Cation field strength effects on high pressure aluminosilicate glass structure: Multinuclear NMR and La XAFS results Kimberly E. Kelsey a, * , Jonathan F. Stebbins a , David M. Singer a , Gordon E. Brown Jr. a,b , Jed L. Mosenfelder c , Paul D. Asimow c a Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA b Stanford Synchrotron Radiation Lightsource, MS 69, SLAC, 2545 Sand Hill Road, Menlo Park, CA 94025, USA c Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA Received 18 September 2008; accepted in revised form 30 March 2009; available online 14 April 2009 Abstract We examined aluminosilicate glasses containing a variety of network modifying to intermediate cations (Li, La, Sc, and Fe), quenched from melts at 1 atm to 8 GPa, to further investigate the role of cation field strength in Al coordination changes and densification. 27 Al Nuclear Magnetic Resonance Spectroscopy (NMR) reveals that the mean Al coordination increases with increasing pressure in the Li-containing glasses, which can be explained by a linear dependence of fractional change in Al coordination number on cation field strengths in similar K-, Na-, and Ca-containing aluminosilicate glasses (K < Na < Li < Ca). Measured recovered densities follow a similar linear trend. In contrast, the La-containing glasses have significantly lower mean Al coordination numbers at given pressures than the cation field strength of La and glass density would predict. La L 3 X-ray absorption fine structure (XAFS) spectroscopy results indicate a significant increase with pressure in average La–O bond distances, suggesting that La and Al may be ‘‘competing” for higher coordinated sites and hence that both play a significant role in the densification of these glasses, especially in the lower pressure range. However, in Na alu- minosilicate glasses with small amounts of Sc, 45 Sc NMR reveals only modest Sc coordination changes, which do not seem to significantly affect the mean Al coordination values. For a Li aluminosilicate glass, 17 O MAS and multiple quantum magic angle spinning (3QMAS) NMR data are consistent with generation of more highly coordinated Al at the expense of non- bridging oxygen (NBO), whereas La aluminosilicate glasses have roughly constant O environments, even up to 8 GPa. Final- ly, we demonstrate that useful 23 Na and 27 Al MAS NMR spectra can be collected for Ca–Na aluminosilicate glasses contain- ing up to 5 wt.% Fe oxide. We discuss the types of structural changes that may accompany density increases with pressure and how these structural changes are affected by the presence of different cations. Ó 2009 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Igneous processes such as partial melting, magma as- cent, magma emplacement, crystal nucleation, and crystal growth are controlled by macroscopic properties of the melt, such as viscosity, density, and diffusivity. These phys- ical properties are in turn controlled by the atomic-scale structure and composition of the melt, including cation coordination environments (Kushiro, 1976; Stolper et al., 1981; Rigden et al., 1984; Scarfe et al., 1987; Xue et al., 1994; Wolf and McMillan, 1995; Kushiro and Mysen, 1992). As melts are subjected to increasing pressure, they undergo structural changes, which may include decreases in bond angles and distances or increases in cation coordi- nation numbers. The link between these types of structural changes and the densification of high pressure melts as a function of composition is important for better understand- ing igneous processes. Due to difficulties associated with studying high pressure melts in situ, it is common to study glasses formed by quickly cooling a melt, which, at least to 0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009.03.040 * Corresponding author. E-mail address: kkelsey@stanford.edu (K.E. Kelsey). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 73 (2009) 3914–3933