Huaiwei Ni

Transport properties of silicate melts

A quantitative description of the transport properties, diffusivity, viscosity, electrical, and thermal conductivity, of silicate melts is essential for understanding melting-related petrologic and geodynamic processes. We here provide a systematic overview on the current knowledge of these properties from experiments and molecular dynamics simulations, their dependence on pressure, temperature, and composition, atomistic processes underlying them, and physical models to describe their variations. We further establish phenomenological and physical links between diffusivity, viscosity, and electrical conductivity that are based on structural rearrangement in the melt. Neutral molecules and network-modifying cations with low electric field strength display intrinsic diffusivity, which is controlled by the intrinsic properties (size and valence) of the species. By contrast, oxygen and network formers with high field strength show extrinsic diffusivity, which is more sensitive to extrinsic parameters including temperature (T), pressure (P), and melt composition (X). Similar T-P-X dependence of diffusivity and electrical conductivity and their quantitative relation reveal the role of intrinsically diffusing species in electrical transport, while viscosity is tied to the extrinsically diffusing species in a similar way. However, the differences in the structural role and mobility of various atomic species diminish with increasing temperature and/or pressure: all transport processes are increasingly coupled, eventually converging to a uniform rate and mechanism. Accurate comprehension of interatomic interactions and melt structure is vital to fully accounting for the compositional dependence of transport properties, and simple polymerization parameters such as nonbridging oxygen per tetrahedrally coordinated cation are inadequate.

In-situ Raman spectroscopic study of sulfur speciation in oxidized magmatic-hydrothermal fluids

Abstract The sulfur compounds released by volcanic eruptions, generally believed to be in the form of SO2 and H2S, may cause global cooling of the atmosphere. However, several recent field and experimental studies suggested that under moderately oxidized conditions hexavalent sulfur species may coexist with SO2 in magmatic fluids and may later be directly emitted at volcanic vents, which contradicts some thermodynamic predictions. We have investigated sulfur speciation in magmatic-hydrothermal fluids by loading different amounts of dilute sulfuric acid into a hydrothermal diamond-anvil cell and performing in situ Raman spectroscopy at temperatures up to 700 °C. Upon heating SO2-4 disappeared beyond 100 °C, and SO2 formed at >250 °C probably due to reduction by the rhenium or iridium gasket. With high-fluid densities (such as >0.9 g/mL), the initial acid and air bubble homogenized into the liquid phase and most sulfur was present in the form of either HSO-4 or H2SO4 (the rest being SO2) within investigated T-P conditions (with pressures up to 10 kb). With low-fluid densities (such as <0.2 g/mL), the system homogenized into the vapor phase and molecular H2SO4 appeared to dominate (with pressures less than 1 kb). These observations strongly suggest that hexavalent sulfur is stabilized by hydration in magmatic fluids.

Electrical conductivity measurements of aqueous fluids under pressure with a hydrothermal diamond anvil cell

Electrical conductivity data of aqueous fluids under pressure can be used to derive the dissociation constants of electrolytes, to assess the effect of ionic dissociation on mineral solubility, and to interpret magnetotelluric data of earths interior where a free fluid phase is present. Due to limitation on the tensile strength of the alloy material of hydrothermal autoclaves, previous measurements of fluid conductivity were mostly restricted to less than 0.4 GPa and 800 °C. By adapting a Bassett-type hydrothermal diamond anvil cell, we have developed a new method for acquiring electrical conductivity of aqueous fluids under pressure. Our preliminary results for KCl solutions using the new method are consistent with literature data acquired with the conventional method, but the new method has great potential for working in a much broader pressure range.

