Isotopic composition of lead and concentrations of uranium, thorium, and lead in volcanic rocks from Dogo of the Oki Islands, Japan

摘要: The isotopic composition of lead and the abundances lead, uranium, thorium in alkali rocks from Dogo Oki Islands, Taka-shima, West Japan, have been determined. uranium contents range 2.4 to 21.7 ppm, 0.4 3.6 2.6 27.1 respectively. They show a slight increase with progress magmatic differentiation large trachyte rhyolite. A μ0 (238U/204Pb) = 8.6∼8.7 is calculated for primary growth curve leads Dogo. observed μ values fall 8.4∼13.5. κ (232Th/238U) are 4.8∼9.8. higher than tholeiites highalumina basalts Japan. relatively deficient 206Pb compared model, suggesting that at least some was derived source low during last 2∼3 b.y. Consequently these volcanic isochron model ages around 0∼500 m.y., which older known age rocks. In addition, olivine isotopically uniform more radiogenic 206Pb/204Pb trachybasalt series differentiates. Either trachybasalts were generated different rest or, if all single source, they subsequently acquired those source. Natural contamination can explain peculiar results islands, where partial assimilation pre-Tertiary basement rocks, probably Precambrian resulted less isotope ratios differentiates islands.