Effect of Water on the Structure and Properties of Albite Melt
Abstract
Aluminosilicate melt with an albite stoichiometry (NaAlSi3O8) is an important endmember component for more complex crustal and deep crustal melts. It is well known that water (H2O) lowers melting temperatures of rocks and affects elastic and transport properties of silicate melts. However, few studies fully capture the effect of water on the structure, elasticity, and transport properties of albitic melts. In this study, we used first-principles molecular dynamics simulations to examine the effect of water on the atomistic-scale structures, equations of state, and transport properties of hydrous albite melts with up to ~5 wt % H2O. The temperatures explored in our study are greater than what is expected in the deep crust. To address this issue, we explored several isotherms to have a better constraint on the pressure and temperature trends in the atomistic scale structures and properties and extrapolated our results to temperatures relevant for deep crustal settings. Our preliminary results show that, at ambient pressures and a ~3000 K isotherm, ~5 wt % water decreases the melt density by ~10 % and the bulk modulus by ~40 %. In contrast, the pressure derivative of the bulk modulus increases by ~20 %. At all explored pressures, we find that the structure of the hydrous melt is depolymerized. The degree of depolymerization also enhances with water content. Depolymerization is often manifested in the abundance of non-bridging oxygen atoms (NBOs) in the melts, and ~5 wt % water enhances the NBO abundance by ~10 to 15 %. This is due to the attachment of H+ ions to bridging oxygen atoms, which forms hydroxyl ions (OH-) that break polymerized aluminosilicate chains and thereby enhances the abundance of NBOs. Hydroxyl ions are the dominant water-related species, and our results indicate that the abundances of both NBOs and OH- decrease with increasing pressure. At 3000 K, we find that all cations (Na+, Si4+, Al3+) and anions (O2-) diffuse at a faster pace in the hydrous albitic melts compared to water-free albitic melts. We are currently exploring how the water-assisted structural changes influence the viscosities and overall mobilities of the melts in deep crustal settings.
Acknowledgement: We acknowledge funding support from NSF EAR1753125 and EAR 1763215.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMV038.0003A
- Keywords:
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- 1033 Intra-plate processes;
- GEOCHEMISTRY;
- 8408 Volcano/climate interactions;
- VOLCANOLOGY;
- 8413 Subduction zone processes;
- VOLCANOLOGY;
- 8416 Mid-oceanic ridge processes;
- VOLCANOLOGY