Melt and fluid inclusions in dunite xenoliths from La Gomera, Canary Islands: tracking the mantle metasomatic fluids

Composite xenoliths in alkali-basaltic lavas on La Gomera (Canary Islands) consist of clinopyroxene-spinel-dunites cut by 3-mm to 2-cm wide clinopyroxenite veins. The xenoliths are crosscut by a complex system of mainly intracrystalline late veinlets, filled by microcrystalline aggregates and glass remnants. Two different types of fluid and melt inclusions occur. Type 1: primary glass inclusions + CO2 fluid inclusions; these contain Cr-spinel and diopside daughter minerals, compositionally similar to those in dunite. CO2 inclusions (L + V at room temperature) have glass rims (10-30 % of the volume), and are always re-equilibrated. Type 1 inclusions are remnants of the magma from which the dunites were formed. Type 2: secondary silicate glass inclusions and mixed silicate glass + carbonate inclusions, occurring together with reequilibrated CO2 inclusions along fractures originating from a complex network of late veinlets. Melt inclusions contain a silicate glass which may include a spherical carbonate droplet. The glass has an ultramafic composition (MgO: 24-38 wt.%, FeO: 5-18 wt.% and SiO2: 33-46 wt.%). A high volatile content (H2O + CO2) is suggested by very low oxide totals (approximate to 85 wt. %) and by high Cl contents (up to 3900 ppm). Bulk analysis of late veinlets show similar compositions, but with higher FeO/MgO ratios. The carbonates are high-Mg calcite or dolomite; the shape of the crystal and the poor crystallinity suggest that they were derived from a carbonate melt (carbonatite). Associated CO2 inclusions, always containing some magnetite, have low densities (0.56-0.19 g/cm(3)). We propose that the fracture-bound ultramafic glass + carbonate inclusions and the CO2 inclusions associated with Type-2 represent a trapping episode of a homogeneous, volatile-rich, CO2-saturated melt which was present in the upper mantle during Canary Islands volcanism. This melt is responsible for mantle metasomatism, but apparently distinct from the erupted lava