Stability of various hydrous phases in CMAS pyrolite-H<Subscript>2</Subscript>O system up to 25 GPa

 We carried out a series of melting experiments with hydrous primitive mantle compositions to determine the stability of dense hydrous phases under high pressures. Phase relations in the CaO–MgO–Al₂O₃–SiO₂ pyrolite with ˜2 wt% of water have been determined in the pressure range of 10–25 GPa and in the temperature range between 800 and 1400 °C. We have found that phase E coexisting with olivine is stable at 10–12 GPa and below 1050 °C. Phase E coexisting with wadsleyite is stable at 14–16 GPa and below 900 °C. A superhydrous phase B is stable in pyrolite below 1100 °C at 18.5 GPa and below 1300 °C at 25 GPa. No hydrous phases other than wadsleyite are stable in pyrolite at 14–17 GPa and 900–1100 °C, suggesting a gap in the stability of dense hydrous magnesium silicates (DHMS). We detected an expansion in the stability field of wadsleyite to lower pressures (12 GPa and 1000 °C). The H₂O content of wadsleyite was found to decrease not only with increasing temperature but also with increasing pressure. The DHMS phases could exist in a pyrolitic composition only under the conditions present in the subducting slabs descending into the lower mantle. Under the normal mantle and hot plume conditions, wadsleyite and ringwoodite are the major H₂O-bearing phases. The top of the transition zone could be enriched in H₂O in accordance with the observed increase in water solubility in wadsleyite with decreasing pressure. As a consequence of the thermal equilibration between the subducting slabs and the ambient mantle, the uppermost lower mantle could be an important zone of dehydration, providing fluid for the rising plumes. Received: 9 September 2002 / Accepted: 11 January 2003 Acknowledgements The authors are thankful to Y. Ito for the assistance with the EPMA measurement, A. Suzuki, T. Kubo and T. Kondo for technical help with the high-pressure experiments and Raman and X-ray diffraction measurements and C.R. Menako for technical support. K. Litasov thanks H. Taniguchi for his continuous encouragement and the Center for Northeast Asian Studies of Tohoku University and the Japanese Society for the Promotion of Science for the research fellowships. This work was partially supported by the Grant-in-Aid of Scientific Research of the Priority Area (B) of the Ministry of Education, Science, Sport, and Culture of the Japanese government (no. 12126201) to E. Ohtani.     

Plastic deformation of wadsleyite: I. High-pressure deformation in compression

 We have studied the plastic deformation of Mg₂SiO₄ wadsleyite polycrystals. Wadsleyite was synthesized from a forsterite powder in a multianvil apparatus. It was then recovered and placed in a second multianvil assembly designed to induce plastic deformation by compression between two hard alumina pistons. After the deformation experiment, the microstructures are characterized by transmission electron microscopy (TEM) and large-angle convergent beam electron diffraction (LACBED). Deformation experiments have been carried out at 15–19 GPa and at temperatures ranging from room temperature to 1800–2000 °C. Five different dislocation types have been identified by LACBED: [100], 1/2〈111〉, [010], 〈101〉 and [001]. The [001] dislocations result from dislocation reactions and not from activation of a slip system. The [010] dislocations are activated under high stresses at the beginning of the experiments and further relax by decomposition into 1/2〈111〉 dislocations or by dissociation into four 1/4[010] partial dislocations. The following slip systems have been identified: 1/2〈111〉{101}, [100](010), [100](001), [100]{011}, [100]{021}, [010](001), [010]{101} and 〈101〉(010). Received: 15 July 2002 / Accepted: 14 February 2003 Acknowledgements High-pressure experiments were performed at the Bayerisches Geoinstitut under the EU IHP – Access to Research Infrastructures Programme (Contract no. HPRI-1999-CT-00004 to D.C. Rubie). P.C. has benefited from a Congé thématique pour recherche from the University of Lille, and would like to thank warmly all the people in Bayreuth who contributed to this work by daily assistance and discussions: Nathalie Bolfan-Casanova, Daniel Frost, Jed L. Mosenfelder and Brent Poe. The quality of the preparation of the TEM specimens by H. Schultze is greatly appreciated.     

