Hydrous minerals and the storage of water in the deep mantle
Introduction
Water transportation from the Earth's surface to the deep interior and its circulation of water on a global scale are important for understanding the evolution and dynamics of the Earth. There are several opinions regarding the water content of the Earth's interior, such as those based on the estimation of the water flux from volcanism on the surface of the Earth, including on the ocean floor, and the return flux of water by slab subduction (e.g., Peacock, 1990, Wallace, 2005) from geophysical measurements of the sound velocity and electrical conductivity (e.g., Koyama et al., 2006), and evidence from natural samples, including glass from mid-ocean ridge basalts, the water content in mantle minerals, and mineral inclusions in diamond (e.g., Wallace, 2005).
Peacock (1990) estimated the mass of water outgassed from the mantle through magmatism along island arcs and mid-oceanic ridges to be 1.4 × 1011 and 0.6 × 1011 kg/year, respectively, giving a total mass of water outgassed from the mantle to the surface of 2 × 1011 kg/year. Peacock (1990) also estimated that the mass of water returned by subducting slabs is approximately 0.7 × 1011 kg/year for pelagic sediments and 8 × 1011 kg/year for the oceanic crust. Therefore, the total mass of water returned to the Earth's mantle is estimated to be 8.7 × 1011 kg/year. Thus, 6.7 × 1011 kg/year of water is transported to the deep mantle by subduction. This quantity is probably a lower limit, because the mass of water stored in the peridotite layer of the slabs is not included in the above estimate.
Geochemical evidence of water cycling in subduction indicates that less water may be transported to the deeper mantle below a depth of 240 km. Wallace (2005) estimated a global flux of 3 × 1011 kg/year of outgassing water associated with arc magmatism from the data on melt inclusions and volcanic gases. Wallace (2005) suggested that there might be an approximate balance between the input of water by slab subduction and the output of water outgassed to the surface through arc volcanism. An estimation of the global water cycle by van Keken et al. (2011) suggests that 7 − 10 × 1011 kg/year of water is recycled into the mantle through slab subduction, and two-thirds of this water stored in the slab is lost through dehydration of the slab, while one-third of the water bound in the slab (i.e., 3 × 1011 kg/year) penetrates to depths exceeding 240 km. Recently, Bodnar et al. (2013) estimated that the mass of water contained in the mantle is comparable to the mass of water held in the ocean (1.37 × 1021 kg). Although the amount of water penetrating to the deep mantle estimated by the above authors is not large and the deep mantle may actually contain more water than that transported by the slab. Some of the water in the deep mantle may originate from primitive mantle reservoirs in the deep mantle.
Despite these uncertainties, it is important to consider the water abundance and the potential water reservoirs in the deep mantle because it is generally believed that even a small amount of water strongly influences the physical properties of minerals and magmas in the upper mantle, although we need more studies on the effect of water on the physical properties of the mantle under extreme conditions.
Seismic tomography studies suggest that subducting slab penetrates into the deep lower mantle and possibly accumulates at the mantle–core boundary region (e.g., Fukao et al., 2001, Grand, 2002). These observations also suggest that water trapped in the slabs may be transported to the core–mantle boundary at depths of approximately 2900 km. However, the water transport capacity of different slabs can vary, depending on the thermal conditions of the slab. In general, hotter and younger slabs do not transport much water into the deep mantle, whereas colder and older slabs may transport a significant amount of water into the deep mantle, although their water transport capability is still a matter of debate (e.g., Poli and Schmidt, 2002, van Keken et al., 2011).
The electrical conductivity of mantle transition zone minerals has been studied as a function of temperature and water content (Dai and Karato, 2009, Manthilake et al., 2009) and has been used to evaluate the water content in the mantle transition zone (Yoshino, 2010, Karato, 2011). Geophysical observations indicate that there are significant heterogeneities in the water content of the mantle, which has been estimated from regional variations in the electrical conductivity and seismic tomography data (e.g., Koyama et al., 2006). According to Koyama et al., water is localized in the mantle transition zone beneath the subduction zones. Khan and Shankland (2012) estimated the water content in the upper mantle from geophysical observations of the electrical conductivity and from laboratory measurements (Dai and Karato, 2009, Manthilake et al., 2009). Dai and Karato and Manthilake et al. argued that the upper mantle is, in general, dry, with a water content of about 0.01 wt.% or less, but they concluded that the transition zone is highly heterogeneous and that the transition zone beneath the United States and northeast China is “wet,” whereas the transition zone beneath Europe is “dry,” supporting earlier conclusions drawn by Koyama et al. (2006) and Utada et al. (2009).
