| Iron is a critical element in soils. It is not only an essential element for almost all organisms, but also has an important role on controling the behavior of other nutrient elements (e.g. P, K, Si, Cu) in soils. Beside, iron transportation processes are governed by the prevailing physical and biochemical conditions, so that it can record many fundamental information in iron cycling.Iron transportation could influence the distribution signature of the iron isotopes in pedosphere. Thus, investigating iron isotopic compositions of soils and it’s variation mechanism is a good way to provide valuable clues on studying soil transformation processes, which occur under different redox conditions.Currently, the available data on iron isotopic composition of soils is definitely limited and the mechanism which is responsible for the variations of iron isotope in soil is still understood up to now. As such, this thesis aims to develope the appropriate analytical method to determine iron isotopes in soil samples and apply iron isotopes as a powerful geochemical tool to study paddy soil systems. We hope to offer additional information of the effects of iron on environment during pedogenic processes. For this purpose, we develop a iron seperating procedure for paddy soil samples. Our results show that this chemical procedure can seperate iron from other matrical elements such as Na, K, Ca, Cr, Mn, Mg etc. Yields of33samples are all over98%and blank contains acceptable iron contents for isotope determination.Equilibrium calcium isotope fractionations between clinopyroxene and orthopyroxene, the two major Ca-bearing minerals in the upper mantle, are calculated with density functional perturbation theory (DFPT). The results suggest that orthopyroxene has higher44Ca/40Ca ratios than clinopyroxene due to smaller coordination numbers (CN) of Ca in orthopyroxene than that in clinopyroxene (6vs.8), showing a good agreement with the observation in natural samples. We further find that Ca concentration of orthopyroxene significantly influences the Ca isotopic composition of it, and hence, Δ44/40Ca OPX-CPX especially when Ca/Mg in orthopyroxene is below1:15. Our results successfully explain the observations of variable Ca isotopic fractionation between coexisting orthopyroxene and clinopyroxene from Kilbourne Hole and San Carlos mantle peridotites, i.e., Δ44/40Ca OPX-CPX increasing from0.36to0.75%o with [CaO]OPX decreasing from1.03to0.75%. This reveals that crystalline environment such as the average Ca-0bond length parameter may be controlled by Ca content in orthopyroxene when Ca is a minor element. Our calculations also suggest that, although σ44Ca of orthopyroxene may increase dramatically with decreasing CaO content, the average Ca isotope composition of the upper mantle is relatively constant because [CaO]cpx is much higher than [CaO]opx. If Ca content and Ca isotope compositions of clinopyroxene and othorpyroxene are known, Ca isotopic fractionation between clinopyroxene and orthopyroxene can be used as a potential two-pyroxene Ca isotope thermometery. Further more, we put forward that change in content of minor elements could effect on the equlibrium isotope fractionation between two phases. |
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