Water In Silicate Melts Research Articles

Thermoelasticity of Water in Silicate Melts: Implications for Melt Buoyancy in Earth's Mantle

AbstractA comprehensive analysis of experimental data and theoretical simulations on the partial molar volume of water in silicate melt indicates that finite strain theory successfully describes the compression of the H2O component dissolved in silicate melt at high pressures and temperatures. However, because of the high compressibility of the water component, a fourth order equation of state fit is required to accurately simulate experimental results on water's volume in silicate melts at a deep upper mantle, transition zone, and lower mantle pressures. Data from previous shock compression experiments on hydrous minerals in which melting occurs along the Hugoniot are used to provide an experimental constraint on the partial molar volume of water in silicate melt at deep mantle temperatures and pressures. The equation of state of the water component indicates that, depending on elastic averaging technique, the amount of water that could be present in neutrally or negatively buoyant mafic/ultramafic melts above the 410 km seismic discontinuity is upper‐bounded at 5.6 wt%: smaller than previously inferred, and consistent with melt being confined to a narrow depth range above the 410 km discontinuity. If melt is predominantly distributed along grain boundaries in low aspect ratio films, extents of melting as low as 2% could produce observed seismic velocity reductions. The ability of the lowermost mantle to contain negatively buoyant hydrous liquids hinges on the trade‐off between iron content and hydration: at these depths, substantially higher degrees of hydration could be present within partial melts.

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

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.

An experimentally-validated numerical model of diffusion and speciation of water in rhyolitic silicate melt

The diffusion of water through silicate melts is a key process in volcanic systems. Diffusion controls the growth of the bubbles that drive volcanic eruptions and determines the evolution of the spatial distribution of dissolved water during and after magma mingling, crystal growth, fracturing and fragmentation, and welding of pyroclasts. Accurate models for water diffusion are therefore essential for forward modelling of eruptive behaviour, and for inverse modelling to reconstruct eruptive and post-eruptive history from the spatial distribution of water in eruptive products. Existing models do not include the kinetics of the homogeneous species reaction that interconverts molecular (H2Om) and hydroxyl (OH) water; reaction kinetics are important because final species distribution depends on cooling history. Here we develop a flexible 1D numerical model for diffusion and speciation of water in silicate melts. We validate the model against FTIR transects of the spatial distribution of molecular, hydroxyl, and total water across diffusion-couple experiments of haplogranite composition, run at 800–1200 °C and 5 kbar. We adopt a stepwise approach to analysing and modelling the data. First, we use the analytical Sauer-Freise method to determine the effective diffusivity of total water DH2Ot as a function of dissolved water concentration CH2Ot and temperature T for each experiment and find that the dependence of DH2Ot on CH2Ot is linear for CH2Ot≲1.8 wt.% and exponential for CH2Ot≳1.8 wt.%. Second, we develop a 1D numerical forward model, using the method of lines, to determine a piece-wise function for DH2OtCH2Ot,T that is globally-minimized against the entire experimental dataset. Third, we extend this numerical model to account for speciation of water and determine globally-minimized functions for diffusivity of molecular water DH2OmCH2Ot,T and the equilibrium constant K for the speciation reaction. Our approach includes three key novelties: (1) functions for diffusivities of H2Ot and H2Om, and the speciation reaction, are minimized simultaneously against a large experimental dataset, covering a wide range of water concentration (0.25≤CH2Ot≤7 wt.%) and temperature (800°C≤T≤1200°C), such that the resulting functions are both mutually-consistent and broadly applicable; (2) the minimization allows rigorous and robust analysis of uncertainties such that the accuracy of the functions is quantified; (3) the model can be straightforwardly used to determine functions for diffusivity and speciation for other melt compositions pending suitable diffusion-couple experiments. The modelling approach is suitable for both forward and inverse modelling of diffusion processes in silicate melts; the model is available as a MATLAB script from the electronic supplementary material.

