Constraints from high-pressure veins in eclogites on the composition of hydrous fluids in subduction zones

Hydrous high-pressure veins formed during dehydration of eclogites in two p aleo-subduction zones (Trescolmen locality in the Adula nappe, central Alps and Munchberg Gneiss Massif, Variscan fold belt, Germany) constrain the ma jor and trace element composition of solutes in fluids liberated during deh ydration of eclogites. Similar initial isotopic compositions of veins and h ost eclogites at the time of metamorphism indicate that the fluids were der ived predominantly from the host rocks. Quartz, kyanite, paragonite, phengi te, zoisite and omphacite are the dominant minerals in the veins. The major element compositions of the veins are in agreement with experimental evide nce indicating that the composition of solutes in such fluids is dominated by SiO2 and Al2O3. Relative to N-MORB, the veins show enrichments of Cs, Rb , Ba, Pb, and K, comparable or slightly lower abundances of Sr, U, and Th, and very low abundances of Nd, Sm, Zr, Nb, Ti and Y. The differential fract ionation of highly incompatible elements such as K, U and Th in the veins, as well as the presence of hydrous minerals in the eclogites rule out parti al melting as a cause for vein formation. These results confirm previous su ggestions that fluids derived from subducted basalt may have low abundances of high field strength elements, rare earth elements and Y. Variable vein- eclogite enrichment factors of incompatible alkalis and to a lesser extent Pb appear to reflect mineralogical controls (phengite, epidote-group minera ls) on partitioning of these elements during dehydration of eclogite in sub duction zones. However, abundance variations of incompatible elements in mi nerals from eclogites suggest that the composition of fluids released from eclogites at temperatures < 700 degrees C may not reflect true equilibrium partitioning during dehydration. Simple models for the trace elements U and Th indicate the relative importance of the basaltic and sedimentary portio ns of subducted oceanic crust in producing the characteristic chemical sign atures of these elements in convergent plate margin volcanism. (C) 1999 Els evier Science B.V. All rights reserved.