Mainak Mookherjee

The effect of water on the electrical conductivity of olivine

It is well known that water (as a source of hydrogen) affects the physical and chemical properties of minerals—for example, plastic deformation and melting temperature—and accordingly plays an important role in the dynamics and geochemical evolution of the Earth. Estimating the water content of the Earth’s mantle by direct sampling provides only a limited data set from shallow regions (<200 km depth). Geophysical observations such as electrical conductivity are considered to be sensitive to water content, but there has been no experimental study to determine the effect of water on the electrical conductivity of olivine, the most abundant mineral in the Earth’s mantle. Here we report a laboratory study of the dependence of the electrical conductivity of olivine aggregates on water content at high temperature and pressure. The electrical conductivity of synthetic polycrystalline olivine was determined from a.c. impedance measurements at a pressure of 4 GPa for a temperature range of 873–1,273 K for water contents of 0.01–0.08 wt%. The results show that the electrical conductivity is strongly dependent on water content but depends only modestly on temperature. The water content dependence of conductivity is best explained by a model in which electrical conduction is due to the motion of free protons. A comparison of the laboratory data with geophysical observations suggests that the typical oceanic asthenosphere contains ∼10-2 wt% water, whereas the water content in the continental upper mantle is less than ∼10-3 wt%.

Hydrous silicate melt at high pressure

The structure and physical properties of hydrous silicate melts and the solubility of water in melts over most of the pressure regime of Earth’s mantle (up to 136 GPa) remain unknown. At low pressure (up to a few gigapascals) the solubility of water increases rapidly with increasing pressure, and water has a large influence on the solidus temperature, density, viscosity and electrical conductivity. Here we report the results of first-principles molecular dynamics simulations of hydrous MgSiO3 melt. These show that pressure has a profound influence on speciation of the water component, which changes from being dominated by hydroxyls and water molecules at low pressure to extended structures at high pressure. We link this change in structure to our finding that the water–silicate system becomes increasingly ideal at high pressure: we find complete miscibility of water and silicate melt throughout almost the entire mantle pressure regime. On the basis of our results, we argue that a buoyantly stable melt at the base of the upper mantle would contain approximately 3 wt% water and have an electrical conductivity of 18 S m-1, and should therefore be detectable by means of electromagnetic sounding.

High-pressure proton disorder in brucite

Abstract In this paper we explore the structure and physical properties of brucite over a wide range of pressures with density functional theory using the variable cell-shape plane wave pseudopotential method in the local density (LDA) and generalized gradient (GGA) approximations. We probe the energetics underlying the structure and dynamics of the proton sub-lattice by performing a series of constrained and unconstrained static calculations based on an energetically stable √3×√3×1 super-cell wherein proton locations are related to the 6i Wyckoff sites as opposed to the ideal 2d site. The displacement of the hydrogen atom from the threefold axis increases with increasing pressure. This means that even in the absence of thermal energy, the protons are frustrated and would be expected to exhibit long-range disorder akin to a spin glass. To shed light on the dynamic nature of the proton hopping between the 6i-like sites, we determined the activation energy barrier for such jumps. We found that the energy barrier increases with compression, possibly indicating a transition from dynamic proton disorder at lower pressures to static disorder at higher pressure. We have also investigated the possibility of proton jumps across the interlayer, by determining the potential energy well along the O···O vector. We infer that proton jumps across the interlayer are either severely limited or highly cooperative since we did not find any evidence for a double well along the O···O vector. The absence of a double well along the O···O vector, the evolution of O-H···O distances with compression, and the gradual transition to a symmetric O-H···O configuration, all argue for weak hydrogen bonding in brucite.

Elasticity and anisotropy of Fe3C at high pressures

Abstract Using static electronic structure calculations, we determine the equation of state, the full elastic constant tensor and the sound wave velocities of cementite (Fe3C) at pressures up to 410 GPa. Fe3C is ferromagnetic (fm) at ambient pressures. Upon compression, the magnetic moment of the Fe atoms are gradually lost and, at around -62 GPa, Fe3C becomes non-magnetic (nm). We find that the pressurevolume results for the Fe3C (fm) phase are well represented by a Vinet equation of state with K0fm = 183 GPa, K0 = 5.9, and V0fm = 151.6 Å3 and that of the Fe3C (nm) phase are well represented by a Vinet equation of state with K0nm = 297 GPa, K0′ = 4.9, and V0nm = 143.2 Å3. A third-order Birch-Murnaghan equation of state formulation for the Fe3C (nm) phase yields similar parameters with K0nm = 304 GPa, K0 = 4.5, and V0nm = 143.3 Å3. At pressures relevant to the Earth’s inner core, the full elastic constant tensor of Fe3C (nm) reveals significant P-wave anisotropy (-10%). A crystal preferred orientation with the [110] directions of Fe3C aligned along the pole axis would be required to explain the inner core anisotropy. Comparing, pure hcp Fe and iron carbides with varying stoichiometry, we find that the shear wave velocity decreases linearly with the increasing C content.

