AF: Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7 Canada
AU: Gagnon, J E
AF: Department of Earth Sciences, University of Windsor, Windsor, ON N9B 3P4 Canada
AU: Fryer, B J
AF: Department of Earth Sciences, University of Windsor, Windsor, ON N9B 3P4 Canada
AB: The experimental work of Murrell et al (1984) on the solubility of U in FeS was carried out at low pressure (< or =1.5GPa). Their work showed that U may combine with CaS in the FeS phase. Later, McDonough (2003) argued that if U entered the Earth^'s core with Ca, then the current Ca/Ti and Ca/Al ratios of the mantle would be difficult to explain. Here we investigate the solubility of U in pure Fe using a mixture of uraninite, Fe and peridotite powder at pressures of 3-7.5GPa and at temperatures of 1900-2000 °C. Our results show that U is soluble in Fe melts. Recovered run products were analyzed by LA-ICP-MS (detection limit for U: 0.0122 ppm) and by Electron Microprobe (EM, detection limit for U: 149 to 167 ppm). The average concentration of U in pure Fe from LA-ICP-MS is: 3GPa: 7 ppm; 5GPa: 55 ppm; 6GPa:102 ppm and 7.5GPa:126 ppm and D_U values, partition coefficient of U (concentration of U in Fe / concentration of U in silicate) at the above pressures are (×10^-3): 0.219; 1.82; 2.69 and 4.59, respectively. The EM data support the trend defined by the LA-ICP-MS data but are too close to the detection limit to provide quantitative results. It can be seen that U in the Fe phase generally increases with pressure. We also found that Si in the Fe phase increases from 0.45 wt% to 2.13wt% from 3GPa to 7.5GPa resulting in a positive correlation between Si and U in the Fe phase. This also can be seen qualitatively from the LA-ICP-MS line scan figures. The concentration of CaO is small (<0.02 wt%) and no relation with U is found, which indicates that U may alloy with Fe directly. If Si concentration in the Fe phase can be used as an indicator of oxygen fugacity (Kilburn and Wood, 1997), then the increase in Si and U with pressure suggests a pressure dependent decrease in oxygen fugacity. This supports U (and possibly also Si) inclusion in the Earth^'s core at the time of core formation.
DE: 1015 Composition of the core
DE: 3924 High-pressure behavior
DE: 8115 Core processes (1507)
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8147 Planetary interiors (5430, 5724)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly