Water in the Deep Earth

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


Everyone is familiar with the importance of water in Earth's hydrosphere. The hydrosphere likely owes its existence to water in rocks, because water is delivered to the Earth's surface by degassing of magmas derived from Earth's interior. We don't know how much water the Earth currently contains, or contained in the past. Based on our present knowledge of the maximum capacity of water in the minerals that make up Earth's mantle, there could be several oceans of water in the mantle, and possibly enough hydrogen dissolved in the core for several more. There may have been much more water during the earliest history of the Earth when deep silicate magma oceans first crystallized. We currently do not know if one or more early, possibly water-rich, atmospheres were lost due to impact erosion during accretion. One or many oceans of water may have been involved in the internal differentiation of Earth by transfer of dissolved materials in hydrous fluid. We all know of the amazing properties of water at the surface. It turns out to have different but equally amazing properties when dissolved in minerals, melts and fluids in the mantle. Even a little bit can change dramatically the strength of a mineral or the melting point of a rock. At high temperatures water as a fluid in the mantle acts like a corrosive solvent and can dissolve rock. Water as a fluid also moves around easily in the rocks of the mantle because of its low density and viscosity, so that it can transport the materials that it dissolves. Water is known to be an important agent of mass transfer in the upper mantle, changing the chemistry and physical properties of the rocks it moves through. Experimentalists have been able to determine much about how water behaves in the rocks and mineral that comprise the Earth's upper mantle. For example, at high temperatures and pressures water and melts are no longer distinct, but instead hot water in the mantle dissolves so much silicate that it much like a melt. This 'supercritical' hydrous fluid may move upward in the mantle and change the chemistry of the rocks above. The base of the silicate mantle is at ~ 2900 km, and currently we know very little about the behaviour of hydrous fluids in the depth range of the lower mantle (~ 660 - 2900 km), which constitutes the largest silicate reservoir in Earth. What we want to do in the research proposed here is to work out how hydrous fluids affect the stability of the lower mantle minerals magnesium and calcium perovskites. We also want to know the chemistry of hydrous fluids that dissolve lower mantle minerals at high P and T. We will make high P-T experiments using a multi-anvil pressure apparatus (~ 24 GPa), as well as a laser-heated diamond anvil cell (24-100 GPa). We will heat water-bearing compositions in systems containing MgO, CaO, SiO2 and H2O. In multi-anvil experiments we can measure the major and trace element composition of the hydrous fluid directly using modern micro-probe analytical techniques. In the diamond anvil cell experiments we will use in situ synchrotron X-ray diffraction techniques to carefully track phase relationships to isolate the major element composition of the fluids. With the data we collect we hope to answer important questions like: Can the movement of hydrous fluid from the lower to upper mantle significantly alter its composition? How do trace elements partition between perovskite phases and the hydrous fluid, and can they be used as tracers of fluid transfer in the mantle? Could the lower mantle have been depleted in MgO (and perhaps CaO) to the extent that it is now nearly 100% perovskite? Could subducted slabs that penetrate into the lower mantle provide enough fluid to have altered mantle chemistry through Earth history? The research we propose will provide the first systematic experimental data set that will allow us to address these important questions about the role of water in the evolution and differentiation of our planet.


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Description The Earth's lower mantle represents the largest potential reservoir for water in the Earth, perhaps capable of storing several oceans worth of water. However, little is known about what phases water might reside at the extreme high pressures and temperatures of this region. We determined phase relations in the MgO-SiO2-H2O and MgO-Al2O3-SiO2-H2O systems throughout Earths lower mantle using the laser-heated diamond anvil cell. We discovered, along with another group, a new water-bearing phase, called phase H, that can potentially transport water to the base of the mantle in subducted lithologies. However, we found that at mantle temperatures, the primary water-bearing phase will be hydrous silicate melt, the fate of which in the mantle remains uncertain.
Exploitation Route Through utilization of published outputs.
Sectors Other