Geometry and connectivity of hydrous-carbonatitic liquids in the mantle: an experimental model

The goal of this work is a quantitative assessment of variables controlling the percolation of hydrous carbonatitic liquids in peridotites, and their bearing on the mobility of melts. The percolation of melts and the interconnectivity of melt pockets are investigated here by means of an experimental geometry where a cylindrical dunite rod is placed against a liquid reservoir. Thick graphite cylindrical inner capsules control the redox conditions and prevent Fe-loss to the outer Pt capsules. As a peridotitic matrix we first use a synthetic dunite, pre-sintered in a Mo capsule starting from natural San Carols olivine previously sieved to 38-64 μm. Sintering has been performed in a single stage piston cylinder apparatus at 0.8 GPa and 1200°C. The liquid is generated from a powder mixture of CaCO3, MgCO3 and FeCO3 on a bulk dolomitic composition, using free water as hydrous source (5 wt.% of mix). Preliminary experiments using Mg(OH)2 as hydrous source invariably led to the precipitation of (Mg,Fe)O grains. We performed time resolved experiments in a end loaded piston cylinder apparatus, at temperature and pressure condition of 1100°C - 1200°C and 2.5 GPa, and duration from 3 to 300 hours. Hydrous carbonatitic melt pockets ( 3 m) were found along olivine grain boundaries. BSE images and X-ray mapping at the microprobe allow a quantification of the apparent dihedral angle between the liquid and olivine. The median of frequency distribution of approximately 400 measurements per experiment gives a dihedral angle of 50°, a value much larger than the dihedral angle found for anhydrous carbonatitic liquids (25° - 28°, Hunter and McKenzie, 1989). Interestingly, fluids show the opposite behaviour, as CO2-rich fluids are less wetting ( 90°) than H2O pure fluids ( 65°). By means of EBSD analysis, grain boundary wetness and the role of faceting are evaluated.