@phdthesis{oai:nagoya.repo.nii.ac.jp:00007943,
 author = {若木, 重行 and WAKAKI, Shigeyuki},
 month = {Mar},
 note = {Abundance of isotopes of an element is not a universal constant in nature. Every element suffers from isotope fractionation during a chemical reaction. Both equilibrium and kinetic chemical reactions fractionate isotopes of an element in a mass-dependent manner. Theoretical investigation on fractionation behavior of isotopes in equilibrium chemical reaction showed that isotope fractionation factor of an isotope exchange reaction is proportional to the relative mass difference between the isotopes of interest, to the inverse of the square of the temperature, and to the difference in strength of chemical bonding between two chemical species involved in the reaction. Mass-dependent fractionation induced variations of isotope composition in nature have been studied widely on “light” elements such as H, O, C, N and S. Large difference of relative mass between isotopes in these elements leads to large and easily measurable variations in their isotope compositions. No natural variations in isotope compositions of high-atomic-number elements are expected beyond analytical error because of the small relative mass differences between isotopes in these elements. However, resent advance in mass spectrometry have opened the door to high-precision stable isotope analysis of intermediate to high-atomic number elements. Stable isotopes of the newly introduced intermediate to high-atomic number elements are referred as “non-traditional” stable isotopes. Studies of the non-traditional stable isotopes have showed that mass-dependent fractionation induced variations of isotope compositions in nature are measurable in every elements investigated. This study has focused on rare earth elements (REEs) as a target for new nontraditional stable isotope study. Rare earth elements are one of the most useful elements in geochemistry. Elemental fractionation pattern of REEs is widely used in geochemical studies to constrain the origin of geological materials. Moreover, abundance of the radiogenic nuclide 143Nd, product of the long-lived Sm-Nd radioactive decay system, provides chronological constraints of the sample. Combining stable isotopes with these two existing methodology, multiple information regarding the age, the origin and the process may be obtained from a single element (or a single group of elements) to constrain the history of geological materials. The target element of this study is set to two light REEs, Nd and Sm, to find out whether variation in stable isotopes of these elements can be used to trace various geochemical reactions. Two mass spectrometric techniques are developed in this study to make high precision stable isotope analysis of REEs possible by thermal ionization mass spectrometry  (TIMS). The first technique is total evaporation normalization (TEN) method developed for the analysis of very small amount of Nd samples. It is a combination of two existing techniques: total evaporation technique and internal normalization technique. Total evaporation technique is developed and used primarily in nuclear industry. Internal normalization technique is used for instrumental mass-bias correction in conventional geochemical Nd isotope ratio measurements. Combination of these two existing techniques allows precise radiogenic Nd isotope ratio measurements of sub-ng Nd samples. The external precision of the 143Nd/144Nd ratio for 0.5 ng Nd sample was 140 ppm. This precision is order of magnitude smaller than that obtained by the conventional measurements. The precision achieved by the TEN method for the measurement of sub-ng Nd sample is sufficient for the application of 143Nd/144Nd ratio as a geochemical tracer. The second technique developed in this study is a combined double-spike TIMS technique for high-precision stable isotope analysis of two REEs, Nd and Sm. Instrumental mass bias is inevitable in mass spectrometric isotope ratio measurements. Expected stable isotopic variation of Nd and Sm in nature is about an order of magnitude smaller than the possible instrumental mass bias effect. Rigorous instrumental mass bias correction is thus essential for stable isotope analysis of these elements. Small expected variations in isotope composition also requires very high precision in the analysis. The double-spike TIMS technique is a method of choice to meet these requirements. Several refinements in the double spike TIMS technique were made to minimize the introduction of possible error during deconvolution. Adjustment of free-parameters such as isotope composition of the double spike and samplespike mixing ratio is important in double-spike analysis, because the degree of error magnification during the deconvolution process is considerably affected by these parameters. These parameters are optimized for Nd and Sm analyses by means of error propagation simulation. Precision of the developed technique is estimated from the analysis of in-house reference materials. The long-term reproducibility of the ε146Nd and ε148Sm values of the inhouse reference materials were ± 0.2 (2SD, n = 44) and ± 1.2 (2SD, n = 44), respectively. This precision is sufficient for the investigation of the possible stable isotope variations in nature. Accuracy of the developed technique is confirmed from the analysis of isotope fractionation behavior during cation exchange chromatography for both elements. In addition, eleven commercial Nd oxide reagents were analyzed for their stable Nd isotope composition. The ε146Nd value (reference to the in-house reference material JNdi-1) of the 11 reagents ranges from -2.5 to +0.3. No correlation was found between ε146Nd value and the purity of the reagents. Therefore, it is not clear whether the observed stable isotopic variation among these reagents reflects the difference of the degree of isotope fractionation during production and purification processes of these reagents or the difference of the isotope composition of their source materials. Various terrestrial materials were analyzed for Nd stable isotopes by the developed double-spike TIMS technique. The stable Nd isotope composition of 8 igneous rocks (including 3 basalts, 2 granites and 3 rhyolites) agreed within analytical error. The average ε146Nd value of the igneous rocks was –0.2 ± 0.4 (2SD). The consistency of stable Nd isotopes in igneous rocks suggests uniform stable Nd isotope composition of the mantle material since the effect of isotope fractionation is negligibly small in high-temperature reactions. Thus, the average Nd isotope composition of igneous rocks is a good estimate of the Nd isotope composition of the bulk silicate earth (BSE). Stable isotope composition of Nd in the modern seawater is estimated from the analysis of Mn nodule and coral. The ε146Nd value of Mn nodule and coral were 0.2 ± 0.2 (2SD, n = 2) and -0.2 ± 0.2 (2SE), respectively. REEs in Mn nodule and coral are of seawater origin. The consistency of the ε146Nd values in Mn nodule and coral implies that no isotope fractionation took place during the REE incorporation from seawater into these materials: if isotope fractionation occurs, the degree of the fractionation will be different among different chemical compounds. Therefore, stable Nd isotope composition of the modern seawater is directly represented by Mn nodule and coral. The theory of equilibrium isotope fractionation predicts a large isotope fractionation in low-temperature reactions. Marine carbonate rocks are good example of the materials formed in low-temperature environments. Large variation in Nd isotope composition was found among marine carbonate rocks: ε146Nd values of 15 marine carbonate rocks range from –0.1 to +2.6. Difference of the REE concentrations between carbonate rocks and organic calcite, precursor material of the carbonate rocks, suggests that REE was concentrated in carbonate rocks via inorganic chemical reaction between calcite and seawater during diagenesis. REE concentration in most of the carbonate rock samples is consistent with the concentration of the calcite equilibrated with modern seawater. The stable isotope composition of Nd in carbonate rocks probably reflects equilibrium or near equilibrium isotope fractionation between seawater and inorganic calcite., 名古屋大学博士学位論文 学位の種類:博士(理学)(課程) 学位授与年月日:平成20年3月25日},
 school = {名古屋大学, Nagoya University},
 title = {Stable isotope geochemistry of light rare earth elements:technical development and isotope fractionation in nature},
 year = {2008}
}