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.%橄榄石的含水量对上地幔的物理化学性质具有重要影响.傅里叶变换红外光谱(简称FTIR) 是目前最常用的测量名义上无水矿物含水量的方法.由于矿物中红外光谱的吸收系数是各向异性的,理论上应使用偏振光来测量橄榄石的含水量,但是该方法需要把橄榄石颗粒分离出来进行定向,非常费时.前人大多使用非偏振光来测量薄片中多个橄榄石的含水量,并使用Paterson (1982) 的校正方法计算样品中橄榄石的平均含水量.非偏振光的使用忽略了颗粒定向造成的影响,会低估橄榄石的含水量.上地幔橄榄石常发育晶格优选定向,为快速准确地获得橄榄石的原位含水量,该文将电子背散射衍射技术(EBSD) 与FTIR 相结合,提出测量薄片中橄榄石含水量的新方法.首先使用Bell 等(2003) 的红外偏振光实验结果,通过橄榄石[100]、[010]和[001]方向的含水量吸收系数建立一个椭球体.然后使用EBSD 测量某一颗粒在薄片中的取向,根据欧拉角计算该方向的含水量吸收系数Ws,再根据Beer-Lambert 公式获得该颗粒的含水量.对加拿大Slave 克拉通Muskox 金伯利岩携带的石榴橄榄岩中橄榄石的含水量研究表明,该方法可用于测量显微尺度的橄榄石原位含水量分布,为橄榄石含水量与组构关系的研究提供基础资料.
展开▼
