Water speciation in oxide glasses and melts

Behrens, Harald

doi:10.1016/j.chemgeo.2020.119850

Cited by 18 publications

(5 citation statements)

References 74 publications

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“…The rest of oxygen form mostly hydroxyls and some molecular water at low pressure (Fig. 6 ), generally consistent with the experimental observations 48 , 49 . In contrast, every oxygen is bonded with two hydrogen atoms in water corresponding to Z OH = 2 (and Z HO = 1) and nearly 100% H 2 O molecules at zero pressure (Fig.…”

Section: Results and Analysissupporting

confidence: 90%

Behavior and properties of water in silicate melts under deep mantle conditions

Karki

Ghosh

Karato

2021

Sci Rep

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Water (H2O) as one of the most abundant fluids present in Earth plays crucial role in the generation and transport of magmas in the interior. Though hydrous silicate melts have been studied extensively, the experimental data are confined to relatively low pressures and the computational results are still rare. Moreover, these studies imply large differences in the way water influences the physical properties of silicate magmas, such as density and electrical conductivity. Here, we investigate the equation of state, speciation, and transport properties of water dissolved in Mg1−xFexSiO3 and Mg2(1−x)Fe2xSiO4 melts (for x = 0 and 0.25) as well as in its bulk (pure) fluid state over the entire mantle pressure regime at 2000–4000 K using first-principles molecular dynamics. The simulation results allow us to constrain the partial molar volume of the water component in melts along with the molar volume of pure water. The predicted volume of silicate melt + water solution is negative at low pressures and becomes almost zero above 15 GPa. Consequently, the hydrous component tends to lower the melt density to similar extent over much of the mantle pressure regime irrespective of composition. Our results also show that hydrogen diffuses fast in silicate melts and enhances the melt electrical conductivity in a way that differs from electrical conduction in the bulk water. The speciation of the water component varies considerably from the bulk water structure as well. Water is dissolved in melts mostly as hydroxyls at low pressure and as –O–H–O–, –O–H–O–H– and other extended species with increasing pressure. On the other hand, the pure water behaves as a molecular fluid below 15 GPa, gradually becoming a dissociated fluid with further compression. On the basis of modeled density and conductivity results, we suggest that partial melts containing a few percent of water may be gravitationally trapped both above and below the upper mantle-transition region. Moreover, such hydrous melts can give rise to detectable electrical conductance by means of electromagnetic sounding observations.

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Section: Results and Analysissupporting

confidence: 90%

Behavior and properties of water in silicate melts under deep mantle conditions

Karki

Ghosh

Karato

2021

Sci Rep

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show abstract

“…The rest of oxygen form mostly hydroxyls and some molecular water at low pressure (Fig. 6), generally consistent with the experimental observations 48,49 . In contrast, every oxygen is bonded with two hydrogen atoms in water corresponding to ZOH = 2 (and ZHO = 1) and nearly 100% H2O molecules at zero pressure (Fig.…”

Section: Speciation Of Hydrous Component and Structure Of Watersupporting

confidence: 90%

Behavior and properties of water in silicate melts under deep mantle conditions

Karki

Ghosh

Karato

2021

Preprint

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Water (H2O) as one of the most abundant fluids present in Earth plays crucial role in the generation and transport of magmas in the interior. Though hydrous silicate melts have been studied extensively, the experimental data are confined to relatively low pressures and the computational results are still rare. Moreover, these studies imply large differences in the way water influences the physical properties of silicate magmas, such as density and electrical conductivity. Here, we invesigate the equation of state, speciation, and transport properties of water dissolved in Mg1 − xFexSiO3 and Mg2(1−x)Fe2xSiO4 melts (for x = 0 and 0.25) as well as in its bulk (pure) fluid state over the entire mantle pressure regime at 2000 to 4000 K using first-principles molecular dynamics. The simulation results allow us to constrain the partial molar volume of the water component in melts along with the molar volume of pure water. The predicted volume of silicate melt + water solution is negative at low pressures but becomes zero above 15 GPa implying ideal mixing at higher pressures. Consequently, the hydrous component tends to lower the melt density to similar extent over much of the mantle pressure regime irrespective of composition. Our results also show that hydrogen diffuses fast in silicate melts and enhances the melt electrical conductivity in a way that differs from electrical conduction in the bulk water. The speciation of the water component varies considerably from the bulk water structure as well. Water is dissolved in melts mostly as hydroxyls at low pressure and as -O-H-O-, -O-H-O-H- and other extended species with increasing pressure. On the other hand, the pure water behaves as a molecular fluid below 15 GPa, gradually becoming a dissociated fluid with further compression. On the basis of modeled density and conductivity results, we suggest that partial melts containing a few percent of water may be gravitationally trapped both above and below the upper mantle-transition region. Moreover, such hydrous melts can give rise to detectable electrical conductance by means of electromagnetic sounding observations.

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“…Bridging O‐H‐O configurations are approached at high pressure, but symmetrization is never achieved, and hydrogen seems to preferentially remain bonded to one oxygen atom. Experiments have shown that the proportion of OH/(OH + H 2 O) in silicate glasses decreases with the total water content (Stolper, 1982) and strongly increases with increasing temperature (Behrens, 2020; Shen & Keppler, 1995; Zhang, 1999). For example, the proportion of OH/(OH + H 2 O) in haplogranitic and sodium alumino‐silicate glasses increases from about 25%–35% at ∼300 K to 65%–75% at ∼1200 K (Nowak & Behrens, 2001; Shen & Keppler, 1995).…”

Section: Resultsmentioning

confidence: 99%

Buoyancy and Structure of Volatile‐Rich Silicate Melts

Solomatova

Caracas

2021

JGR Solid Earth

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The early Earth was marked by at least one global magma ocean. Melt buoyancy played a major role for its evolution. Here we model the composition of the magma ocean using a six-component pyrolite melt, to which we add volatiles in the form of carbon as molecular CO or CO 2 and hydrogen as molecular H 2 O or through substitution for magnesium. We compute the density relations from firstprinciples molecular dynamics simulations. We find that the addition of volatiles renders all the melts more buoyant compared to the reference volatile-free pyrolite. The effect is pressure dependent, largest at the surface, decreasing to about 20 GPa, and remaining roughly constant to 135 GPa. The increased buoyancy would have enhanced convection and turbulence, and thus promoted the chemical exchanges of the magma ocean with the early atmosphere. We determine the partial molar volume of both H 2 O and CO 2 throughout the magma ocean conditions. We find a pronounced dependence with temperature at low pressures, whereas at megabar pressures the partial molar volumes are independent of temperature. At all pressures, the polymerization of the silicate melt is strongly affected by the amount of oxygen added to the system while being only weakly affected by the specific type of volatile added.

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Water speciation in oxide glasses and melts

Cited by 18 publications

References 74 publications

Behavior and properties of water in silicate melts under deep mantle conditions

Behavior and properties of water in silicate melts under deep mantle conditions

Behavior and properties of water in silicate melts under deep mantle conditions

Buoyancy and Structure of Volatile‐Rich Silicate Melts

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