HR: 1330h
AN: V42D-0389    [PDF]
TI: Ion Microprobe U-Pb Dating and REE Analysis of Apatite from Kerogen-rich Silica Dike from North Pole Area, Pilbara Craton, Western Australia
AU: * Nishizawa, M
EM: nishizawa@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, The University of Tokyo, Minamidai 1-15-1, Nakanoku, Tokyo, 164-8639 Japan
AB: In order to provide a time constraint on the 13C-depleted kerogen in silica dikes that intruded 3.5 Ga greenstone from Pilbara Craton in Western Australia, we have carried out an ion microprobe U-Pb dating and rare earth element (REE) analysis of apatite from the dike. Two types of apatite were identified in the dikes based on their occurrences. One is stick-shape apatites (Type 1) in secondary silica micro-veins that cut the silica dike. The other is granular apatites (Type 2) that occurs in matrix of the dike. Occurrence in the secondary micro-veins (Type 1), non-igenous chondrite normalized REE patterns (Type 1 and 2), chemical zoning (some of Type 1 and 2), and presence of mineral inclusion that is composed of Fe and S (some of Type 2) suggest that both Type 1 and 2 apatites were crystallized in the silica dike. Ion microprobe U-Pb dating of Type 1 apatite did not give a meaningful age, while Type 2 apatite yields a Tera-Wasserburg concordia intercept age of 3214 +/- 140 Ma (95 per cent confidence level, MSWD = 0.6) in a three-dimensional 238U/206Pb-207Pb/206Pb-204Pb/206Pb diagram, and a 204Pb/206Pb-207Pb/206Pb isochron age of 3191 +/- 150 Ma (95 per cent confidence level, MSWD = 0.5). It is difficult to judge whether the U-Pb and Pb-Pb age of Type 2 apatite is crystallization age or metamorphic age, since the estimated range of closure temperature of U-Pb system in the apatite and that of metamorphic temperature is partly overlapped. In either case, it can be safely concluded that the minimum age of the dike and kerogen is 3.0 Ga. These ages might allow the interpretation that the kerogen was produced by biological carbon fixation and/or abiological reaction (such as Fischer-Tropsch Type reaction) at least before 3.0 Ga.
DE: 1035 Geochronology
DE: 1065 Trace elements (3670)
DE: 9619 Precambrian
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting