Water-melt interaction in hydrous magmatic systems at high temperature and pressure
© Mysen; licensee Springer. 2014
Received: 3 December 2013
Accepted: 10 March 2014
Published: 22 April 2014
Experimental data on the structure and properties of melts and fluids relevant to water-melt interaction in hydrous magmatic systems in the Earth's interior have been reviewed. Complex relationships between water solubility in melts and their bulk composition [Al/Si-ratio, metal oxide/(Al + Si) and electron properties of metal cations] explain why water solubility in felsic magmas such as those of rhyolite and andesite composition is significantly greater than the water solubility in basalt melts. The silicate solubility in aqueous fluid is also significantly dependent on composition with metal oxide/(Al + Si) and electron properties of the metal cations, the dominant variables. Hydrogen bonding is not important in hydrous fluids and melts at temperatures above 500°C to 550°C and does not, therefore, play a role in hydrous magmatic systems. The properties of hydrous melts and aqueous solutions are governed by how the silicate speciation (Q n species, where n is the number of bridging oxygen in an individual species) varies with bulk composition, silicate composition, temperature, and pressure. The reactions that describe the interactions are similar in melts, fluids, and supercritical fluids. The degree of melt polymerization caused by dissolved water varies with melt composition and total water content. Silicate- and alkali-rich felsic magmatic melts are more sensitive to water content than more mafic magmas. Transport and configurational properties of hydrous magmatic melts can be modeled with the aid of the Q n speciation variations. Liquidus and melting phase relations of hydrous systems also can be described in such terms, as can minor and trace element partition coefficients. Stable isotope fractionation (e.g., D/H) can also be rationalized in this manner. Critical to these latter observations is the high silicate concentration in aqueous fluids. These components can enhance solubility of minor and trace elements by orders of magnitude and change the speciation of H and D complexes so that their fractionation factors change quite significantly. Data from pure silicate-H2O systems cannot be employed for these purposes.
The principal mass and energy transport agents in the Earth are magmatic melts and water-rich fluids. Their transport properties are sensitive to their water and silicate content. Experimental characterization of solubility and solution mechanisms of water and silicate components is, therefore, central to our understanding of magmatic and metasomatic processes in the Earth’s interior.
Properties of magmatic melts are sensitive to their water content as first noted by Spallanzani (1798). Perhaps the most well-known among these effects is a several hundred degree centigrade temperature depression at high pressure of solidii and liquidii of silicate phase assemblages (including natural mineral assemblages) (e.g., Kushiro et al. 1968a, b; Grove et al. 2012). Liquidus phase relations (including partial melt compositions), transport, and volume properties of hydrous melts also vary in important ways with variations in water content (Kushiro 1972; Richet et al. 1996; Ochs and Lange 1999; Grove et al. 2003).
Aqueous solutions in the Earth's interior are efficient solvents of oxide components (Zhang and Frantz 2000; Manning 2004). Several mol% of silicate components dissolve under conditions corresponding to those of the deep continental crust and upper mantle. In the upper mantle, there can be complete miscibility between H2O and silicate (Bureau and Keppler 1999; Mibe et al. 2007). Major element solutes in aqueous fluids (silicate components) can also enhance the solubility of other components by up to orders of magnitude (Pascal and Anderson 1989; Antignano and Manning 2008; Mysen 2012a, b; Ayers and Watson 1993). Transport, volume, and mixing properties of silicate-rich aqueous fluids differ in important ways from those of pure H2O (Audetat and Keppler 2004; Hunt and Manning 2012; Hack and Thompson 2012; Foustoukos and Mysen 2013).
The property behavior of melts and fluids in hydrous silicate systems at the high temperatures and pressures can be traced to the relationships between fluid and melt structure and their properties. Most experimental and theoretical studies have focused on the behavior in chemically simpler systems in order to isolate the effects of individual intensive and extensive variables. With the information from chemically simpler systems, we can model melt and fluid behavior in systems relevant to natural processes. In this review, these relationships will be presented and their applications to natural systems will be discussed.