Plastic deformation of wadsleyite: II. High-pressure deformation in shear

 We have studied the dislocation microstructures that develop in (Mg_0.9Fe_0.1)₂SiO₄ wadsleyite deformed by simple shear at high pressure. The experiments were performed in a multianvil apparatus with the shear assembly designed by Karato and Rubie (1997). The samples were synthesized in a separate experiment from high-purity oxides. The deformation experiments were carried out at 14 GPa and 1300 °C with time durations ranging from 1 to 8 h leading to plastic shear strains of 60 and 73%, respectively. The microstructures investigated by transmission electron microscopy (TEM) show that dislocation glide is activated under these conditions over the whole experimental time. The easy slip systems at 1300 °C involve 1/2<111> dislocations gliding in {101} as well as [100] dislocations gliding in (010) and {011}. Received: 15 July 2002 / Accepted: 14 February 2003 Acknowledgements High-pressure experiments were performed at the Bayerisches Geoinstitut under the EU IHP — Access to Research Infrastructures Programme (Contract no. HPRI-1999-CT-00004 to D.C. Rubie). The quality of the preparation of the TEM specimens by H. Schultze is greatly appreciated.     

Mg-vacant structural modules and dilution of the symmetry of hydrous wadsleyite, β-Mg2–xSiH2xO4 with 0.00≤x≤0.25

The crystal structures of the two hydrous wadsleyite crystals with formulae, Mg_1.75SiH_0.50O₄ (0.5H–β) and Mg_1.86SiH_0.28O₄ (0.3H–β) have been analyzed in this study. The single-crystal X-ray diffraction data showed that the unit cells of the 0.3H–β and the 0.5H–β are metrically monoclinic with a slight distortion from the orthorhombic cell but their intensity distributions conform to the orthorhombic symmetry within the limit of experimental errors. The Fourier and the difference Fourier syntheses were calculated. Small but significant Fourier peaks were found at the site, Si2, in a normally vacant tetrahedral void adjacent to Mg3 site as reported for the monoclinic hydrous wadsleyite by Smyth et al.. From the comparison of the hydrous and anhydrous wadsleyite structures, the Mg-vacant structural modules were found to be the building units for the structure of hydrous wadsleyite. The dilution of symmetry from orthorhombic to monoclinic in the hydrous wadsleyite structure is interpreted qualitatively due to lack of mirror perpendicular to the a axis in the module. The mode of arrangement of the Mg-vacant structural modules interprets the symmetry and hydrogen content of the hydrous wadsleyite and gives the structural relationship between hydrous wadsleyite and hydrous ringwoodite. Received: 8 May 1998 / Revised, accepted: 3 October 1998     

Anelasticity and microcreep in polycrystalline MgO at high temperature: an exploratory study

 The frequency dependence of the shear modulus and dissipation in polycrystalline MgO has been determined at high temperature using both microcreep (ɛ = 10⁻⁴) and seismic frequency forced-oscillation (ɛ = 10⁻⁵) measurements. The frequency-dependent and time-dependent data have been described in terms of the elastic, anelastic and viscous components of deformation using the Andrade model. The forced-oscillation measurements show that for temperatures above 700 °C the shear modulus begins to decrease dramatically and the modulus becomes frequency-dependent with increasing temperature. This is accompanied by an increase in dissipation, which also becomes frequency-dependent. The microcreep measurements resolve this frequency-dependent behaviour into an anelastic regime from 700–1050 °C, and a viscoelastic regime from 1100–1300 °C. At 1300 °C, the seismic frequency shear wave speed is ∼60% of the extrapolated low-temperature frequency-independent value, and the dissipation has risen to Q ⁻¹ = 10⁻¹ from 10⁻³ at temperatures below 600 °C. The mechanism by which this frequency-dependent rheology occurs appears to be diffusional creep, which produces viscous slip on the grain boundaries. It is proposed that the anelastic behaviour is due to viscous slip occurring on segments of grain boundaries, with the viscous deformation being accommodated by elastic distortion of adjacent unslipped regions of the grain boundary. At higher temperatures, slippage occurs across the entire grain boundary and viscoelastic behaviour begins to occur. Received: 11 April 2002 / Accepted: 9 January 2003 Acknowledgements Samples were precision-ground by Andrew Wilson, and polished sections prepared by Harri Kokkonen, who also did the SEM work. Uli Faul calculated the volume fraction of grain sizes. The density measurements were done by Lara Weston.     

