Deep water cycle

Though the water cycle is commonly defined as a process that occurs above and on the surface of Earth, there is also a deep water cycle (or geologic water cycle) that involves exchange of water with the mantle, with water carried down by subducting oceanic plates and returning through volcanic activity. Some of the water makes it all the way to the lower mantle and may even reach the outer core. Mineral physics experiments show that hydrous minerals can carry water deep into the mantle in colder slabs and even "nominally anhydrous minerals" can store several oceans' worth of water.

Introduction

Main article: Water cycle

In the conventional view of the water cycle (also known as the hydrologic cycle), water moves between reservoirs in the atmosphere and Earth's surface or near-surface (including the ocean, rivers and lakes, glaciers and polar ice caps, the biosphere and groundwater). However, water plays an important role in geological processes in the crust and mantle. Water content in magma determines how explosive a volcanic eruption is; hot water is the main conduit for economically important minerals to concentrate in hydrothermal mineral deposits; and water plays an important role in the formation and migration of petroleum.

Schematic of tectonic plate boundaries. Discussed in the text are a subducting plate (5); an island arc (15) overlying a mantle wedge; a mid-ocean ridge (12); and a hotspot (3).

Water is not just present as a separate phase in the ground. Seawater percolates into oceanic crust and hydrates igneous rocks such as olivine and pyroxene, transforming them into hydrous minerals such as serpentines, talc and brucite. In this form, water is carried down into the mantle. In the upper mantle, heat and pressure dehydrates these minerals, releasing much of it to the overlying mantle wedge, triggering the melting of rock that rises to form volcanic arcs. However, some of the "nominally anhydrous minerals" that are stable deeper in the mantle can store small concentrations of water in the form of hydroxyl (OH^-), and because they occupy large volumes of the Earth, they are capable of storing at least as much as the world's oceans.

The conventional view of the ocean's origin is that it was filled by outgassing from the mantle in the early Archean and the mantle has remained dehydrated ever since. However, subduction carries water down at a rate that would empty the ocean in 1–2 billion years. Despite this, changes in the global sea level over the past 3–4 billion years have only been a few hundred metres, much smaller than the average ocean depth of 4 kilometres. Thus, the fluxes of water into and out of the mantle are expected to be roughly balanced, and the water content of the mantle steady. Water carried into the mantle eventually returns to the surface in eruptions at mid-ocean ridges and hotspots. This circulation of water into the mantle and back is known as the deep water cycle or the geologic water cycle.

Estimates of the amount of water in the mantle range from 1/4 to 4 times the water in the ocean. Constraints on water in the mantle come from mantle mineralogy, samples of rock from the mantle, and geophysical probes.

Storage capacity

See also: Mineral physics

Dependence of temperature on depth in Earth's upper 500 kilometers (black curve).

An upper bound on the amount of water in the mantle can be obtained by considering the amount of water that can be carried by its minerals (their storage capacity). This depends on temperature and pressure. There is a steep temperature gradient in the lithosphere where heat travels by conduction, but in the mantle the rock is stirred by convection and the temperature increases more slowly (see figure). Descending slabs have colder than average temperatures.

Phase transformations of olivine moving through the upper mantle, transition zone, and lower mantle. In the core, water might be stored as hydrogen bound to iron.

The mantle can be divided into the upper mantle (above 410 km depth), transition zone (between 410 km and 660 km), and the lower mantle (below 660 km). Much of the mantle consists of olivine and its high-pressure polymorphs. At the top of the transition zone, it undergoes a phase transition to wadsleyite, and at about 520 km depth, wadsleyite transforms into ringwoodite, which has the spinel structure. At the top of the lower mantle, ringwoodite decomposes into bridgmanite and ferropericlase.

The most common mineral in the upper mantle is olivine. For a depth of 410 km, an early estimate of 0.13 wt.% was revised upwards to 0.4 wt.% and then to 1 wt.%. However, the carrying capacity decreases dramatically towards the top of the mantle. Another common mineral, pyroxene, also has an estimated capacity of 1 wt.% near 410 km.

In the transition zone, water is carried by wadsleyite and ringwoodite; in the relatively cold conditions of a descending slab, they can carry up to 3 wt.%, while in the warmer temperatures of the surrounding mantle their storage capacity is about 0.5 wt.%. The transition zone is also composed of at least 40% majorite, a high pressure phase of garnet; this only has capacity of 0.1 wt.% or less.

The storage capacity of the lower mantle is a subject of controversy, with estimates ranging from the equivalent of 3 times to less than 3% of the ocean. Experiments have been limited to pressures found in the top 100 km of the mantle and are challenging to perform. Results may be biased upwards by hydrous mineral inclusions and downwards by a failure to maintain fluid saturation.

At high pressures, water can interact with pure iron to get FeH and FeO. Models of the outer core predict that it could hold as much as 100 oceans of water in this form, and this reaction may have dried out the lower mantle in the early history of Earth.

