The behavior of rare-earth elements (REE) in the Oklo natural nuclear reactors was studied. Isotopic ratios of all REE in the Oklo uranium ore samples were measured with a thermal ionization mass spectrometer, and the REE abundances were determined by the stable isotope dilution method. On the basis of these data, the behavior of REE in the process of formation of ore body and during and/or after the operation of the natural nuclear reactor was investigated. The total amount for each of the REE is allotted to natural and fissiongenic fractions by a calculation based on the nuclear data. The pattern for pre-reactor natural REE explicitly endorses the participation of water in formation of this ore deposit. Meanwhile, a fission yield curve involving Nd, Sm and Gd is found to be generally smooth. But deviations in La and Ce, and shifts among the fissiongenic segments for Nd, Sm and Gd are observed. These features can be accounted for as reflecting the systematic differential partial loss of fissiongenic light REE. The partial loss of progenitors of 140Ce and 142Ce is found to be unlikely.
A vacuum ball mill was devised to extract volatiles from fluid inclusions in minerals. A special feature of the crushing mode is the horizontal oscillation of the mill, which enhances crushing efficiency and overcomes the fragility of the mill, which is made of Pyrex glass. This ball mill is equipped with a cold finger trap cooled with liquid nitrogen to reduce the influence of adsorption of once extracted volatiles on to mineral powder during the crushing process. Amounts and isotope ratios were determined for water extracted from the fluid inclusins of quartz and halite. The determinations were made for CO2 as well in fluid inclusions of quartz. The influence of adsorption on amounts and isotopic ratios of volatiles was evaluated by comparing the results of repeated extractions with and without the liquid nitrogen trap during the crushing process. In a single run of extraction, two kinds of water were obtained: one was water trapped in the cold finger trap during the crushing, the other was desorbed water given by heating of the sample after crushing. A simple summation of the isotope ratios of the desorbd and the trapped waters gives an erratic result in comparison with the original isotope ratio. As for the fluid inclusions in hydrothermal quartz, the original δ18O and δD values for water in the inclusions can be estimated by applying Rayleigh's equation to the adsorption process. In the case of halite samples, however, the estimation of the original δ18O and δD values is much more difficult because of the formation of hydrate salts. However, in the special case where the brine in inclusions is a pure NaCl solution, the original inclusion water is simply the sum of the desorbed and trapped waters. For CO2 analysis of inclusion fluids, the adsorption during the crushing process is negligibly small, not affecting appreciably the results on δ13C and δ18O values.
Rb-Sr whole rock isochron ages have been determined on Late Cretaceous granites occurring in the northern Shikoku, Shodo-shima and Okayama regions. They are 93.1 ± 2.9 Ma (Takanawa Peninsula), 82.1 ± 3.0. Ma (East Sanuki and Shodo-shima), 93 ± 28 Ma (Shirotori granite in East Sanuki) and 84.0 ± 3.7 Ma (Okayama region). Compilation of radiometric age data shows that igneous activity of the age from 120-90 Ma covered almost the whole area of the Chugoku and northern Shikoku Province, and that succeeding activity of the age from 86-80 Ma gradually migrated eastward from the mid Chugoku-northern Shikoku area. On the other hand, igneous activity of the age from 75-60 Ma took place in a narrow zone along the Japan Sea coast, suggesting that an abrupt change in geological situation of igneous activity occurred at around 75 Ma. The field of igneous activities gradually narrowed toward the central area of the Japan Sea coast from Late Cretaceous to Paleogene. This migration of igneous activity may be assumed to have started around 75 Ma, when the field of igneous activities abruptly changed.
Submarine hydrothermal activity can hardly be detected by measuring water temperature or found by conventional methods like 3He, CH4 and/or metal ions measurements except through time consuming procedures. We applied a rapid and ultra-sensitive colorimetric method to measure the concentration of lipopolysaccharide (LPS), an indicator of bacterial biomass, in seawater around the Izu-Ogasawara Arc, northwestern Pacific, where the presence of hydrothermal activity is suggested. High anomalies of LPS at ca. 50 m-water layer above the sea bottom were discovered in a submarine caldera along the volcanic front and in a back-arc depression. Many extraordinarily large-sized bacteria were also discovered in the sample containing high LPS concentration. It takes only about an hour to analyze LPS after obtaining a sample, and a mere 100 μl of seawater sample is enough for this method. This method should be very effective for the purposes of prospecting the hydrothermal activity.