Extending the applicability of the Raman carbonaceous-material geothermometer using data from contact metamorphic rocks
Extending the applicability of the Raman carbonaceous-material geothermometer using data from contact metamorphic rocks
复制标题
DOI：
10.1111/j.1525-1314.2010.00896.x
复制
发表时间：
2010-12-01
影响因子：
3.4
通讯作者：
Wallis, S.
中科院分区：
文献类型：
--
作者：
Aoya, M.;Kouketsu, Y.;Wallis, S.
碳质材料的石墨化度(CM)已被广泛用作变质程度的指标。前人的工作表明，区域变质岩的峰值变质温度(T)可以通过CM拉曼光谱中识别的峰的面积比(R2)来估计。通过对日本两个接触变质晕(Daimonji和Kasuga地区)的样品进行拉曼光谱分析，验证了该方法在低压接触变质作用中的适用性。合适的测量程序允许测试地温计对样品类型(薄片、芯片)和激光束相对于CM c轴的入射角的依赖关系。两个重要的一般结果是：(I)除了标准薄片外，芯片也适用于光谱分析；(Ii)激光束的入射角度对温度估计没有显著影响，即地质温度计的光谱测量可以不受晶体取向的影响。这项研究使用了532 nm的激光波长，而之前的一项独立研究使用了514.5 nm的激光波长。比较表明，与514.5 nm激光相比，使用532 nm激光产生的R2比略大，但系统地更大。考虑到这一影响，我们的结果表明，接触变质岩和区域变质岩的R2-T关联式存在着微小但明显的差异：在相同的温度下，前者的结晶略好于后者(R2值略低)。这种差异被解释为是由于相关变形的程度造成的。尽管有微小的差异，但本研究的结果与先前提出的拉曼CM地温计的估计误差在+/-50摄氏度内一致，从而证明了该方法在接触变质作用中的适用性。为了便于更准确地估计接触变质作用区域的温度，导出了一种使用532 nm激光进行分析的新定标。另一个重要的观察结果是，泥质岩和砂岩中变质CM的R2比相对于单个样品是高度不均匀的。为了获得可靠的温度估计，必须使用大量的测量结果(通常为N&gT；50)来确定平均R2值，以充分反映样品的异质性范围。使用这种方法(使用532 nm激光)并采用我们的新定标，接触变质岩的拉曼CM地温计的误差降至类似于+/-30℃。
The degree of graphitization of carbonaceous material (CM) has been widely used as an indicator of metamorphic grade. Previous work has demonstrated that peak metamorphic temperature (T) of regional metamorphic rocks can be estimated by an area ratio (R2) of peaks recognized in Raman spectra of CM. The applicability of this method to low-pressure (< 3 kbar) contact metamorphism was tested using Raman spectroscopic analyses of samples from two contact-metamorphic aureoles in Japan (Daimonji and Kasuga areas). A suitable measurement procedure allows the dependence of the geothermometer on sample type (thin section, chip) and incident angle of laser beam relative to the c-axes of CM to be tested. Two important general results are: (i) in addition to standard thin sections, chips are also suitable for spectral analysis; and (ii) the incident angle of the laser beam does not significantly affect the temperature estimation, i.e. spectral measurements for the geothermometer can be carried out irrespective of the crystallographic orientation. A laser wavelength of 532 nm was used in this study compared with 514.5 nm in an independent previous study. A comparison shows that the use of a 532-nm laser results in a slightly, but systematically larger R2 ratio than that of a 514.5-nm laser. Taking this effect into account, our results show that there is a slight but distinct difference between the R2-T correlations shown by contact and regional metamorphic rocks: the former are slightly better-crystallized (have slightly lower R2 values) than the latter at the same temperature. This difference is interpreted as due to the degree of associated deformation. Despite the slight difference, the results of this study coincide within the estimated errors of +/- 50 degrees C with those of the previously proposed Raman CM geothermometer, thus demonstrating the applicability of this method to contact metamorphism. To facilitate more precise temperature estimates in regions of contact metamorphism, a new calibration for analyses using a 532-nm laser is derived. Another important observation is that the R2 ratio of metamorphosed CM in pelitic and psammitic rocks is highly heterogeneous with respect to a single sample. To obtain a reliable temperature estimate, the average R2 value must be determined by using a substantial number of measurements (usually N > 50) that adequately reflects the range of sample heterogeneity. Using this procedure (with 532-nm laser) and adapting our new calibration, the errors of the Raman CM geothermometer for contact metamorphic rocks decrease to similar to +/- 30 degrees C.