ABSTRACT

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