关键词: 橄榄石, 含水量, 傅里叶红外光谱, 电子背散射衍射, 晶格优选定向

Abstract: Water contents in olivine have significant effects on physical and chemical properties of the upper mantle. Water
concentration in nominally anhydrous minerals is mainly determined by Fourier transform infrared (FTIR) spectroscopy based on the
absorption coefficients of infrared light, which vary along different crystallographic directions in anisotropic minerals. Ideally, water
contents in olivine should be analyzed on oriented grains using the polarized infrared light. However, this method is very time
consuming because it requires mineral separation and orientation. Previous studies usually used unpolarized infrared light and
calculated the average water content of many olivine grains in a thin section by Paterson (1982) calibration. However, application of
unpolarized light assumes random orientation of mineral grains, which may underestimate water contents in olivine. Olivine in the
upper mantle often develops lattice preferred orientation. In order to obtain water contents in olivine in a more effective and accurate
way, this paper presents a new method to measure in situ water contents of olivine in a thin section using the combined electron
backscatter diffraction (EBSD) and FTIR analysis. Based on results of Bell et al. (2003), an ellipsoid of the water absorption coefficient(W) is established using W values along three crystallographic axes [100], [010] and [001]. Then based on the EBSD-derived orientation
of an olivine grain, the specific water absorption coefficient along a certain direction (Ws) in the ellipsoid is determined. Finally, water
content of the olivine grain is calculated using the Beer-Lambert equation. This method is applied to measure water concentration of
olivine from garnet peridotite xenoliths brought by Muskox kimberlites in the Slave Craton (Canada). The results demonstrate that this
method allows us to determine in situ water distribution of olivine in microscale and investigate the relationship between water contents
and fabrics of olivine.

Key words: olivine, water content, Fourier transform infrared, electron backscatter diffraction, lattice preferred orientation