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

本研究发展了两种质谱技术，使热电离质谱法（TIMS）对稀土元素进行高精度稳定同位素分析成为可能。第一种技术是总蒸发归一化（TEN）方法，用于分析非常少量的Nd样品。它是两种现有技术的组合：总蒸发技术和内部归一化技术。这两种现有技术的结合允许精确的放射性成因Nd同位素比测量的亚ng Nd样品。对0.5ng钕样品的~（13）<143>Nd/~<144>（13）Nd比值的外精密度为140 ppm。该精度比常规测量获得的精度小一个数量级。TEN方法测量亚ng级Nd样品的精度足以满足^<143>Nd/^<144>Nd比值作为地球化学示踪剂的应用。 ...更多信息 Nd和Sm两种稀土元素的同位素分析表。双峰TIMS技术是满足这些要求的首选方法。在双尖峰TIMS技术中进行了一些改进，以最大限度地减少反卷积过程中可能引入的误差。自由参数的调整，如同位素组成的双穗和样品-穗混合比是重要的双穗分析，因为在反褶积过程中的误差放大的程度是相当大的影响，这些参数。通过误差传播模拟，这些参数被优化用于Nd和Sm分析。开发的技术的精密度估计从内部参考材料的分析。室内标准物质的ε^<146>Nd和ε^<148>Sm值的长期重现性分别为±0.2（2SD，n=44）和±1.2（2SD，n=44）。所开发的技术的准确性，证实了从分析的同位素分馏行为在阳离子交换色谱两种元素。此外，11个商业氧化钕试剂的稳定Nd同位素组成进行了分析。11<146>种试剂的ε^ Nd值（参考内部参比物质JNdi-1）范围为-2.5至+0.3。ε^<146>Nd值与试剂纯度无相关性。因此，目前尚不清楚这些试剂之间观察到的稳定同位素的变化是否反映了这些试剂的生产和纯化过程中的同位素分馏程度的差异，或其源材料的同位素组成的差异。8种火成岩（包括3种玄武岩、2种花岗岩和3种流纹岩）的Nd稳定同位素组成在分析误差范围内一致。火成岩的ε^<146>Nd值平均为-0.2 ~0.4（2SD）。岩浆岩中稳定Nd同位素的一致性表明地幔物质的稳定Nd同位素组成是均匀的，因为在高温反应中同位素分馏的影响是微不足道的。因此，岩浆岩的平均Nd同位素组成可以很好地估算块状硅酸盐地球（BSE）的Nd同位素组成。通过对锰结核和珊瑚的分析，估算了现代海水Nd的稳定同位素组成。锰<146>结核和珊瑚的ε^ Nd值分别为0.2±0.2（2SD，n=2）和-0.2 ~0.2（2SE）。锰结核和珊瑚中的稀土元素主要来源于海水。锰<146>结核和珊瑚中ε^ Nd值的一致性表明，海水中稀土元素掺入这些物质的过程中没有发生同位素分馏：如果发生同位素分馏，则不同化合物的分馏程度不同。碳酸盐岩与碳酸盐岩前体有机质方解石的稀土元素含量差异表明，碳酸盐岩在成岩过程中通过方解石与海水的无机化学反应富集稀土元素。大多数碳酸盐岩样品中的稀土元素浓度与现代海水平衡的方解石浓度一致。碳酸盐岩中Nd的稳定同位素组成可能反映了海水与无机方解石之间的平衡或近平衡同位素分馏。少

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