Tracing volatile cycling during progressive subduction in the Mariana Forearc

Lead Research Organisation: Durham University


Plate boundaries, where tectonic plates slide past each other, are the principal regions on Earth where the greatest and most rapid geological changes occur, including catastrophic earthquakes and volcanic eruptions. At one type of boundary, subduction zones, the colder, denser oceanic tectonic plate plunges beneath the hotter more buoyant plate into the mantle below. This process controls the plate tectonic cycle and drives the resurfacing of the ocean floor every ~200-270 million years. During this process materials from the Earth's surface are transported to the mantle, which makes up almost all of the Earth's volume. As this material is pulled into the mantle it induces changes in the surrounding mantle rocks, facilitating melting that drives volcanism in arcs, such as the Andes. Volcanism above subduction zones, and where new crust is formed at ocean spreading centres is the main route for material in the mantle to return to the Earth's surface.

Society is increasingly concerned about rising levels of CO2 and its effect on global climate. Indeed, the study of transfers of carbon from different global reservoirs is at the forefront of investigations into the ability of the Earth System to regulate climate. The timescales over which these controls operate depend upon the rate of cycling between, and the size of different reservoirs of carbon on Earth. The balance of these processes and transfers between the surface and the deep Earth has controlled the evolution of the composition of our atmosphere and moderated climate over Earth's history. The mantle and core of the Earth are estimated to hold ~90% of Earth's carbon, but the ability of the Earth System to lock away carbon in the mantle is not well understood. We can measure how much carbon is coming out of volcanoes above subduction zones, and this value is about half of what is initially pulled down into the mantle. Carbon may escape from the subducting plate before it reaches the zone of melting below arc volcanoes, in some cases this carbon is stabilised as carbonate minerals in the mantle directly above the subducting plate. Evidence for this comes from rocks that were originally made of the same material as mantle rocks that have now become 100% carbonated.

As one tectonic plate subducts beneath the other the resulting temperature and pressure increase causes seawater in pore spaces to be squeezed out followed by the breakdown of minerals that contain water. This water is expelled into the mantle rocks that the subducting plate slides through. The mantle is far from equilibrium with the Earth's surface such that the release of these waters drive serpentinisation (the hydration of mantle rocks), which forms a new group of minerals called serpentine which is less dense than the other mantle rocks. The density discrepancy drives movement of this newly formed rock, driving it to move upwards where it erupts from mud volcanoes on the seafloor. The process of serpentinisation generates energy that may drive deep microbial life, and environments like the one studied here may be where life first began on Earth.

Presently there are ~62,000 km of subduction zone on Earth, and there is variability in temperature of the subducting slab, and angle at which they subduct; these factors control the conditions under which chemical reactions occur at depth. This study will investigate the conditions where carbon and water are released from the subducting plate and sampled through modern day mud volcanoes above a subduction zone in the Mariana Forearc in the Pacific Ocean. By investigating the conditions that favour carbon storage in the mantle above subducting plates, and quantifying releases of carbon from the subducting slab, we will identify the global significance of this process and ultimately its role in the global carbon budget.


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