Electrical conductivity of hydrous andesitic melts pertinent to subduction zones

Andesitic magmatism and rocks are widespread at convergent plate boundaries. Electrically conductive bodies beneath subduction zone arc volcanoes, such as the Uturuncu Volcano, Bolivia, may correspond to active reservoirs of H2O-bearing andesitic magma. Laboratory measurements of electrical conductivity of hydrous andesitic melts are required to constrain the physicochemical conditions of these magma reservoirs in combination with magnetotelluric data. This experimental study investigates electrical conductivity of andesitic melts with 0.01–5.9 wt % of H2O at 1164–1573 K and 0.5–1.0 GPa in a piston cylinder apparatus using sweeping-frequency impedance spectroscopy. Electrical conductivity of andesitic melt increases with increasing temperature and H2O concentration but decreases with pressure. Across the investigated range of H2O concentration, electrical conductivity varies by 1.2–2.4 log units, indicating stronger influence of H2O for andesitic melt than for rhyolitic and dacitic melts. Using the Nernst-Einstein equation, the principal charge carrier is inferred to be Na in anhydrous melt but divalent cations in hydrous andesitic melts. The experimental data are regressed into a general electrical conductivity model for andesitic melt accounting for the pressure-temperature-H2O dependences altogether. Modeling results show that the conductive layer at >20 km depths beneath the surface of the Uturuncu Volcano could be interpreted by the presence of less than 20 vol % of H2O-rich andesitic melt (with 6–9 wt % H2O).

Elasticity of single‐crystal superhydrous phase B at simultaneous high pressure‐temperature conditions

We investigated the combined effect of pressure and temperature on the elasticity of single-crystal superhydrous phase B (Shy-B) using Brillouin scattering and X-ray diffraction up to 12 GPa and 700 K. Using the obtained elasticity, we modeled the anisotropy of Shy-B along slab geotherms, showing that Shy-B has a low anisotropy and cannot be the major cause of the observed anisotropy in the region. Modeled velocities of Shy-B show that Shy-B will be shown as positive velocity anomalies at the bottom transition zone. Once Shy-B is transported to the topmost lower mantle, it will exhibit a seismic signature of lower velocities than topmost lower mantle. We speculate that an accumulation of hydrous phases, such as Shy-B, at the topmost lower mantle with a weight percentage of ~17–26% in the peridotite layer as subduction progresses could help explain the observed 2–3% low shear velocity anomalies in the region.

Compositional dependence of alkali diffusivity in silicate melts: Mixed alkali effect and pseudo-alkali effect

Abstract Compositional dependence of alkali diffusivities in silicate melts is modeled by explicitly considering mixed alkali and “pseudo-alkali” effects. The well-known mixed alkali effect describes that the presence of light alkalis (Li, Na) retards diffusion of heavy alkalis (K, Rb, Cs) and vice versa, which can be attributed to stronger interaction between dissimilar alkalis and reduction of favorable sites for the alkali in question. Due to the same reasons, Ca, which has similar ionic radius as Li and Na but carries more charge and greater field strength, also impedes the migration of light alkalis. In this regard Ca behaves as a “pseudo-alkali,” and I refer to the influence caused by Ca on Li and Na diffusion as the “pseudo-alkali effect,” which belongs to the category of mixed cation effects in glass literature. Pseudo-alkali effect is expected to exist also between other size-matching divalent cations and alkali cations, such as Ba blocking K diffusion. Mixed alkali and pseudo-alkali effects are manifested mainly by an increase in the activation energy for diffusion. Sodium diffusivities for melt compositions ranging between albite, orthoclase, and anorthite reveal that the amplitude of the pseudo-alkali effect is much larger than that of the mixed alkali effect. Furthermore, the activation energy increases more rapidly as more Ca substitutes for Na, but the mixed alkali effect approaches saturation as K substitutes for sodium. A general model based on mixed alkali effect and pseudo-alkali effect, together with a special treatment relating Cs diffusion to melt polymerization, successfully characterizes existing experimental data on alkali diffusion in silicate melts. Enthalpies of mixing between end-member melts (e.g., Ab-Or melts and Ab-An melts) inferred from the model agree well with previous experimental investigations.

