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 Isla nds) 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 a ggregates and glass remnants. Two different types of fluid and melt in clusions occur. Type 1: primary glass inclusions + CO2 fluid inclusion s; these contain Cr-spinel and diopside daughter minerals, composition ally similar to those in dunite. CO2 inclusions (L + V at room tempera ture) have glass rims (10-30 % of the volume), and are always re-equil ibrated. Type 1 inclusions are remnants of the magma from which the du nites were formed. Type 2: secondary silicate glass inclusions and mix ed silicate glass + carbonate inclusions, occurring together with reeq uilibrated CO2 inclusions along fractures originating from a complex n etwork of late veinlets. Melt inclusions contain a silicate glass whic h may include a spherical carbonate droplet. The glass has an ultramaf ic 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 to tals (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 incl usions, always containing some magnetite, have low densities (0.56-0.1 9 g/cm(3)). We propose that the fracture-bound ultramafic glass + carb onate inclusions and the CO2 inclusions associated with Type-2 represe nt a trapping episode of a homogeneous, volatile-rich, CO2-saturated m elt which was present in the upper mantle during Canary Islands volcan ism. This melt is responsible for mantle metasomatism, but apparently distinct from the erupted lava.