The First Terrestrial Pb-isotope Paradox and the Core Formation

Details The First Terrestrial Pb-isotope Paradox and the Core Formation The First Terrestrial Pb-isotope Paradox and the Core Formation

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmmr41a..05m&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #MR41A-05

Physics

8115 Core Processes (1507), 7207 Core And Mantle, 3924 High-Pressure Behavior, 1212 Earth'S Interior: Composition And State (8105), 1507 Core Processes (8115)

Scientific paper

The first terrestrial Pb-isotope paradox refers to the fact that the Pb isotope compositions of most marine sediments and MORB plot well to the right of the area defined by meteorites in 207Pb/204Pb vs. 206Pb/204Pb space. This paradox requires at least one reservoir formed in the early Earth's history that plots to the left of the meteorite isochron. The core is one candidate that have been widely discussed in the past. In order to quantify possible fractionation into the metallic core, we have done different set of multi-anvil experiments at high pressure (up to 20 GPa) and high temperature (1900°C). The partitioning of Pb between liquid metal and silicates is investigated at different oxygen fugacity by using different metallic starting compositions. The oxygen fugacity in most experiments has been calculated by using two independent buffers : iron/wüstite (IW) and SiO2/Si, allowing to link consistently the Fe contents in silicates, the Si contents in metal and the temperatures of the experiments. The concentration of Pb in metal and in silicates has been measured by electron microprobe and ?PIXE. At oxygen fugacities 3 log units below IW, we have found partition coefficients between metal and silicate melt ranging from 6 to 9 depending on the starting composition of the metal. A first conclusion for this study is that Pb would remain siderophile even with a metal without sulphur. These first results heighten the importance of :(1) the hypothesis that the fisrt Pb-paradox could be the result of the Pb incorporation into a metallic phase at the time of core formation, (2) doing more experimental work at different pressures, temperatures and fO2 relevant to the early Earth.

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