- © The Mineralogical Society Of America
Hydrogen traces change the physical properties of mantle minerals to an extent that is far out of proportion to its low concentration. These properties include mechanical strength, melting behavior, diffusion rate, electrical conductivity, viscosity and rheology. Besides minerals of the pyroxene group (as discussed by Skogby 2006, this volume), the close-packed mineral structures of olivine, garnet and some accessory minerals offer important storage sites for hydrogen traces in the Earth’s mantle. However, the water content stored in olivine and mantle garnet is quite low compared to that in pyroxenes.
Based on chemical considerations, but also on information obtained from infrared (IR) data, Martin and Donnay (1972) proposed the existence of hydroxyl in nominally anhydrous minerals (NAMS) occurring in the upper mantle, especially in pyroxene and olivine. The review articles of Bell and Rossman (1992a), Skogby (1999), and Ingrin and Skogby (2000) reveal a wide range of water contents for mantle-derived pyroxenes, olivines and garnets, which are derived from IR spectroscopic data. Fourier transform infrared (FTIR) spectroscopy provides an extremely sensitive method for detecting trace hydrogen bonded to oxygen in the structures of various NAMS (Beran 1999; Beran and Libowitzky 2003; Libowitzky and Beran 2004). As this method is not self-calibrating, attempts have been made to calibrate the IR spectra with independent absolute methods. These methods include hydrogen manometry and measurement of thermally released water in an electrolytic cell or by Karl Fischer titration. 1H Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS NMR), Secondary Ion Mass Spectrometry (SIMS) and Nuclear Reaction Analysis (NRA) are encouraging but experimentally demanding and expensive methods (as discussed by Rossman 2006, this volume). To overcome these problems, approximations such as that proposed by Paterson (1982) and refined by Libowitzky and Rossman (1997), attempt to provide a way to deal with …