New High‐Pressure Phase of CaCO3 at the Topmost Lower Mantle: Implication for the Deep‐Mantle Carbon Transportation

In this study, we have investigated the stability of CaCO3 at high pressures and temperatures using synchrotron X-ray diffraction in laser-heated diamond anvil cells. Our experimental results have shown that CaCO3 in the aragonite structure transforms into CaCO3-VII (P21/c) at 27 GPa and 1,500 K with a negative Clapeyron slope of 4.3(9) MPa/K. CaCO3-VII is stable between 23 and 38 GPa at 2,300 K and transforms into post-aragonite at 42 GPa and 1,400 K. Furthermore, it reacts with stishovite, an abundant form of SiO2 in subducted oceanic crust, forming CaSiO3-perovskite. The occurrence of CaSiO3-perovskite via the reaction of CaCO3-VII and stishovite provides an explanation for the observation of the high concentrations of CaSiO3-perovskite and some amount of CaCO3 in deep-mantle inclusions. CaCO3-VII is thus an important carbon-bearing phase at the topmost lower mantle and may provide necessary carbon to produce deep-mantle diamonds.

The relationship between apparent equilibrium temperature and closure temperature with application to oxygen isotope geospeedometry

Oxygen isotope fractionation between coexisting minerals in slowly cooled rocks conveys information about their cooling history. By using the fast grain boundary (FGB) model to simulate closed-system diffusive exchange of oxygen isotopes between coexisting minerals, I show that the apparent equilibrium temperatures (Tae) by the mineral pair with the largest isotopic fractionation (PLIF) always lies between the closure temperatures (Tc) of those two minerals. Therefore, when the rate of oxygen diffusion and hence Tc for the PLIF chance to be comparable (such as in the case of quartz and magnetite), Tae will serve as a good approximation of Tc regardless of variation in mineral proportions. The specialty of the PLIF in constraining Tae within their Tc range can be generalized to other stable isotope systems and element partitioning. By approximating Tc with Tae and inverting Dodson’s equation, the cooling rate of plutonic or metamorphic rocks can be inferred.

Oxygen isotope thermometry, speedometry, and hygrometry: Apparent equilibrium temperature versus closure temperature

Rather than indicating formation/peak temperature, oxygen isotope fractionations preserved in mineral assemblages of slowly cooled plutonic and metamorphic rocks yield apparent equilibrium temperatures (Tae). The isotopic fractionations and Tae values deliver information about cooling history, as the extent of diffusive exchange of oxygen isotopes during cooling is controlled by the cooling time scale or cooling rate. Despite that several models, such as the Fast Grain Boundary (FGB) model, have been developed to simulate oxygen isotope exchange between coexisting minerals during cooling, extraction of cooling rate remains far from straightforward. On the other hand, there is a well-defined quantitative relationship between the Dodson closure temperature (Tc) and the cooling rate, but Tc cannot be directly measured. Based on simulation results of existing models for a variety of rock systems, including open systems (with an infinite fluid reservoir), closed systems (with negligible fluid participation) and semi-open systems (with moderate fluid participation), this study demonstrates that Tae of the mineral pair with the largest equilibrium isotope fractionation (PLEIF) is always bounded by their Tc values, regardless of how mineral proportions vary or how significant a role fluid has played in isotopic exchange. If the two Tc values happen to be similar, Tae will serve as a good approximation of both Tc, provided that the equilibrium fractionation factor has been precisely determined as a function of temperature. One such pair is quartz-magnetite. By contrast, a mineral pair with similar Tc but relatively small fractionation is susceptible to the disturbance from other minerals, hence does not always have Tae confined within their Tc range. The relationship of Tae-Tc correspondence for PLEIF with similar Tc can be used to constrain either cooling rate (i.e., as a speedometry method) or oxygen isotope diffusivity if one of them has been independently determined. In the latter case, the inferred oxygen diffusivity may be an index of water fugacity (i.e., as a hygrometry method) when compared with experimental diffusivity values measured under different fluid conditions.