Deep water cycle
The deep water cycle, or geologic water cycle, involves exchange of water with the mantle, with water carried down by subducting oceanic plates and returning through volcanic activity, distinct from the water cycle process that occurs above and on the surface of Earth. 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.
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, in addition to the surface cycle, water also plays an important role in geological processes reaching down into 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.
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. There are 1.37×1018 m3 of water in the seas, therefore, this would suggest that there is between 3.4×1017 and 5.5×1018 m3 of water in the mantle. Constraints on water in the mantle come from mantle mineralogy, samples of rock from the mantle, and geophysical probes.
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.
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 percentage of water by weight (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.
Basalts formed at mid-ocean ridges and hotspots originate in the mantle and are used to provide information on the composition of the mantle. Magma rising to the surface may undergo fractional crystallization in which components with higher melting points settle out first, and the resulting melts can have widely varying water contents; but when little separation has occurred, the water content is between about 0.07–0.6 wt%. (By comparison, basalts in back-arc basins around volcanic arcs have between 1 wt% and 2.9 wt% because of the water coming off the subducting plate.)
Mid-ocean ridge basalts (MORBs) are commonly classified by the abundance of trace elements that are incompatible with the minerals they inhabit. They are divided into "normal" MORB or N-MORB, with relatively low abundances of these elements, and enriched E-MORB. The enrichment of water correlates well with that of these elements. In N-MORB, the water content of the source mantle is inferred to be 0.08–0.18 wt%, while in E-MORB it is 0.2–0.95 wt%.
Another common classification, based on analyses of MORBs and ocean island basalts (OIBs) from hotspots, identifies five components. Focal zone (FOZO) basalt is considered to be closest to the original composition of the mantle. Two enriched end-members (EM-1 and EM-2) are thought to arise from recycling of ocean sediments and OIBs. H1MU stands for "high-μ", where μ is a ratio of uranium and lead isotopes (μ = 238U/234Pb). The fifth component is depleted MORB (DMM). Because the behavior of water is very similar to that of the element cesium, ratios of water to cesium are often used to estimate the concentration of water in regions that are sources for the components. Multiple studies put the water content of FOZO at around 0.075 wt%, and much of this water is likely "juvenile" water acquired during the accretion of Earth. DMM has only 60 ppm water. If these sources sample all the regions of the mantle, the total water depends on their proportion; including uncertainties, estimates range from 0.2 to 2.3 oceans.
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%.
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 (FeO2Hx) aligns with expectations of the ULVZs. Researchers believe that iron and water could react to form FeO2Hx 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.
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. In cooler slabs, some of the released water may also be stable as Ice VII.
- Bodnar, R.J.; Azbej, T.; Becker, S.P.; Cannatelli, C.; Fall, A.; Severs, M.J. (2013). "Whole Earth geohydrologic cycle, from the clouds to the core: The distribution of water in the dynamic Earth system" (PDF). In M.E., Bickford (ed.). The Web of Geological Sciences: Advances, Impacts, and Interactions: Geological Society of America Special Paper 500. The Geological Society of America. pp. 431–461. doi:10.1130/2013.2500(13). ISBN 9780813725000. Retrieved 19 April 2019.
- Peacock, Simon M.; Hyndman, Roy D. (15 August 1999). "Hydrous minerals in the mantle wedge and the maximum depth of subduction thrust earthquakes". Geophysical Research Letters. 26 (16): 2517–2520. doi:10.1029/1999GL900558.
- Rüpke, L; Morgan, Jason Phipps; Hort, Matthias; Connolly, James A. D. (June 2004). "Serpentine and the subduction zone water cycle". Earth and Planetary Science Letters. 223 (1–2): 17–34. doi:10.1016/j.epsl.2004.04.018.
- Bell, D. R.; Rossman, G. R. (13 March 1992). "Water in Earth's Mantle: The Role of Nominally Anhydrous Minerals". Science. 255 (5050): 1391–1397. doi:10.1126/science.255.5050.1391. Retrieved 23 April 2019.
- Keppler, Hans (2013). "Volatiles under high pressure". In Karato, Shun-ichiro; Karato, Shun'ichirō (eds.). Physics and chemistry of the deep Earth. John Wiley & Sons. pp. 22–23. doi:10.1002/9781118529492.ch1. ISBN 9780470659144.
- Hirschmann 2006, p. 646
- Rüpke, Lars; Morgan, Jason Phipps; Dixon, Jacqueline Eaby. "Implications of Subduction Rehydration for Earth's Deep Water Cycle" (PDF). doi:10.1029/168GM20. Retrieved 20 April 2019. In Jacobsen & Van Der Lee 2006, pp. 263–276.
- Magni, Valentina; Bouilhol, Pierre; van Hunen, Jeroen (November 2014). "Deep water recycling through time". Geochemistry, Geophysics, Geosystems. 15 (11): 4203–4216. doi:10.1002/2014GC005525.
- 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.
- Hirschmann 2006, pp. 630–634
- Turcotte, Donald L.; Schubert, Gerald (2002). "4-28 Mantle geotherms and adiabats". Geodynamics (2nd ed.). Cambridge University Press. pp. 185–188. ISBN 978-0-521-66624-4.
- Christensen, U.R. (1995). "Effects of phase transitions on mantle convection". Annu. Rev. Earth Planet. Sci. 23: 65–87. Bibcode:1995AREPS..23...65C. doi:10.1146/annurev.ea.23.050195.000433.
- Smyth, Joseph R.; Jacobsen, Steven D. "Nominally anhydrous minerals and Earth's deep water cycle" (PDF). doi:10.1029/168GM02. Retrieved 23 April 2019. In Jacobsen & Van Der Lee 2006, pp. 1–12.