Thermodynamic modeling of hydrous-melt–olivine equilibrium using exhaustive variable selection

Water in silicate melt influences the phase relations of a hydrous-melt system. Given the importance of water in silicate melts, a quantitative thermodynamic understanding of the non-ideality of hydrous melt is necessary to properly model natural magmatic processes. This paper presents a novel method for quantitative thermodynamic modeling of hydrous-melt–olivine equilibrium. Specifically, a machine learning method, exhaustive variable selection (ES), is used to model the non-ideality of hydrous melts. Using the ES method, we quantitatively validate the predictive capacities of all possible combinations of variables and then adopt the combination with the highest predictive capacity as the optimal model equation. The ES method allows us to obtain the underlying thermodynamic relationship of the hydrous-melt–olivine system, such as the relative importance of different variables to the thermodynamic equilibrium, as well as to construct a robust and generalized model. We show that the combination of a linear term and a squared term of the total water concentration of melt is significant for describing the hydrous-melt–olivine equilibrium. This result is interpreted in terms of the microstructural changes related to the dissociation of water in silicate melt. Calculations using the optimal model reproduce the experimentally determined effects of water on the olivine liquidus and the distribution coefficient for Mg between olivine and hydrous melt. Our study demonstrates that the ES method yields a thermodynamic equilibrium model that captures the essential thermodynamic relationship explaining the high-dimensional and complex experimental data.

The influence of water in silicate melt on aluminium excess in plagioclase as a potential hygrometer

Measuring water contents of magmas is fundamental to resolving a number of geological questions, such as the mechanisms of silicic magma evolution, the triggering of volcanic eruptions, and the formation of porphyry copper deposits. This study focuses on the correlation between apparent deviations from stoichiometry of plagioclase crystals and high water concentration in the magmatic melt from which they grew. We considered this relationship as a potential geo-hygrometer (water activity indicator). To test and potentially calibrate this new technique, a range of natural and experimental plagioclase crystals were analysed, with particular care taken to identify and avoid analytical bias and artefacts. In contrast to recently published material, we found no systematic aluminium excess in plagioclase, irrespective of the water concentration of the silicate melt it crystallised from. This suggests that aluminium excess in plagioclase cannot serve as a geo-hygrometer. The high likelihood of misinterpreting analytical artefacts (due to alkali migration and imprecise standardisation) as small deviations from stoichiometry, also requires its application as a mineral exploration tool to be treated with caution.

Water and magmas: insights about the water solution mechanisms in alkali silicate melts from infrared, Raman, and 29Si solid-state NMR spectroscopies

Degassing of water during the ascent of hydrous magma in a volcanic edifice produces dramatic changes in the magma density and viscosity. This can profoundly affect the dynamics of volcanic eruptions. The water exsolution history, in turn, is driven by the water solubility and solution mechanisms in the silicate melt. Previous studies pointed to dissolved water in silicate glasses and melts existing as molecules (H2Omol species) and hydroxyl groups, OH. These latter OH groups commonly are considered bonded to Si4+ but may form other bonds, such as with alkali or alkaline-earth cations, for instance. Those forms of bonding influence the structure of hydrous melts in different ways and, therefore, their properties. As a result, exsolution of water from magmas may have different eruptive consequences depending on the initial bonding mechanisms of the dissolved water. However, despite their importance, the solution mechanisms of water in silicate melts are not clear. In particular, how chemical composition of melts affects water solubility and solution mechanism is not well understood. In the present experimental study, components of such information are reported via determination of how water interacts with the cationic network of alkali (Li, Na, and K) silicate quenched melts. Results from 29Si single-pulse magic-angle spinning nuclear magnetic resonance (29Si SP MAS NMR), infrared, and Raman spectroscopies show that decreasing the ionic radius of alkali metal cation in silicate melts results in decreasing fraction of water dissolved as OH groups. The nature of OH bonding also changes as the alkali ionic radius changes. Therefore, as the speciation and bonding of water controls the degree of polymerization of melts, water will have different effects on the transport properties of silicate melts depending on their chemical composition. This conclusion, in turn, may affect volcanic phenomena related to the viscous relaxation of hydrous magmas, such as for instance the fragmentation process that occurs during explosive eruptions.

