HR: 15:15h
AN: V42E-07    [PDF]
TI: Chemical Order in Silicate Melts: Implications for Microscopic Origins of Mantle Melting Behavior
AU: Mysen, B O
EM: mysen@gl.ciw.edu
AF: Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015 United States
AU: * Lee, S
EM: s.lee@gl.ciw.edu
AF: Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015 United States
AU: Cody, G D
EM: cody@gl.ciw.edu
AF: Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015 United States
AU: Fei, Y
EM: fei@gl.ciw.edu
AF: Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015 United States
AB: Configurational thermodynamic properties of silicate melts (e.g. activity coefficient of silica) at high pressure govern composition of melts in equilibrium with mantle mineral assemblages. These properties are controlled by the distribution of framework units (e.g. $^{[4]}$Si, $^{[5,6]}$Si, $^{[4]}$Al), and the disorder among network-modifying cations (e.g. Ca$^{2+}$, Mg$^{2+}$, Na$^{+}$) in the melts (Lee, Fei, Cody, \& Mysen, Geophys. Res. Lett., 2003, 30, 1845; Lee and Stebbins, Geochim. Cosmochim. Acta., 2003, 67, 1699). Spectroscopic data obtained in the diamond anvil cell (DAC) together with quantum chemical simulations, allow us to measure details of distributions of framework units and network modifying cations with varying pressure, temperature and compositions. Here we report structural details of model basaltic melts (sodium silicate and aluminosilicates with varying degree of polymerization) mainly using solid state NMR, vibrational spectroscopy and synchrotron X-ray with DAC. These results highlight the tendency for chemical ordering resulted from cation mixing in silicate melts and glasses at ambient as well as high pressure (6-10 GPa). The chemical ordering among framework units leads to the formation of $^{[5,6]}$Si-O-$^{[4]}$Si in silicates and $^{[5,6]}$Al-O-$^{[4]}$Si in aluminosilicates, contributing to the total negative deviation of silica activity from ideal solution in silicate melts at high pressure. Network-modifying cations also prefer to form dissimilar pairs (e.g. Ca-Na and Mg-Ba). These results indicate that there will be a further reduction in the activity coefficient of silica in multi-component melts. We also present modeling results of configurational enthalpy and entropy of multi-component silicate melts derived from the spectroscopic analysis and calculated the effect of degree of chemical order in melt properties. Increasing chemical ordering among framework units leads to a decrease in configurational entropy and enthalpy of melts, and also contributes to the decrease of silica activity coefficient in melts. Structural ordering in the melts, together with extensive mixing among framework units, strongly affect the composition of the partial melts. The alkali or silica content in equilibrium with mantle peridotite can increase as a results of these structural effects, thus manifesting the strong links between melt structures, properties and magmatic processes.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting