Elsevier

Earth and Planetary Science Letters

Volume 401, 1 September 2014, Pages 12-17
Earth and Planetary Science Letters

Stability of a hydrous δ-phase, AlOOH–MgSiO2(OH)2, and a mechanism for water transport into the base of lower mantle

https://doi.org/10.1016/j.epsl.2014.05.059Get rights and content

Highlights

  • Hydrogen is stored in hydrous δ-phase AlOOH–MgSiO2(OH)2 in the lower mantle.

  • Hydrous δ-phase coexists with perovskite and post-perovskite in the lower mantle.

  • Hydrous δ-phase has a high bulk modulus due to the O–H–O bonding symmetrization.

  • Hydrous δ-phase in the slabs transports hydrogen to the lower mantle and core.

Abstract

The global water cycle in the Earth is one of the most important issues in geodynamics, because water can affect the physical and rheological properties of the mantle. However, it is still a matter of debate whether water can be transported into the lower mantle and core. Here we report a new reaction between aluminous perovskite and water to form alumina-depleted perovskite and hydrous δ-phase AlOOH–MgSiO2(OH)2 along the mantle geotherm in the lower mantle. Chemical analysis of the coexisting phases showed that the perovskite and post-perovskite phases were depleted in Al2O3, whereas hydrous δ-phase contains at least 44 mol% of MgSiO2(OH)2 component at 68 GPa and 2010 K, and 23 mol% of this component at 128 GPa and 2190 K. The present experiments revealed that hydrous δ-phase AlOOH–MgSiO2(OH)2 can coexist with alumina-depleted MgSiO3 perovskite or post-perovskite under the lower mantle conditions along the slab geotherm. Thus this hydrous phase in the slabs can transport water into the base of the lower mantle.

Introduction

It has been considered that water can be transported to at least the deep upper mantle and the transition zone through slab subduction. Peacock (1990) estimated that the total amount of water transported by a sediment layer and an oceanic crust layer in a subducting slab is approximately 8.7×1011 kg/yr. Of transported water, an amount of 2×1011 kg/yr is outgassed from the mantle through magmatism in arcs and mid-oceanic ridges (Peacock, 1990), and residual water may be transported into the deep mantle by subducting slabs.

Recent global seismic tomography revealed significant high-velocity anomalies in the lower mantle down to the core–mantle boundary (CMB), which represent pieces of slab materials collapsing down to the CMB (e.g., Zhao, 2012). Such subducting slabs retain water stored in hydrous and nominally anhydrous minerals. Therefore, it is important to determine the stability fields of hydrous minerals at high pressure and temperature corresponding to Earth's mantle.

Various studies have been conducted to clarify the stability of hydrous phases in sediment, oceanic crust, and peridotite mantle compositions. Ono (1998) investigated phase relations in hydrated natural sediment and mid-ocean ridge basalt (MORB) compositions from 6 to 15 GPa and from 973 to 1673 K. According to this study, hydrous phase egg, AlSiO3(OH), is stable at pressures greater than 12 GPa in a subducting sediment layer. Phase egg decomposes into hydrous δ-phase AlOOH and stishovite at pressures greater than 23 GPa at temperatures below 1473 K (Sano et al., 2004). Hydrous δ-phase containing 86 mol% Al2O2(OH)2 and 14 mol% MgSiO2(OH)2 is first synthesized at 21 GPa and 1273 K (Suzuki et al., 2000), and the end-member δ-AlOOH is stable at pressures from 19 to 134 GPa in the temperature range 1273–2300 K, suggesting that this hydrous mineral could transport hydrogen into the CMB in the sediment layer of a subducting slab (Sano et al., 2008).

