Crystallization conditions of carbonatites and associated alkaline silicate rocks

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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.