Water and properties of hydrous magma
Melting phase relations
Properties of hydrous silicate melts
Most other physical properties of silicate melts show analogous relations with water content and silicate composition. Examples include cation and water diffusion in melts (Watson 1994; Behrens and Nowak 1997), effects of water on glass transition temperature (Richet et al. 1996, 1997; Whittington et al. 2000), and electrical conductivity (e.g., Takata et al. 1981; Satherley and Smedley 1985). These effects exist because of the functional relationships that exist between these transport properties (Nernst 1888; Einstein 1905; Eyring 1935a, b).
Water solubility in magmatic melts
Given the complex relationships between melt composition and water solubility, it is not surprising that the solubility varies significantly in different magmatic systems. In general, water solubility in felsic magmas such as those of rhyolite and andesite composition is significantly greater than the solubility in basalt melts (Hamilton et al. 1964; Dixon and Stolper 1995; Behrens and Jantos 2001; Zhang 1999). This would be expected because of the higher alkali/alkaline earth and Si/Al ratios in rhyolite and andesite melt compared with melts of basaltic composition. These solubility relationships have been modeled with a variety of empirical models (e.g., Spera 1974; Burnham 1975; Dixon and Stolper 1995; Behrens and Jantos 2001). However, quantitative linkage between solubility behavior in chemically simple melts and more complex natural magma compositions await detailed structural characterization of the water solution mechanisms in simple and complex magmatic melts.
Silicate solubility in aqueous fluids
Melt and glass
Most of the experimental data on solubility and solution behavior of water in silicate melts at high temperature have been obtained by analysis of the glass formed by temperature quenching from a high-temperature/high-pressure hydrous melt. Temperature-dependent structure may change during such a cooling process and eventually gets frozen in at the temperature of the glass transition (Dingwell and Webb 1990). By definition, the glass transition temperature (actually a small temperature range), therefore, is that below which the material is not relaxed on the time scale of a measurement (glass), whereas above that temperature, the material is relaxed (liquid). In the temperature interval between a liquidus and glass transition temperature, the material is a supercooled liquid with the same property behavior as that of the melt above the melting temperature. These distinctions are important when a property measured on a glass is applied to the property behavior of its melt.
Principles of silicate melt structure
where XT and XO are the atomic proportions of tetrahedrally coordinated cations and oxygen, respectively.
where n is the number of bridging oxygen and mol fraction of individual Q n species. However, Q n species abundance obviously cannot be calculated from NBO/T values alone.
Solution mechanisms of silicate and water in fluids and melts in hydrous magmatic systems
Most structural data from hydrous silicate melts have been obtained from analysis of samples quenched from high temperature and pressure to ambient conditions prior to chemical and structural analysis. These data reflect, therefore, the melt structural environment near the glass transition temperature. Temperature-dependent structural features cannot be captured in such studies. That notwithstanding, many important principles have been derived from studies of quenched melt.
The behavior of an aqueous fluid in equilibrium with molten or crystalline silicates at high temperature and pressure may not be addressed by examination of quenched materials because most, perhaps all, of their properties (including the structure itself) cannot be determined by examination of the high-temperature/high-pressure fluid after quenching to ambient conditions. Fluid structure studies require, therefore, examination while the sample is at the temperature and pressure of interest. However, before addressing such experimental environments, structural data from quenched melts will be discussed.
Hydrous melts quenched from high temperature at high pressure
Water is dissolved in silicate melts in the form of water molecules, H2O0, and structurally bound hydroxyl groups, OH. The OH groups can form bonds with Si4+ and Al3+ as well as with other metal cations (Mysen and Virgo 1986; Xue and Kanzaki 2004; Cody et al. 2005). In either case, water dissolved in the form of OH groups in silicate melts affects their structure.
In chemically more complex systems, the OH-forming process also is more complex as will be discussed further below.
