HR: 10:20h
AN: V52A-01    [Abstracts]
TI: Aluminum coordination and density in high-pressure aluminosilicate glasses: Significance for the structural changes and densification in basaltic magmas
AU: * Allwardt, J R
EM: allwardt@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Dept. of GES, Bldg. 320, Stanford, CA 94305-2115 United States
AU: Stebbins, J F
EM: stebbins@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Dept. of GES, Bldg. 320, Stanford, CA 94305-2115 United States
AU: Schmidt, B C
EM: Burkhard.Schmidt@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universit„t Bayreuth, Bayerisches Geoinstitut Universit„t Bayreuth, Bayreuth, D-95440 Germany
AU: Frost, D J
EM: Dan.Frost@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universit„t Bayreuth, Bayerisches Geoinstitut Universit„t Bayreuth, Bayreuth, D-95440 Germany
AU: Withers, A C
EM: withe012@tc.umn.edu
AF: Department of Geology and Geophysics, 310 Pillsbury Dr SE, Minneapolis, MN 55455-0219 United States
AU: Hirschmann, M M
EM: Marc.M.Hirschmann-1@umn.edu
AF: Department of Geology and Geophysics, 310 Pillsbury Dr SE, Minneapolis, MN 55455-0219 United States
AB: Densities of aluminosilicate melts are strongly pressure-dependent, owing in part to atomic-scale structural changes. Glasses are commonly used as a first approximation for melts because the structure of a glass is that of the liquid at the glass transition temperature. This study used $^{27}$Al MAS NMR to determine the speciation of aluminum ions in glasses quenched from simple basalt-like melts at pressures up to 10 GPa. The spectra establish that high field strength (i.e., Ca) modifier cations induce more high-coordinated Al than lower field strength (i.e., Na and K) cations. The amount of high-coordinated Al gradually increases with increasing synthesis pressure to percentages as high as 81 % for the Ca-aluminosilicate glass at 10 GPa, which yields an average Al-coordination of 5.25. Densities of glasses synthesized at 10 GPa are 8 % (K-aluminosilicate) to 16 % (Ca-aluminosilicate) higher than those quenched at ambient pressure. Additionally, a rapidly decompressed ($\sim$1 second) Ca-aluminosilicate glass (5GPa) was densified by about 14 % relative to the ambient pressure sample, compared to 11 % for the conventionally decompressed 5 GPa glass ($\sim$14 hours). Previous density measurements show that MORB melts at pressures between 5 and 10 GPa are roughly 20 to 30 percent more dense than the ambient pressure melt, which suggests that rapidly decompressed glasses retain much or even most of the total densification and structural changes that occur in the high-pressure melts. The observed increases in Al coordination and density show linear trends, which suggests that this structural change is a major part of the densification mechanism of high-pressure melts. In detail, the data reveal that other changes, such as the compression of modifier cation sites and/or decreased network bond angles, must also contribute to the densification of the melt, especially at low pressure ($<$ 3 GPa).
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
DE: 3924 High-pressure behavior
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting