Quantifying Ridge Flank Hydrothermal Alteration on the Juan de Fuca Ridge: IODP Expedition 327

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The formation of ocean crust is an integral component of the plate tectonic cycle, repaving the ocean floor continuously (the oldest ocean floor today is 180 Myr). The ocean crust is composed of igneous rocks that form along a chain of underwater volcanoes called the mid ocean ridges. Mid ocean ridges are located at the edges of plate boundaries where two tectonic plates move away from each other, allowing the mantle to upwell beneath the ridge and partially melt. New ocean crust forms and moves away from the ridge and eventually will be subducted back into the mantle. The ocean crust is also the largest aquifer on Earth, with seawater percolating down into the volcanic crust through fractures and becoming heated and chemically reacted in a process called hydrothermal circulation. Near the mid ocean ridges, heat from the crystallising magma drives this circulation and modifies the chemistry of fluids that exit the crust at black smoker chimneys. On the ridge flanks, hydrothermal circulation is driven by the cooling of the crust as it ages. Hydrothermal fluid temperatures are considerably lower during ridge flank circulation (10's oC) and the chemical changes smaller, but the volume of fluid circulation is far greater. Consequently, ridge flank circulation represents a larger proportion of the chemical fluxes from hydrothermal circulation into the oceans and quantifying these fluxes is important for understanding global geochemical cycles.Direct sampling of ridge flank hydrothermal fluids is difficult because they are generally diffuse and the geographical area where they are present is too large to map extensively. Indirect sampling of the ridge flank hydrothermal system is possible by studying the crustal rocks the fluids have reacted with and circulated through. Hydrothermal alteration of the ocean crust manifests as replacement of primary igneous minerals and as secondary minerals filling open porosity (fractures, vesicles). The distribution, composition and relative timing of secondary hydrothermal minerals within the rock provide the only direct insight into the processes operating in the subsurface during the circulation of seawater.In this project we will investigate ridge flank hydrothermal circulation through the Juan de Fuca Ridge Flank using samples recovered by the Integrated Ocean Drilling Program (Expedition 327). The Juan de Fuca plate is located in the NE Pacific, and formed at Juan de Fuca Ridge at a spreading rate of 30 mm/yr. The seafloor in this area is characterised by lava flows overlain by sediments, abyssal hill topography, high angle faulting and basement outcrops. The Juan de Fuca Ridge Flank experiences unusually high sedimentation rates because of the abundant supply of young glacial sediments from the North American continental margin, creating hydrologically and thermally isolated young (1 Ma) ocean crust. The Juan de Fuca Ridge Flank has been the focus of a decadal program of ODP/IODP drilling, with 12 ODP/IODP drill sites located along both ridge perpendicular and ridge parallel transects, ranging from 0.6 to 3.6 Ma.Being able to map out the patterns of low temperature alteration in the sections of crust that are available to us will allow an overall characterization of the effects of fluid flow through the oceanic crust. A representative range of alteration types, flow types, and other features will be sampled so that the maximum possible combinations of geologic features are accounted for. Chemical analyses of powered whole rock samples will determine how changed the basalt is as a result of hydrothermal alteration. Measurements of the levels of oxidised iron (Fe3+) will give an indication of the past conditions of alteration. Isotopic analyses document the signature that the hydrothermal fluids have imparted to the ridge flank and allow determination of the fluid origin.