Hydrous minerals and the storage of water in the deep mantle

Abstract

Water is transported into the deep mantle via hydrous minerals in subducting slabs. During subduction, a series of minerals in these slabs such as serpentine or chlorite, Mg–sursassite and/or the 10 Å phase, and phase A can be stable at different pressures within the slab geotherms, and may transport significant amount of water into the Earth's interior. The transition zone has a large water storage capacity because of the high solubility of water in wadsleyite and ringwoodite. The recent discovery of hydrous ringwoodite and phase Egg as inclusions in ultra deep diamonds from Juina, Brazil suggests that the transition zone may indeed contain water. Seismic tomographic studies and electrical conductivity observations suggest that the transition zone may contain large amount of water, at least locally, beneath the subduction zones. The discovery of a new hydrous phase H, MgSiO₂(OH)₂, and its solid solution with isostructural phase δ-AlOOH, suggests that a significant amount of water could be stored in this hydrous magnesium silicate phase which is stable down to the lower mantle. Water may be transported into the bottom of the lower mantle via phase H–δ solid solution in descending slabs. This new high pressure hydrous phase solid solution has a high bulk modulus and sound velocity owing to strong O–H bonding due to hydrogen bond symmetrization in the lower mantle. Therefore, water stored in this hydrous phase would not reduce the seismic wave velocity in the lower mantle, and is seismically invisible. Dehydration melting could then occur at the base of the lower mantle, providing a potential explanation for the ultralow-velocity zone at the core–mantle boundary. When this hydrous magnesium silicate phase or hydrous melt makes contact with the metallic outer core at the core–mantle boundary, then hydrogen is likely to dissolve into the core.

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-Q_μ 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.