High-pressure Raman spectroscopic study of Fo90 hydrous wadsleyite

Raman spectra of monoclinic Fo₉₀ hydrous wadsleyite with 2.4 wt% H₂O have been measured in a diamond-anvil cell with helium as a pressure-transmitting medium to 58.4 GPa at room temperature. The most intense, characteristic wadsleyite modes, the Si–O–Si symmetric stretch at 721 cm⁻¹ and the symmetric stretch of the SiO₃ unit at 918 cm⁻¹, shift continuously to 58.4 GPa showing no evidence of a first order change in the crystal structure despite compression well beyond the stability field of wadsleyite in terms of pressure. The pressure dependence of these two modes is nearly identical for Fo₉₀ hydrous and Fo₁₀₀ anhydrous wadsleyite. A striking feature in the high-pressure Raman spectra of Fo₉₀ hydrous wadsleyite is the appearance of new Raman modes above 9 GPa in the mid-frequency range (300–650 cm⁻¹ at 1-bar and shifted to 500–850 cm⁻¹ at 58.4 GPa) accompanied by a significant growth in their intensities under further compression. In the OH stretching frequency range Fo₉₀ hydrous wadsleyite exhibits a larger number of modes than the Mg end-member phase. The higher number of modes may be due to either additional protonation sites or simply that we observe a different subset of all possible OH modes for each sample. The high-pressure behaviour of the OH stretching modes of Fo₉₀ and Fo₁₀₀ hydrous wadsleyite is consistent: OH stretching modes with frequencies <3,530 cm⁻¹ decrease with increasing pressure whereas the higher-frequency OH modes show a close to constant pressure dependence to at least 13.2 GPa. The approximately constant pressure dependence of the OH modes above 3,530 cm⁻¹ is consistent with protons being located at the O1···O edges around M3.     

A quantum-mechanical study of the (110) surface of sphalerite (ZnS) and its interaction with Pb<Superscript>2+</Superscript> species

 The structure of the (110) surface of sphalerite has been studied using mainly quantum-mechanical (QM) methods. The experimentally observed puckering of the surface is well reproduced by periodic plane wave density functional calculations. Water adsorption on this surface is modelled using this QM approach and is used to judge the accuracy of less computationally demanding cluster methods. A cluster model, thus validated, is used to study the possible modes of interaction of Pb²⁺ ions with the (110) surface. It is found that adsorption of hydrated PbO and PbOH⁺ is energetically feasible and leads to Pb–O distances compatible with REFLEXAFS data. Received: 22 April 2002 / Accepted: 23 October 2002 Acknowledgements We thank EPSRC for support of this research.     

Systematic study of hydrogen incorporation into Fe-free wadsleyite

The H₂O content of wadsleyite were measured in a wide pressure (13–20 GPa) and temperature range (1,200–1,900°C) using FTIR method. We confirmed significant decrease of the H₂O content of wadsleyite with increasing temperature and reported first systematic data for temperature interval of 1,400–1,900°C. Wadsleyite contains 0.37–0.55 wt% H₂O at 1,600°C, which may be close to its water storage capacity along average mantle geotherm in the transition zone. Accordingly, water storage capacity of the average mantle in the transition zone may be estimated as 0.2–0.3 wt% H₂O. The H₂O contents of wadsleyite at 1,800–1,900°C are 0.22–0.39 wt%, indicating that it can store significant amount of water even under the hot mantle environments. Temperature dependence of the H₂O content of wadsleyite can be described by exponential equation C_{\textH2 \textO} = 6 3 7.0 7 \texte ^{- 0.00 4 8T} , C_{{{\text{H}}_{2} {\text{O}}}} = 6 3 7.0 7 {\text{e}}^{ - 0.00 4 8T} , where T is in °C. This equation is valid for temperature range 1,200–2,100°C with the coefficient of determination R ² = 0.954. Temperature dependence of H₂O partition coefficient between wadsleyite and forsterite (D _wd/fo) is complex. According to our data apparent D_wd/fo decreases with increasing temperature from D _wd/fo = 4–5 at 1,200°C, reaches a minimum of D _wd/fo = 2.0 at 1,400–1,500°C, and then again increases to D _wd/fo = 4–6 at 1,700–1,900°C.     

