Imperial College, United Kingdom

During movement of the oceanic crust away from mid-ocean ridges, hydrothermal alteration of the oceanic crust results in a distinct elemental and isotopic signal. The change in isotopic composition compared to unaltered MORB is an important tracer for the involvement of fluids released from the altered oceanic crust at the base of arc volcanoes during subduction. Any isotopic system not fractionated during dehydration reaction and inert to diffusion is therefore a valuable tracer to shallow or deep recycling of subducted material. Recent studies showed that Mo isotopes could be such a novel proxy [1, 2, 3] as measured Mo isotope values in arc and ocean island basalts differ significantly from mantle values (estimated to be similar to continental crust 0ppm delta (super 97/95) Mo). This study investigates the change in Mo isotopes through the altered oceanic crust to better constrain the input into the subduction zone. Samples of the altered oceanic crust across the Pacific Ocean have been measured for Mo isotopes and Mo concentrations (ODP Sites 1179, 1301, 1256). Combined the studied samples show a decreasing down-hole trend varying over 1 ppm delta (super 97/95) Mo. The heavier isotope ratio in the upper part of the crust might be the source region for the heavy isotope ratio seen in the Mariana arc [2] and the lighter isotope ratio further down-hole might be the source for the light isotope ratio seen in ocean islands basalts [1, 3]. To further constrain this down-hole trend over a complete section of the altered oceanic crust (through pillow basalts, sheeted dyke complex, and gabbros), additional Mo isotope and concentration measurements on the deepest ODP hole (Site 1256) are currently in process.