Two-thirds of Earth’s surface is made up of oceanic crust, which forms at mid-ocean ridges and is recycled into the underlying mantle via subduction at convergent plate boundaries. During each phase of its ~200 million year lifecycle the oceanic crust plays a key role in global geochemical cycles, including the carbon cycle. The inaccessibility of the present-day seafloor makes it logistically difficult to study. However, fragments of ancient oceanic lithosphere (crust + uppermost mantle) that have been tectonically emplaced on continental margins provide access to complete cross-sections of seafloor. These exposed sections of oceanic lithosphere are called ophiolites. The Samail Ophiolite, in Oman and the United Arab Emirates, is the largest, best-exposed section of oceanic lithosphere in the World. The Oman Drilling Project is a comprehensive drilling program that will sample the whole ophiolite sequence, from crust through to upper mantle, in a series of diamond- and rotary-drilled boreholes. Data collection will include analysis of rock core, geophysical logging, fluid sampling, hydrological measurements and microbiological sampling. More than 40 natural scientists from a broad spectrum of disciplines will use these new datasets to address a diverse range of scientific questions relating to the formation, hydrothermal alteration and biotic and abiotic weathering of oceanic lithosphere.
|Geological map of the southwesten Samail Ophiolite showing drillsite locations. Map data from Nicolas, Boudier & co-workers.|
The overarching goal of scientific drilling in the Samail ophiolite is to understand the full spectrum of processes that create and modify oceanic crust and shallow mantle, involving mass and energy transfer between the mantle, the crust, the hydrosphere, the atmosphere and the biosphere over a range of temperatures from ~ 1350 to 20°C, depths from the surface to 10 or 20 km below the paleo-seafloor, and tectonic settings from spreading ridges to the deep ocean to surficial weathering to subduction zones.
The Oman DP will address long-standing unresolved questions regarding formation of oceanic lithosphere at mid-ocean ridges, hydrothermal alteration of the sea floor and subsequent mass transfer between the crust and the oceans and recycling of volatile elements in subduction zones. Furthermore, the science team will undertake frontier exploration of subsurface weathering processes that lead to natural uptake of CO2 from surface waters and the atmosphere, and the nature of the subsurface biosphere in areas where these processes are occurring.
The scientific objectives are to:
(1) Quantify the nature and timing of solid upwelling beneath a spreading ridge using crystal shape and lattice preferred orientation data systematically collected on core from the periphery of a mantle diapir.
(2) Quantify the nature and structural relationships of melt transport features in the shallow mantle, to evaluate mechanisms that focus transport from a melting region 100’s of kilometers wide into a zone 2 kilometers wide where igneous oceanic crust is formed.
(3) Quantify chemical variability deformation structures in the crust-mantle transition zone and plutonic lower crust, to determine the depth of crystallization, the nature of ductile flow, and mechanisms of melt transport.
(4) Quantify hydrothermal alteration and cooling of the plutonic lower crust using mineral compositions, diffusion profiles, and stable isotopes to determine the importance of hydrothermal convection in heat and mass transfer.
(5) Investigate processes in the critical “dike-gabbro transition” via study of cross-cutting igneous relationships, metamorphic mineral assemblages, and geochemical alteration.
(6) Quantify mass transfer from subducted sediments into overlying peridotite at the “leading edge of the mantle wedge” via petrologic and geochemical studies, with special focus on carbon cycling.
(7) Systematic and detailed study of ongoing, subsurface alteration of mantle peridotite, including fluid compositions, flow rates and hydrology, characterization of fracture and vein spacing, studies of mineral assemblages formed by carbonation, hydration (serpentinization) and oxidation and resulting mass transfer, and characterization of the subsurface microbial biosphere that derives energy from catalysis of low temperature alteration in this unique and fundamentally important environment.