New laboratory constraints on the water content of the mantle

We have long known that the nominally anhydrous mantle minerals, such as can take substantial amounts of water into them (eg 3% for wadsleyite and ringwoodite). We also know that water has a drastic effect on the physical and chemical properties of these minerals, changing phase relations by hundreds of Kelvin and causing order-of-magnitude variations in viscosity, electrical conductivity and vacancy populations. This has created a whole genre of speculation about the water content in the mantle and its effects on mantle dynamics (eg; 'The mid-mantle water filter'; Bercovici and Karato 2003: 'Nine oceans of water in the transition zone'; Smyth et al 1994: 'No water in the transition zone'; Green at al 2010). We have, however, no firm understanding of the water content of the mantle deeper than about 100 km and these papers will remain speculative until we place firm constraints on the water content of the deep mantle. We can do this by combining geophysical inversions for electrical conductivity of the geomagnetic response function with laboratory measurements of the effect of water content on electrical conductivity. This is what the current project will do using cutting-edge designer-diamond anvil cell conductivity measurements with simultaneous infra-red measurements of water content. These new data will be used in Monte Carlo simulations of the geomagnetic response function which invert directly for water content.

1. Who will benefit from this research?
a/ The main beneficiaries form this reaserch are academics as outlined in the academic beneficiaries section. Beyond the immediate area of reserach interest of the proposal (Solid Earth Geophysics) the results will provide constraints on the present water content of the Earth's mantle. This is important for informing the discussion regarding the long-term hydrological cycle: Currently we do not even know if the mantle is loosing water to the surface or if it is extracting water from the surface.

b/ We have identified an area of potential interest for schools outreach in demonstrating high-pressure synthesis and polymorphism: 'Diamonds from Dirt'. This outreach project links in with existing outreach by the Pi and colleagues from UCL Chemistry (Prof. Andrea Sella; EPSRC outreach professor).

2. How will they benefit from the research?
a/ The new constraints on the water content of the mantle will provide boundary conditions for geodynamical and hydrological models. They will also help to constrain the chemistry of the mantle for interpretations of geophysical observables such as seismic wave speed and attenuation. The best way to disseminate the results will be via conference presentations and publications in high-impact journals. In addition, the UCL ES website highlights important results as they are published.

b/ The 'Diamonds from Dirt' program will involve 2 components: 1) school visits to explain the concepts of polymorphism, high-pressure and materials synthesis. 2) this will be supplemented with experiments in which diamond will be grown from 'dirt' of various sorts and a web cast of the experiments and recovered diamond will be developed for use in the school visits.