Studies on geochemical cycle using stable and radiogenic isotopes of rare earth elements

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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. Combination of these two existing techniques allows precise radiogenic Nd isotope ratio measurements of sub-ng Nd samples. The external precision of the ^<143>Nd/^<144>Nd ratio for 0.5ng Nd sample was 140ppm. 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 ^<143>Nd/^<144>Nd ratio as a geochemical tracer.The second technique developed in this study is a combined double-spike TIMS technique for high-precision s … More table isotope analysis of two REEs, Nd and Sm. 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 sample-spike 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 ε^<146>Nd and ε^<148>Sm values of the in-house reference materials were ±0.2 (2SD, n=44) and ±1.2 (2SD, n=44), respectively. 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 ε^<146>Nd 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 ε^<146>Nd 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 ε^<146>Nd 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 ε^<146>Nd 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 ε^<146>Nd 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.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. Less