大野 剛

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.

In this study, we propose a new method for elucidating zircon growth in granitic plutons, based on the variations in three-dimensional (3D) cathodoluminescence (CL) patterns, U[sbnd]Pb age, titanium concentration, and Th/U. We focused on the zircon growth in the Okueyama granite (OKG) in central Kyushu, Japan, to obtain interpretations of magma chamber processes that result in the formation of granitic plutons. Three lithofacies of the OKG were sampled: biotite granite, hornblende granite, and hornblende granodiorite. To determine the 3D internal structure and growth pattern of zircon, we performed CL observations for multiple crystals that were sectioned in depth intervals of 5–10 μm. The zircon U[sbnd]Pb ages and Ti concentrations for the center sections of the crystals were simultaneously determined. The 3D distribution of the internal structure of zircon crystals comprises the following five textures: 1) oscillatory zoned (OZ), 2) porous, 3) chaotic, 4) locally disturbed, and 5) crystals with inherited cores. The 3D distribution of the OZ can be used to approximate the location of zircon nucleation. The simultaneous determination of zircon U[sbnd]Pb ages and Ti concentrations of the granite samples indicates the time–temperature (t–T) history of granitic magma before its solidification. The three lithofacies record similar cooling histories among the temperatures of zircon crystallization. The magma chamber cooled from 900 °C to 650 °C at approximately 13 Ma and then cooled to the biotite K[sbnd]Ar closure temperature. The simultaneous determination of zircon U[sbnd]Pb age and Ti concentration in the granite also enabled us to correlate the Th/U with apparent temperature. A decrease in zircon Th/U with decreasing temperature indicates progressive fractional crystallization during the cooling of the magma chamber. This decrease in zircon Th/U with decreasing temperature may be related to the co-crystallization of accessory monazite and zircon. The relationship between Th/U and temperature indicates that fractional crystallization in the magma chamber at temperatures above 770 °C was more progressive than that at temperatures below 770 °C. The three lithofacies demonstrate common tendencies in the relationship between Th/U and temperature, which indicate the same path of fractional crystallization among the three lithofacies in the cooling magma chamber.