Compression mechanisms of coesite

 The structure of coesite has been determined at ten pressures up to a maximum of 8.68 GPa by single-crystal X-ray diffraction. The dominant mechanism of compression is the reduction of four of the five independent Si–O–Si angles within the structure. There is no evidence of the fifth linkage, Si1–O1–Si1, deviating from 180°. Some Si–O bond distances also decrease by up to 1.6% over the pressure range studied. The pattern of Si–O–Si angle reduction amounts to a rotation of the Si2 tetrahedron around the [001] direction. This rotation induces significant internal deformation of the Si1 tetrahedron. Comparison of the experimental data with rigid-unit distance least-squares simulations of coesite suggests that this pattern of compression, the anomalous positive values of both s₂₃ and K′′ in the equation of state of coesite, its high elastic anisotropy and the unusual straight Si1–O1–Si1 linkage within the structure are all consequences of the connectivity of the tetrahedral framework. Received: 11 July 2002 / Accepted: 14 January 2003 Acknowledgements The help of Christian Baerlocher of ETH Zurich in providing both the DLS-76 software and advice in its use is gratefully acknowledged, as are discussions with Paul Ribbe of Virginia Tech and the comments of two anonymous reviewers. The data analysis was supported by the National Science Foundation under grant EAR-0105864 to N.L. Ross and R.J. Angel.     

Impact of hydrology on the Port-Harcourt–Patani-Warri Road

 The paper explores the impact of hydrology on the Port-Harcourt–Patani-Warri Road. It reviews the hydrometeorological, drainage and terrain peculiarities of the area against the backdrop of the design, alignment and performance of the road. The paper ascribes the poor performance of the road to: 1.  The southeast-northwest orientation of the road in a region with predominately northeast-southwest surface and subsurface flow, in which the road acts like a dam; 2. The inferior construction aggregate composition; 3. Changes in pavement condition due to interaction of local road aggregates with water. Received: 21 September 1998 · Accepted: 16 November 1999     

The influence of marine inflows on the chemical composition of groundwater in small islands: the example of the Cyclades (Greece)

 Marine contamination of groundwater may be caused by seawater intrusion and by salt spray. The role of both processes was studied in the Cyclades archipelago on four small islands (45–195 km²) whose aquifers consist essentially of fractured, weathered metamorphic rocks. Annual rainfall ranges from 400 to 650 mm and precipitation has high total dissolved solids contents of 45–223 mg l^–1. The chemical characteristics of the groundwater, whose salinity is from 0.4 to 22 g l^–1, are strongly influenced by seawater intrusion. However, the effect of atmospheric input is shown in certain water sampling locations on high ground elevation where the dissolved chloride contents may attain 200 mg l^–1. Received: 14 November 1995 · Accepted: 9 September 1996     

The effect of energy on the adoption of conservation tillage in the United States

 The impact of energy on the adoption of conservation tillage is of special importance in addressing concerns about the effect of agricultural production on the environment in the United States. It is the subject of this paper. After establishing that a relationship exists between the price of energy and the adoption of conservation tillage via cointegration techniques, the relationship is quantified. It is shown that while the real price of crude oil, the proxy used for the price of energy, does not affect the rate of adoption of conservation tillage, it does impact the extent to which it is used. Finally, there is no structural instability in the relationship between the relative use of conservation tillage and the real price of crude oil over the period 1963–1997. Received: 6 February 1998 · Accepted: 4 May 1998     

A physical basis for Pauling's definition of bond strength

 The average strength, s, of the bonded interactions comprising a cation containing oxide anion coordination polyhedron and the value of the electron density, ρ(r _c ), at the bond-critical points are inversely correlated with bond length. In each case, the observed bond lengths, R, were modeled with power-law expressions defined in terms of s/r and ρ(r _c )/r, respectively, where r is the Periodic Table row number of the cation involved in the bonded interaction. On the basis of the close connection between bond strength and the value of the electron density at the bond-critical point, we conclude that bond strength is a direct measure of bond type; the greater its value, the greater the localization of electron density in the binding region and the greater the shared–electron covalent character of the bonded interaction. Received: 15 October 2002 / Accepted: 17 February 2003 Present address:G. V. Gibbs in care of M. Spackman Department of Chemistry, University of New England, Armidale 2351, Australia Acknowledgements The NSF is thanked for supporting this study with grant EAR–9627458. The paper was written while GVG was a Visiting NSF Scholar at The University of Arizona. The faculty and graduate students of the Department of Geosciences and Bob Downs and Marelina Stimpf in particular are thanked for making the visit great fun.     

