Cenozoic alkali basalts (1-12 Ma) in the Chugoku district, SW Japan, are divided into two types: 1) MF-type basalts characterized by high MgO, FeO and TiO2, and 2) SA-type basalts rich in SiO2 and Al2O3. They are also distinguished in terms of the TiO2/MnO/P2O5 tectonomag matic discriminant diagram and the ƒÃNd-87Sr/86Sr systematics. The MF-type basalts are similar in petrochernistry to oceanic island alkali basalts, while most of the SA-type basalts show the features of arc basalts. The isotopic data and trace element abundances of the rocks indicate that the SA-type basalts are not derivatives from the MF-type basalt magma. These two types of the alkali basalts should be produced from the different mantle sources. The distribution of the SA-type basalts with arc signature is restricted to the back-arc side (Japan Sea side) of the Chugoku district. The spatial variation in Sr and Nd isotopic composition of the MF-type basalts show a reverse tendency to that shown by the Quaternary volcanics from NE Japan. These facts cannot be ascribed to subduction effect of the Pacific plate or of the Philippine Sea plate. The Cenozoic alkali basalts with oceanic island basalt signature from the Eastern Asia, including the MF-type basalts, show the highest 87Sr/86Sr and lowest 143Nd/144Nd ratios in the southern Japan Sea, and these ratios become lower and higher with distance from this area to both sides, respectively.
Greenstones, representing remnants of paleo‐oceanic crust, occur in Permian and Jurassic accretionary complexes of the Inner Zone in the Southwestern Japan arc. The formation age of most of the greenstones is early Carboniferous, based on fossil ages for overlying limestones and Sm‐Nd isotope ages of the greenstones themselves. The geochemistry of such greenstones is similar to those of present‐day oceanic islands. Greenstones of the Permian accretionary complex (Akiyoshi belt) are alkalic and tholeiitic in composition. Some alkali basalts show peculiar features from an EM‐1 mantle source, such as the Gough Island and Tristan da Chunha basalts in the South Atlantic. Greenstones of the Jurassic accretionary complex (Tamba belt) are also alkali and tholeiitic basalts with both basalt types in the northern part of the Tamba belt coming from strongly depleted characters similar to a mid‐ocean ridge basalt source mantle. The variable geochemistry of the oceanic basalts is explained by hypothesis on existence of a Carboniferous mantle plume below the spreading ridge which divides the Farallon and Izanagi plates. The Akiyoshi belt seamounts and/or oceanic islands of the Farallon plate and Tamba belt seamounts and/or oceanic islands of the Izanagi plate formed simultaneously by the upwelling of the thermal plume. Some part of the Akiyoshi belt basalts originated locally from an EM‐1 mantle source, while basalts from the northern parts of the Tamba belt have a normal‐type mid‐ocean ridge basalt (N‐MORB) source component. Existence of an N‐MORB signature is consistent with the presence of a spreading center in a Carboniferous ‘Pacific Ocean’ that caused separation of the Farallon and Izanagi plates. Disparity in accretion ages of the basaltic rocks in the Permian and Jurassic may have been caused by differences in the relative motion of the two plates.
The alpha radioactive components in the Hemp-palm of Bontenchiku were determined with emphasis on the measurement low-level 237Np by alpha-ray spectrometry after chemical separation. Bontenchiku is a kind of fishing gear for long-line fishing used by the Fifth Fukuryu-Maru (Lucky Dragon). This gear was exposed to fallout from the second thermonuclear test explosion (Bravo) at Bikini Atoll in March 1954. The 237Np content in the Bontenchiku sample was determined to be 11.5 +/- 0.8 mBq g(-1), with an activity ratio of 237Np:239,240Pu and an atom ratio of 237Np:239Pu estimated to be (2.2 +/- 0.2) x 10(-3) and 0.42 +/- 0.04, respectively. The data showed the existence of a chain reaction of 238U and its ratio to be 237Np:239Pu, as well as the presence of 237U at the time of fallout from Bravo event in March 1954.
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