Analysis of the attenuation of seismic shear waves revealed a large low-Qμ region in the shallow lower mantle beneath eastern Asia, suggesting the possible existence of a wide region of hydrogen enrichment beneath eastern Asia (Lawrence and Wysession, 2006). Additional strong evidence for the existence of water in the mantle transition zone and the lower mantle comes from the studies of inclusions in ultra deep diamond from the deep mantle. Wirth et al. (2007) discovered hydrous phase Egg (AlSiO3OH) as inclusions in ultra deep diamond from Juina, Brazil. More recently, Pearson et al. (2014) discovered hydrous ringwoodite as an inclusion in diamonds from the same place, which is the first discovery of terrestrial ringwoodite with a high water (most likely OH and not H2O) content around 1.0 wt.%. These observations of deep-seated diamonds imply that the transition zone, where the diamond crystallized and captured the high-pressure mineral as inclusions, contains a high water content, indicating the existence of a local wet transition zone.
Water (or hydrogen) plays an important role in the dynamics of the Earth's interior, because it influences the melting temperature, the transport properties of mantle rocks such as the viscosity, the diffusivity, and the strain rate. Water also influences mantle convection and the dynamics of ascending plumes, because a trace amount of water lowers the viscosity of the mantle through a process known as “hydrolytic weakening” (e.g., Mei and Kohlstedt, 2000). Hot plumes under wet conditions may be partially molten because of the depression of the melting temperature (e.g., Litasov and Ohtani, 2003a, Ohtani et al., 2004). Water also enhances the diffusion of elements and the kinetics of phase transformations, such as the olivine–wadsleyite transformation in Mg2SiO4 (Kubo et al., 1998). Water influences the position of the pressure and temperature phase boundaries, such as the olivine–wadsleyite transformation, and decomposition of ringwoodite into bridgmanite and ferropericlase in the Mg2SiO4–Fe2SiO4 system (e.g., Litasov et al., 2005, Frost and Dolejs, 2007, Ghosh et al., 2013a, Ghosh et al., 2013b).
In this work, I review recent studies on potential water reservoirs and the water storage capacity of the upper mantle, transition zone, lower mantle, and the core, and the stability and properties of OH-bearing hydrous minerals in the mantle. I also discuss potential mechanisms for the transport of water into the deep mantle.
Section snippets
Nominally anhydrous minerals in the mantle
A small amount of water can dissolve into the structure of anhydrous mantle minerals as an impurity. Such an amount of water cannot be negligible when we consider the water budget of the Earth. There have been various works on this topic (e.g., Keppler and Smyth, 2006). It has been reported that transition zone minerals, such as wadsleyite and ringwoodite, can accommodate a large amount of water, up to 2–3 wt.%, in their crystal structures at temperatures around 1000 °C (Inoue et al., 1995,
Discovery of a new hydrous phase H
An Mg- and Si-bearing hydrous phase δ-AlOOH has been reported by Suzuki et al. (2000). The importance of this phase as a water carrier in slabs descending into the lower mantle has been suggested by Ohtani (2005). Shieh et al. (1998) reported that the decomposition of hydrous phase D to form bridgmanite, periclase, and fluid water occurs around 50 GPa, based on laser-heated diamond anvil cell experiments. Shieh et al. suggested the existence of one or more unknown phases that may be
Reaction of a new hydrous phase and metallic iron and hydrogen transport into the core
Dehydration melting could occur at the base of the lower mantle as discussed above because of the rapid increase in temperature at the core–mantle boundary. Partitioning of hydrogen between metallic iron and hydrous magma indicates that hydrogen strongly prefers metallic iron (Okuchi, 1997). The hydrogen partitioning between FeNi alloy and hydrous phase δ–H solid solution is also an important process to evaluate, to determine the possible preference of hydrogen in the metallic core at the
Summary
Water stored in hydrous minerals is transported into the transition zone and lower mantle via subducting slab. There are a series of reactions that can transport water into the transition zone in the slabs, i.e., water can be transported by a series of hydrous minerals, from serpentine or chlorite through to Mg-sursassite (Bromiley and Pawley, 2002) and/or the 10 Å phase (Fumagalli et al., 2001) to hydrous phase A (Bose and Ganguly, 1994). The transition zone has a large water storage capacity
Acknowledgments
The author thanks I. Ohira, I. Mashino, Y. Amaike, A. Suzuki, S. Kamada, T. Sakamaki, M. Murakami, K. D. Litasov, S. V. Rashchenko, and N. Sobolev for the useful discussions on this manuscript. This work was supported by a JSPS KAKENHI grant no. 22000002 awarded to EO and also supported partly by a grant from the Ministry of Education and Science of the Russian Federation, Project 14.B25.31.0032, awarded to EO.
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