In this example, the network-modifying Na+ bonding with nonbridging oxygen in a Q3 species (with NBO/Si = 1) in an anhydrous melt reacts with H2O to form NaOH complexes in hydrous Na2O-SiO2 melts. This interaction causes the nonbridging oxygens bonded to Na+ in anhydrous Q3 to be transformed to bridging oxygens resulting in the formation of the more polymerized Q4 species. Silicon-29 MAS NMR data from melts quenched from 1,400°C along the Na2O-SiO2 join show that this is exactly the situation and that the abundance of NaOH complexes increases with increasing Na/Si (Cody et al. 2005). Similar conclusions have been reported for alkaline earth silicate glasses wherein Ca..OH and Mg..OH groups were formed in CaO-MgO-SiO2 melts (Xue and Kanzaki 2004).
From a compositional perspective, aluminosilicate melts are more akin to natural magmatic melts than metal oxide silicate melts. At pressures less than 5 to 6 GPa, Al3+ is in tetrahedral coordination where it is charge-balanced with alkalis or alkaline earths in a manner conceptually similar to that observed in crystalline aluminosilicates such as feldspars (see Lee et al. 2004; for high-pressure structural data). The aluminate groups (AlO2 -) in aluminosilicate melts can interact with dissolved water to form either Al-OH or metal-OH bonding (metal can be alkali metal or alkaline earth), or both, in addition to Si-OH bonding (Mysen and Virgo 1986; Schmidt et al. 2001). The extent to which aluminate interaction takes place is correlated with the Si/Al ratio of the melt (Mysen and Virgo 1986). The nature of the Al3+ charge balance probably also affects the solution mechanism because the Al-O bond strength depends on the electronic properties of the charge-balancing cation (Roy and Navrotsky 1984).
Additional complexity may exist because in melts with mixed M m+ cations and H+, the H+, because of its size for steric reasons, exhibits preference for forming OH groups in the silicate portion of the network by reacting with the nonbridging oxygen in Q n species with the largest number of nonbridging oxygens (Cody et al. 2005).
The decreasing ∂(NBO/T)/∂ with increasing water content, , reflects the decreasing rate of change of the abundance ratio, , as the total water content increases. This evolution, in turn, reflects the diminishing rate by which the Q n species changes with increasing concentration of water in the melt (Figure 16). These composition-dependent solution mechanisms of water in aluminosilicate melts also explain why the solubility of water in silicate melt at any temperature and pressure is significantly dependent on the bulk chemical composition of the melt itself.
Hydrous melts and aqueous fluids at high temperature and pressure
Experimental protocols have recently been implemented for examination of fluids and melts in hydrous silicate systems at deep crustal and mantle pressures and temperatures in situ while the sample is at the desired pressure and temperature conditions. Structural data obtained under such conditions are, therefore, increasingly available from all regions of silicate-H2O phase diagrams (Figure 4). In considering such data, commonly obtained in so-called hydrothermal diamond anvil cells (e.g., Bassett et al. 1994), the experiments usually are conducted in such a manner that pressure is a variable dependent on temperature. This means that increasing temperature normally is associated with increasing pressure. In the following discussion, this is the case unless otherwise indicated.
The system SiO2-H2O is too simple for modeling natural processes because neither Al2O3 nor alkali metals and alkaline earths are involved. The system Na2O-Al2O3-SiO2 is more realistic even though alkaline earths, in particular, have not yet been addressed. The Na2O-Al2O3-SiO2 system is also useful for characterization of the chemical interaction between nonbridging oxygen and important network-modifying cation (Na+) and protons (H+).
describes the equilibrium. Here, M denotes an alkali metal and where (M) and (H) indicate where alkali metals and protons form bonding with the relevant nonbridging oxygens, respectively. The tetrahedrally coordinated cations forming the Q n species can be either Al3+ or Si4+. The ∆H for this equilibrium (350 to 400 kJ/mol) is the same, within uncertainty for melt fluids and supercritical fluids (Mysen 2010a). With increasing Al/(Al + Si), equilibrium (11) likely shifts to the left (∆H decreases) because Al3+ will preferably occupy the most polymerized of available Q n species (Merzbacher and White 1991). In Equation 11, that species is Q3, but for other more polymerization melt compositions, Q4 species likely also would be involved. The principles outlined in equilibrium (11) may also be applied to alkaline earths, but quantitative information awaits further experiments. These are all considerations necessary for the application of the experimental data to hydrous magmatic systems. However, the necessary experimental data are at present insufficiently comprehensive for quantitative application.
