MSA DANA MEDAL LECTURE: MINERALOGICAL AND PETROLOGICAL CONSTRAINTS ON THE EARTH'S DEEP WATER CYCLE

The presence of liquid water at the Earth's surface through its entire history is crucial to develop and sustain life on our planet. Earth doesn't only have a water cycle linking hydrosphere and atmosphere, but also a deep water cycle, acting over millions of years, where the hydrosphere is interacting with the lithosphere. How much water is stored in the Earth’s interior? How is water transported and redistributed among different reservoirs? How does water influence the physical and chemical properties of solid and fluid phases? Is it possible that oceans are drained by subduction? Is the deep water cycle in a secular equilibrium? I will use a combined petrological and mineralogical approach using examples from the field, experiments and from thermodynamic modelling to address some of these questions, focussing on the deep water cycle in ultramafic rocks.

Water is incorporated into hydrous minerals when oceanic lithosphere reacts with seawater-derived fluids close to the ocean floor. As the hydrous phases are dragged down the subduction channel they break down due to increasing pressure and temperature. The P-T stability fields of antigorite and chlorite with respect to slab geotherms will be discussed in detail as this provides a first-order constrain on how much water is returned to the Earth’s surface through subduction-related magmatism and how much water can be transferred to the deeper mantle beyond 200 km depth.

The water liberated from the breakdown of hydrous phases interacts with altered oceanic crust and sediments. It will be discussed at which conditions aqueous fluids, hydrous melts or supercritical fluids with intermediate composition form and how this influences the mass transport of major and trace elements from the slab into the mantle wedge.

Beyond the stability of hydrous phases, H is present in trace amounts in nominally anhydrous minerals. Experimental results provide constraints on how H is incorporated in the main upper mantle mineral olivine. Results from experiments and natural rocks will be compared to investigate the water transfer from hydrous minerals to olivine at the slab-mantle wedge interface. This is a critical step to evaluate the amount of water that can be dragged down to the deeper mantle.