Nathalie Bolfan-Casanova

Water in the Earth’s mantle

Abstract Water plays an important role in most processes within the Earths mantle, e.g. transport phenomena, differentiation and seismic properties. This paper reviews the various aspects of water in the mantle, with a special emphasis on the water content in nominally anhydrous minerals, especially lower-mantle minerals. The saturation of the upper mantle with respect to water is calculated as a function of pressure, based on available water-solubility data obtained for upper-mantle minerals. The result indicates that the upper mantle is saturated at pressures between 2 and 4 GPa for bulk water contents of 250 to 700 ppm wt., as retrieved from measurements on mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs). Whereas up to 4000 ppm wt. of H2O could be dissolved in the upper mantle at pressures corresponding to 410 km depth, such a value is less than the 1.5-2.5 wt.% solubility stored in the hydrous phases of the transition zone. Water solubility in mantle perovskite is still controversial, because of the difficulty of synthesizing samples free of impurities. Reported data indicate that water solubility in perovskite decreases with increasing temperature and Al content. Water partitions preferentially into ferropericlase rather than into perovskite, and its water solubility increases with the incorporation of trivalent cations.

Equation of state of lower mantle (Al,Fe)-MgSiO3 perovskite

The compression behavior of various (Al,Fe)-bearing MgSiO3 perovskites was investigated by powder X-ray diffraction up to 70 GPa on the ID30 beamline of ESRF (Grenoble, France). Using diamond anvil cell coupled with CO2 laser-heating, we obtained a reliable equation of state up to typical lower mantle pressures. In contrast to Fe which essentially increases the room pressure unit cell volume (V0), the effect of Al is to increase the bulk modulus of silicate perovskite. This result contrasts with previous determinations performed at pressures below 10 GPa on samples synthesized in the multi-anvil press. Such a difference can be explained by a change in the substitution mechanism of Al in MgSiO3 with increasing pressure and temperature, in agreement with recent ab-initio calculations. Our results confirm that the Earth’s lower mantle (Mg+Fe)/Si ratio is greater than unity, because of the high stiffness of silicate perovskite.

Melting of subducted basalt at the core-mantle boundary

The geological materials in Earths lowermost mantle control the characteristics and interpretation of seismic ultra–low velocity zones at the base of the core-mantle boundary. Partial melting of the bulk lower mantle is often advocated as the cause, but this does not explain the nonubiquitous character of these regional seismic features. We explored the melting properties of mid-oceanic ridge basalt (MORB), which can reach the lowermost mantle after subduction of oceanic crust. At a pressure representative of the core-mantle boundary (135 gigapascals), the onset of melting occurs at ~3800 kelvin, which is ~350 kelvin below the mantle solidus. The SiO2-rich liquid generated either remains trapped in the MORB material or solidifies after reacting with the surrounding MgO-rich mantle, remixing subducted MORB with the lowermost mantle. Mid-ocean ridge basalt entrained in the lowermost mantle is consistent with seismic ultra–low velocity zones [Also see Perspective by Williams] Delving deeper into the lower mantle Earths lower mantle is an enigmatic region, a transition zone between slowly churning solids and a liquid outer core. Large seismic structures and discontinuities in this region are probably due to sharp gradients in temperature, composition, or mineralogy. Teasing apart the precise effects of these factors requires experiments at lower mantle temperatures and pressures (see the Perspective by Williams). Zhang et al. found that the major mineral phase of the lower mantle decomposes into two minerals. Andrault et al. show how the melting of subducted basalt from the oceanic crust will form pile-like structures on top of the core/mantle boundary. Science, this issue p. 877, p. 892; see also p. 800.

Uranium in the Earth's lower mantle

[1] The distribution of the radiogenic heat sources strongly influences the geodynamics and thermal behaviour of the Earth. About 11 TW is produced by the radioactive decay of uranium (25% of the total heat flux at Earth surface), and 55% of this energy comes from the lower mantle. Here we report the first experimental evidence that aluminous CaSiO 3 perovskite is the major, or even the only, host of uranium in the Earth lower mantle, since such a phase is able to incorporate up to 35 wt% UO 2 (or 4 at% of U). The aluminous Ca-perovskite could be the main U-bearing constituent of a dense and radiogenic reservoir proposed in a recent model and located in the bottom half of the lower mantle.

