FOCUSED FLUID-FLOW DURING SUBDUCTION - OXYGEN-ISOTOPE DATA FROM HIGH-PRESSURE OPHIOLITES OF THE WESTERN ALPS

During subduction, hydrothermally altered oceanic crust undergoes exte nsive dehydration. The water released migrates upwards and may escape along the slab-mantle interface, or continue rising to hydrate the man tle overlying the subducting slab. Oxygen isotope ratios from eclogiti c ophiolites of the western Alps correlate closely with lithology. Sam ples with little-altered basalt as a protolith have delta(18)O values of 5.1-6.0 parts per thousand. Material representing low-temperature a lteration zone rocks have delta(18)O values of 5.9-6.6 parts per thous and and rocks originating in high-temperature alteration zones have de lta(18)O values of 4.6-5.3 parts per thousand. These isotopic values c losely approach those of the ocean crust and unmetamorphosed ophiolite s. These data suggest that the Alpine rocks have not experienced signi ficant fluid flow during metamorphism, implying that the escape of flu id from subducted ocean crust is highly focused, probably along struct urally controlled pathways to the top of the slab. The fluid may then continue largely along the slab-mantle interface, although a portion o f it rises further to hydrate the overlying mantle.