Accretion of the lower oceanic crust at fast-spreading ridges: a rock drill and near-bottom seafloor survey in support of IODP drilling in Hess Deep.

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Generation of ocean lithosphere by seafloor spreading at mid-ocean ridges is one of the fundamental geological processes operating on Earth. One of the most important yet most intractable problems is to understand how the magma reservoir beneath ridges generates the lower crust, especially at fast spreading rates. Gabbroic rocks from fast-spread lower crust are normally inaccessible; only in the Hess Deep rift valley (E Pacific) is a substantial section known to be exposed. We propose to survey this unique section using a remotely-operated vehicle in order to satisfy site survey requirements for the highly-ranked IODP proposal #551. In addition we plan to conduct our own sampling programme using the British Geological Survey's 'BRIDGE' seabed rock drill to obtain large numbers of short (1 m) orientated cores in transects through the gabbro section. Constraining heat and mass flux from the Earth's mantle to crust and crust to ocean, and testing conflicting models for the functioning of mid-ocean ridge magma chambers, can only be tested by direct observation and sampling. Fully oriented cores are imperative to address many of the outstanding questions and these capabilities of the BRIDGE drill remain unique. This survey will not only augment the IODP proposal but will in its own right also answer many critical questions that IODP operations cannot tackle.