First-principles investigation of hydrous post-perovskite

https://doi.org/10.1016/j.pepi.2015.03.010Get rights and content

Highlights

  • We have predicted a stable hydrogen defect structure in post-perovskite, stable to 150 GPa.

  • The shear-wave velocity of ppv with 1.6 wt.% water is identical to that of dry bridgmanite.

  • The bulk sound velocity is insensitive to hydration of ppv; slowing by 3.5 m/s per wt.% water.

Abstract

A stable, hydrogen-defect structure of post-perovskite (hy-ppv, Mg1−xSiH2xO3) has been determined by first-principles calculations of the vibrational and elastic properties up to 150 GPa. Among three potential hy-ppv structures analyzed, one was found to be stable at pressures relevant to the lower-mantle D region. Hydrogen has a pronounced effect on the elastic properties of post-perovskite due to magnesium defects associated with hydration, including a reduction of the zero-pressure bulk (K0) and shear (G0) moduli by 5% and 8%, respectively, for a structure containing ∼1 wt.% H2O. However, with increasing pressure the moduli of hy-ppv increase significantly relative to ppv, resulting in a structure that is only 1% slower in bulk compressional velocity and 2.5% slower in shear-wave velocity than ppv at 120 GPa. In contrast, the reduction of certain anisotropic elastic constants (Cij) in hy-ppv increases with pressure (notably, C55, C66, and C23), indicating that hydration generally increases elastic anisotropy in hy-ppv at D pressures. Calculated infrared absorption spectra show two O–H stretching bands at ∼3500 cm−1 that shift with pressure to lower wavenumber by about 2 cm−1/GPa. At 120 GPa the hydrogen bonds in hy-ppv are still asymmetric. The stability of a hy-ppv structure containing 1–2 wt.% H2O at D pressures implies that post-perovskite may be a host for recycled or primordial hydrogen near the Earth’s core-mantle boundary.

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

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