Evidence for channelized external fluid flow and element transfer in subducting slabs (Raspas Complex, Ecuador)

Fluids released at great depth by dehydrating subducting plates play a major role in mass transport and are responsible for metasomatism and partial melting in the overlying mantle wedge. To investigate the fluid-flow regime and element mobility within a subducting oceanic lithosphere, the Raspas Complex in Ecuador has been selected where an ophiolite association of blueschists, eclogites, metapelites and ultramafitites is exposed. This ophiolite association stands out by high-pressure zoisite veins, associated metasomatized zoisite eclogites and deserpentinized, pseudo-spinifex textured chlorite harzburgites indicating fluid flow and element mobility at a depth of about 60 km. Oscillatory zoning in vein zoisite and garnet rims in the metasomatized eclogites is explained by cyclic fluid expulsion during high-pressure deserpentinization after overstepping of the antigorite breakdown reaction. Fluid inclusion studies reveal a homogeneous low-salinity aqueous fluid composition with minor CH4 and CO2 which is in accord with open-system fluid infiltration derived from an external source. The deserpentinized chlorite harzburgite is a potential source for the low-salinity aqueous fluid. The association of zoisite veins with zoisite eclogites which are enriched in LREE, MREE, Pb, Sr, HFSE, Th, and U, compared to MORB-type eclogites, documents the metasomatic effects and the fluid mobility of a large range of trace elements. In addition, the fluid-mobile trace elements B, Rb, Pb, and Sr are enriched in fluid inclusions in omphacite. Most trace elements cannot be derived from the serpentinites but might be explained by leaching from metabasites and metapelites in zones of intense fluid–rock interaction. A garnet–amphibole rock, deficient in LREE, MREE and Sr, could represent such a leached metabasite. B, Rb, Th and U could be derived from metapelites. By channelized fluid flow and high fluid flux, the fluid-mobile trace elements might be transported into the mantle wedge.

► High-P zoisite veins and metasomatized eclogites indicate fluid flow at 70 km depth.
► External homogeneous low-salinity H2O–NaCl–CH4–CO2 fluid infiltrates eclogites.
► Trace elements (REE, Pb, Sr, Th, U) are transported in high-pressure zoisite veins.
► Deserpentinized spinifex-textured chlorite harzburgite is the potential fluid source.
► Dehydration fluid leached metabasites and metapelites by fluid–rock interaction.