Origin of 'ultra' - rocks

After decades of studies, it seems generally accepted that the Earth’s lithospheric mantle cannot be characterized by a homogeneous composition, either from a mineralogical, geochemical and isotopic point of view. It would rather be affected by re-fertilization and metasomatic processes. Indeed, the constant subduction and the recycling of crustal material, either continental and oceanic, triggered by the Plate Tectonics, would cause a local enrichment of the previously depleted peridotitic mantle (Stracke, 2021). The upper mantle heterogeneity is clearly reflected in the variegated igneous lithologies that crop out on the surface of our planet, which show a wide chemical compositional range, as their formation would be related to partial melting events, resulting after a long interaction and assimilation among these different lithologies (Stracke, 2021). In particular, partial melting of veined lithospheric mantle sources may explain the presence of exotic liquids, whose formation cannot be ascribed to a classical four-phase mantle assemblage (olivine, orthopyroxene, clinopyroxene, spinel or garnet; Foley, 1992a). Indeed, Davis et al. (2011) demonstrated that low degree of partial melting of a lherzolite source, which did not undergo any metasomatic events, cannot lead to the generation of melts with total alkalis content higher than 3-4 wt%. Even if the rocks produced by these exotic melts have always attracted the attention of the scientific community, today many rare lithologies represent matter of discussion, as their mantle sources, their petrogenesis, and even their classification and nomenclature, are only poorly constrained. A detailed and systematic study of these debated rocks might allow to reach a better knowledge of the mantle conditions, i.e., its thermal and chemical state, and of the processes responsible for their formation. The goal of this thesis is to gain a deeper understanding of different groups of exotic and peculiar lithologies, mainly ultrabasic to basic, ultramafic and/or ultra-alkaline in composition, through a multidisciplinary investigation, based on petrographic, mineral chemical (SEM-EMP analyses), whole-rock geochemical data, together with isotopic analyses (either on radiogenic systematics, as Sr-Nd-P, plus conventional, as C and O, and unconventional stable isotopes, as B) and petrological experiments at different pressure (from 1 to 5 GPa) and temperature (from sub-solidus to supra-liquidus T) conditions. The three main rock-types that have been accurately analysed are kamafugites, carbonatites and ultramafic lamprophyres. In particular, kamafugite samples come from south-eastern Brazil (Alto Paranaiba Igneous Province), central Italy (Intra-Apennine Province) and Uganda (Toro Ankole Volcanic Province); carbonatites from Uganda (Toro Ankole Volcanic Province) and from western Germany (Eifel Volcanic Field); and ultramafic lamprophyres are from eastern Antarctica (Beaver Lake). Together with these samples, minor melilitites (Montefiascone Volcanic Complex, central Italy), leucitite and nephelinite (Toro Ankole Volcanic Province) have been also studied. Exotic samples have been collected from many sampling sites found in different geodynamic environments, and they have been chosen with variable petrographic, geochemical and isotopic characteristics, as we believe that a detailed comparison would highlight similarities and differences helpful to infer the reason of such heterogeneities, but also to better constrain the mantle sources and processes involved in their formation. All the analyses that have been carried out highlight the exotic nature of the samples. Petrography and mineral chemistry point out the presence of rare phases, as primary carbonate (found not only in the carbonatite samples, but also in the groundmass of kamafugites, ultramafic lamprophyres and melilitites), kalsilite, melilite and perovskite (diffuse in kamafugite, ultramafic lamprophyres and melilitite samples). Whole-rock geochemistry indicates strongly ultrabasic (carbonatites), ultrabasic (Ugandan and Brazilian kamafugite, ultramafic lamprophyres) and basic (Italian kamafugites) compositions, mostly characterized by a potassic or ultrapotassic affinity (K2O up to 8.36 wt% in Italian kamafugites, K2O/Na2O < 34). Moreover, all samples show a variable, but often marked, enrichment in trace elements. Also the isotopic data indicates strongly radiogenic 87Sr/86Sr and 206Pb/204Pb, coupled with low values of 143Nd/144Nd isotopic ratios. Only two carbonatites represent an exception, having 87Sr/86Sr < BSE (present-day Bulk Silicate Earth). 11B values roughly fall in the mantle range (-8.3 to -3.3 ‰; Agostini et al., 2021), with the exception of few Ugandan rocks, having heavier values (11B = -1.9 to 6.6 ‰). Even if samples cover a large compositional spectrum, multi-stage refertilization events must have occurred and must have strongly modified all the lithospheric mantle sources. Recycling of various lithologies, as marine limestone, caused by more or less old subductions, could explain either the carbonated metasomatism, necessary in all the analysed volcanic districts, but also the phlogopite and diopsitic clinopyroxene-rich veins, necessary in the formations of kamafugites, ultramafic lamprophyres and melilites.