Deep Water: Hydrous Silicate Melts and the Transition Zone Water Filter

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

Abstract

Earth is a wet planet, and its habitability is intrinsically tied to water at the surface. Water also plays a key role inside the Earth because it has the affect of drastically lowering the melting point of mantle rocks, and indeed, the water at the surface ultimately comes from the interior through magmatism. Water is returned to the interior at subduction zones, and over geological time the surface and mantle water inventories are regulated by a deep water cycle. Nearly all of the volcanism we see at the Earth's surface is caused by melting in the shallow mantle, especially above subduction zones, and this constitutes an important part of the deep water cycle. However, water is also transported deeper down into the mantle, and what happens to it, and the controls it has on interior processes, is a mystery.

Deep mantle rocks are generally too cool to melt, but there are regions in the mantle, most notably at about 400 and 700 km, where seismic signals are interpreted to represent partial melting. Water reduces the solidus of the mantle by hundreds of degrees in the upper and lower mantle, but much less so in the mantle transition zone (410-660 km), and this is because water is soluble in the main minerals that make up this region, wadsleyite and ringwoodite, but it is not soluble in the minerals that constitute the regions above and below the transition zone. This means that if there is water in the transition zone, and it potentially can store a couple of oceans worth, when mantle is transported from the transition zone into either the upper or lower mantle it is expected to melt when water is released. This concept was originally applied to the region above the transition zone by Bercovici and Karato in a landmark paper in 2003, and was called the 'transition zone water filter' because of the important predicted affects on mantle geochemistry.

Interestingly, since then seismic evidence has been mounting indicating melted regions above and below the transition zone. If these signals do indeed represent the presence of hydrous melt in these regions, then the transition zone may act as a double-sided mid-mantle water filter, and the melting that occurs at its boundaries could have modulated the chemistry and geodynamics of the mantle throughout its history. Currently we cannot adequately test this model or understand its implications because we do not know accurately the composition of hydrous silicate melts of the mantle at these depths, nor do we know their physical properties, such as the density and viscosity. Because of this, we are currently unable to model accurately the seismic response expected for hydrous silicate melting at these depths, and we cannot model the dynamic behavior of such melts should they exist in these locations. Here we propose to collect this data.

We propose to make high P-T experimental measurements to determine the compositions of hydrous silicate melts in the mantle at depths corresponding to the deep upper mantle, transition zone and upper part of the lower mantle. We will also use novel experiments where we combine diamond anvil cell techniques with synchrotron X-ray scattering methods to determine melt densities. Simultaneously we will use first principles molecular dynamics methods to calculate the physical and seismic properties of these melts, supplemented with experiments to measure their wetting properties. With these data, we will be able for the first time to develop dynamic and seismic models to explicitly test the transition zone water filter model, and make predictions about its chemical and dynamical affects on the mantle, and on the deep water cycle, throughout geological time.

Planned Impact

Outputs from this research will be disseminated to academic beneficiaries in the normal way, through publication of results in high-calibre international journals. We will disseminate results to the public via current successful University and Departmental schemes, existing contacts with the local and national media and our web presence. As part of this proposal we will target public outreach to School-aged children by linking in with the Geobus that is part of the Deep Volatile consortium being run by Bristol, UCL and Oxford. We will provide materials for the Geobus in the form models for the Earth's interior that are accessible to School aged children, emphasizing the links between the deep water cycle and the long-term habitability of our planet.

Publications

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Description We have made significant progress on this grant. We have performed computer simulations on liquids consisting of mixtures of MgO, SiO2 and water, designed to simulate the compositions that you would expect to form in the Earth's mid and lower mantle, regions where seismologists see evidence for the existence of molten rock. These simulations have allowed us to determine the viscosity and density of these liquids and thus estimate what will happen to them. We are reasonably confident now that they will be buoyant and will rapidly move up toward the surface. However, we need to answer some additional questions using experiments before we can be certain, and this is what we are working on now. At the same time we are developing an internal resistive heating method so that we can trap these melts inside a very small electrically heated filament inside a diamond anvil cell pressure generating device so that we can directly measure the liquid density using X-ray diffraction at the UK synchrotron, Diamond. This method is still at the prototype stage. However, to test the diffraction part of the strategy, we have performed experiments on liquid Ga using older external resistive heating technology, that is not capable of reaching the necessary temperatures. Gallium is convenient because it has a very low melting point - almost room temperature at ambient pressure. This allowed us to make density measurements of the liquid at high pressure, proving that the method works at the UK synchrotron, and showing us several technical problems that we will need to overcome to be able to do these experiments on hydrous silicate melts.
Exploitation Route The finding that hydrous melts might not be stable in the Earth' mantle due to their low viscosity and density is an important discovery that will lead to new models of water transport in the deep Earth, as well as invalidating some existing models. This will also help the seismologists to interpret their measurement. Our development of internal resistive heating, though risky, would be revolutionary is successful, because it would allow us to access a range of P-T conditions currently only accessible by laser heating, which suffers from several technical issues, including thermal gradients that make results hard to interpret.
Sectors Environment

 
Title Internal Resistive Heating 
Description As a part of this award, we are developing internal resistive heating the the DAC, using a combination of laser milling and sputter coating to produce heating filaments small enough to fit into the sample chambers of diamond anvil cell experiments. The first prototypes were successful, and we are refining our methodologies. 
Type Of Material Improvements to research infrastructure 
Year Produced 2018 
Provided To Others? No  
Impact None yet. 
 
Description Laser heating at I15 
Organisation Diamond Light Source
Country United Kingdom 
Sector Academic/University 
PI Contribution I have been providing support and advice to the staff at beamline I15 (extreme conditions) as they design and install an in situ laser heating system which will be of great utility to a broad range of geoscientists and materials scientists in the UK, and will make Diamond competitive with other synchrotrons which already have this capability. This system is now operational; I was also part of the team which performed the first laser heating experiment with in situ X-ray diffraction at I15 in December 2015.
Collaborator Contribution The staff ay I15 have designed and built the system; we expect to use it regularly from now onwards through the usual beam-time application process.
Impact The laser heating system at I15 is now operational and available to all to use.
Start Year 2015