On the other hand, lawsonite, the hydrous mineral in natural MORB, breaks down above 9 GPa, causing hydration of the peridotite layer in and above the subducting slab (Ohtani et al., 2004). In the peridotite layer, a number of hydrous minerals are stable along the slab geotherm. However, such hydrous minerals decompose at pressures below 44 GPa, corresponding to a depth of 1250 km (Shieh et al., 1998, Ohtani et al., 2004). Thus, water in subducting slabs at depths greater than 1250 km may exist as hydrous δ-phase AlOOH in a sediment layer and fluid H2O generated by the decomposition of hydrous minerals in slabs. Recently, a new hydrous phase H, MgSiO2(OH)2, was predicted by ab-initio calculation and synthesized at pressures around 50 GPa (Tsuchiya, 2013, Nishi et al., 2014). Tsuchiya (2013) reported that this phase H has a narrow stability field at around 50 GPa and temperature below 1500 K. This phase has a structure similar to hydrous δ-phase AlOOH, and δ-phase clearly dissolves a Mg Si component in its structure as was already reported previously by Suzuki et al. (2000). Therefore, it is very important to study the stability field of δ-phase and phase H solid solution, and to study partitioning of aluminum and hydrogen between the coexisting phases in the lower mantle.

Ono et al. (2002) reported that water originating from dehydration of hydrous minerals can be trapped in the interstitials of garnet grains in the oceanic crust because of garnet–fluid dihedral angles greater than 60°. Thus, hydrous δ-phase AlOOH and fluid water are important in discussing water transport into the deep lower mantle and the core–mantle boundary. However, previous studies on formation of hydrous δ-phase in wet slab and mantle peridotite are limited in the pressure–temperature conditions up to the mantle transition zone (Ohtani et al., 2001).

Here we report on the phase relations and mineral chemistry of aluminous MgSiO3 perovskite and hydrous δ-phase AlOOH–MgSiO2(OH)2 in the MgSiO3–Al2O3–H2O system by in situ X-ray diffraction (XRD) measurements at high pressure and temperature, and on chemical composition analysis using a scanning transmission electron microscope (STEM) with an energy-dispersive X-ray spectroscopy (EDS) detector system.

Section snippets

Starting materials

Gels with the composition of 70 mol% MgSiO3–30 mol% Al2O3 synthesized by the sol-gel method using reagent-grade of Mg(NO3)26H2O, Al(NO3)39H2O and TEOS((C2H5O)4Si) (Hamilton and Henderson, 1968, Kawamura, 1994) were used as starting materials in the present experiments. The water content was measured by the ignition loss of the gel material, which was 7.0 wt.%. The gels containing less than 7.0 wt.% water were synthesized by heating the same gel again at 700 or 800 °C for an additional day in

Results

In situ XRD measurements were conducted in the pressure range from 43 to 128 GPa and the temperature range from 1580 to 2430 K in a symmetrical diamond anvil cell with beveled anvils or without bevels. Synthetic gels containing 1.5, 6.0 and 7.0 wt.% water with the composition of 70 mol% MgSiO3–30 mol% Al2O3 were used as starting materials. In all runs, the samples were compressed at room temperature to the target pressure and then heated to target temperature. Each sample was heated for a

Discussion

The stability of hydrous phase H, MgSiO2(OH)2, a pure end-component of hydrous δ-phase solid solution at around 4052 GPa at 0 K has been predicted by ab-initio calculation (Tsuchiya, 2013). This calculation suggests that the phase has relatively a narrow stability field and it eventually decomposes to perovskite and water at higher pressures above 52 GPa. The present experiments together with this theoretical calculation (Tsuchiya, 2013) indicate that hydrous δ-phase has a wide range of solid

Conclusions

The hydrous δ-phase which coexists with perovskite and post-perovskite contains a significant amount of phase H component, MgSiO2(OH)2 with minor SiO2. This hydrous phase is stable at high temperature of the normal mantle geotherm in the lower mantle. Chemical analyses of the coexisting phases showed that perovskite and post-perovskite phases are depleted in Al2O3, whereas hydrous δ-phase contains at least 44 mol% of a MgSiO2(OH)2 component at 68 GPa and 2010 K and 23 mol% of this component at

Acknowledgments

We thank A. Suzuki, M. Murakami and T. Sakamaki for useful discussions during this work. We also thank S. Kamada for his experimental assistance at SPring-8. This work was supported by KAKENHI Grant of Japan Society for the Promotion of Science (JSPS), Grant numbers 18194009, 2200002 awarded to E.O. This work was also supported partly by the Ministry of Education and Science of Russian Federation, project 14.B25.31.0032 to E.O. This work was conducted as part of the Global Center of Excellence

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