| In recent years, high-precision stable isotope analytical methods have acquired a great breakthrough with the wide applications of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), resulting in a rapid development of non-traditional stable isotope geochemistry. Barium is an alkaline earth metal element that is highly incompatible during mantle melting, and thus it is more enriched in the crust than in the mantle. As is a water-soluble element, Ba will be released from the subducting plate to the fluid, and then added into the mantle wedge. Therefore, Ba can be used to track fluid activities associated with the subduction. More applications of Ba include constraining the paleo-oceanic export productivity by Ba accumulation rates in the marine sediments.Significant Ba isotope fractionation has been found in both natural samples and experimental studies, indicating that Ba isotopes have the potential to serve as a new tool for tracing both high- and low-temperature geochemical processes. Establishment of a high-precision Ba isotope analytical method and determination of the Ba isotope compositions of important Earth's reservoirs are prerequisite for geogical applications of Ba isotopes. However, there are still some problems existing in the existing in the available studies. One is the lacking a uniform international "?-0" standard for Ba isotope and a large number of repeated measurements of different reference standard materials, which results in extra difficulties in comparing data between different laboratories evaluating the data quality. Another is lacking constraints of Ba isotope characteristics of important Earth's reservoirs. In this thesis, we first systematically introduced the elemental geochemical properties of Ba and Ba isotopes, and then reviewed the research status of Ba isotope geochemistry. Subsequently, we presented our new Ba isotope analytical method in detail, and our comprehensive studies on the Ba isotope compositions of the upper continental crust and the oceanic basalts.We improved the chemical separation of Ba, taking the complexity of geological samples into account. Barium was separated from matrix elements using 2mL of AG50W-X12 resin to achieve a more pure Ba solution. Ba isotope compositions were measured by MC-ICP-MS, and using the sample-standard bracketing method was used to correct the instrumental bias. We tested various parameters that might affect the precision and accuracy of Ba isotope measurement, including the acidity mismatch and Ba concentration mismatch between samples and bracketing standards, and the matrix effect. Ba isotope composition was expressed relative to the pure Ba standard material SRM3104a as ?137/134Ba (%?)= [137/134Basample / 137/134BasRM3104a - 1] Χ 1000. The average ?137/134Ba of synthetic solutions made by doping SRM3104a with matrix elements is 0.005 ± 0.047 (2SD,n = 36), consistent with the recommended value with in the error range. Using the established method, we determined the ?137/134Ba of one Ba carbonate standard IAEA-CO-9, eight international rock reference materials and two late Mesozoic basalt samples. Their ?137/134Ba are giving below: IAEA-CO-9,0.017±0.049 (2SD, n=130); BCR-2, 0.050 ± 0.039 (2SD, n=13); BHVO-2, 0.047± 0.028 (2SD,n=22); JB-2, 0.085 ? 0.035 (2SD,n=19); W-2, 0.035 ± 0.022(2SD,n=11);AGV-1,0.047 ± 0.040 (2SD,n=11);JA-2,0.038 ± 0.048(2SD,n=17);RGM-1, 0.142 ± 0.030 (2SD,n= 15); GSP-2,0.013 ± 0.046 (2SD,n=15). The?137/134Ba of the two basalts (MZ815 and MZ834) were -0.132 ± 0.020 and 0.001 ±0.034 , respectively. Collectivelly, the total Ba isotope variation of all igneous rock standards and samples is 0.274, significantly larger than the long-term external precision of ?137/134Ba (0.05, 2SD). The recommended values of Ba isotope composition of international reference materials yielded in our work are of important significance for data comparation between different laboratories.Based on the established method, we investigated the Ba isotopic characteristics of the upper continental crust (UCC) by analyzing 71 samples (including granites,granodiorites, loess, glacial diamictites, and river sediments). Nine I-type granites from the Fogang batholith exhibit variable ?137/134Ba (-0.16 to 0.01), which may reflect isotopic fractionation during late stage magmatic differentiation. The ?137/134Ba of nine S-type granites from Darongshan-Shiwandashan (-0.03 to 0.11) correlate with ?Nd(t), likely reflecting mixing of heterogeneous crustal source materials. Five A-type granites from Nankunshan have remarkably low ?137/134Ba (-0.47 to -0.33), which possibly arose from magmatic differentiation or assimilation of crustal materials with light Ba isotopic compositions. Loess from northwestern China have homogeneous?37/134Ba with an average of 0.00 ± 0.03 (2SD, n = 18),regardless of geography and CIA values, suggesting that loess can be used to constrain the average Ba isotopic composition of the UCC. Three river sediments from northern China have similar?137/134Ba as loess. Glacial diamictites display a large range of ?137/134Ba (-0.1919 to 0.35). The highly variable ?137/134Ba in samples with low Ba concentration and high CIA suggests that Ba isotopes may be fractionated during chemical weathering. In summary, the UCC has highly heterogeneous Ba isotopic compositions,with R137/134Ba ranging from -0.47 to 0.35. Based on the samples investigated in this study, the weighted average ?137/134Ba of the UCC is estimated to be 0.00 ± 0.03 (2SD/?n,n=71),which is similar to the average Ba isotopic composition of the upper mantle, but significantly lighter than ocean waters.To constrain the Ba isotope characteristics of the mantle, we measured the Ba isotope composition of mantle derivatives, including MORB, BABB and OIB. The?137/134Ba of twenty-six fresh MORB samples from different oceanic ridges range from-0.05 to 0.08 . According to the La/Sm ratio, these samples were subdivided into nineteen N-MORB and seven E-MORB. Their ?137/134Ba vary from -0.03 to 0.08and from -0.05 to 0.08, respectively. Five BABB samples from the Lau basin have?137/134Ba ranging from 0.01 to 0.06. Large variations of ?137/134Ba (-0.08 to 0.15) occur in sixty Hawaii OIB samples. Because Ba is extremely incompatible during mantle melting, >99.9% of Ba will be partitioned in the melt after 1% partial melting. Therefore, Ba isotope fractionation during mantle melting is negligible. No correlations are shown between the ?137/134Ba of these oceanic basalts and the indicators representing magma differentiation, such as MgO content and CaO/Al2O3, indicating that Ba isotope fractionation during fractional crystallization can be ignored. Therefore,Ba isotope compositons of the oceanic basalts investiaged in this work mainly reflect the characteristics of their mantle sources. The relation between ?137/134Ba and Ba/Th and 87Sr/86Sr suggestes that addition of crustal materials (e.g., subducted oceanic crust and sediments) with variable Ba isotope compositions to the mantle may affect the?137/134Ba of the mantle reservoir. We thus considered the average ?137/134Ba of the mantle is 0.04±0.03 (2SD), based on unmodified N-MORB samples. The estimated average ?137/134Ba of the mantle is similar to that of the upper continental crust, but lower than those of seawaters. |
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