- Ohtani, Eiji; Litasov, Konstantin; Hosoya, Tomofumi; Kubo, Tomoaki; Kondo, Tadashi (June 2004). "Water transport into the deep mantle and formation of a hydrous transition zone". Physics of the Earth and Planetary Interiors. 143–144: 255–269. doi:10.1016/j.pepi.2003.09.015.
- Thomas, Sylvia-Monique; Wilson, Kathryn; Koch-Müller, Monika; Hauri, Erik H.; McCammon, Catherine; Jacobsen, Steven D.; Lazarz, John; Rhede, Dieter; Ren, Minghua; Blair, Neal; Lenz, Stephan (12 May 2015). "Quantification of water in majoritic garnet". American Mineralogist. 100 (5–6): 1084–1092. doi:10.2138/am-2015-5136.
- Bolfan‐Casanova, Nathalie; Mccammon, Catherine A.; Mackwell, Stephen J. "Water in Transition Zone and Lower Mantle Minerals". doi:10.1029/168GM06.
- Hirschmann 2006, p. 644
- Williams, Quentin; Hemley, Russell J. (May 2001). "Hydrogen in the Deep Earth". Annual Review of Earth and Planetary Sciences. 29 (1): 365–418. doi:10.1146/annurev.earth.29.1.365. Retrieved 23 April 2019.
- Karato, Shun-ichiro (January 2011). "Water distribution across the mantle transition zone and its implications for global material circulation". Earth and Planetary Science Letters. 301 (3–4): 413–423. doi:10.1016/j.epsl.2010.11.038. Retrieved 24 April 2019.
- Ulrich, Marc; Hémond, Christophe; Nonnotte, Philippe; Jochum, Klaus Peter (June 2012). "OIB/seamount recycling as a possible process for E-MORB genesis". Geochemistry, Geophysics, Geosystems. 13 (6): Q0AC19. doi:10.1029/2012GC004078.
- Stracke, Andreas; Hofmann, Albrecht W.; Hart, Stan R. (May 2005). "FOZO, HIMU, and the rest of the mantle zoo". Geochemistry, Geophysics, Geosystems. 6 (5): n/a–n/a. doi:10.1029/2004GC000824.
- Pearson, D. G.; Brenker, F. E.; Nestola, F.; McNeill, J.; Nasdala, L.; Hutchison, M. T.; Matveev, S.; Mather, K.; Silversmit, G.; Schmitz, S.; Vekemans, B.; Vincze, L. (2014). "Hydrous mantle transition zone indicated by ringwoodite included within diamond". Nature. 507 (7491): 221–4. Bibcode:2014Natur.507..221P. doi:10.1038/nature13080. PMID 24622201.
- Tschauner, O; Huang, S; Greenberg, E; Prakapenka, VB; Ma, C; Rossman, GR; Shen, AH; Zhang, D; Newville, M; Lanzirotti, A; Tait, K (9 March 2018). "Ice-VII inclusions in diamonds: Evidence for aqueous fluid in Earth's deep mantle". Science. 359 (6380): 1136–1139. Bibcode:2018Sci...359.1136T. doi:10.1126/science.aao3030. PMID 29590042.
- "Water in Earth's transition zone directly measured". Deep Carbon Observatory. 13 March 2014. Retrieved 24 April 2019.
- Alden, Andrew (12 June 2014). "New Evidence of Earth's Deep Water Cycle Reveals A Virtual Buried Ocean". KQED. Retrieved 24 April 2019.
- Schmandt, B.; Jacobsen, S. D.; Becker, T. W.; Liu, Z.; Dueker, K. G. (2014). "Dehydration melting at the top of the lower mantle". Science. 344 (6189): 1265–8. Bibcode:2014Sci...344.1265S. doi:10.1126/science.1253358. PMID 24926016.
- Liu, Jin; Hu, Qingyang; Young Kim, Duck; Wu, Zhongqing; Wang, Wenzhong; Xiao, Yuming; Chow, Paul; Meng, Yue; Prakapenka, Vitali B.; Mao, Ho-Kwang; Mao, Wendy L. (2017). "Hydrogen-bearing iron peroxide and the origin of ultralow-velocity zones". Nature. 551 (7681): 494–497. Bibcode:2017Natur.551..494L. doi:10.1038/nature24461. PMID 29168804.
- Kawamoto, T. (1 January 2006). "Hydrous Phases and Water Transport in the Subducting Slab". Reviews in Mineralogy and Geochemistry. 62 (1): 273–289. doi:10.2138/rmg.2006.62.12.
- Webb, Graham A. (2003). Annual reports on NMR spectroscopy. Volume 56. Elsevier Academic Press. p. 324. ISBN 9780124079052.
- Mainprice, David; Ildefonse, Benoit (2009). "Seismic Anisotropy of Subduction Zone Minerals–Contribution of Hydrous Phases". In Lallemand, Serge; Funiciello, Francesca (eds.). Subduction zone geodynamics. Springer Science & Business Media. pp. 65–67. doi:10.1007/978-3-540-87974-9_4. ISBN 9783540879749. Retrieved 24 April 2019.
- Bina, Craig R.; Navrotsky, Alexandra (December 2000). "Possible presence of high-pressure ice in cold subducting slabs". Nature. 408 (6814): 844–847. doi:10.1038/35048555.
- Ivanov, Alexei V.; Litasov, Konstantin D. (30 July 2013). "The deep water cycle and flood basalt volcanism". International Geology Review. 56 (1): 1–14. doi:10.1080/00206814.2013.817567.
- 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. Lay summary – ScienceDaily (November 14, 2018).
- 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 Sciences. 34: 629–653. doi:10.1146/annurev.earth.34.031405.125211. Retrieved 17 April 2019.
- 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.
- 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.
- 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). "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.
- 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.