Thermal response of structure and hydroxyl ion of phengite-2M1an in situ, neutron diffraction and FTIR study

The thermal dependence of the structure of natural phengite-2M 1 with chemical formula (K 0.95 Na 0.05 )(Al 0.76 Fe 0.14 Mg 0.10 2 (Si 3.25 Al 0.75 )O 10 (OH 1.96 F 0.04 ) has been studied by in situ neutron diffraction. The short-range correlated behaviour of the hydroxyl group was probed by FTIR spectroscopy while the long-range correlated hydroxyl structure was studied by neutron diffraction. Changes in long-range ordering of Si and Al on the tetrahedral sites were not observed from neutron diffraction. Structure refinement of the neutron diffraction data by the Rietveld method suggested that the apparent average hydroxyl bond length decreases on heating. The infrared data show a decrease in the stretching frequency of hydroxyl group, however. Possible explanations for these results are explored. It seems most likely that the apparent shortening of the hydroxyl bond length may be an artefact due to an increase in its vibrational amplitude. The anisotropic vibration of the hydroxyl bond as revealed by the anisotropic displacement parameters of H, increases so much that the average length (shown by the neutron refinement) appears to decrease at high temperatures while the local length of the bond, as indicated by FTIR results, increases.

A high-temperature Fourier transform infrared study of the interlayer and Si–O-stretching region in phengite-2M1

Abstract A natural phengite-2M1 of composition (K0.95Na0.05)(Al0.76Fe0.14Mg0.10)2 (Si3.25Al0.75)O10(OH1.96F0.04) [a = 5.2173(1) Å , b = 9.0493(2) Å , c = 19.989 (1) Å and β = 95.734(4)°] was studied using in situ high-temperature FTIR. Correlations to structural changes were made using previously-reported neutron diffraction data from the same sample. Correlations have been made between the microscopic atomic displacements (arising from thermal effects) and analogous macroscopic properties, such as bond strain and ditrigonal distortions. Spectra were collected in the far-infrared region to study the behaviour of the interlayer (K+) cation and also in the mid-infrared region to distinguish the Si-O stretching modes. We found anisotropic thermal expansion of the interlayer site. The K-O bond length is divided into K-Oouter and K-Oinner, and the K-Oinner bond length is correlated with the far-infrared spectra. The thermal dependence of the correlation between K-O bond length and corresponding far-infrared stretching frequency is different from the effect of the chemical composition. We also found that the K-O bond strain could be successfully resolved into the sum of inner strain and lattice strain. The Si-O stretching mode, obtained from the mid-infrared measurements, showed only weak changes. However, the neutron refinement data showed different thermal behaviour for distinct crystallographic T-sites.

Unusually large shear wave anisotropy for chlorite in subduction zone settings

Using first principle simulations we calculated the elasticity of chlorite. At a density ρ~ 2.60 g cm−3, the elastic constant tensor reveals significant elastic anisotropy: VP ~27%, VS1 ~56%, and VS2 ~43%. The shear anisotropy is exceptionally large for chlorite and enhances upon compression. Upon compression, the shear elastic constant component C44 and C55 decreases, whereas C66 shear component stiffens. The softening in C44 and C55 is reflected in shear modulus, G, and the shear wave velocity, VS. Our results on elastic anisotropy at conditions relevant to the mantle wedge indicates that a 10-20 km layer of hydrated peridotite with serpentine and chlorite could account for the observed shear polarization anisotropy and associated large delay times of 1-2 s observed in some subduction zone settings. In addition, chlorite could also explain the low VP/VS ratios that have been observed in recent high-resolution seismological studies.

Orientational order-disorder of ND4+/NH4+ in synthetic ND4/NH4-phlogopite: a low-temperature infrared study

The dynamic behaviour of the ammonium ion in synthetic ND 4/NH4-phlogopite has been studied as a function of temperature by infrared spectroscopy, from room temperature to 20 K. Ammonium occupies the interlayer site in phlogopite. Orientational order-disorder of NH 4 + is a well-established phenomenon in ammonium chloride (NH 4Cl) and other ammonium halides and ammonium salts. Although ammonium chloride has higher symmetry than ND 4/NH4-phlogopite, we anticipate similar dynamic behaviour of ammonium in the interlayer site of the phlogopite structure. Infrared spectra show noticeable changes on cooling. Using the autocorrelation analysis method we find distinct changes in the line width of the Gaussian profile fitted to the central autocorrelation peak. This is attributed to a transition of the ND 4 + group, which is orientationally disordered at higher temperaturesandbecomesrelativelyorderedattemperaturesbelowthetransitiontemperatureofaround130K.Theorderparameter for the transition appears to follow classical second-order behaviour as a function of temperature.

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.

Crystal structure, equation of state, and elasticity of phase H (MgSiO4H2) at Earth's lower mantle pressures.

Dense hydrous magnesium silicate (DHMS) phases play a crucial role in transporting water in to the Earth’s interior. A newly discovered DHMS, phase H (MgSiO4H2), is stable at Earth’s lower mantle, i.e., at pressures greater than 30 GPa. Here we report the crystal structure and elasticity of phase H and its evolution upon compression. Using first principles simulations, we have explored the relative energetics of the candidate crystal structures with ordered and disordered configurations of magnesium and silicon atoms in the octahedral sites. At conditions relevant to Earth’s lower mantle, it is likely that phase H is able to incorporate a significant amount of aluminum, which may enhance the thermodynamic stability of phase H. The sound wave velocities of phase H are ~2–4% smaller than those of isostructural δ-AlOOH. The shear wave impedance contrast due to the transformation of phase D to a mixture of phase H and stishovite at pressures relevant to the upper part of the lower mantle could partly explain the geophysical observations. The calculated elastic wave velocities and anisotropies indicate that phase H can be a source of significant seismic anisotropy in the lower mantle.