Dissolved water and melt properties
The evolution of Q n species abundance of a melt with water content can be used to characterize how dissolved water governs phase relations and mixing properties of melts. For example, the rapid abundance increase of depolymerized species at the expense of polymerized species with water content of the melt enhances the stability of depolymerized relative to more polymerized liquidus phases. An example of this effect can be seen in the silica/pyroxene liquidus boundary of the Mg2SiO4-CaMgSi2O6-SiO2 system where the silica polymorph is more polymerized than pyroxene (their NBO/Si are 0 and 2, respectively) (Figure 2A). The magnitude of liquidus boundary shifts will reflect the size of the NBO/Si difference between the minerals coexisting along a liquidus boundary. The greater the NBO/Si difference, the greater is the effect of water on the shift of the boundary as illustrated, for example, by the different shifts of olivine/pyroxene and pyroxene/silica liquidus boundaries (Kushiro 1969). Effects such as these also explain why hydrous melts in equilibrium with peridotite mineral assemblages is more silica rich (quartz normative andesitic melts) than under anhydrous conditions (olivine normative tholeiitic melts) during melting and crystallization in the upper mantle.
where T g is glass transition temperature, T is the temperature of interest (T > T g), and C p conf(T g) is the configurational heat capacity at the glass transition temperature. This configurational heat capacity of hydrous magmatic melts is a systematic function of the water content of the melt (Richet et al. 1996; Bouhifd et al. 2006).
Water and fluid-melt equilibria
Stable isotope fractionation
The temperature and pressure effects in Figure 24 reflect the changes in silicate concentration and speciation in the coexisting aqueous fluid and silicate melts. Furthermore, different D/H values for different fluid densities again reflect the concentration and type of Q n species because the different density trajectories reflect pressure/temperature trajectories of the diamond cell experiments that followed different pressure paths (Mysen 2013). It follows from the different trajectories of D/H ratios in fluids and melts that the D/H partitioning between fluids and melts will also be dependent on temperature and pressure (Figure 25). Finally, in silicate mineral/aqueous fluid or silicate mineral/hydrous melt environments, temperature- and pressure-dependent partitioning will also take place because of the temperature/pressure dependence of the D/H fractionation with the fluid and melt phases.
Understanding the role of hydrous melts and silicate-rich fluids in transport processes depends sensitively on how chemical composition, temperature, and pressure govern the physicochemical properties of these materials (viscosity, diffusion, thermodynamics of mixing, element partitioning between phases, phase relations, etc.; see, for example, Mysen and Richet 2005, for review of experimental and theoretical information of structure-property relations of silicate glasses and melts). Attainment of this objective requires well-determined solubilities in and partitioning between melts, fluids, and crystalline phases, detailed understanding of the oxide solution mechanisms in melts and fluids, and determination of how fluid and melt structure governs those properties.
Structural information cannot be obtained directly on chemically complex natural systems because the resolving power of spectroscopic methods employed for such purposes diminishes rapidly with increasing chemical complexity. However, structural data from simpler binary and ternary systems can be used to describe the more complex systems. This objective requires, however, detailed characterization of silicate speciation in fluids and melts as a function of Al/Si ratio, the type of charge balance for tetrahedrally coordinated cations, and the type and proportion of network-modifying metals (alkali metals and alkaline earths). Currently, a combination of structural information and empirical relationships can be applied to describe liquidus phase relations and fluid/mineral/melt element and stable isotope partitioning. Transport properties can be understood and sometimes quantified in terms of configurational properties of individual Q n species. However, the experimental database used for these purposes dominantly is from alkali silicate and alkali aluminosilicate systems. This permits application to felsic magmatic systems. However, the lack of much information in alkaline earth aluminosilicate system makes quantitative application to hydrous basaltic less quantitative.
Many of the ideas and conclusions discussed in this review were developed during the preparation of invited lectures at Tohoku University, Japan in 2013. Portions of the reviewed research were supported by grants from the National Science Foundation (US) (EAR-0707861, EAR1250449, EAR12151931 and EAR-1212754).
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