A general agreement on the origin of carbonatite magma has not yet reached after more than sixty years of hot discussion. The diatribe is mostly focused on four main topics: 1) the origin of carbonatites and their relation with silicate magmas; 2) the origin of incompatible element enrichment in the most abundant carbonatitic magma (i.e., Ca-carbonatites); 3) the geodynamic implication of carbonatitic activity in terms of plate tectonics, and 4) the difficulty to distinguish true carbonatites from carbothermal/carbohydrothermal carbonatites or simple skarns.
In the last twenty years several experimental petrology studies have demonstrated that carbonatitic melts can be generated not only from partial melting of a peridotitic (i.e., olivine-rich) sources or by immiscibility from ultrabasic to intermediate silicate melts, but also from subducted carbonated eclogite (i.e., olivine-rich to olivine-poor) lithologies. These subducted lithologies are efficient carriers of carbon in the upper mantle and, depending on the thermobaric evolution of the recycled slabs, they can experience different types of interaction with the peridotiti matrix. Partial melts of recycled carbonated eclogites Several experimental studies have demonstrated that partial melts of recycled carbonated eclogites share major oxide content of classical oceanic basaltic rocks (with compositions from nephelinite/melilitite to basanite and alkali basaltic.
The general aim of the project is to propose a petrological model to explain the origin of different ultrabasic rocks (kamafugites, ultramafic lamprophyres, melilitites, leucitites, mela-nephelinites and similar exotic compositions). These magmas, often volatile-rich, are commonly associated to high amounts of carbonate material of not certain origin, alternatively interpreted as carbonatites, pseudo-carbonatites and skarns. A deeper comprehension of the mantle source features and the processes responsible for the generation of these quite rare ¿ but often economically important ¿ lithologies is highly required. In particular, the definition of specific terms and the choice of mineralogical, geochemical or geo-tectonic tools to use need profound rethinking. The specific object of this project is a petrological, geochemical and isotopic comparison between products from different sampling sites (Italy, Uganda, South Africa, Australia, Canada and Brazil) that will be used to highlights similarities, if present, that could be useful to reach the general petrogenetic model.
A deep analysis of peculiar rocks from different geodynamic settings may help to highlight the relation with the upper mantle structures and with particular tectonic events, if present. Moreover, the achievement a better understanding of the formation of exotic magmas will help to reach a more comprehensive knowledge of the upper mantle. All these new data could help to clarify the general background in which exotic rocks are framed, outlining more appropriated classification parameters and significant differences between the various groups. A classical petrological approach (with investigation of thin sections, whole-rock geochemical study, mineral chemistry and Sr-Nd-Pb isotopic survey) will be paired by a not yet developed Ca-Mg isotopic study, in collaboration with the CNR laboratories of Pisa (IGG Institute).