The effects of sulfur, silicon, water, and oxygen fugacity on carbon solubility and partitioning in Fe-rich alloy and silicate melt systems at 3 GPa and 1600 °C: Implications for core–mantle differentiation and degassing of magma oceans and reduced planetary mantles

The partition coefficient of carbon between Fe-rich alloy melt and silicate melt, DCmetal/silicate and solubility of C–O–H volatiles in reduced silicate melts are key parameters that need to be quantified in order to constrain the budget and origin of carbon in different planetary reservoirs and subsequent evolution of volatiles in magma oceans (MO) and silicate mantles. In this study, three sets of graphite-saturated experiments have been performed at 3 GPa and 1600 °C to investigate the effects of oxygen fugacity (fO2), sulfur, silicon, and water on the dissolution and partitioning of carbon between Fe-rich alloy melt and silicate melt. The results show that the presence of 0–5 wt% sulfur in alloy melt does not have considerable effect on carbon solubility (∼5.6 wt%) in alloy melt, determined by electron microprobe, whereas the presence of 0–10 wt% silicon decreases the carbon solubility from ∼5.6 wt% to 1.8 wt%. Carbon solubility (11–192 ppm) in silicate melt, determined by SIMS, is strongly controlled by fO2 and the bulk water content. Decreasing log⁡fO2 from IW-0.6 to IW-4.7 or increasing bulk water content from 0.07 to 0.55 wt% results in significant increase of carbon solubility in silicate melt. Raman and FTIR spectroscopic analyses of silicate glasses show that the carbon species is mostly methane, which is further confirmed by the strong, positive correlation between the non-carbonate carbon and non-hydroxyl hydrogen in silicate melt. The DCmetal/silicate ranging from 180 to 4600 decreases with decreasing fO2 or increasing bulk water in silicate melt. In addition, increasing Si in alloy melt also decreases DCmetal/silicate.Our results demonstrate that fO2 and bulk water contents in silicate melt play an important role in determining the fractionation of carbon in planetary MO. A reduced, hydrous MO may have led to a considerable fraction of carbon retained in the silicate mantle, whereas an oxidized, dry MO may have lost almost its entire carbon into the core. If delivery of bulk Earth carbon predominantly occurred after >90% of accretion, i.e., in a relatively oxidized MO (IW-2 to IW-1), then with applicable DCmetal/silicate>1000, most early Earth carbon would also enter the segregating core. Finally, the predominance of methane in reduced silicate melt with fO2 below IW-1 also indicates that degassing of a hydrous, solidifying MO may have created a reduced early atmosphere, and degassing from lunar and Martian mantle may have released much more methane than carbon dioxide.

The density of andesitic melts and the compressibility of dissolved water in silicate melts at crustal and upper mantle conditions

Many magmatic processes, including the extraction of partial melts, fractional crystallization and buoyancy overpressure, depend on the density of the silicate melt phase. Because of its low molecular weight, water has a disproportionately large effect on melt density and accurate data on the effect of water on melt density is required to model density-dependent processes. However, the volumetric properties of the hydrous component at crustal and mantle conditions remain debated. In this study, we determined the density of dry and hydrous (0–9 wt% H2O) haplo-andesitic melts from 0.7 to 4.0 GPa and 1600 to 1930 K using the X-ray absorption method in a Paris–Edinburgh press to determine the partial molar volume of water at high pressure. Our in situ data were combined with atmospheric pressure data to derive the first experimental equation of state for andesitic melts valid for crustal and subduction zone conditions, with KT=17.2 GPa and K′¯=6.3−2.5+3.3 at 1273 K. In addition, the partial molar volume and compressibility of dissolved water in andesitic melts were derived from the in situ data, with V0¯=19.2−1.9+2.4 cm3/mol, K¯T=2.7−2.2+7.6 GPa and K′¯=3.6−2.7+13.5 at 1273 K. Combined with available literature data for other melt compositions, our results support a compositionally independent partial molar volume of water in silicate melts, at least within the currently attainable precision of the in situ measurements. This observation greatly simplifies the construction of future models for magma density.

