The subduction of tectonic plates is one of the fundamental processes of plate tectonics, and the most potent source of earthquakes near the Earth's surface. The process occurs at convergent boundaries where one tectonic plate moves under another tectonic plate by sinking into the Earth's mantle. Despite the wealth of research that has been dedicated to understanding the role of subduction in earthquake generation (seismogenesis) direct sampling of the seismogenic region of subduction zones has been beyond the reach of direct sampling and observation.

I sailed as part of an international team of scientists on Integrated Ocean Drilling Program (IODP) Expedition 344 to recover seafloor sediments and underlying volcanic rocks that make up the subducting oceanic plate (Cocos-Nazca) and the upper plate (Caribbean) of the Costa Rica Arc. Expedition 344 made important progress in determining the geological nature of subducting sediments and basement, the rate of subduction of the Cocos plate into the Costa Rican margin, the nature of fluid-rock interactions within the Costa Rican margin, and how the forces acting on the subduction system change as the subduction system enters the seismogenic zone.
At one site, volcanic rock beneath sediment was sampled. The volcanic rocks recovered during Expedition 344 originally formed during the eruption of sub-sea volcanoes along mid-ocean ridges, but have since cooled, moved by ocean spreading, and are now entering the Costa Rican subduction zone.

Throughout the lifetime of oceanic crust, thermally driven seawater/rock interaction known as hydrothermal alteration, results in exchange of elements and compounds to form new minerals within voids and veins, or locally replacing the original minerals in the rocks. Fluids that form these minerals are sourced from seawater that has interacted with oceanic crust at a low temperature. As oceanic crust subducts, temperatures and pressures increase such that water previously bound in secondary minerals migrates upwards and that it may contribute the hydrothermal activity at or near subduction zones. Our research, based on secondary mineral calcium carbonate will be used to determine if subduction-related fluids have influenced the hydrothermal system of imminently subducting basement.

The element Strontium (Sr), present in calcium carbonate, is extremely useful since its isotopic composition in seawater through time is well known. We will use the Sr-isotopic composition in calcium carbonate to estimate how much seawater has mixed with mid-ocean ridge basalt, which in turn has a very different but well defined isotopic ratio. In addition we will use Sr isotopes to partially constrain the timing of calcium carbonate formation.

Temperature is known to control the preference of Oxygen (O) isotopes uptake during calcium carbonate formation. We will therefore use O-isotopes in carbonates to determine the temperature of formation to estimate the thermal environment of formation. Knowing the temperature will provide insight into whether some or all of the fluids have come from subduction settings. We will see if later episodes of veining (as the crust moves towards the subduction zone) increase in temperature. This will indicate that subduction processes are influencing incoming ocean crust.

Trace elements from fluids are incorporated in to calcium carbonate during its formation, therefore we can measure the concentrations of these elements to estimate the composition of the fluid. Trace element compositions will then be compared against seawater compositions and pore-fluid compositions from other subduction zone settings to estimate where fluids may have come from.

Our research will offer new insight into the nature of alteration within imminently subducting ocean crust, and therefore will contribute to our understanding of the composition and physical properties of material that enters subduction zones.

Potential Impact:
The proposed research to be conducted on calcium carbonate samples acquired during IODP Expedition 344, will greatly benefit my immediate research partners and a broad range of researchers across several disciplines worldwide. Specifically this research will provide detailed information regarding the relative timing of late stage hydrothermal alteration, the composition and origin of the hydrothermal fluid from which carbonate precipitated from, and the evolution of the thermal conditions of alteration within imminently subduction upper oceanic crust along the Costa Rican margin.
Benefits to: IODP
Our work helps addresses one of the fundamental objectives in the Initial Science Plan of the IODP that is to explore the seismogenic zone within a convergent margin (subduction zone). This research helps address two key objectives of Expedition 344: 1) Characterising the hydrogeological regime of incoming basement which will ultimately help determine 2) how the incoming oceanic crust affects seismogenesis. Our research will be publicised in internationally recognised peer-reviewed journals that will help to assert IODP's position of being at the forefront of pioneering high impact science.

Benefits to Academic Researchers:
Through this grant, our work will have significant impact in a wide range of researchers in many disciplines that are focussed on understanding the processes of subduction, seismogenesis, and hydrothermal activity at convergent margins. Our research will impact on geophysicists and structural geologists who will use our findings to understand the controls on physical properties of igneous basement rocks, which in turn impacts on studies of the stress regime and geometry of subduction systems. Our research will be fundamental to other petrologists and geochemists that seek to understand subduction related volcanism and the hydrogeological processes surrounding subduction zones.

Benefits to Public:
The goal of CRISP (to which Exp 344 forms a major component) is to learn about the nature of earthquakes at subduction zones. Given that large portions of our society live within earthquake prone regions, our contribution to CRISP through Expedition 344 is of great societal relevance. Our links to school teachers (Teacher at Sea Program), journalists, and museums will be utilized to ensure that wider society is both informed and inspired.

How does the proposed research generate impact?
Isotopic and geochemical analysis of calcium carbonate veins in this study combined with detailed documentation of the nature and extent of hydrothermal alteration and crustal composition, together with inferences on the hydrogeological regime from Expedition 344 will be central to assessing the impact of ocean basement-influenced properties such as permeability, fluid pressure, and composition on seismogenic zones. All of which are major aims of Expedition 344 and the wider CRISP project.

Understanding the hydrogeological regime of incoming basement will be essential for understanding the chemical and physical properties of basement entering the subduction system, which in-turn likely affects the seismogenic behaviour along the subduction megathrust.
IODP Expedition 344 is the second stage of a major project that ultimately aims to sample an erosive subduction system from the aseismic zone and into the seismogenic region. 90% of Earth's seismogenesis takes place at convergent margins and of these convergent margins, at least 50% are erosive. Due to the wealth of previous study at the central American margin, the erosive nature of the Costa Rican subduction zone, and the relative ease of access to the seismogenic zone, our collective efforts during Expedition 344 will represent a major step forward in achieving the long standing goal of understanding the nature of seimogeneisis at subduction zones.