Detecting melt in the deep mantle with seismic anisotropy and attenuation

Lead Research Organisation: University of Bristol
Department Name: Earth Sciences

Abstract

Melting is a critically important component of the Earth system. The chemistry of the crust and upper mantle is controlled by melting and crystallisation at mid-ocean ridges, hotspots and island arcs. Plate tectonics, volcanos and heat flow, and many other Earth processes are all affected by shallow melting. However, melting may be equally important at the other 'end' of the mantle - the core-mantle boundary (CMB). This boundary is, objectively speaking, the most significant in the Earth system as it represents a huge contrast in most physical properties (e.g., temperature, density, chemistry, viscosity).

The presence of a melt phase in the lowermost mantle would significantly affect whole mantle thermodynamics and chemistry. The presence of melt would alter the viscosity at the base of the mantle and could affect the generation of plumes. It could provide a persistent hidden reservoir for primordial chemical components 'missing' from the upper mantle. These include incompatible radiogenic elements which might raise the temperature at the base of the mantle, altering the heat-flow out of the core. This would have consequences for the energy available to power the geodynamo: the rapid convection of liquid iron in the Earth's core which generates its magnetic field. It could also potentially sequester volatile phases like water and CO2, altering our picture of the evolution of the abundance of these near the surface through deep time.

Experiments have provided plausible candidates for such a melt phase. These include the melting of basalts that have formed at mid-ocean ridges (MORB) which have descended to the core mantle boundary in subducting slabs. Another possibility is that melt is left over from the time when the entire Earth was molten (billions of years ago). While these have been shown experimentally to be possible, we would like to be able to observe their presence directly - a long standing challenge in the Earth Sciences.

Seismology provides the only direct probe of the deepest parts of the Earth. The lowermost mantle shows a range of interesting seismic features, including a strong signature of seismic anisotropy (the variation of seismic wavespeed with direction). This is generally ascribed to the deformation of lower mantle minerals but can also be caused by the preferred alignment of an included melt phase. In order to distinguish between these two mechanisms, we propose a new technique which includes measurements of another parameter: seismic attenuation. We have a large dataset of seismic waveforms which image the lowermost mantle across the world, to which we will apply our new methodology. This will allow us to test for the presence of melt across a broad swath of D''. The map of melt we aim to generate will allow us to assess its effect on the broader Earth system, provide insights into the dynamics and structure of the base of the mantle, and probe the origins of deep melting.

Planned Impact

There is quite significant scope for aspects of the research to be of wider societal value. We have identified two primary avenues for this: industrial applications of the imaging technology we will develop, and public engagement with the wider implications of the science we produce.

Industrial applications for new seismic imaging technology
A primary user of seismic imaging is the hydrocarbons industry. Constraining seismic anisotropy in such contexts can be used for improving seismic images, quantifying important rock phenomena such as fracturing and compaction, and monitoring fluid migration in the subsurface. This also has significant application in the geothermal industry. Incorporating attenuation into anisotropy measurement will enable improved understanding of the subsurface; we envisage this to be a promising avenue for continued research.
There are two primary mechanisms which we will use to facilitate knowledge exchange with the industry. The first is through the Bristol University Microseismicity Project (BUMPS). This is a consortium sponsored by ten hydrocarbon companies (both primary and service) based in the Geophysics group in Bristol. We will present this work (focussing on the technical and methodological aspects). The second avenue to develop such partnerships is the Marie Sklodowska-Curie Innovative Training Network CREEP (www.creep-itn.eu). This project has several industrial partners including representatives from the hydrocarbon, glass, salt and geothermal industries. There are 6-monthly meetings of the CREEP project, and we will present this work at one of these.

Knowledge exchange opportunities.
While the deep mantle is not directly relevant in the daily life of the public, we find that Deep Earth studies are often appealing to a general audience (similarly to astronomy, it speaks to a human need to understand the broadest extent of the world around them). Furthermore, the project we propose will enable new images of the dynamic Earth which quite naturally lend themselves to communication. A project on the scale of this one is best served by providing materials and information to on-going, proven, larger scale efforts rather than developing lower impact individual activities. Thus, we will leverage current successful NERC, University and School schemes. In this project we will focus on public engagement with Schools.
Secondary Schools: We will engage with the University of Bristol's Centre for Public Engagement, which is working in partnership with local schools to inspire the next generation of researchers and enhance and enrich the school curriculum. The School-University Partnerships Initiative (SUPI), which is funded by Research Councils UK, supports direct engagement between researchers and secondary school students and enables teachers to work alongside our researchers to bring contemporary and inspirational research contexts into formal and informal learning. For example, the School of Earth Sciences offers a one-week work experience programme for year ten and eleven students from local schools. We will provide content based on our study of deep Earth dynamics for one of these days, alongside other related projects which are on-going in the School.
Primary Schools: The NERC-funded programme grant on the Volatiles, Geodynamics & Solid Earth Controls on the Habitable Planet, which is managed by University College London, runs a GeoBus that is a free educational outreach project specifically targeting primary schools (see http://www.deepvolatiles.org/impact.html). It launched in September 2016, and is visiting primary schools in southern England. We will provide materials for a Deep Earth exhibit based on, for example, the knowledge this project will generate.

Publications

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