Hydrothermal alteration of an intact section of the upper oceanic crust formed at a superfast spreading rate

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The mid-ocean ridges form a chain of mountains in the oceans that circuit the Earth like seams on a baseball. These ridges are constructive plate boundaries where new ocean crust is formed by plate tectonic spreading and the eruption of magma from the mantle. This is the major process by which the Earth releases its internal heat, and ~60% of the Earth's surface was formed in the past 180 million years. Because magma is erupted onto and intruded into the ocean crust at ~1200 deg C, seawater that percolates into the crust becomes vigorously heated, commonly resulting in submarine geysers known as black smokers that disgorge >350 deg C, sulfide-rich fluids. There is a very close relationship between magma and hydrothermal 'hot water' circulation. Because the ocean ridges are mostly beneath >2000 m of water many of the processes related to their formation remain poorly understood. Scientists using submersibles can observe only the most recently erupted lavas. Geophysicists can make measurements of the ocean crust using seismic velocities (the speed that waves travel through rocks), but their results must be calibrated against actual rocks to be understood. Sampling the sub-surface of the ocean crust requires deep drilling and the recovery of basement rock cores has been a major goal of scientific ocean drilling since the 1960s. Ancient oceanic rocks preserved on land, known as ophiolites, suggest that the ocean crust is made of three basic layers: erupted lavas, commonly with pillowed shapes, that overly vertical intrusions known as sheeted dikes, which in turn overly, coarse-grained gabbroic rocks that are crystallized magma chambers. Until very recently, only one drill hole had penetrated the lavas into the dikes and no hole had sampled the dike-gabbro boundary even though that zone is the most critical in terms of heat exchange between magma and seawater, because those phases are separated by only a thin thermal contact layer. IODP Expeditions 309-312 recently returned to Hole 1256D, a deep drill hole in the eastern Pacific, and deepened that hole to >1500 m, completely through the lavas, dikes, and into the gabbros. Hole 1256D was drilled into crust that formed at a very fast spreading rate (>200 mm/yr), because gabbros were predicted to be at their shallowest there. Although only ~20% of the global ridge axis is spreading at fast spreading rates (>80 mm/yr full rate), 60% of the current ocean basins and ~30% of the Earth's surface was formed by fast spreading. Because crust formed at fast spreading rates should be relatively uniform, drilling at a single location can be reasonably extrapolated to describe a significant portion of the Earth's crust. The cores from Hole 1256D are a unique, hitherto unavailable, resource for understanding the igneous construction of the ocean crust and interactions between magma and seawater. By making careful descriptions linked with chemical analyses we will be able to work out the physical conditions of fluid-rock interaction and estimate the chemical and isotopic composition of the fluids. From the extent of chemical exchange we can make estimates of how much seawater passed through the crust. Using this new information for Hole 1256D we can compare our results with estimates of hydrothermal fluxes from global chemical budgets (such as poorly known estimates of river discharge), other less complete sections of ocean crust, and ophiolites. The amount of seawater that cools the ocean crust, and the temperatures to which it is heated, greatly influences the location and size of mid-ocean ridge magma chambers. Once we have worked out the conditions and fluxes of hydrothermal alteration, we will use these constraints in computer simulations that couple magma intrusion and crystallization with hydrothermal circulation. These computer models will allow us to establish what size and arrangements of magma chambers and hydrothermal fluid circulation are physically plausible.