Alteration and hydrothermal circulation in the lower oceanic crust (IODP Expedition 345 Hess Deep Plutonic Crust)

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Considering that oceanic crust covers about 2/3 of our planet, we still know remarkably little about it! This is because it is extremely difficult to sample anything other than the uppermost volcanic layer of the crust, especially in the Pacific where the spreading rate is fast (10-15 cm/yr) and the topography is smooth with few exposures of deeper parts of the crust in fault scarps. One place where the lower crust CAN be sampled is Hess Deep, a tectonic window into the Pacific ocean floor situated near the equator and about 800 km west of the Galapagos Islands. IODP expedition 345 aims to drill into the deeper part of the gabbroic lower crust exposed on the south slope of the fault-bounded interrift ridge.There are many objectives for the expedition, but this proposal focuses on alteration, metamorphism and hydrothermal cooling in the lower oceanic crust. The depth, timing, temperature conditions and extent of water penetration are all unknown in the lower crust, and can be constrained by shipboard investigation using thin sections and chemical analyses, and by later work onshore through: 1) strontium and oxygen isotope analysis, 2) characterisation of mineral textures and chemistry using the electron microscope and the electron probe microanalyser, 3) study of the chemistry of fluid inclusions trapped in minerals 4) numerical modelling of cooling rates.