First-principles investigation of hydrous post-perovskite
Introduction
Seismic investigations of the lowermost several hundred kilometers of the mantle (called the D″ region) have revealed a heterogeneous region with large-scale structures including large low-shear-velocity provinces (LLSVPs) and ultralow-velocity zones (ULVZs) overlying the core (Garnero and McNamara, 2008). The bridgmanite (brg) to post-perovskite (ppv) phase transition of MgSiO3 has been invoked to explain some of the features within the D″ region (Murakami et al., 2004, Tsuchiya et al., 2004, Oganov and Ono, 2004, Wookey et al., 2005, Nowacki et al., 2010). The composition and mineralogy of D″ remains unresolved due to uncertainties in core-mantle boundary (CMB) temperature (Nomura et al., 2014), spatial heterogeneity of D″ material (e.g. slab graveyards) (Garnero and McNamara, 2008), and the effect of major element substitution on physical properties of the brg/ppv phase boundary for candidate lower mantle compositions (Grocholski et al., 2012).
Previous studies have investigated the effect of major-element substitution on the bridgmanite to post-perovskite phase transition and on physical properties of post-perovskite (Murakami and Hirose, 2005, Mao et al., 2006, Grocholski et al., 2012). In Al-free systems, increasing Fe2+ decreases the pressure of the phase boundary, whereas increasing Fe3+ and Al-content suppresses the phase boundary to higher pressures (greater depths) (Grocholski et al., 2012). The brg to ppv transition should occur above the CMB in harzburgite and MORB but potentially below the CMB conditions in pyrolite (Grocholski et al., 2012). However, the influence of hydrogen on ppv structure and physical properties has not been determined.
The bulk H2O content of the mantle is among the least well constrained compositional parameters of the Earth, with estimates varying by orders of magnitude due to uncertainty in the bulk mantle and core hydrogen content (e.g. Williams and Hemley, 2001). The water storage capacity of the uppermost mantle varies with depth, but in the peridotite system olivine and pyroxene can contain about 0.1 wt.% H2O at 400 km depth (Tenner et al., 2012, Ferot and Bolfan-Casanova, 2012). The transition zone water storage capacity is likely much higher because wadsleyite and ringwoodite can incorporate 1–2 wt.% H2O into their structures (Bolfan-Casanova et al., 2000, Inoue et al., 2010, Kohlstedt et al., 1996). The recent discovery of a hydrous ringwoodite inclusion in diamond containing ∼1.5 wt.% H2O suggests the transition zone may be very hydrous, at least locally (Pearson et al., 2014). The H2O storage capacity of the lower mantle remains highly uncertain due to conflicting estimates of H2O storage capacity of bridgmanite, which range from about 0.001 wt.% (Bolfan-Casanova et al., 2003) to 0.4 wt.% (Murakami et al., 2002) and values in between (Litasov et al., 2003). A recent computational investigation by Hernandez et al., 2013 calculated the hydrogen partition coefficient between ringwoodite, ferropericlase, and bridgmanite and estimated that bridgmanite may contain up to 1000 ppm (0.1 wt.%) water. Contrast in the H2O storage capacity between ringwoodite and bridgmanite may lead to dehydration melting below the 660 km discontinuity and provide evidence for regional scale hydration of the transition zone (Schmandt et al., 2014).
In contrast to bridgmanite, the post-perovskite structure is potentially more accommodating of hydrogen because both oxygen sites of the structure are slightly under-bonded. Magnesium is coordinated to eight oxygens with interatomic distances less than 2 Å and to two oxygens with distances slightly longer than 2 Å (Zhang et al., 2013). If the two longer oxygens are excluded from the Pauling bond strength sum, both O1 and O2 have the potential to protonate with charge balance achieved by an Mg-site vacancy. To test the idea that ppv may store seismically detectable amounts of hydrogen at D″ pressures, we have investigated several potential hydrous post-perovskite structures using density functional theory (DFT). We describe the most favorable hy-ppv structure and calculate its elastic and vibrational properties under static conditions in order to determine its mechanical stability, single-crystal and bulk-elastic wave velocities, and infrared absorption spectra.
