title: A Combined Study on Earth's Deep Water Cycle using Numerical Modelling and Laboratory Experiments
Creators: name: Marzotto, Enrico
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Deep Earth Water Cycle
subject: Geodynamics
subject: Numerical Modelling
subject: StagYY
subject: Finite Differences
subject: Thermal Conductivity
subject: Ringwoodite
subject: Tectonic Regimes
subject: Mantle Transition Zone
subject: Slab Stagnation
subject: 500

description: A Combined Study on Earth's Deep Water Cycle using Numerical Modelling and Laboratory Experiments. The distinctive feature of Earth’s surface compared to other known planets is the abundance of liquid water. This water was delivered during the accretion stage of the planet by rocky asteroids, with minor contributions from comets and protosolar nebular gas. Experimental evidence shows that water can be incorporated into many of the minerals that make up Earth’s interior. When water is hosted in the crystalline structures, it alters the physical properties of minerals, thereby enhancing deformation processes. Therefore, water-bearing rocks are less dense and weaker compared to their dry counterparts. Geophysical observations and natural samples reveal that water is indeed present in Earth’s mantle, mostly concentrated in the mantle transition zone. The region is bounded by two seismic discontinuities, at 410 km and 660 km depth, and is characterized by the presence of two minerals with high water solubility: wadsleyite and ringwoodite. Water is carried into Earth’s interior by the subduction of oceanic lithosphere, i.e. slabs. This water-delivery mechanism is also known as the ‘deep water cycle,’ and might represent the key element for the onset of plate tectonics on Earth. One essential tool to explore the role of water in plate tectonics is numerical modelling. With this technique, it is possible to reproduce many physical phenomena occurring on Earth by self-consistently simulating mantle convection. This is achieved by solving the governing equations of mass, momentum and energy conservation, also known as Stokes equations. The complexity of these equations requires the use of approximation methods, like Finite Difference (FD), to solve the derivatives over time and space. The feedback loop between mineral physics constraints, numerical modelling and geophysical observations represents the best strategy to unravel Earth’s interior. However, despite the efforts of geoscientists, many questions regarding the deep Earth water cycle remain so far unanswered. This thesis focuses on the hydration state of the MTZ, with three aims addressing different aspects of the topic: (1) provide mineral physics measurements on the effect of water on ringwoodite thermal conductivity; (2) produce a model featuring an Earth-like mobile lid while minimizing the effects of numerical parameters; and (3) analyse the parameters that allow for the stagnation of a slab in the MTZ, which may lead to the water enrichment in this region In project (1), hydrous ringwoodite crystals were synthesized with multi-anvil experiments and characterized by X-ray diffraction, electron-microprobe analysis, and infrared spectroscopy. The samples were loaded into a diamond anvil cell to perform measurements at the high-pressure conditions of the MTZ. The thermal conductivity of ringwoodite, Λ(Rw) was measured with the time-domain thermo-reflectance method. It was found that the presence of 1.73 wt% water reduces Λ(Rw) by 40%. From this analysis, it was possible to derive a parameterized equation to extrapolate Λ(Rw) as a function of pressure and water content. With this tool, the large-scale thermal evolution of a slab was studied. The calculations were performed by assuming a slab stagnating in the MTZ, then being progressively heated by the warm ambient mantle. A 1D FD numerical code was designed to solve the heat diffusion equation, and the derived equation for Λ(Rw) was included into the physical model of the slab. The results reveal that hydrous ringwoodite hinders the heating of the slab, thus promoting the survival of water-bearing minerals. In project (2), a global-scale model to reproduce self-consistently plate-like behaviour was designed. The models were computed with StagYY in a 2D spherical annulus geometry. The tectonic regime of a planet is controlled by the yield strength of the lithosphere τy. Four main tectonic regimes can be identified in nature: micro-plate dripping, plate-like, episodic resurfacing, and stagnant lid. It was found that the tectonic regime in the models is heavily influenced by the grid resolution used for the discretization. The modelled lithosphere is weakened by reducing the horizontal grid spacing ∆w, while it becomes stronger when reducing the vertical grid spacing ∆r. These effects are numerical in nature, and are the consequence of the interpolation of the Stokes equations. It was found that the best results are achieved by accurately resolving the lithosphere, i.e. ∆w ≤40 km and ∆r ≤15 km. In project (3), the interactions between the cold descending slab and the 660 km discontinuity were analysed. The models were computed with StagYY in a 2D spherical annulus geometry. From the models it can be inferred that the stagnation of a slab in the MTZ is controlled by three parameters: (i) the density jump ∆ρ, which enhances the slab pull, and affects the latent heat absorbed by the post-spinel reaction; (ii) the negative Clapeyron slope Υ, which causes a downward deflection of the phase transition; and (iii) the viscosity jump ∆η, which decelerates the descent of the slab.