Crystallization conditions of carbonatites and associated alkaline silicate rocks

大多数已知的碳酸盐岩产状在空间和时间上与碱性至过碱性硅酸盐岩有关。碱性-过碱性硅酸盐岩石结晶过程中的氧化还原条件表现出相当大的变化。碳酸岩的氧化还原条件估计，然而，似乎是相对有限的，虽然他们与碱性-过碱性岩石硅酸盐岩石的组合是如此典型。本项目将研究（1）通过研究世界范围内几个合适的碳酸岩产状，研究碳酸岩结晶过程中的普遍氧化还原条件;（2）碳酸岩与空间和时间相关的碱性岩之间氧化还原条件的相似性或差异，重点是两个典型的深成和次火山碳酸岩-碱性岩组合（Kaiserstuhl和Kovdor）。将通过显微镜、电子探针、氧同位素和穆斯堡尔谱等方法详细研究待研究样品中的硅酸盐-氧化物-硫化物组合，以确定它们在fO 2、T、aSiO 2、预期的结果将（1）限制碳酸盐熔体结晶过程中的主要氧化还原条件和可能的变化;阐明碳酸岩和相关的碱性岩石在特定的地方是否在相似或不同的氧化还原条件下结晶。在更普遍的意义上，该项目的结果预计将有助于我们的知识在岩石圈地幔氧化还原条件的变化（碳酸岩和碱性硅酸盐岩石是合适的探针）和物理化学过程中所涉及的含碳酸盐地幔熔融碳酸岩和相关的碱性岩石的演变。

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.