Water-melt interaction in hydrous magmatic systems at high temperature and pressure

Bjorn Mysen

doi:10.1186/2197-4284-1-4

Abstract

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 (Qn 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 Qn 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.

Highlights

The principal mass and energy transport agents in the Earth are magmatic melts and water-rich fluids
Liquidus phase relations, transport, and volume properties of hydrous melts vary in important ways with variations in water content (Kushiro 1972; Richet et al 1996; Ochs and Lange 1999; Grove et al 2003)
Depolymerization takes place because as the tetrahedrally coordinated Al3+ interacts with H2O to form Al-OH bonds, an equivalent proportion of the charge-balancing cation becomes a network-modifier, or charge-balancing Na+ for Na-OH bonds leading to network-modifying Al3+, or both

Summary

Introduction

The principal mass and energy transport agents in the Earth are magmatic melts and water-rich fluids. The extent of the solidus temperature depression depends on silicate composition and increases, for example, with increasing silica content (Figure 1B) In natural systems, this effect helps explain why the liquidus depression caused by H2O of a felsic composition (e.g., granitic composition) is greater than that of peridotite (Figure 1A). Melt viscosity decreases by several orders of magnitude by a solution of as little as 1 wt% H2O (Richet et al 1996) This effect, is strongly non-linear and diminishes with increasing water content (Figure 3A). These complex relations likely reflect several effects on the solubility of H2O of individual silicate components.

24 Na2O-SiO2 22 20 18

40 Melt 35

Discussion

Conclusions