Reinvestigation of the MgSiO3 perovskite structure at high pressure

Abstract High-pressure single-crystal X-ray diffraction experiments of MgSiO3 perovskite have been carried out up to 15 GPa in a diamond-anvil cell using synchrotron radiation. Precise crystal structural parameters, including the anisotropic displacement parameters of every atom in MgSiO3, are determined under high pressure. In the pressure range up to 15 GPa, the most important responses of the structure are the compressions of SiO6 and MgO8 polyhedra and an increase in tilting of SiO6 octahedra represented by the decrease in angles between octahedra (both Si-O2-Si angle in the a-b plane and Si-O1-Si angle in the b-c plane decrease). The degree of the change in both angles in the a-b and b-c planes is the same. The amplitude of mean square displacement for the Mg atom has the largest value in the structures and its thermal vibration is significantly anisotropic at ambient pressure. Under high pressure, all atoms in the structure have obvious anisotropy of thermal vibration and the largest amplitudes of thermal vibration for Mg, Si, and O2 atoms are directed toward vacant space in the structure. Anisotropy of the structure increases with pressure.

Water in Transition Zone and Lower Mantle Minerals

Wadsleyite and ringwoodite have the potential to contain up to 3.2 weight percent of water as structurally bound hydroxyl (OH). However, at transition zone pressures the solubility of water in these phases decreases with increasing temperature to ∼2 wt% H 2 O at 1460°C for wadsleyite and to ∼1 wt% H 2 O at 1600°C for ringwoodite, the approximate temperatures of the 410 and 660 km discontinuities, respectively. Majorite garnet can only contain up to ∼0.1 wt% water or less, and is therefore not likely to control water storage in the transition zone. In the lower mantle, water solubility in magnesium silicate perovskite and in ferropericlase are much lower. OH solubility in ferropericlase increases with pressure and water fugacity, with up to 20 ppm wt H 2 O in (Mg 0.93 Fe 0.07 )O at 25 GPa and 1200°C, and new results show that it does not seem to depend on iron content. The solubility of water in mantle perovskite is still controversial, and depends on interpretation of the infrared data. Maximum water contents reported are ∼0.2 wt% H 2 O, but could be overestimated. Contrary to previous expectations, the solubility of water in silicate perovskite probably does not increase with Al content; Al-rich perovskites synthesized in the MORB system appear to contain less water than peridotitic perovskite. Similar to transition zone minerals, water solubility in perovskite and (Mg,Fe)O decreases with increasing temperature. A partition coefficient of 710(±180) between coexisting ringwoodite and perovskite+ferropericlase in the Al-free system implies that ringwoodite will be stabilized to greater depths in the presence of water.

XANES study of the oxidation state of Cr in lower mantle phases: Periclase and magnesium silicate perovskite

Abstract Cr K-edge X-ray absorption near-edge structure (XANES) spectra were recorded on Cr:MgO periclase and Cr:(Mg,Fe)O ferropericlase synthesized at different pressures (4 and 12 GPa) and temperatures (1200 to 1400 °C) at reducing oxygen fugacity conditions (~iron-wüstite buffer IW to IW - 2), and on Cr:MgSiO3 perovskite with 0.5 wt% Cr2O3. 57Fe Mössbauer spectra were collected on the Fe-containing samples. The aim of the study was to determine the Cr oxidation state in phases found in the Earth.s lower mantle, and to examine the possible relationship with the Fe oxidation state in the same materials. To calculate the amount of Cr2+, the intensity of the shoulder at the low-energy side of the edge crest was quantified using the area of the corresponding peak in the derivative XANES spectra (Berry and O.Neill 2004). In Cr:(Mg,Fe)O the relative Cr2+ content reached at most 12.5% but results from Mössbauer spectroscopy combined with chemical composition data suggest that some Cr2+ oxidized during cooling through the reaction Cr2+ + Fe3+ → Cr3+ + Fe2+. In iron-free Cr:MgO, the Cr2+ content is much higher and reaches ~40%. In Cr:MgSiO3 perovskite with 0.006 Cr pfu (similar to estimated lower mantle abundance), chromium is mainly divalent.

