Objective
Carbonatites are magmatic rocks that mostly occur on the continents. These rocks are characterised by very high concentrations of some elements like strontium (Sr) and the rare earth elements (REE), much higher than in average continental crustal rocks, so that crustal contamination has probably been negligible during magma emplacement. Moreover, carbonatite melts are also volatile-rich so that they can carry volatile components, including carbon dioxide and the noble gases, also without considerable contamination. These volatiles can be trapped and retained by minerals during crystallisation. The strontium (Sr), neodymium (Nd) and noble gases (He, Ne, Ar) isotopic compositions of carbonatites thus reflect their subcontinental mantle-source region. Worldwide, most of the carbonatites have isotopic compositions reflecting a mantle-source comparable to the one, in the oceans, of oceanic island basalts. It will be very useful to compare the geochemical features of the mantle zone beneath the Kola craton with the sub-oceanic mantle.
The lead (Pb) content of carbonatites is in the range of crustal values and the Pb isotopic composition can then be used to decipher crustal contamination processes which might have occurred during emplacement.
This research project will study some of the well-known carbonatites of the Kola Peninsula (Russia) which forms part of the widespread alkaline magmatic province (24 intrusions) of Middle to Late Devonian age. The relations between these carbonatites and the inclusions (pyroxenite, hornblendite) they sometimes contain, as well as their relations with the associated alkaline silicate rocks (pyroxenites, nepheline syenites), will also be examined. This study will also give some interesting new information on the genesis of the mineralisations related to some carbonatite intrusions like the magnetite (source of iron) and phlogopite (insulator) ores in Kovdor and the apatite-rich cumulates (source of phosphorus) of Khibiny.
Through the Sr, Nd and noble gases isotope geochemistry, the project will provide information on the geochemical features of the subcontinental mantle beneath the Baltic cratonic zone during the Paleozoic. The whole rock Pb isotope composition, on the other hand, will be used to trace contamination processes of the mantle-derived magma during its emplacement in the continental crust. The in situ Pb isotope analyses on single crystal grain by the ISOLAB mass spectrometry will enable the definition of the behaviour of Pb during the late regressive stages. This study will eventually lead to a better understanding of the mode of formation of the mineralised zones such as magnetite, phlogopite and apatite, related to this alkaline magmatism of Devonian age.
Topic(s)
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1050 Bruxelles
Belgium