The understanding of magma generation processes includes characterisation of the source region that undergoes partial melting. Except for in situ partial melting during high-grade metamorphism (migmatites), our knowledge about the magma source region / residue and their phase assemblages is based mostly on indirect approaches, such as major and trace element bulk rock composition and experimental studies on liquidus relationships. Still, some alkaline magmas appear to be able to transport the solid fragments of the source region and the residue to the Earth’s surface, allowing a direct study of these materials.
Alkaline mafic and ultramafic magmas are considered to rise rapidly to the Earth’s surface. Their ascent can be accompanied by the entrainment of significant amounts of disintegrated rocks from the walls of the magma conduit as xenoliths and xenocrysts. Typical magmas that bear such debris are kimberlites, olivine melilitites, olivine nephelinites and alkali basalts. At the same time, there are kimberlite, melilitite, foidite and alkali basalt rocks almost devoid of xenoliths but containing frequent macrocrysts of olivine (kimberlite) or olivine and clinopyroxene (olivine melilitite, olivine melilitites and nephelinite, basanite), some of these minerals exhibiting indications of solid-state deformation such as kink-bands, undulose extinction, recrystallization and internal cracks. Can these minerals be products of fragmentation of common mantle rocks? Dunite lithologies are rare among kimberlite-hosted xenoliths; thus, mantle dunites cannot account for the abundance of olivine macrocrysts in kimberlites. Wehrlite, which can be a source of olivine and clinopyroxene macrocrysts in olivine melilitites and nephelinites, also does not represent a common mantle rock. The occurrence of wehrlite in the upper mantle can be related to carbonatite metasomatism and formation of the source region of carbonated olivine melilitite and nephelinite magmas. This raises the question: if the macrocrysts are not the products of fragmentation of common mantle lithologies, due to the lack of other minerals typical of such lithologies, can they be entrained directly from the source region / residue of the host magma. If so, they could provide constraints on the phase and trace element composition and, potentially, history of formation and melting of the source region.
The present project aims to study such macrocrysts to clarify their origin. Major and trace element data on the macrocrysts, including zonation and domain textures, will be combined with a careful characterisation of the inclusions with the aid of a Raman spectrometer, electron microprobe, LA-ICPMS and ion microprobe.
The study of inclusions is crucial for the project. Fluid, melt and solid inclusions in cumulate and in residual / source region phases are not necessarily the same, particularly if the source material melts fast and without complete re-equilibration with the melt (possibly the case of carbonated ultramafic magmas), and when a time gap between the residue and the cumulates exists. Our previous study shows that processes related to the formation of the source region of carbonated olivine nephelinite leave a unique suite of inclusions in the clinopyroxene macrocrysts. These inclusions represent carbonates, solidified carbonatite melt, olivine, Al-spinel, alkali aluminosilicate glass and a fluid, most of them being absent in ‘true’ clinopyroxene phenocrysts from the same rock.
The inclusions contained in the macrocrysts are usually small, most of them ranging from several to several tens of micrometers in size. The diagnostics of such inclusions is a considerable analytical challenge. Larger inclusions can still be characterised by optical microscopy, while smaller and inhomogeneous inclusions can only be studied on a SEM/electron microprobe and on a Raman spectrometer. The latter is especially useful for the study of glass and fluid bearing inclusions. In particular, the phase diagnostics, including liquid and vapour phases in the inclusions, as well as the study of the residual stress in the host macrocrysts is facilitated by the application of Raman spectrometry.
This project is a quest for the residual and source region material in macrocrysts from igneous rocks supposedly bringing such material to the surface. It is also an analytical project aimed at collecting more information about the macrocrysts. Detailed, inclusions-oriented data remain scarce in the literature. The outcome of this project will be a better understanding of the origin of primitive alkaline magmas, such as kimberlite, melilitite and nephelinite.