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 consist of clinopyroxene-spinel dunites cut by clinopyroxenite veins; the xenoliths are crosscut by a complex system of intracrystalline late veinlets, filled by microcrystalline aggregates and glass remnants. A study of the fluid and melt inclusions indicate their occurrence at various stages of the evolution of the lavas. Earliest melts, remnants from the magmas from which the olivine crystallized, were already CO (sub 2) -saturated, but their composition is unknown. Shortly before eruption, the xenoliths were permeated by a complex metasomatic fluid, rich in CO (sub 2) and H (sub 2) O, which evolved through magmatic immiscibility into distinct ultrabasic and carbonate melts. CO (sub 2) was present mainly as an immiscible oxidizing fluid phase, whereas H (sub 2) O and Cl remained dissolved in the ultramafic, Mg- and Fe-rich silicate melt. The initial metasomatic magma was probably carbonate-rich (carbonatite composition). The petrographic modifications caused by these melts remained minor in La Gomera, but seem to have been more important elsewhere in the Canary Islands. An hypothesis of several metasomatic episodes is proposed.