Constraints on volatile concentrations of pre-eruptive lunar magma

Until recently, the Moon had long been thought to be depleted of volatiles such as H2O, S, COx and Cl2. Researchers have recently measured volatile concentrations in the minerals, glasses and inclusions of lunar samples, and the results show that lunar rocks contain volatiles that are more similar to terrestrial materials than was previously thought. Mare basalts are located on the Earth-facing hemisphere in large impact basins, and they are not representative of the feldspathic highlands. Thus, it is likely that the density of lunar mafic magma exceeds that of the highland rocks based on buoyancy alone. According to this observational fact, we calculate the density of mare basalt to give a constraint for the maximum amount of water mare basalt can contain because water can effectively decrease the density of mare magma. Our result shows that water contained in the pre-eruptive magma could not have been more than 1000ppm; otherwise, the density of very-low-Ti basaltic magma would be less than that of the highland rocks. Additionally, if magma contains other species of volatiles such as C–O, S, F, or Cl2, the water in the pre-eruptive magma would have to be much less than 1000ppm because volatiles such as CO2 can effectively decrease the solubility of water in silicate melts. Based on these calculations on densities and a comparison with water in MORB, we conclude that the moon’s water is not as great as has been recently suggested.

The amphoteric behavior of water in silicate melts from the point of view of their ionic-polymeric constitution

Dissociation of water into protons and hydroxyl ions is a fundamental feature of aqueous solutions. Although it exerts a profound influence on properties of magmas, this autoprotolysis reaction has been hitherto neglected for water dissolved in silicate melts. As made here with an acid–base model, in fact one has to deal with molecular water (H2Omol) and two kinds of hydroxyl groups, bonded or not to network-forming cations (OH and OH−, respectively) in hydrous silicate melts. By mixing cations and anions on distinct sublattices and quantifying the disproportionation of water dissolved in silicate melts into its ionic products, H+ and OH−, we reconcile spectroscopic determinations of water speciation, and highlight the main compositional features involving chemical exchanges between H2Omol, TOH and MOH groups (T and M being a former and a modifier, respectively). In particular, water addition to depolymerized systems, such as basalts, determine a relative predominance of OH− with respect to silicic systems, such that the increase of water concentration tends to immediately limit depolymerization rather than enhance it. This opens new perspectives to the understanding of the chemical control of hydrous magmas and their physical properties, as well as the attainment of saturation in hydrous minerals such as amphiboles or micas, particularly in depolymerized melts.

Static compression of hydrous silicate melt and the effect of water on planetary differentiation

High pressure experiments using the sink/float method have bracketed the density of hydrous iron-rich ultrabasic silicate melt from 1.35 to 10.0 GPa at temperatures from 1400 to 1860 °C. The silicate melt composition was a 50–50 mixture of natural komatiite and synthetic fayalite. Water was added in the form of brucite Mg(OH) 2 and was present in the experimental run products at 2 wt.% and 5 wt.% levels as confirmed by microprobe analyses of total oxygen. Buoyancy marker spheres were olivines and garnets of known composition and density. The density of the silicate melt with 5 wt.% water at 2 GPa and 1500 °C is 0.192 g cm − 3 less than the anhydrous form of this melt at the same P and T. This density difference gives a partial molar volume of water in silicate melt of ∼ 7 cm 3 mol − 1 , which is similar to previous studies at high pressure. The komatiite–fayalite liquids with 0 and 2 wt.% H 2O, have extrapolated density crossovers with equilibrium liquidus olivine at 8 and 9 GPa respectively, but there is no crossover for the liquid with 5 wt.% H 2O. These results are consistent with the hypothesis that dense hydrous melts could be gravitationally stable atop the 410 km discontinuity in the Earth. The results also support the notion that equilibrium liquidus olivine could float in an FeO-rich hydrous martian magma ocean. Extrapolation of the data suggests that FeO-rich hydrous melt could be negatively buoyant in the Earth's D″-region or atop the core–mantle-boundary (CMB), although experiments at higher pressure are needed to confirm this prediction.