Determination of the concentration of water dissolved in glasses and minerals using nuclear microprobe

In Earth Sciences, the global water cycle is of fundamental importance. For this reason, the H2O content of volcanic glass and mantle minerals must be analysed: usually by micro-infrared spectroscopy (FTIR) or secondary ion mass spectrometry (SIMS). However, both of these methods require calibration using standards of known water content. To avoid matrix effects, the standards and unknowns must be otherwise identical in composition. In this study we have determined the water content of geological samples, in the range 10 ppm–5wt.%H2O, using an absolute analytical technique: a combination of elastic recoil detection analysis (ERDA) and Rutherford backscattering spectrometry (RBS). We compared the results obtained by this method to data obtained by FTIR on the same samples. We discuss the limitations of the method and use the results to calibrate IR extinction coefficients for FTIR spectroscopy.

Study of partial melting at high-pressure using in situ X-ray diffraction

The high-pressure melting behavior of different iron alloys was investigated using the classical synchrotron-based in situ X-ray diffraction techniques. As they offer specific advantages and disadvantages, both energy-dispersive (EDX) and angle-dispersive (ADX) X-ray diffraction methods were performed at the BL04B1 beamline of SPring8 (Japan) and at the ID27-30 beamline of the ESRF (France), respectively. High-pressure vessels and pressure ranges investigated include the Paris–Edinburgh press from 2 to 17 GPa, the SPEED-1500 multi-anvil press from 10 to 27 GPa, and the laser-heated diamond anvil cell from 15 to 60 GPa. The onset of melting (at the solidus or eutectic temperature) can be easily detected using EDX because the grains start to rotate relative to the X-ray beam, which provokes rapid and drastic changes with time of the peak growth rate. Then, the degree of melting can be determined, using both EDX and ADX, from the intensity of diffuse X-ray scattering characteristic of the liquid phase. This diffuse contribution can be easily differentiated from the Compton diffusion of the pressure medium because they have different shapes in the diffraction patterns. Information about the composition and/or about the structure of the liquid phase can then be extracted from the shape of the diffuse X-ray scattering.

Dehydration of chlorite explains anomalously high electrical conductivity in the mantle wedges.

Development of interconnected magnetite during chlorite dehydration explains anomalous high conductivity at shallow mantle wedges. Mantle wedge regions in subduction zone settings show anomalously high electrical conductivity (~1 S/m) that has often been attributed to the presence of aqueous fluids released by slab dehydration. Laboratory-based measurements of the electrical conductivity of hydrous phases and aqueous fluids are significantly lower and cannot readily explain the geophysically observed anomalously high electrical conductivity. The released aqueous fluid also rehydrates the mantle wedge and stabilizes a suite of hydrous phases, including serpentine and chlorite. In this present study, we have measured the electrical conductivity of a natural chlorite at pressures and temperatures relevant for the subduction zone setting. In our experiment, we observe two distinct conductivity enhancements when chlorite is heated to temperatures beyond its thermodynamic stability field. The initial increase in electrical conductivity to ~3 × 10−3 S/m can be attributed to chlorite dehydration and the release of aqueous fluids. This is followed by a unique, subsequent enhancement of electrical conductivity of up to 7 × 10−1 S/m. This is related to the growth of an interconnected network of a highly conductive and chemically impure magnetite mineral phase. Thus, the dehydration of chlorite and associated processes are likely to be crucial in explaining the anomalously high electrical conductivity observed in mantle wedges. Chlorite dehydration in the mantle wedge provides an additional source of aqueous fluid above the slab and could also be responsible for the fixed depth (120 ± 40 km) of melting at the top of the subducting slab beneath the subduction-related volcanic arc front.