EXPERIMENTAL STUDY OF INTERCONNECTIVITY AND GRAIN BOUNDARY WETNESS OF HYDROUS CARBONATITIC LIQUIDS IN MANTLE PERIDOTITE

Carbon-bearing solids, fluids, and melts in the Earth's deep interior play an important role in the long-term carbon cycle. Carbonatite magmas have been suggested as important agents of mantle metasomatism and yet, their physical features are expected to control the mobility from the source region to shallow Earth. Carbonatites are known to form at relatively low temperatures and are very mobile, as controlled by their low viscosities and their ability to form an interconnected grain-edge melt at low melt fraction. The factors promoting migration and infiltration are the minimization of interfacial energy, the density and chemical gradients, the thermal diffusion. However, the mobility and infiltration rates of carbonatitic melts, together with their influence on the annealing of mantle peridotites are poorly constrained processes. Although natural carbonatitic melts are complex chemical systems with C-O-H species as a major component, previous work has been performed in anhydrous model systems. Here we present a quantitative laboratory simulation of variables and processes controlling the ascent, mobility and connectivity of carbonatites in a model mantle material investigating the dihedral angle of hydrous carbonatitic liquids. We aim at comparing the texturally equilibrated volume proportions of volatile-rich carbonatitic melts with silicate melts in a partially molten peridotite, and and we examine whether carbonatitic liquids are always more wetting than silicate melts. The percolation of carbonatitic liquids and the interconnectivity of melt pockets are investigated by placing a cylindrical dunite rod against a liquid reservoir. As peridotitic matrix we used a synthetic dunite starting from natural San Carlos olivine powder. Sintering has been performed in a single stage piston-cylinder apparatus at 0.8 GPa and 1200°C P-T conditions. The liquid reservoir has a dolomitic composition (Ca0.541, Mg0.389, Fe0.069) CO3 and uses free water as hydrous source (5 wt.% and 30% of the starting material). Time resolved infiltration experiments were performed employing an end loaded piston-cylinder apparatus, at T= 1200°C and P = 2.5 GPa. In order to account for the different roles of gravity, chemical diffusion and Ludwig-Soret diffusion we used two opposite capsule geometries. Hydrous carbonatitic melt pockets were found along olivine grain boundaries; image analysis on electron back scattered and X-ray maps allow us quantifying the apparent dihedral angles between the liquid and olivine and to calculate the grain boundary wetness. Experiments performed at 5 wt. % of water content and 3, 30 and 300 hours of run durations result in dihedral angles evolving from ∼ 31° for 3 hours run, to ∼ 41° for 300 hours run through ∼ 34° for 30 hours run. The volume of liquid fraction infiltrated provides values of 10 vol.%, 8 vol.% and 2 vol.% for short, medium and long run duration experiments respectively. Experiments carried out at 30 wt. % of water content and 48 hours of durations show a dihedral angle values of almost 50° with a range of volume infiltrated melts between 4 to 9 vol. %. The experimental results indicate that dihedral angles progressively increase with increasing water dissolved from 25°-28° in anhydrous carbonatitic liquids up to 50° in water-rich carbonatitic liquids, and, as expected, the volume of interstitial liquid decreases with water increasing. The increase of wetting angles is representative of a sintering process of the solid matrix, which evolves with time in the development of channels of pores, as highlighted relating the grain boundary wetness with fraction of liquid infiltrated. We suggest that the low grain boundary wetness measured may be due to a relatively low liquid-solid interfaces which develop with channelized liquid, and that channelization is promoted by chemical gradient, as established by a carbonatitic segregate in the silicate matrix. If H2O is available, we expect that H2O strongly partitions into carbonatitic liquids. As a result, their dihedral angle may evolve up to 50°, a value which is significantly higher than that characterizing silicate melts at similar mantle conditions.