Accretion of the lower oceanic crust: Reconciling evidence of hydrothermal fluid fluxes with mineral cooling rates from ODP Hole 1256D, IODP Exp335

Ocean crust covers ~ two thirds of the Earth's surface and is constantly recycled through the plate tectonic cycle. New ocean crust is created along mid ocean ridges, a submarine chain of volcanoes that exist at the boundaries between two tectonic plates, and will eventually be returned to the mantle at subduction zones. Much of the ocean crust produced today is forming at fast spreading ridges, where the two tectonic plates are moving away from each other at rates >80 mm/yr. Ocean crust formed at these fast spreading ridges has a relatively simple stratigraphy. The upper crust, the top 1-2 km of ocean crust (total thickness ~ 6-7km), is composed of erupted lava flows that overlie intrusive feeder dikes. The lower crust (~5 km thick) is made up of plutonic rocks called gabbros that represent crystallised magma chambers. The magmatic processes that generate the lower crust (~5 km thick) are not well understood primarily due to the sparse sampling of the lower crust.

From studies of ophiolites, pieces of the ocean crust that are now emplaced on the continents, two end member models for the accretion of the lower crust have been proposed, the "gabbro glacier" and "sheeted sills" models. They primarily differ in the location of melt intrusion and crystallisation. The removal of the heat within the melt has to be effectively achieved within a few kilometres of the ridge axis, and places strict thermal constraints on the feasibility of the accretion models. Heat from the lower crust can be extracted by conduction into the surrounding rock and by heating seawater-derived hydrothermal fluids that percolate into the crust and convect heat away to the seafloor. The hydrothermal fluids are recorded in the igneous rocks by fluid-rock chemical reactions that create new secondary minerals. These minerals are present both replacing primary igneous minerals and filling fractures to form hydrothermal veins. The thermal feasibility of the two accretion models is intimately linked to the magnitude and distribution of hydrothermal fluids in the ocean crust, with the multiple sills model requiring extensive hydrothermal cooling in the lower crust. Samples recovered from an intact section of the lower crust will provide opportunity to test these models.

The interface between the upper and lower crust is the principal boundary over which magmatic heat from lower crust is transferred to the convecting hydrothermal fluids in the upper crust, and is called the conductive boundary layer. A complete section of upper ocean crust and the upper/lower crust transition has only been sampled once in modern ocean crust, in ODP/IODP Hole 1256D by the Ocean Drilling Program and Integrated Ocean Drilling Program and required ~6 months of continuous drilling to reach this boundary. In this borehole, the complex interplay of magmatism and hydrothermal processes are recorded in the igneous rocks recovered. In this study, the magnitude of hydrothermal fluid fluxes in the conductive boundary layer will be calculated using geochemical tracers of fluid rock reaction. Sr isotope are ideal for this task and have been used extensively and successfully in several studies. These results will then be combined into a thermal model that will use the magmatic observation from Hole 1256D as boundary conditions. The model will include magmatic intrusions into the conductive boundary layer and calculate the heat flux across the boundary and ultimately will be used to test accretion models.

This project will make significant scientific advancement towards understanding the role of hydrothermal circulation during the accretion of the lower oceanic crust. This project will primarily benefit the extensive ocean crust scientific community, both those working on modern ocean crust and ophiolites, as outlined in the Academic Beneficiaries section.

Benefits to: IODP
The proposed research will contribute to addressing 2 of the proposed objectives in the new science plan for IODP; 'Test 3-D models for the formation of oceanic crust' and 'Decipher the record of seawater-rock exchange and quantify its role in global geochemical cycles'. Our research will contribute to these by modeling the hydrothermal fluid and heat fluxes at the transition between upper/lower crust, a critical boundary for both hydrothermal systems and in the end member models for the accretion of the lower crust. Publication of our research in internationally recognized peer-reviewed journals will help to emphasize IODP's position at the forefront of pioneering high impact science.

Benefits to: Academic Researchers
Our research will be of significance to a broad range of disciplines concerned with understanding the interactions between seawater and ocean crust and the accretion of the lower crust. These include: geophysicists, geologists and geochemists, that are addressing key scientific questions about the thermal evolution of the ocean crust, the formation of the ocean crust, hydrothermal alteration of the crust and the aging of the ocean crust, and hydrothermal fluid fluxes into the oceans and their impact on ocean chemistry.

Benefits to: Public
The ultimate goal of the ocean crust community, to drill down to the mantle, and the achievements leading up to this goal (e.g., the technical abilities of scientific ocean drilling to recover hard rock samples, the dynamic nature of the ocean crust and spectacular hydrothermal vent systems) make this project of great interest to the general public, in particular to school children. This has been recently shown by articles published in Nature and National Geographic and exposure of this project in the UK national press. Involvement in University Open Day events, IODP press releases, and publishing results in articles accessible to the wider public will be used to ensure the public are informed and engaged with the ambitions of the ocean crust community.

How does the proposed research generate impact?
Our research will be used to test theoretical models for the accretion of the lower oceanic crust. The development of these end member models involved reconciling geophysical observations of ocean crust with geological records from ophiolites. In addition, by characterizing the heat fluxes that drive hydrothermal circulation in the upper crust, the longevity and potential for these hydrothermal systems to generate metal rich deposits.
Our results will provide a crucial link between the geophysical and ophliolite observations, as for the first time geological observations from intact modern ocean crust will be directly combined with thermal models for the evolution of the dike/gabbro transition zone. This will impact our understanding for how much of the Earth's surface forms, and drive the evolution of the models for how ocean crust forms.

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 will result in publishing our research in peer reviewed journals within 5 years of sailing on IODP Expedition 335. 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.