Section snippets
Methods
Post-perovskite is orthorhombic with space group Cmcm (Murakami et al., 2004, Tsuchiya et al., 2004, Oganov and Ono, 2004). The structure contains alternating layers of corner-sharing SiO6 octahedra and Mg polyhedra in eight coordination to oxygen (Murakami et al., 2004, Zhang et al., 2013). DFT calculations were carried out using the PWSCF code, part of the Quantum ESPRESSO package using the Perdew–Ernzerhof–Burke generalized gradient approximation (Hohenberg, 1964, Kohn and Sham, 1965, Perdew
Results and discussion
Three potential OH-defect structures of post-perovskite were studied by positioning hydrogen in a magnesium vacancy of the ppv supercell using the electron localization function (Gibbs et al., 2003) to identify initial H positions. The first model (hy-ppv1) features one O1–H group and one O2–H group, the second (hy-ppv2) features two approximately symmetric O2–H groups, and the third model (hy-ppv3) features two asymmetric O2–H groups. After calculation of phonons and enthalpy under static
Summary
Recent estimates of the return flux of water to the mantle via subduction suggest that perhaps several hundreds of ppm water may be retained in the slab beyond the depth of magma generation (Parai and Mukhopadhyay, 2012, Garth and Rietbrock, 2014). If slabs carry water through the lower mantle and into D″, then ppv, the major silicate mineral in D″, may be a potential host for water.
Primordial components, such as noble gases, in some ridge basalts may imply that magmas deriving from the deep
Acknowledgements
JPT was supported by the EAPSI Program of the U.S. National Science Foundation (NSF) Grant Number 1209633 and the Japan Society for the Promotion of Science, and by the Premier Research Institute for Ultrahigh-pressure Sciences (PRIUS) joint research program carried out at the Geodynamics Research Center, Ehime University. This research was supported by NSF Grants EAR-1452344 (SDJ), EAR-0847951 (CRB), the Carnegie/DOE Alliance Center (CDAC), the David and Lucile Packard Foundation, and by the
References (55)
- et al.
Water partitioning between nominally anhydrous minerals in the MgO-SiO2-H2O system up to 24 GPa: implications for the distribution of water in the Earths mantle
Earth Planet. Sci. Lett.
(2000) - et al.
Water storage capacity in olivine and pyroxene to 14 GPa: implications for the water content of the Earth’s upper mantle and nature of seismic discontinuities
Earth Planet. Sci. Lett.
(2012) - et al.
The incorporation of water into lower-mantle perovskites: a first-principles study
Earth Planet. Sci. Lett.
(2013) - et al.
Water partitioning in the Earths mantle
Phys. Earth Planet. Inter.
(2010) - et al.
Elastic properties of hydrous ringwoodite (-phase) in Mg2SiO4
Earth Planet. Sci. Lett.
(1998) - et al.
Sound velocities and elastic constants of iron-bearing hydrous ringwoodite
Phys. Earth Planet. Inter.
(2004) - et al.
Structure and elasticity of hydrous ringwoodite: a first principle investigation
Phys. Earth Planet. Inter.
(2009) - et al.
Water solubility in Mg-perovskites and water storage capacity in the lower mantle
Earth Planet. Sci. Lett.
(2003) - et al.
How large is the subducted water flux? New constraints on mantle regassing rates
Earth Planet. Sci. Lett.
(2012) - et al.
The effect of temperature on the seismic anisotropy of the perovskite and post-perovskite polymorphs of MgSiO3
Earth Planet. Sci. Lett.
(2005)
Phase transition in MgSiO3 perovskite in the earth’s lower mantle
Earth Planet. Sci. Lett.
Ab initio investigations of native and protonic point defects in Mg2SiO4 polymorphs under high pressure
Earth Planet. Sci. Lett.
Phonons and related crystal properties from density-functional perturbation theory
Rev. Mod. Phys.
Incorporation of water in iron-free ringwoodite: a first-principles study
Am. Mineral.
Water partitioning at 660 km depth and evidence for very low water solubility in magnesium silicate perovskite
Geophys. Res. Lett.
Dynamical Theory of Crystal Lattices
An ab initio study of hydrogen in forsterite and a possible mechanism for hydrolytic weakening
J. Geophys. Res.
Helium isotope ratios in yellowstone and Lassen park volcanic gases
Geophys. Res. Lett.
Effect of iron on the compressibility of hydrous ringwoodite
Am. Mineral.
Structure and dynamics of Earth’s lower mantle
Science
Order of magnitude increase in subducted H2O due to hydrated normal faults within the Wadati–Benioff zone
Geology
QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials
J. Phys.: Condens. Matter
The electron localization function: a tool for locating favorable proton docking sites in the silica polymorphs
Phys. Chem. Miner.
Non-equilibrium degassing and a primordial source for helium in ocean-island volcanism
Nature
Mineralogical effects on the detectability of the postperovskite boundary
Proc. Natl. Acad. Sci.
The elastic behavior of a crystalline aggregate
Proc. Phys. Soc. A
Inhomogeneous electron gas
Phys. Rev.
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