Incompatible element-rich fluids released by antigorite breakdown in deeply subducted mantle

We present first trace element analyses of the fluid produced during breakd own of antigorite serpentine, a major dehydration reaction occurring at dep th within subducting oceanic plates. Microinclusions filled with crystals+a queous liquid are disseminated within olivine and orthopyroxene grown at pr essures and temperatures beyond the stability field of antigorite. Despite hydrogen loss and significant major element changes that have affected the analyzed inclusions, their trace element composition still reflects charact eristics of the subduction fluid released during serpentinite dehydration. The fluid is enriched in incompatible elements indicating either (1) intera ction with fluids derived from crustal slab components, or (2) dehydration of altered (serpentinized) oceanic mantle previously enriched in incompatib le elements. Several features of the analyzed fluid+mineral inclusions (hig h Pb/Th, Pb/U and Pb/Ce) are in agreement with available experimental work, as well as with the geochemical signatures of most arc lavas and of severa l ocean island basalt mantle sources. The trace element patterns of the flu id+mineral inclusions do not display relative enrichment in large ion litho phile elements compared to high field strength elements, thus suggesting th at the latter elements may become soluble in natural subduction fluids. (C) 2001 Elsevier Science BN. All rights reserved.