Calculating the input to the Mariana Subduction Factory: Integration of geochemical and logging data from ODP Leg 185, Site 801C.

The main objective of ODP Leg 185 was to determine the geochemical composition of the inputs to the West Pacific Subduction Factory for use in calculations of elemental mass balance across the subduction zone. To understand such elemental budgets, it is critical to know the lithological diversity and chemical characteristics of the down going oceanic plate. Site 801C, located ocean-ward of the Mariana island arc system, which provide the most complete section of old oceanic crust to date offers an unique opportunity to address this problem. Drilling provides samples, which give us clear constraints on the structure and composition of the oceanic crust. However, core recovery in basement holes is often poor and induces a biasing, such that the less altered material is preferentially recovered. This produces erroneous estimates of the lithologies and bulk compositions input into mass balance calculations. This contribution presents results from core-log integration, using resistivity, density, porosity, velocity, image logs (Formation Micro Scanner) and U and K concentrations derived from the natural gamma ray tool, in conjunction with core description, digital core images and shipboard physical properties measurements. Based on core description, the lithological sequence in Hole 801C consists of pre-dominantly aphyric, massive, fractured or brecciated basalt that are intercalated with thin layers of sediment and two hydrothermal deposits. Recovery average 47%, but over half the lithological sequence is still missing. Downhole logging data together with core material were used to predict the lithology where recovery is incomplete. We defined criteria to identify the different lithologies (massive basalt units, fragmented pillowed basalt units and brecciated units). Detailed study of FMS images allowed us to relocate many samples within the 9.5m drilled for each core. Such relocation is critical in order to compare directly logging data, mainly K abundances extracted from the natural gamma ray tool, and geochemical data measured on discrete samples. We demonstrate that K concentrations extracted from the natural gamma ray tool measurements are accurate and can be used as a proxy for K concentrations downhole. We also show that K abundances recycled within the Mariana Subduction Factory are, at least, twice higher when calculated using log-derived lithological proportions.