HR: 11:05h
AN: DI42A-04    [Abstracts]
TI: Mineral Specific IR Molar Absorption Coefficients for Routine Water Determination in Olivine, SiO2 polymorphs and Garnet
AU: * Thomas, S
EM: smthomas@gfz-potsdam.de
AF: GFZ Potsdam, Telegrafenberg, Potsdam, 14473, Germany
AU: Koch-Mueller, M
EM: mkoch@gfz-potsdam.de
AF: GFZ Potsdam, Telegrafenberg, Potsdam, 14473, Germany
AU: Reichart, P
EM: patrick.reichart@unibw.de
AF: Universitaet der Bundeswehr, LRT 2, Neubiberg, 85577, Germany
AU: Rhede, D
EM: rhede@gfz-potsdam.de
AF: GFZ Potsdam, Telegrafenberg, Potsdam, 14473, Germany
AU: Thomas, R
EM: thomas@gfz-potsdam.de
AF: GFZ Potsdam, Telegrafenberg, Potsdam, 14473, Germany
AB: Conventionally applied Infrared (IR) calibrations [1, 2] for quantitative water analyses in solids are established on hydrous minerals and glasses with several wt% water. These calibrations are based on a negative correlation between the IR molar absorption coefficient (ε) for water and the mean wavenumber of the corresponding OH pattern. The correlation reflects the dependence of the OH band position on the appropriate O- H...O distances and thereby the magnitude of the dipole momentum which is proportional to the band intensity. However, it has been observed that these calibrations can not be adopted to nominally anhydrous minerals (NAMs) [3].
To study the potential dependence of ε on structure and chemistry in NAMs we synthesized olivine and SiO₂ polymorphs with specific isolated hydroxyl point defects, e.g. quartz, coesite and stishovite with B³⁺+H⁺=Si⁴⁺ and/or Al³⁺+H⁺=Si⁴⁺ substitutions. Experiments were performed with water in excess in piston cylinder and multi-anvil presses. Single crystal IR spectra demonstrate that we successfully managed to seperate generally complex OH patterns as e.g. observed in natural quartz and synthetic coesite. We quantified sample water contents of both natural samples and our run products by applying proton-proton-scattering [4], confocal microRaman spectroscopy [5] and Secondary Ion mass spectrometry. Resulting water concentrations were used to calculate new mineral specific εs.
For olivine with the mean wavenumber of 3517 cm^-1 we determined an ε value of 41,000±5,000 lmol^-1_H2Ocm^-2. Quantification of olivine with the mean wavenumber of 3550 cm^-1 in contrast resulted in an ε value of 47,000±1,000 lmol^-1_H2Ocm^-2. Taking into account previous studies [6, 7] there is evidence to suggest a linear wavenumber dependent correlation for olivine, where ε increases with decreasing wavenumber.
In case of the SiO₂ system it turns out that the magnitude of ε within one structure type is independent of the liable OH point defect and therewith the wavenumber of the observed band position. Consequently, one single mean ε of 68,000±5,000 lmol^-1_H2Ocm^-2 could be determined for a suite of quartz samples with varying OH point defects. In contrast, ε varies with the structure itself. For polymorphic coesite we calculated a different ε of 214,000±8,000 lmol^-1_H2O}cm^-2, that is in good agreement with earlier established data [8]. Quantification data of stishovite resulted in an even higher value of ε=867,000±29,000 lmol^-1_H2Ocm^-2, similar to that determined by [9]. First data on natural garnet give an ε value of 40,000±2,000 lmol^-1_H2Ocm^-2, that confirms prior suggested values [10].
Our results demonstrate that not using mineral specific calibrations for quantitative water analyses in NAMs leads to overestimation of sample water concentrations, that are required for modelling the earth's deep water cycle. [1]Paterson, M. S. (1982), Bull. Min., 105, 20-29. [2]Libowitzky, E., Rossman, G. R. (1997), Am. Min., 82, 1111- 1115. [3]Rossman, G. R. (2006), Rev. Mineral., 62, 1-28. [4]Reichart et al. (2004), Science, 306, 1537-1540. [5]Thomas et al. (2006), Am. Min., 91, 467-470. [6]Bell et al. (2003), JGR, 108, (B2), 2105-2113. [7]Koch-Mueller et al. (2006), PCM, 33, 276-287. [8]Koch-Mueller et al. (2001), PCM, 28, 693-705. [9]Pawley et al. (1993), Science, 261, 1024-1026. [10]Maldener et al. (2003), PCM, 30, 337-344.
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3621 Mantle processes (1038)
DE: 3630 Experimental mineralogy and petrology
DE: 3694 Instruments and techniques
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting