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

10 25 50
publication icon
Liu H (2020) From Molten Calcium Aluminates through Phase Transitions to Cement Phases. in Advanced science (Weinheim, Baden-Wurttemberg, Germany)

publication icon
Drewitt J (2019) The fate of carbonate in oceanic crust subducted into earth's lower mantle in Earth and Planetary Science Letters

 
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 now confident that they will be buoyant and will rapidly move up toward the surface due to their very low viscosity. We are in the process of parameterising this as a function of P, T and composition. At the same time we have developed 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 rapidly maturing. We have performed successful samples on solid and liquid samples up to 50 GPa and 3000 K, far beyond the condiitons originally envisaged. 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. The remaining steps are to fully encapsulate the sample, and improve th XRD measurements so we can reliable record the diffuse scattering from the liquid.
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, is revolutionary, because it allows 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. We are writing a technical paper; we expect other groups to copy the technology.
Sectors Other

 
Description The non academic impacts of this award are really spin offs from the technology used to develop internal resistive heating. This has required us to learn how to mill, using lasers, very small, very precise parts from metals and ceramics. This has led to others in the School of Earth Sciences using our milling technology and facilities to aid in studies of a wide range of materials, maby of which have potential economic or industrial importance. These include: 1. Novel high temperature superconductors (with the School of Physics) 2. Olivine grains from meteorites (with the Geochemistry Group in the School of Earth Sciences) 3. CVD grown nanocrystalline diamonds (with the School of Physics) 4. Carbon fibre materials using in aviation (with the Faculty of Engineering)
First Year Of Impact 2019
Sector Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics,Energy
Impact Types Economic

 
Title Internal Resistive Heating 
Description As a part of this award, we have developed internal resistive heating in 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. Since the first successful prototype in 2018, we have now dramatically improved the technique. In a recent experiment, we performed in situ X-ray diffraction of a sample inside a 10 micron diameter hole, itself inside the filament, at 50 GPa and 3000 K. In addition, we have demonstrated that the technique is stable and remarkably forgiving; we can heat samples for long periods (hours) with minimal drift in temperature. We have not yet found the upper P-T limit, and I am conifdent that with further development, the technique could access the whole of the mantle geotherm. 
Type Of Material Improvements to research infrastructure 
Year Produced 2020 
Provided To Others? Yes  
Impact This will be game changing and is enabling experiments that were impossible with standard laser heating. 
 
Title P-T dependence of the viscosity and density of hydrous melts 
Description Based on our computational work on hydrous melts at mantle conditions, we are developing a simple parameterisation for viscosity and density as a function of P, T and composition. This is being written in Python, and a paper is being prepared. A Jupyter notebook containing the parameterisation will be published to the web, allowing other scientists to enter P, T and X and extract density and viscosty. 
Type Of Material Computer model/algorithm 
Year Produced 2020 
Provided To Others? No  
Impact None yet. 
 
Description Laser heating at I15 
Organisation Diamond Light Source
Country United Kingdom 
Sector Private 
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