Application of the Raman Spectroscopy to Identification of Titanomagnetites

The titanomagnetite-magnetite series serves as important magnetic carriers in paleomagnetic studies. Commonly Curie points are used to determine the composition of these magnetic phases. However, Curie points generally reflect bulk rock properties and do not provide insight for individual grains. Determination of individual Fe-Ti oxide grains can be done by petrography or with electron microprobe techniques. In contrast to these methods, which require special sample preparation, Raman spectroscopy can be done with minimal preparation. We have therefore investigated the Raman spectra for magnetite, TM20, TM40, and TM60, so that we can identify titanomagnetites with different Ti content in rocks. The samples were prepared following methods of Wanamaker and Moskovitz (1994). The Ti concentrations were verified by thermomagnetic analysis (Gilder and LeGoff, 2005). The Raman spectra were obtained with WITec Raman Confocal Microscope 200 using a green laser. Slight shifts and changes in relative intensities of the peaks at the characteristic wavelengths of the Raman spectra that correspond to different molecular vibrations were observed. These changes can serve to identify the composition of individual titanomagnetite grains. It also enables mapping of the variation of the composition within grains and the distribution of compositions of grains within a rock. We have applied the technique to synthetic Martian samples and found relatively uniform compositions between different grains. In contrast, variable oxide compositions are discerned using the Raman spectroscopy in natural basalts erupted from East Maui volcano.