High-resolution seismic constraints to reveal mid-mantle processes

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The dynamics of Earth's mantle, the 2900 km layer sandwiched between the crust and core, have shaped the Earth's surface, as we know it today. For example, upwelling material in the mantle, known as mantle plumes, causes localized, increased volcanism at the surface, forming most of today's ocean islands. The initiation and early formation of continents has also been attributed to mantle plumes, along with more recent continent-sized volcanic outflows known as 'large igneous provinces'. Processes in the deep mantle have also been hypothesized to control the pattern of plate tectonics, and the supercontinent cycle over the history of the Earth. We currently do not have a full picture of the dynamical history of the mantle that explains all these observations. For example, we do not know whether the mantle convects as one layer or two layers, and thus if mantle plumes directly connect processes at the core-mantle boundary to the surface, and if the mantle is well mixed over time. We also do not know the nature of heterogeneity in the deeper mantle, nor how this influences the overall dynamics. Much of our knowledge of the deep Earth's structure and dynamics comes from global seismic tomography, which uses earthquake waves to make an image of seismic velocity variations in the mantle. The images of seismic tomography show features with fast seismic wave speeds, interpreted as cold downgoing slabs, and features of slow seismic wave speeds, interpreted as hot upwelling mantle plumes. Resolution of these images has been ever improving with the burgeoning increase in data and computational power. One of the most remarkable recent discoveries has been the ponding of some slabs and mantle plumes around a depth of 1000 km in the mid-mantle. An unanswered question lies here: What is happening at this depth that affects the convective motion?The downside of seismic tomography is that it broadens imaged features and underestimates their true amplitudes. This is partly because the periods of the waves used are relatively long, thus reducing the resolution of the heterogeneity imaged at depth. Here we propose targeted studies using higher-frequency waves than can be incorporated in seismic tomography to image the small-scale heterogeneities around 1000 km in the mid-mantle. Specifically, we will use waves that are reflected or converted by these heterogeneities and therefore have strong sensitivity to the boundaries of these features. The unique sensitivities of the different phases allow us to map the size, shape, velocity contrast, density contrast and sharpness of the anomalous heterogeneities. In a preliminary study using converted seismic waves beneath Europe, we mapped broad patches of heterogeneity consistently at 1000 km depth. We will expand this technique to map these features on a global scale and understand how they relate to the observed slabs and mantle plumes and to what degree they are clustered around 1000 km. Next, we need to target the heterogeneities with a combination of different reflected and converted waves. The questions about the nature and role of these mantle heterogeneities are fundamentally interdisciplinary. We will combine these high-resolution seismological constraints with experiments and calculations of the thermo-elastic behaviour of specific compositions under high pressures and temperatures. In this manner we will test a number of key hypotheses on deep Earth structure: Do the heterogeneities originate from the surface and are they introduced by subducting slabs? Or do they represent primordial material, either brought up from the deeper mantle or stagnating at this depth throughout the history of the Earth? Are there different types of heterogeneities present?By understanding the composition of the observed heterogeneities through targeted deep Earth imaging, we can determine its role in controlling the overall mantle dynamics.