Transition elements in water-bearing silicate glasses/melts. part II. Ni in water-bearing glasses

The local coordination environment around Ni(II) in a series of sodium trisilicate (NS3) and albitic (ALB) glasses has been evaluated by using high-resolution XANES and anharmonic EXAFS spectroscopies. The glasses contain ∼1000 to 4000 ppm of Ni and from 0 to 8.2 wt.% water. They were synthesized at pressures between 2.2 and 5 kbars and temperatures between 1050 and 1350 K. The bulk glasses were characterized by using X-ray diffraction, transmission electron microscopy, Raman, and ultraviolet-Vis-NIR spectroscopies. Both hydrous NS3 and ALB glasses show dominant amounts of Ni(II) in relatively regular 6-coordinated environments, in contrast with their anhydrous counterparts, where 5-coordinated Ni dominates. There are also significant differences in the average medium-range environment (2–3.5 Å) around Ni between the anhydrous and hydrous glasses. In the ALB glasses, the presence of water in amounts >2 wt.% induces the formation of nanocrystallites, with an average diameter of ∼40 Å and an atomic arrangement similar to that of nepouite ( [6]Ni 3Si 2O 5(OH) 4) or Ni-talc ( [6]Ni 3Si 4O 10(OH) 2) or a related hydrous Ni-silicate. The presence of Ni-bearing nanocrystallites is thought to be due to the relatively slow quench rate of the high-temperature–high-pressure synthesis apparatus used. These nanophases are difficult to detect by using conventional characterization methods and can cause misleading interpretations of the glass structure if not detected. In contrast, there is no evidence for Ni-rich, nanocrystalline domains in NS3 glasses containing high water contents (up to 8.2 wt.%); instead, two to three Si second neighbors are observed around Ni in all NS3 glasses (and in ALB glasses with water contents <4 wt.%). The bonding of Ni to oxygens in the tetrahedral framework is inconsistent with the presence of large amounts of Ni(H 2O) 6 2+ complexes in these glasses. However, Ni(II) may form Ni(OH) n O 6− n (4+ n)− (n ∼ 6) complexes in hydrous glasses. Our results for Ni combined with results from other studies of 3-d divalent transition metal cations in hydrous silicate glasses suggest that water in silicate melts helps these cations form their preferred coordination environments [6-coordinated for Mn(II), Fe(II), and Ni(II)]. Ni(II) may occur in natural hydrous silicate melts dominantly in 6-coordinated environments, rather than dominantly in 4-coordinated environments, as in anhydrous melts and supercritical aqueous fluids, explaining the compatible behavior of Ni in magmas. However, in situ experiments are required to test this suggestion.

The dissolution mechanisms of water in silicate melts; a synthesis of recent data

AbstractDissolved water has significant effects on the physical and chemical properties of silicate melts. Some of the different approaches towards understanding these effects are reviewed here. Spectroscopic measurements on hydrous glasses quenched from melts provide good models for the structure of the melts at the glass transition temperature (Tg). Such measurements suggest that the mechanism of dissolution of water in silicate melts varies strongly with the bulk composition. In particular framework aluminosilicate compositions seem to have very different dissolution mechanisms from Al-free compositions. The water speciation reactions are temperature dependent, with some of the molecular water which is present in glasses at room temperature being converted to hydroxyl at high temperatures. This conversion probably occurs only above Tg. Data on the kinetics of the speciation reaction and the dynamics of microscopic processes in hydrous silicate melts are also discussed. Finally some important aims of future work on hydrous silicate melts are suggested

In situ IR Spectroscopy on Hydrous Albitic and Rhyolitic Glasses and its Implications for Water Speciation and Water Species Reactions in Silicate Glasses and Melts

