Crystallization conditions of carbonatites and associated alkaline silicate rocks

大多数已知的碳酸盐岩赋存在空间和时间上与碱性至过碱性硅酸盐岩有关。碱性和过碱性硅酸盐岩石结晶过程中的氧化还原条件有较大的差异。然而，碳酸盐岩的氧化还原条件估计似乎相对有限，尽管它们与碱性-过碱性岩石硅酸盐岩石的关联是如此典型。本项目将研究：(1)通过研究世界范围内几种合适的碳酸盐岩产状，研究碳酸盐岩结晶过程中普遍存在的氧化还原条件；(2)以两个典型的深部和次火山碳酸盐岩-碱性岩石组合（Kaiserstuhl和Kovdor）为重点，研究碳酸盐岩与时空相关的碱性岩石之间氧化还原条件的相似性或差异性。将通过显微镜，EMPA，氧同位素和Mössbauer工作详细研究待调查样品中的硅酸盐-氧化物-硫化物组合，以确定其在fO2， T， aSiO2和fS2方面的结晶条件。预期的结果将(1)限制碳酸盐岩熔体结晶过程中普遍的氧化还原条件和可能的变化；(2)阐明某一地区的碳酸盐岩和伴生碱性岩石是否在相似或不同的氧化还原条件下结晶。从更广泛的意义上说，本项目的结果有望有助于我们了解岩石圈地幔中氧化还原条件的可变性（碳酸盐岩和碱性硅酸盐岩是合适的探针），以及从含碳酸盐的地幔熔体到碳酸盐岩和伴生碱性岩的演化过程中所涉及的物理化学过程。

Most of the known carbonatite occurrences are spatially and temporally associated with alkaline to peralkaline silicate rocks. The redox conditions during the crystallization of alkaline to peralkaline silicate rocks show considerable variation. The redox conditions estimated for carbonatites, however, seem to be relatively restricted, although their association with alkaline-peralkaline rocks silicate rocks is so typical. This project will investigate (1) the prevailing redox conditions during the crystallization of carbonatitic rocks in general by studying several suitable carbonatite occurrences world-wide and (2) the similarity or difference of redox conditions between carbonatites and spatially and temporally associated alkaline rocks focusing on two typical plutonic and subvolcanic carbonatite-alkaline rock associations (Kaiserstuhl and Kovdor). The silicate-oxide-sulfide assemblages in the samples to be investigated will be studied in detail by means of microscopy, EMPA, oxygen isotope and Mössbauer work to determine their crystallization conditions in terms of fO2, T, aSiO2, and fS2.The expected results will (1) constrain the prevailing redox conditions and possible variations during the crystallization of carbonatitic melts and (2) clarify whether carbonatites and associated alkaline rocks at a given locality crystallize under similar or different redox conditions. In a more general sense, the results of this project are expected to contribute to our knowledge on the variability of redox conditions in the lithospheric mantle (for which carbonatites and alkaline silicate rocks are suitable probes) and the physico-chemical processes involved during the evolution from carbonate-bearing mantle melts towards carbonatites and associated alkaline rocks.