Water from the mantle

The carrying capacity of the mantle is only an upper bound, and there is no compelling reason to suppose that the mantle is saturated. Further constraints on the quantity and distribution of water in the mantle comes from a geochemical analysis of erupted basalts and xenoliths from the mantle.

Diamond inclusions

Diamond from Juína, Brazil with ringwoodite inclusions suggests presence of water in the transition zone.

Mineral samples from the transition zone and lower mantle come from inclusions found in diamonds. Researchers have recently discovered diamond inclusions of ice-VII in the transition zone. Ice-VII is water in a high pressure state. The presence of diamonds that formed in the transition zone and contain ice-VII inclusions suggests that water is present in the transition zone and at the top of the lower mantle. Of the thirteen ice-VII instances found, eight have pressures around 8-12 GPa, tracing the formation of inclusions to 400–550 km. Two inclusions have pressures between 24-25 GPa, indicating the formation of inclusions at 610–800 km. The pressures of the ice-VII inclusions provide evidence that water must have been present at the time the diamonds formed in the transition zone in order to have become trapped as inclusions. Researchers also suggest that the range of pressures at which inclusions formed implies inclusions existed as fluids rather than solids.

Another diamond was found with ringwoodite inclusions. Using techniques including infrared spectroscopy, Raman spectroscopy, and x-ray diffraction, scientists found that the water content of the ringwoodite was 1.4 wt.% and inferred that the bulk water content of the mantle is about 1 wt.%.

Geophysical evidence

Seismic

Both sudden decreases in seismic activity and electricity conduction indicate that the transition zone is able to produce hydrated ringwoodite. The USArray seismic experiment is a long-term project using seismometers to chart the mantle underlying the United States. Using data from this project, seismometer measurements show corresponding evidence of melt at the bottom of the transition zone. Melt in the transition zone can be visualized through seismic velocity measurements as sharp velocity decreases at the lower mantle caused by the subduction of slabs through the transition zone. The measured decrease in seismic velocities correlates accurately with the predicted presence of 1 weight % melt of H₂O.

Ultra low velocity zones (ULVZs) have been discovered right above the core-mantle boundary (CMB). Experiments highlighting the presence of iron peroxide containing hydrogen (FeO₂H_x) aligns with expectations of the ULVZs. Researchers believe that iron and water could react to form FeO₂H_x in these ULVZs at the CMB. This reaction would be possible with the interaction of the subduction of minerals containing water and the extensive supply of iron in the Earth’s outer core. Past research has suggested the presence of partial melting in ULVZs, but the formation of melt in the area surrounding the CMB remains contested.

Subduction

As an oceanic plate descends into the upper mantle, its minerals tend to lose water. How much water is lost and when depends on the pressure, temperature and mineralogy. Water is carried by a variety of minerals that combine various proportions of magnesium oxide (MgO), silicon dioxide )(SiO₂), and water. At low pressures (below 5 GPa), these include antigorite, a form of serpentine, and clinochlore (both carrying 13 wt% water); talc (4.8 wt%) and some other minerals with a lower capacity. At moderate pressure (5–7 GPa) the minerals include phlogopite (4.8 wt%), the 10Å phase (a high pressure product of talc and water, 10–13 wt%) and lawsonite (11.5 wt%). At pressures above 7 GPa, there is topaz-OH (Al₂SiO₄(OH)₂, 10 wt%), phase Egg (AlSiO₃(OH), 11–18 wt%) and a collection of dense hydrous magnesium silicate (DHMS) or "alphabet" phases such as phase A (12 wt%), D (10 wt%) and E (11 wt%).

The fate of the water depends on whether these phases can maintain an unbroken series as the slab descends. At a depth of about 180 km, where the pressure is about 6 gigapascals (GPa) and the temperature around 600 °C, there is a possible "choke point" where the stability regions just meet. Hotter slabs will lose all their water while cooler slabs pass the water on to the DHMS phases. Some of the released water may also be stable as Ice VII.

See also

• Hydrous components in nominally anhydrous minerals
• Hydrous minerals of a subducting slab