The interpretation of spectroscopic and experimental data on speciation of water in silicate melts and glasses has been controversial. There are two experimental approaches using IR absorption spectroscopy for studying the equilibrium speciation of H20 as a function of temperature (and pressure) and total water content. One method is by holding the silicate glass at high temperature to reach equilibrium, then rapidly quenching it to room temperature and measuring IR spectra at room temperature (e.g. Zhang et al., 1991). In the other method IR spectra are measured in situ as the sample is held at high temperature and pressure (Nowak and Behrens, 1995; Shen and Keppler, 1995). The in situ spectra show systematic changes in band intensities with temperature even in the glass state. Assuming that the molar extinction coefficients do not depend on temperature the authors of the in situ studies have inferred that (i) the speciation varied in the glass state (25-400~ and (ii) the equilibrium constant for the speciation reaction depends more strongly on temperature than inferred from data using the quench technique. It is important to resolve whether the difference of the results from the two techniques is owing to the inability to quench species concentrations even from 400~ for the quench technique, or to the temperature dependence of extinction coefficients or other physical properties of the glass (e.g. density) for the in situ technique. In order to resolve the apparent contradictions of the two experimental approaches we have measured near-infrared spectra of hydrous albitic glass (0.5-9.2 wt.% total water) and rhyolitic glasses (0.2-6.0) in situ in a wide temperature range ( -192 to +600~ depending on water content) as well as after quenching from high temperature. Because different baseline corrections have been applied to determine the concentrations of OH groups and molecular H20 from the combination bands at 4500 and 5220 cm -1, respectively, we have tested the equivalence of these methods for the albitic composition (9 samples). The baselines chosen are (i) a linear baseline for the band at 5220 cm -1, extension of this line to low frequency combined with a gaussian fitted to the band at 4000 cm -1 as the baseline for the band at 4500 cm -1 (Behrens et al., 1996), (ii) linear baselines for both bands (Shen and Keppler, 1995) and (iii) flexicurve baselines for both bands (Newman et al., 1986). If the molar extinction coefficients were independent on total water, OH and H20 concentrations differ by at most 3 % relative if absorbances (peak heights) are evaluated (for baselines i, ii and iii) or if integrated intensities (peak areas) are evaluated (for baselines i and ii). However, integrated intensities gives systematically lower OH and higher H20 (up to 10% relative) at low total water and higher OH and lower H20 (up to 7 % relative) at high total water. We suggest that the systematic differences may be due to the variation of the linear molar extinction coefficient with water content in accordance with results of Zhang et al. (1997). IR spectra were measured in situ using a heating/ cooling stage fitted to an IR microscope. Fig.1 display an example which is typical for the albitic and rhyolitic glasses. Systematic variation with temperature even close to liquid nitrogen temperature are observed for both absorbances and integrated intensities. For the H20 band the absorbance has a maximum close to room temperature whereas the integrated intensity is continously decreasing with temperature. For the OH band both the absorbances and the integrated intensities are continously increasing with temperature. For albitic glasses the T-dependence of the integrated intensity of the H20 band is almost independent on water content (about 3% per 100K) and only is weakly effected by the type of baseline correction. In contrast, the OH band is

Direct observation of complete miscibility in the albite–H2O system

There are essentially two main types of mobile phases in the Earth's crust and mantle: silicate melts and hydrous fluids. Near the surface of the Earth, the physical and chemical properties of these phases are fundamentally different. But with increasing pressure, the solubility of water in silicate melts and the solubility of silicate materials in hydrous fluids increase. This has led to the suggestion that, above a certain critical pressure, the compositions of the two phases in mutual equilibrium become indistinguishable1–5. Here we report the direct visual observation of this phenomenon in the albite–H2O system using an externally heated diamond anvil cell. Our results suggest both that there should be complete miscibility between silicate melts and hydrous fluids in the deeper parts of the upper mantle and that, in the presence of a hydrous fluid, a melting temperature for this part of the mantle can no longer be defined.

