Dating mineral formation during mid-ocean ridge flank hydrothermal circulation: Evidence from the Juan de Fuca Ridge, IODP Expedition 327

New ocean crust is formed from volcanic eruptions along a chain of volcanoes under the oceans called mid ocean ridges. This chain of volcanoes represents the tectonic boundary where two oceanic plates diverge away from each other. This process represents the major mechanism for the exchange of heat from the mantle to the crust. The ocean crust covers ~60% of the Earth's surface and all formed during the past 180 million years. After new ocean crust has formed, seawater percolates down into ocean crust and reacts with the rocks. Near the ridge axis the heat from the magma chamber that formed the crust drives the circulation of the seawater, and results in the heating and chemical modification of seawater that then exit the ocean crust from black smoker chimneys as focused high temperature hydrothermal fluids (~400 deg C metal rich fluids). The percolation of seawater into the ocean crust continues beyond the ridge axis onto the ridge flank region for tens of millions of years. In the ridge flank region, the circulation of seawater occurs at much lower temperatures (a few 10's deg C) and is driven by the cooling of the oceanic plate as it ages. The volume of seawater that circulates in the ridge flank region is far greater than in the axial region and has more impact on the chemistry of the oceans for some elements and is thus important for understanding global geochemical cycles.

The inaccessibility and difficulty in locating and sampling hydrothermal fluids as they exit the ocean crust in the ridge flank region has resulted in limited direct observations of this important hydrogeologic reservoir. An alternative to direct sampling of the fluids is to sample hydrothermal veins; these are fractures in the ocean crust that have been filled with secondary minerals that are precipitated as the fluid migrates and reacts with the ocean crust. The distribution, composition and relative timing of these hydrothermal veins provide the only direct insight into the processes operating in the subsurface during the circulation of seawater.

This project will investigate the Juan de Fuca plate located in the NE Pacific, where ocean crust formed at 30 mm/yr. This area is characterised by lava flows overlain by sediments, abyssal hill topography, high angle faulting and basement outcrops. This regions experiences high sedimentation rates because of the abundant supply of young glacial sediments derived from the North American continental margin that bury relatively young oceanic crust. This creates hydrologically and thermally isolated young (1 Ma) ocean crust with strong lateral pressure and temperature gradients, ideal for the study of ridge flank hydrothermal processes. The Juan de Fuca Ridge Flank has been the focus of a decadal long program of ODP/IODP drilling, where the recent completion of IODP Expedition 327 took the total to 12 ODP/IODP drill sites located along both ridge perpendicular and ridge parallel transects, ranging from 0.6 to 3.6 Ma. Volcanic rocks recovered during Exp 327 from Hole U1362A, one of the deeper boreholes in this region (496 metres below seafloor), contains over 1200 hydrothermal veins. A range of mineral compositions is present within these veins that are dominated by clay minerals and oxidised iron minerals. Calcium carbonate veins are also present. For the first time in this region hydrothermal minerals characteristic of higher temperatures and mixing of fluids were also recovered.

By documenting the distribution and relationships between different vein types, insight into the pathways of fluid flow can be achieved. Chemical analyses of the vein minerals themselves will document the chemical composition and evolution of the fluids they formed from, that in turn reflect the extent of reaction between the fluid and the ocean crust. Dating of the different secondary minerals will provide direct constraints on the timing of hydrothermal ridge flank fluids.

This project will make significant scientific advancement towards understanding the evolution of ridge flank hydrothermal circulation and its contribution to global geochemical cycles. This project will principally benefit the vigorous research community engaged in researching the Juan de Fuca Ridge Flank hydrothermal systems (e.g., Becker, Davis, Fisher, Wheat). These benefits are outlined in the Academic Beneficiaries section.

Benefits to: IODP
The proposed research will contribute to addressing two of the proposed objectives in the new science plan for IODP to; 'Understanding the physical and chemical limits to life in the subseafloor' and 'Decipher the record of seawater-rock exchange and quantify its role in global geochemical cycles'. The key objective of the decadal ODP/IODP program on the Juan de Fuca ridge flank is 'to evaluate formation-scale hydrogeologic properties (transmission and storage) within oceanic crust', and our research will contribute to addressing this by constraining the duration of oxidizing fluid circulation in upper oceanic crust. Publication of our research in internationally recognised peer-reviewed journals will help to emphasize IODP's position at the forefront of pioneering high impact science.

Benefits to: Academic Researchers
The research undertaken in this grant will be of significance to a broad range of disciplines concerned with trying to understand the interactions between seawater and ocean crust and the hydrothermal fluxes into global geochemical cycles. These include: hydrologists, petrologists, geochemists and palaeoclimate scientists, that address key scientific objectives relating to fluid pathways in the subsurface, the aging of the ocean crust, past ocean chemistry and past climate records.

Benefits to: Public
The technical abilities of scientific ocean drilling, sub-seafloor hydrology, the dynamic nature of the ocean crust, and spectacular hydrothermal vents on the seafloor make this Earth science subject of great interest to the general public, especially school children. This research will help to address important questions relating to changes in past oceans and the processes controlling the sub-seafloor biosphere. These topics are at the forefront of public interest.

How does the proposed research generate impact?
Our research will be used to test long standing models on the transition of ridge flank hydrothermal fluids from oxidizing to reducing compositions, and will provide independent evidence on connectivity within the ridge flank crustal aquifer. This will benefit hydrogeologists that routinely use seafloor heatflow measurements to infer fluid movement within oceanic basement.
The Juan de Fuca Ridge Flank is the most intensively studied ridge flank system because it is the best "natural observatory" for these processes. In addition to the investment by ODP/IODP, the subseafloor observatories within the existing boreholes will be connected to the Neptune Canada array, a regional scale underwater ocean observatory network that will allow real-time observation of subseafloor processes. Understanding and quantifying ridge flank hydrothermal circulation in this region will allow the extrapolation to a global scale where ridge flanks cover much of the Earths surface.

What will be done (pathway to impact)?
To ensure that our research benefits other academic researchers, and to meet the publication obligations to IODP, our proposed research be published in peer reviewed journals within 5 years of sailing on IODP Expedition 327. Opportunities will be taken to present research at relevant conferences, and during public engagement events. We will engage, inform, and inspire the wider public through participation in University Open Day events, the NOCS Geochemistry Group webpages, IODP and University of Southampton press releases, and publishing results in articles accessible to the wider public.