大野 剛

ABSTRACT Owing to high resistance to alteration, detrital zircons retain information about their formation ages and parental magmas for a long period of time. Many geochemical researchers have proposed various indicators for zircon to constrain tectonic settings and to identify source rock. Because most detrital zircons analyzed by geochronologic studies are derived primarily from granitoids, we focus on the classification of zircon within granitoids. In the style of alphabetical classification scheme (Igneous, I; Sedimentary, S; and Alkaline, A types), some discrimination diagrams have been proposed. To improve the database and enhance discriminating studies, we examined trace‐element compositions of zircons extracted from some Cenozoic granitoids exposed in the Japan Islands. The zircons showed systematic differences in Nb, Ta, Ce, and P contents. Zircons in Oceanic Arc I‐type granite are poor in Nb and Ta, and these signatures clearly reflect those elements in their parental bodies. Despite their low abundance at the whole‐rock level, zircons in Oceanic Arc I‐type granite are characterized by high Ce content. This is attributable to the relatively oxidizing conditions of Oceanic Arc I‐type magma. Zircons in S‐type granite are characterized by high P and low Ce contents. The former can be explained by high apatite solubility in Al‐rich magma, whereas the reducing environment of S‐type magma is accountable for the latter. The zircon crystallized at the later stage during S‐type granite solidification is slightly depleted in Nb and Ta. This is attributable to the depletion of these elements in the magma by Ti‐bearing minerals such as ilmenite prior to zircon crystallization. In analogy with whole‐rock composition, zircons in transitional I‐A‐type granite have intermediate composition between I‐type and A‐type zircons. On the basis of the updated database, we demonstrated that the Nb/P–Ce/P or Ta/P–Ce/P crossplots are the most useful for discriminating zircons in Oceanic Arc I‐type, I‐type, S‐type, and A‐type granites.

Experimental results are presented showing the variation in the relationship between odd isotopes of tin (Sn) in mass-independent fractionation caused by the magnetic isotope effect (MIE), which has previously only been observed for mercury. These results are consistent with the trend predicted from the difference between the magnitudes of nuclear magnetic moments of odd isotopes with a nuclear spin. However, the correlation between odd isotopes in fractionation induced by the MIE for the reaction system used in this study (solvent extraction using a crown ether) was different from that reported for the photochemical reaction of methyltin. This difference between the two reaction systems is consistent with a theoretical prediction that the correlation between odd isotopes in fractionation induced by the MIE is controlled by the relationship between the spin conversion time and radical lifetime. The characteristic changes in the correlation between odd isotopes in fractionation induced by the MIE observed for Sn in this study provide a guideline for quantitatively determining fractionation patterns caused by the MIE for elements that have multiple isotopes with a nuclear spin. These results improve our understanding of the potential impact of the MIE on mass-independent fractionation observed in natural samples, such as meteorites, and analytical artifacts of high-precision isotope analysis for heavy elements.

We examine the historical changes of 236U/238U and 235U/238U in a sediment core collected in Tokyo Bay and elucidate the anthropogenic sources of uranium in the 1960s-2000s. Uranium-236 was detected in samples deposited in the 1960s-2000s, and the 236U/238U ratio of the sediment core shows peak values in the 1970s. The 235U/238U isotopic ratios in samples deposited in the early 1960s are almost identical to that of natural uranium, implying that the 236U might have originated from global fallout. A decrease in 235U/238U was observed in the late 1960s-2000s, suggesting that depleted uranium from nuclear fuel reprocessing increased the 236U/238U ratios in the sediment. The 236U/238U values in sediments from the 1980s-2000s were lower than those in the 1970s but considerably higher than those in the 1960s, suggesting that the main source of depleted uranium still remains around Tokyo Bay. Our results demonstrated that the depleted uranium released in the 1970s should be considered as an important end-member when using uranium isotopic ratios as environmental tracers in closed aquatic environments around industrial cities.

We examine the temporal changes of 236U/238U and 235U/238U in atmospheric deposition from samples collected in Tokyo and Akita from 1963 to 1979 and elucidate the spatial distribution and historical changes of the anthropogenic sources of uranium in Japan. The 236U/238U ratio of atmospheric deposition in Tokyo peaked in 1963 and again during the 1970s, while the corresponding 235U/238U ratios of atmospheric deposition during the second peak period were lower than that of natural uranium. The 236U/238U ratios of atmospheric deposition in Akita samples peaked in 1963. The 235U/238U ratios in Akita samples were almost identical to that of the natural uranium ratios. These results suggest that the peak of 236U/238U in 1963 corresponds to what is recognized as representative for global fallout. The increase of 236U/238U and the decrease of 235U/238U observed simultaneously in the 1970s indicate that depleted uranium has subsequently been released into the environment around Tokyo. The cumulative deposition density of 236U for atmospheric fallout samples collected in Tokyo from 1968 to 1979 is an order of magnitude larger than that of the global fallout, suggesting that the depleted uranium in the 1970s is a major component of 236U in Tokyo and should be considered as an end-member when using 236U as an environmental tracer in the industrial city. This knowledge can facilitate future research using 236U as an effective environmental tracer.

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Simultaneous determination of zircon U-Pb age and titanium concentration for a single analysis spot gives both the crystallization age and temperature. In laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis, it is challenging to quantitatively analyse a low level of titanium concentration. Two approaches were employed using a quadrupole mass spectrometer equipped with a collision/reaction cell (CRC). In the first approach, the MS/MS mass-shift mode with oxygen reaction gas provided reliable and consistent measurement of titanium as 48Ti16O+. In the second approach, the titanium concentration was determined quantitatively from the signal intensity of 49Ti in the non-gas mode (without the inflow of collision/reaction gas into the CRC). The methods were applied to zircon samples of the Kurobegawa granite (KRG), the Okueyama granite (OKG), the Toki granite (TKG), and the Tono plutonic complex (TCP). The biotite K-Ar geochronology were employed for rock samples of the KRG, OKG, and TPC (N = 3) of which the zircon crystals were analysed. The obtained titanium concentrations of the zircon crystals can lead to the crystallization temperatures through Ti-in-zircon geothermometer.

A short course on inductively coupled plasma-mass spectrometry (ICP-MS) was held from 6th to 7th September, just before the Goldschmidt Conference at Kurashiki. This short course covered the entire range of ICP-MS techniques, including basic principles of ICP-MS, laser ablation sample introduction techniques, and various applications to the Earth and planetary science. The course also covered basic theoretical aspects of the mechanism of isotopic fractionation. Lectures given by Dr. Detlef Gunther (ETH, Switzerland), Dr. Alex Halliday (ETH, Switzerland), Dr. Toshiyuki Fujii (Kyoto Univ., Japan), Dr. Simon E. Jackson (GEMOC, Australia) and Dr. Keith O'Nions (Oxford Univ., UK) will be reviewed in this paper.