Molybdenum isotopes a novel tracer for recycling?

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 of the altered oceanic crust compared to unaltered MORB is an important tracer to identify the involvement of fluids/melts released from the subducted slab in the formation of arc volcano basalts. Any isotopic system not fractionated during dehydration reaction/melting and inert to diffusion is therefore a valuable tracer to identify shallow or deep recycling of subducted material. Recent studies showed that Mo isotopes could be such a novel proxy [e.g. 1] as measured Mo isotope values in arc and ocean island basalts differ significantly from mantle values (estimated to be similar to continental crust 0% δ^98/95Mo). This study investigates the change in Mo isotopes through the different units of an altered oceanic crust to better constrain the input into the subduction zone. A profile through the upper altered oceanic crust (passing pillow basalts, sheeted dyke complex, and gabbros) at ODP Site 1256D has been sampled and Mo isotopes and Mo concentrations have been measured. The studied samples show high variability down-hole (over 1.5% δ^98/95Mo), with a tendency to increase with depth. The heterogeneous dehydration of the oceanic crust might lead to extraction of the heavier isotope ratio in the source of arc volcanoes while the lighter isotope ratio might be the source for the light isotope ratio found in ocean islands basalts. To further constrain the importance of Mo isotopes for recycling, altered ultramafic samples from ODP Site 1272A are studied to understand if the observed isotopic systematic of altered oceanic basalts can also be applied to serpentinized peridotites. As both reservoirs are important players in the fluid/melt regions of arc volcanoes, it is necessary to investigate them in details and constrain their importance against each other. In general it seems the altered oceanic crust has an isotopic ratio slightly heavier then average continental crust and mantle, highlighting the potential of Mo to fingerprint recycling of subducted material. [1] Willbold et al. (2009) GCA 73 suppl., A1444.