Acceptance speech for the 1993 Clarke Award

Thank you, Ed (and Dave, as it was then), for the kind words. I am deeply honored to receive this unexpected award from the Geochemical Society and overwhelmed to have been placed in the company of Clarke medalists, many of whom 1 know and admire. Two of them, Dave Walker and Ed Stolper, have had a profound influence on my career. It humbles me to think about so many worthy peers and to know that even some great minds of geochemistry have not received this award. It is indeed a privilege and honor just to do sciences, not only because you are paid for doing what interests you, but also because you are not restricted in the choices of research projects. I have certainly enjoyed this freedom and have worked on a diverse variety of problems related to diffusion in silicate melts, glasses and minerals, the degassing history of the Earth, island arc petrogenesis, and volcanic eruptions. The progress of science is always built upon previous work. This is especially true in my case. Let me give you an example. Water diffusion in silicate melts and glasses is an interesting problem that has attracted many workers. I was especially interested in the project because it was the first diffusion problem in which the concentration of the several species involved in the diffusion of a component can be measured directly, and hence it provides an ideal case to study the role of speciation during diffusion. When I started working on this, much had already been learned about water in silicate melts and glasses, and therefore the time was ripe to gain a deeper understanding of this problem. For example, the diffusivity of water in rhyolitic melts had been found by Shaw, Jambon, Karsten, and Delaney, and other workers to increase with total water concentration. The work of Stolper and his group, including Newman, Silver, and others using infrared spectroscopy had established the presence of two species of dissolved water, OH groups and H20 molecules. They had also calibrated the infrared technique so that measuring the concentration of the two species in a small area could be done in a routine manner. We knew by then that OH groups and H20 molecules interconvert in the structure and that the ratio of molecular H20 to OH groups increases with total water concentration. The early work of Doremus and the more recent work of Wasserburg had proposed that molecular Hz0 is the diffusing species for water diffusion in silicate melts and glasses, even though some other workers disagreed. It was then that I arrived at Caltech to study water diffusion and to attack the more general problem of the role of speciation during diffusion. From our experimental data and theoretical analysis, Ed Stolper, Jerry Wasserburg, and 1 were able to show that molecular H20 is the diffusing species for water diffusion in our rhyolitic glasses. We were also able to

Diffusion of Water in Silicate Melts

The transport of water in silicate melts is examined from a theoretical point of view, taking into account the presence of both H2O and OH-1 species. It was assumed that only H2O molecules may diffuse and that a constitutive relationship between the number density \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$n_{{\mathrm{H_{2}O}}}$$\end{document} of H2O molecules and the total water content ($$n_{\Sigma}$$) exists such that \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$n_{{\mathrm{H_{2}O}}} = g(n_{\Sigma})$$\end{document}. This is related to dissociation or to chemical reactions such as H2O molecular (melt) + Oxygen (melt) ⇄ 2 OH (melt). This approach leads to diffusion transport equations that are intrinsically nonlinear. Three regions are naturally defined: (I) high water content with a normal linear diffusion equation $$\partial n_{\Sigma}/\partial \tau = D\nabla^{2}n_{\Sigma}$$, where D is the intrinsic diffusion coefficient of a H2O molecule in the silicate; (II) an intermediate or transitional region where the transport equation is $$\partial n_{\Sigma}/\partial\tau = D\nabla^{2}g(n_{\Sigma})$$; and (III) a region of low water concentration where the transport equation is $$\partial n_{\Sigma}/\partial \tau = D^{\ast}\nabla^{2}n_{\Sigma}^{2}$$. In the latter region, the equation is strongly nonlinear. $$D^{\ast}$$ is a constant diffusion coefficient related to D. It follows that the dissociation of water molecules to form OH-1 in silicates automatically implies a nonlinear transport behavior due to the storage of dissociated water molecules in immobile sites. This study of the kinetics of transport will aid in our understanding of speciation and structure in silicate melts.

