Characterising devolatilisation beneath the Mariana Forearc

Lead Research Organisation: University of Southampton
Department Name: School of Ocean and Earth Science

Abstract

There are two types of crust that make up the plates of the World; oceanic and continental, the former is thin and dense and underlies the oceans of the world; and the latter is considerably thicker and more buoyant and makes up the continents. Oceanic crust forms at divergent plate boundaries known as mid ocean ridges, chains of underwater volcanoes. As the newly formed crust ages and moves further from the ridge it cools, becomes denser and sinks. Where a convergent plate boundary occurs between oceanic and continental crust the denser oceanic crust sinks and subducts beneath the continental crust. Where convergence occurs between two oceanic plates, the older, denser plate sinks and subducts. Such boundaries are responsible for the deepest oceanic trenches on Earth, for example the Mariana Trench (deepest point 11,033 m).

Subduction zones are the principal areas on Earth where crustal materials are transported into the mantle and therefore represent the fundamental recycling process in plate tectonic theory. Geological processes at these convergent margins control seismicity, plutonism and associated volcanism, and geochemical cycling between the ocean, crust and mantle. These margins commonly generate large, tsunamigenic earthquakes, thus understanding the physical properties controlling seismogenesis at the subduction interface is key to elucidating plate boundary behaviour. Sediments and crustal rocks experience changing pressures and temperatures as they subduct, minerals become unstable and breakdown to form new, denser minerals, and in doing so release fluids that are buoyant and rise into the faults above where earthquakes nucleate and interact with overlying mantle rocks. The chemical and physical changes that the subducting plate undergoes prior to the onset of melting controls earthquake behaviour, geochemical cycling and magmatism at these plate boundaries.

To study these processes old pieces of subduction zones that have been uplifted onto the present day continental crust have been studied, but the chemical and physical changes that occurred during the transport of these sections of oceanic crust onto the continents are poorly constrained, and there are large errors on estimated pressures and temperatures of formation. Studying these processes in situ is difficult because they occur deep in the crust and in the some of deepest stretches of water on Earth. Subduction zones that are covered by thick piles of accreted sediments have been successfully drilled, but the thick sediment piles obscures signatures of deep fluids and so the deep subduction zone processes cannot be studied.

At the Mariana convergent margin, the Pacific Plate subducts under the Mariana Plate where there is little sediment overburden. The Mariana Plate is faulted and these faults are associated with mud volcanoes. The mud volcanoes originate from the passage of fluids liberated from the subducting plate and carry material from this plate and chemically altered overlying mantle with them. This provides a unique opportunity to sample fluids generated during subduction and materials from the subducting plate without having to drill deep.

We will use samples from the IODP Expedition 366 drilling into mud volcanoes in the Mariana forearc to analyse waters generated during subduction. Previous work shows that fluid chemistry changes as the plate moves deeper. We will measure the isotopic ratios of different chemical tracers in the fluids to document the evolution of these processes and metamorphic transformations in the downgoing rocks and the chemical reactions the fluids facilitate in the overlying mantle. This information will be combined with analyses of rocks that are transported with the fluids to quantify what is carried into the deeper subduction zone and what is liberated to the crust and ocean above. These analyses will help to constrain the reactions controlling the strength of materials where earthquakes nucleate.

Planned Impact

This project will make a significant contribution to the understanding of processes that control the chemical and physical evolution of the downgoing slab and mantle in the forearc of the Mariana subduction zone. Our findings will be relevant to other subduction zones of the world as we expect similar chemical reactions to control slab and mantle evolution in many settings. This project will benefit the scientific community working on mantle geochemistry, metamorphic petrology, volcanology, microbiology and seismogenesis.

Benefits to: IODP
The proposed research will contribute to three of proposed challenges in the 2013-2023 IODP Science Plan: Challenge 8 "What are the composition, structure and dynamics of the Earth's upper mantle?"; Challenge 11 "How do subduction zones initiate, cycle volatiles, and generate continental crust?"; Challenge 14 "How do fluids link subseafloor tectonic, thermal, and biogeochemical processes?". This research will contribute to these challenges by documenting fluid evolution of a downgoing slab; quantifying a geochemical and fluid budget; and identifying water-rock interactions in the forearc mantle of an active subduction zone. Publication of results in international, peer-reviewed journals will publicise the importance of IODP in generating high impact science by bringing together international multidisciplinary scientists to answer fundamental questions in geoscience.

Benefits to Academic Researchers:
The proposed research will be of interest to a wide range geoscientists working on the subduction zone factory, mantle geochemistry, and the evolution of life. Specifically these include geophysicists, geologists, geochemists and microbiologists that are addressing key scientific questions regarding plate boundary geodynamics, arc volcanism, serpentinisation, and global geochemical cycling between the crust, mantle and ocean.

Benefits to: Public
This research will contribute to the understanding of the evolution of subduction zone plate boundaries, the evolution of the mantle, arc volcanism, and the evolution of life in extreme geochemical environments. By integrating the findings of this study with companion studies addressing these topics we anticipate great advancement in these areas, particularly those that are hazard related such as the seismogenic behaviour of these plate boundaries and the controls that the chemistry of the downgoing plate may have on volcanism in the arc above. Involvement with University of Southampton outreach days such as "Ocean and Earth Day" will expose the public to this work and its principal importance to the wider society.

How does the proposed research generate impact?
The proposed research will place constraints on the chemical reactions controlling the physical properties of the forearc in an active subduction zone and their effects on geochemical cycling between the crust, mantle and oceans, the evolution of the downgoing slab and water-rock interactions controlling serpentinisation of the forearc mantle. These processes are principal driving factors for many geohazards generated at this type of plate boundary; tsunamigenic earthquakes and explosive arc volcanism, and so by advancing understanding of subduction zone processes we address fundamental questions in earth science that is in the interest of the public. Recent work by microbiologists investigating the evolution of life in extreme environments has highlighted serpentinisation as a key process that that may have initiated life. Our work will constrain the geochemical conditions under which serpentinisation occurs in this tectonic setting and will shed light on the conditions under which life may have evolved.