Computational study of tetrahedral Al–Si and octahedral Al–Mg ordering in phengite

 As part of a wider study of the nature and origins of cation order–disorder in micas, a variety of computational techniques have been used to investigate the nature of tetrahedral and octahedral ordering in phengite, K₂ ^[6](Al₃Mg)^[4](Si₇Al)O₂₀(OH)₄. Values of the atomic exchange interaction parameters J _n used to model the energies of order–disorder were calculated. Both tetrahedral Al–Si and octahedral Al–Mg ordering were studied and hence three types of interaction parameter were necessary: for T–T, O–O and T–O interactions (where T denotes tetrahedral sites and O denotes octahedral sites). Values for the T–T and O–O interactions were taken from results on other systems, whilst we calculated new values for the T–O interactions. We have demonstrated that modelling the octahedral and tetrahedral sheets alone and independently produces different results from modelling a whole T–O–T layer, hence justifying the inclusion of the T–O interactions. Simulations of a whole T–O–T layer of phengite indicated the presence of short-range order, but no long-range order was observed. Received: 8 August 2002 / Accepted: 14 February 2003 Acknowledgements The authors are grateful to EPSRC (EJP) and the Royal Society (CIS) for financial support. Monte Carlo simulations were performed on the Mineral Physics Group's Beowulf cluster and the University of Cambridge's High Performance Computing Facility.     

Thermodynamic properties and phase stability of wadsleyite II

The thermodynamical stability of a newly observed wadsleyite II phase in the Mg₂SiO₄ system is studied by the density functional theory. The wadsleyite II equation of state has been derived. The phase boundaries of Mg₂SiO₄ polymorphs: wadsleyite, wadsleyite II and ringwoodite are studied using the quasi-harmonic approximation at high external pressures. Clapeyron slopes determined for wadsleyite II–ringwoodite and wadsleyite–wadsleyite II boundaries are 0.0047 and 0.0058 GPa/K, respectively. It is shown that the wadsleyite II phase is not thermodynamically preferred in the pure Mg₂SiO₄ system and will probably not occur between wadsleyite and ringwoodite phases.     

Hydraulic parameters and solute velocities in triple-porosity karstic-fissured-porous carbonate aquifers: case studies in southern Poland

 Rock and flow parameters of three karstic-fissured-porous aquifers in the Krakow-Silesian Triassic formations were measured using various methods and compared. Though cavern and fissure porosities are shown to be very low (cavern porosity below 0.5% and fracture porosity below 0.2%), they contribute dominantly to the hydraulic conductivity (from about 1.3×10^–6 to about 11×10^–6 m/s). Matrix porosity (2–11%) is shown to be the main water reservoir for solute transport and the main or significant contributor to the specific yield (<2%). Though the matrix porosity is shown to be much larger than the sum of the cavern and fissure porosities, its contribution to the total hydraulic conductivity is practically negligible (hydraulic conductivity of the matrix is from about 5×10^–11 m/s to about 2×10^–8 m/s). On the other hand, the matrix porosity (for neglected cavern and fissure porosities) when combined with tracer ages (or mean travel times) is shown to yield proper values of the hydraulic conductivity (K) by applying the following formula: K≅(matrix porosity×mean travel distance)/(mean hydraulic gradient×mean tracer age). Confirming earlier findings of the authors, this equation is shown to be of great practical importance because matrix porosity is easily measured in the laboratory on rock samples, whereas cavern and fracture porosities usually remain unmeasurable. Received: 21 February 1997 · Accepted: 13 May 1997     