Further reading

• Cai, Chen; Wiens, Douglas A.; Shen, Weisen; Eimer, Melody (2018). "Water input into the Mariana subduction zone estimated from ocean-bottom seismic data". Nature. 563 (7731): 389–392. Bibcode:2018Natur.563..389C. doi:10.1038/s41586-018-0655-4. PMID 30429549. {{cite journal}}: Unknown parameter |laydate= ignored (help); Unknown parameter |laysource= ignored (help); Unknown parameter |laysummary= ignored (help)
• Condie, Kent C. (2015). Earth as an evolving planetary system (2nd ed.). Elsevier/Academic Press. pp. 114–115. ISBN 978-0-12-803709-6.
• Conrad, C. P. (28 June 2013). "The solid Earth's influence on sea level" (PDF). Geological Society of America Bulletin. 125 (7–8): 1027–1052. doi:10.1130/B30764.1. Retrieved 24 April 2019.
• Faccenda, Manuele (February 2014). "Water in the slab: A trilogy". Tectonophysics. 614: 1–30. doi:10.1016/j.tecto.2013.12.020.
• Harte, B. (5 July 2018). "Diamond formation in the deep mantle: the record of mineral inclusions and their distribution in relation to mantle dehydration zones". Mineralogical Magazine. 74 (02): 189–215. doi:10.1180/minmag.2010.074.2.189. Retrieved 24 April 2019.
• Hirschmann, Marc M. (2006). "Water, melting, and the deep Earth H₂O cycle". Annual Review of Earth and Planetary Science. 34: 629–653. doi:10.1146/annurev.earth.34.031405.125211. Retrieved 17 April 2019. {{cite journal}}: Invalid |ref=harv (help)
• Houser, C. (August 2016). "Global seismic data reveal little water in the mantle transition zone". Earth and Planetary Science Letters. 448: 94–101. doi:10.1016/j.epsl.2016.04.018.
• Jacobsen, Steven D.; Van Der Lee, Suzan, eds. (2006). Earth's deep water cycle. American Geophysical Union. ISBN 9781118666487. {{cite book}}: Invalid |ref=harv (help)
• Keppler, Hans; Smyth, Joseph R. (2006). Water in nominally anhydrous minerals. Mineralogical Society of America. ISBN 978-0-939950-74-4.
• Khan, A.; Shankland, T.J. (February 2012). "A geophysical perspective on mantle water content and melting: Inverting electromagnetic sounding data using laboratory-based electrical conductivity profiles". Earth and Planetary Science Letters. 317–318: 27–43. doi:10.1016/j.epsl.2011.11.031.
• Korenaga, J. (10 December 2011). "Thermal evolution with a hydrating mantle and the initiation of plate tectonics in the early Earth" (PDF). Journal of Geophysical Research. 116 (B12). doi:10.1029/2011JB008410. Retrieved 24 April 2019.
• Nomura, R; Hirose, K; Uesugi, K; Ohishi, Y; Tsuchiyama, A; Miyake, A; Ueno, Y (31 January 2014). "Low core-mantle boundary temperature inferred from the solidus of pyrolite". Science. 343 (6170): 522–5. Bibcode:2014Sci...343..522N. doi:10.1126/science.1248186. PMID 24436185.
• Ohtani, Eiji; Amaike, Yohei; Kamada, Seiji; Ohira, Itaru; Mashino, Izumi (2016). "21. Stability of Hydrous Minerals and Water Reservoirs in the Deep Earth Interior". In Terasaki, Hidenori; Fischer, Rebecca A. (eds.). Deep earth : physics and chemistry of the lower mantel and core. John Wiley & Sons. doi:10.1002/9781118992487.ch21. ISBN 9781118992500.
• Roberts Jr., Glenn (15 March 2018). "Diamonds From the Deep: Study Suggests Water May Exist in Earth's Lower Mantle". News Center (Press release). Berkeley Lab. Retrieved 27 March 2019.
• Rollinson, Hugh R. (2009). Early Earth Systems : a Geochemical Approach. John Wiley & Sons. pp. 177–180. ISBN 9781444308945.
• Smith, Evan M.; Shirey, Steven B.; Richardson, Stephen H.; Nestola, Fabrizio; Bullock, Emma S.; Wang, Jianhua; Wang, Wuyi (1 August 2018). "Blue boron-bearing diamonds from Earth's lower mantle". Nature. 560 (7716): 84–87. doi:10.1038/s41586-018-0334-5. Retrieved 24 April 2019.
• Suetsugu, D.; Steinberger, B.; Kogiso, T. (2013). [www.earthdynamics.org/steinberger/papers/encyclopedia-2005.pdf "Mantle Plumes and Hotspots"] (PDF). Reference Module in Earth Systems and Environmental Sciences. Elsevier. doi:10.1016/b978-0-12-409548-9.02868-2. ISBN 978-0-12-409548-9. Retrieved 24 April 2019. {{cite book}}: Check |chapter-url= value (help)
• University of California – Riverside (19 October 2010). "Earth's deep water cycle needs revision, geophysicists claim". ScienceDaily (Press release). Retrieved 27 March 2019.
• Yoshino, Takashi; Katsura, Tomoo (30 May 2013). "Electrical Conductivity of Mantle Minerals: Role of Water in Conductivity Anomalies". Annual Review of Earth and Planetary Sciences. 41 (1): 605–628. doi:10.1146/annurev-earth-050212-124022.
• Woo, Marcus (11 July 2018). "The Hunt for Earth's Deep Hidden Oceans". Quanta Magazine. Retrieved 27 March 2019.