The Influence of Melt Composition and Dissolved Water on the Melt-Vapor Surface Tension of Dacite and Rhyolite Magma
Abstract
Melt-vapor surface tension (σ ) is a poorly known quantity, yet it plays a critical role in many volcanic processes. In this study, we provide new data for σ of hydrous natural dacite and rhyolite melt at 200 MPa, 800-1055° C, and 4.8-7.7 wt% H2O using results from high temperature and pressure decompression experiments (this study; Mangan and Sisson, 2000; Mourtada-Bonnefoi and Laporte, 2002; 2004) and classical nucleation theory. The solutions give values for σ that vary inversely with dissolved H2O by 0.025 (± 0.009) J/m2 per wt% H2O. Combining our results with data from hydrous haplogranite and rhyodacite (Epel'baum et al., 1973; Bagdassarov et al., 2000) reveals that melt-vapor surface tension also varies inversely with the concentration of mafic melt components (e.g., CaO, FeOtotal, MgO). At constant H2O content, σ increases by 0.70 (± 0.53) J/m2 per tenth increment increase in the melt felsic index (FI = Na2O+K2O/Na2O+K2O+CaO) from FI ∼ 0.75 (dacite) to FI ∼ 1.0 (haplogranite). To understand these results, we consider the thermodynamic definition of σ , i.e., the work per unit surface area to create an interface between two phases. It can be expressed as: σ dA = ω dissociation - ω interaction. The ω dissociation term is the work done to dissociate molecules from the bulk melt for incorporation in a diffuse, lower density interfacial zone only a few molecular layers thick. In contrast, the work of interaction, ω interaction, is work made available as a result of across-interface attractions between gas and melt molecules. Increasing the concentration of dissolved H2O and other network modifiers (e.g., Ca, Fe, and Mg) influence the balance of work terms as (1) the ω dissociation declines due to lowered cohesion of the disrupted melt structure, and (2) the ω interaction increases because enhanced molecular diversity supports more numerous and varied interfacial zone attractions. The net effect of decreased ω dissociation and increased ω interaction is a decrease in σ . Additional, systematic data on the variation of σ in natural hydrous melts are needed before the volcanic implications can be fully explored. Nevertheless, it is evident from the data in hand that surface tension, much like melt density and viscosity, can and should be treated as a variable. Cooling, crystallization, and vapor exsolution impart a time-dependency to σ that must be accounted for in modeling volcanic processes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFM.V51D..07M
- Keywords:
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- 8414 Eruption mechanisms;
- 8439 Physics and chemistry of magma bodies;
- 8450 Planetary volcanism (5480);
- 3630 Experimental mineralogy and petrology