Raman spectroscopic study of hydrous wadsleyite (β-Mg2SiO4) to 50 GPa

 Raman spectra of hydrous β-Mg₂SiO₄ (1.65 wt% H₂O) have been measured in a diamond-anvil cell with helium as a pressure-transmitting medium at room temperature to 50 GPa. We observe three OH-stretching modes, a doublet with components at 3329 and 3373 cm⁻¹, which decrease linearly with pressure, and a single mode at 3586 cm⁻¹, which remains nearly constant up to 24 GPa before decreasing at higher pressures. Assessment of the mode frequencies and their pressure dependence, together with previous results from X-ray and IR data, are consistent with protonation of the O1 site in agreement with previous studies. Strict assignment of Raman activity awaits detailed structural models. The nature of the protonation in wadsleyite may require more specific experimental probes for full solution of the hydrogen-site problem. Received: 18 July 2000 / Accepted: 22 November 2000     

Experimentally derived thermochemical data for pargasite and reinvestigation of its stability with quartz in the system Na2O–CaO–>MgO–Al2O3–SiO2–H2O

 The stability of pargasite in the presence of excess quartz has been determined in the range of 0.5–6.0 kbar and 500–950 °C in the system Na₂O– CaO–MgO–Al₂O₃–SiO₂–H₂O, using synthetic minerals. The experimental results from this study indicate the presence of two distinct mineral assemblage regions: (1) a high temperature supersolidus region containing tremolitic amphibole+melt+quartz; (b) a low temperature subsolidus region consisting of Al-rich amphibole+plagioclase+enstatite+quartz. Compositional reversals have been determined for the following three equilibria: (a) 2 pargasite+9 quartz=tremolite+4 plagioclase (An₅₀)+1.5 enstatite+H₂O, (b) 2 pargasite+10 quartz=tremolite+4 plagioclase (An₅₀)+talc, and (c) pargasite+diopside+5 quartz=tremolite+2 plagioclase (An₅₀). These experiments indicate a continuous change of amphibole composition from pargasite to tremolite with increasing temperature, and an opposite effect with increasing pressure. The third equilibria is used to constrain a site-mixing model for the pargasitic amphiboles, which favor a single-coupled Na^A-Al^T1 site mixing. The thermochemical data for pargasite estimated from the reversal data of the three equilibrium reactions is estimated as for ΔG ⁰ _f ,Pg=−12022.11±5.2 kJ mole^-1, and S ⁰ _Pg=591.7 ±7.9 JK^-1 mole^-1. Received: 31 July 1995/Accepted: 3 June 1996     

The phase boundary between wadsleyite and ringwoodite in Mg2SiO4 determined by in situ X-ray diffraction

The phase boundary between wadsleyite and ringwoodite in Mg₂SiO₄ has been determined in situ using a multi-anvil apparatus and synchrotron X-rays radiation at SPring-8. In spite of the similar X-ray diffraction profiles of these high-pressure phases with closely related structures, we were able to identify the occurrence of the mutual phase transformations based on the change in the difference profile by utilizing a newly introduced press-oscillation system. The boundary was located at ~18.9 GPa and 1,400°C when we used Shim’s gold pressure scale (Shim et al. in Earth Planet Sci Lett 203:729–739, 2002), which was slightly (~0.8 GPa) lower than the pressure as determined from the quench experiments of Katsura and Ito (J Geophys Res 94:15663–15670, 1989). Although it was difficult to constrain the Clapeyron slope based solely on the present data due to the kinetic problem, the phase boundary [P (GPa)=13.1+4.11×10⁻³×T (K)] calculated by a combination of a P–T position well constrained by the present experiment and the calorimetric data of Akaogi et al. (J Geophys Res 94:15671–15685, 1989) reasonably explains all the present data within the experimental error. When we used Anderson’s gold pressure scale (Anderson et al. in J Appl Phys 65:1535–1543, 1989), our phase boundary was located in ~18.1 GPa and 1,400°C, and the extrapolation boundary was consistent with that of Kuroda et al. (Phys Chem Miner 27:523–532, 2000), which was determined at high temperature (1,800–2,000°C) using a calibration based on the same pressure scale. Our new phase boundary is marginally consistent with that of Suzuki et al. (Geophys Res Lett 27:803–806, 2000) based on in situ X-ray experiments at lower temperatures (<1,000°C) using Brown’s and Decker’s NaCl pressure scales.