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Effect of Water on the Equation of State of Nominally Anhydrous Minerals

Department of Geological Sciences Northwestern University Evanston, Illinois, 60208, U.S.A., e-mail: steven@earth.northwestern.edu

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
It is possible that the majority of Earth’s H₂O budget is present as hydroxyl (OH) structurally incorporated into the major nominally anhydrous minerals (NAMs) of the mantle (e.g., Martin and Donnay 1972). Ringwood (1966) thought as much as five times the surface H₂O-mass could be present in the mantle, amounting to ~0.2 wt% H₂O if distributed throughout the entire mantle (Harris and Middlemost 1969). We know now the (Mg,Fe)₂SiO₄ polymorphs of the upper mantle and transition zone can incorporate up to several weight percent of water in their structures (e.g., Smyth 1987; Inoue et al. 1995; Kohlstedt et al. 1996; Bolfan-Casanvoa et al. 2000; Mosenfelder et al. 2006). The possibility of a deep-Earth water cycle leads naturally to the question, is "water" in the mantle, whether regional or globally distributed, detectible seismically?

In order to address this question, it is necessary to know, quantitatively, the effects of water (or more precisely structurally bound hydroxyl) on the elastic moduli of mantle minerals. Also required are pressure and temperature derivatives of the elastic moduli for more direct comparison with seismological observation. This chapter will review what is known about the elastic properties of "hydroxylated" NAMs from experimental studies. From a crystal chemical perspective, hydrated NAMs are defect structures because hydrogen is usually incorporated through charge balance by cation vacancies. Therefore, small variations in water content can have a dramatic effect on thermoelastic parameters, more so than any other major geochemical substitution such as iron or aluminum. For example, at one atmosphere the addition of ~1 wt% H₂O into ringwoodite has a similar effect on the shear modulus as raising the temperature by 800–1000 °C (Wang et al. 2003a; Jacobsen et al. 2004). However, due to elevated pressure . . . [Full Text of this Article]

This article has been cited by other articles:

				E. Libowitzky and A. Beran The Structure of Hydrous Species in Nominally Anhydrous Minerals: Information from Polarized IR Spectroscopy Reviews in Mineralogy and Geochemistry, January 1, 2006; 62(1): 29 - 52. [Full Text] [PDF]

				J. R. Smyth Hydrogen in High Pressure Silicate and Oxide Mineral Structures Reviews in Mineralogy and Geochemistry, January 1, 2006; 62(1): 85 - 115. [Full Text] [PDF]