| Co-Investigator(Kenkyū-buntansha) |
TANAKA Tsuyoshi Geological Survey of Japan, Senior Reseacher, 主任研究官 (00236605)
NAKAI Shun'ichi The University of Tokyo, Faculty of Science, Research Associate, 理学部, 助手 (50188869)
AKAGI Tasuku The University of Tokyo, Faculty of Science, Research Associate, 理学部, 助手 (80184076)
SHIMIZU Hiroshi The University of Tokyo, Faculty of Science, Lecturer, 理学部, 講師 (60090544)
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| Research Abstract |
two new geochronometers, <^(138)La> - <^(138)Ce> and <^(138)La> - <^(138)Ba> systems, have been developed in this study, with some technical and/or scientific achievments in other geochronometers, Re-Os, Rb-Sr and Sm-Nd.
1. La-Ce systematics: (1) The <beta^-> decay constant of <^(138)La> was calculated to be 2.94x <10^(-12)> <yr^(-1)> by fitting of La-Ce ages to Sm-Nd ages for the Amitsoq gneiss, Greenland, and the Mustikkamaki pegmatite, Finland. (2) Ce isotopic study suggested that the positive Ce anomaly observed for lunar sample 14310 is considered to have been subjected to some oxic (hydrothermal/aqueous/ice) alteration in the early stage of the lunar history. (3) The Archean Amitsoq gneisses have been analyzed for Ce and Nb isotopic ratios and REE abundances. It was suggested that parent material of the Amitsoq gneisses had Ce and Nd isotopic ratios close to chondritic values at the time of their formation (ca. 3,800 Ma). (4) <^(138)Ce> / <^(142)Ce> and <^(143)Nd> / <^(144)Nd> rat
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ios in nineteen terrestrial rocks including MORB and metamorphic rocks from continental crust have been determined. It was disclosed that the rocks from continental crust were derived directly from the source with chondritic or liquid-type REE patterns. (5) Nd and Ce isotopic ratios for ferromanganese nodules from various Pacific regions indicate that the major source of Nd in these ferromanganese nodules is continental, while Ce is supplied mainly from abyssal volcanics. 2. La-Ba systematics: The geochronological utility of the <^(138)La> - <^(138)Ba> decay has been demonstrated for REE-rich minerals such as allanite, monazite and epidote. Fitting of La-Ba ages to Sm-Nb ages for Korsnas galena mine samples and Mustikkamaki pegmatite samples yielded the <lambda> <_(EC)> <^(138)La> value of 4.15x <10^(-12)> <yr^(-1)> . 3. Re-Os systematics: Osmium isotopes in three Os reagents, four terrestrial Os-Ir alloys (iridosmines) and five iron meteorites have been measured on the inductively coupled plasma mass spectrometer, with the precision of around 1% for <^(187)Os> / <^(192)Os> . Our studies on iron meteorites suggest that the formation ages for iron meteorites merit re-examination. Further, Re-Os ages were reported for molybdenite samples. 4. Rb-Sr systematics: Rb-Sr age for diogenite have been determined to be 4,400Ma which is 100Ma younger than the formation age of eucrite. Less
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