Water in silicate melts

Water is one of the more important volatile species in magmas, both in terms of its abundance and its influence on the properties of a given magma. Many workers in the geological sciences have measured, modeled, and speculated on the interaction of water with silicate melts as a function of pressure. At the same time, glass and materials scientists have collected a considerable body of data on the effect of water on the properties of liquid and glassy silicates at 1 atmosphere (1.01325×105 N m−2) and below. A special session on “Solubility and Transport Properties of Water in Silicate Melts” was held during the 1983 AGU Spring Meeting, May 30–June 3, in Baltimore. The session had three main objectives: (1) review the present data base and discuss the status of current models in order to identify areas where further work is needed; (2) introduce interested geologists to the large body of work being carried out in the glass and materials sciences; and (3) consider static properties, such as thermodynamic relations, structure of hydrous melts, and dynamic properties including diffusion and viscosity. This report summarizes the major topics discussed. More detailed information may be found in the published abstracts (Eos, May 3, 1983, pp. 338–343).

Generation of arc basalt magmas and thermal structure of the mantle wedge in subduction zones

The chemical compositions of primary magmas of olivine tholeiite (OTB), high‐alumina basalt (HAB), and alkali olivine basalt (AOB) are obtained by the olivine maximum fractionation model for Quaternary magnesian basalts from the Northeastern Japan arc. These basalts are assumed to have fractionated only olivine crystals before eruption. The melting phase relations for three primary basalt compositions have been determined under both anhydrous and water‐undersaturated conditions. The AOB melt coexists with olivine, orthopyroxene, and clinopyroxene at 17 kbar and 1360°C under anhydrous conditions and at 23kbar and 1320°C in the presence of 3wt % water. The HAB melt also coexists with the above three phases at 15 kbar and 1340°C under anhydrous conditions and at 17 kbar and 1325°C in the presence of 1.5 wt % water. The OTB melt, on the other hand, coexists with olivine and orthopyroxene at 11 kbar and 1320°C under anhydrous conditions. The water contents in arc basalt magmas are estimated to be about 3, 1.5, and nearly O wt % for the AOB, HAB, and OTB, respectively, on the basis of the solubility limit of water in silicate melts. Based on these estimates and the experimental results, the AOB, HAB, and OTB magmas are suggested to segregate from the mantle at about 1320°C and at 23, 17, and 11 kbar, respectively. As the temperatures at the segregation of the magmas given above appear to be too high for a stable mantle geotherm, the mantle diapir is the most probable mechanism for the magma production in a subduction zone. Considering the heat of formation of melt in the diapir, the region with temperatures higher than 1400°C has to be present in the mantle wedge.

The speciation of water in silicate melts

Previous models of water solubility in silicate melts generally assume essentially complete reaction of water molecules to hydroxyl groups. In this paper a new model is proposed that is based on the hypothesis that the observed concentrations of molecular water and hydroxyl groups in hydrous silicate glasses reflect those of the melts from which they were quenched. The new model relates the proportions of molecular water and hydroxyl groups in melts via the following reaction describing the homogeneous equilibrium between melt species: H 2O molecular (melt) + oxygen (melt) = 2OH (melt). An equilibrium constant has been formulated for this reaction and species are assumed to mix ideally. Given an equilibrium constant for this reaction of 0.1–0.3, the proposed model can account for variations in the concentrations of molecular water and hydroxyl groups in melts as functions of the total dissolved water content that are similar to those observed in glasses. The solubility of molecular water in melt is described by the following reaction: H 2O (vapor) = H 2O molecular (melt). These reactions describing the homogeneous and heterogeneous equilibria of hydrous silicate melts can account for the following observations: the linearity between f H 2O and the square of the mole fraction of dissolved water at low total water contents and deviations from linearity at high total water contents; the difference between the partial molar volume of water in melts at low total water contents and at high total water contents; the similarity between water contents of vapor-saturated melts of significantly different compositions at high pressures versus the dependence on melt composition of water solubility in silicate melts at low pressures; and the variations of viscosity, electrical conductivity, the diffusivity of “water,” the diffusivity of cesium, and phase relationships with the total dissolved water contents of melts. This model is thus consistent with available observations on hydrous melt systems and available data on the species concentrations of hydrous glasses and is easily tested, since measurements of the concentrations of molecular water and hydroxyl groups in silicate glasses quenched from melts equilibrated over a range of conditions and total dissolved water contents are readily obtainable.