5:15 PM - 7:15 PM
[SVC33-P06] Using Polarizer into Raman Spectroscopy for Knowing Amount of Dissolved H2O in Melt Inclusion in Opx
Keywords:Raman Spectroscopy, Polarizer, Melt Inclusion, Dissolved Water
The water content of magma attracts everyone’s interest because it greatly affects its viscosity and foaminess. However, magma undergoes various disturbances near the surface, such as degassing and reaction with groundwater. Hence it does not preserve subsurface information. Therefore, the water content of melt inclusions in ejecta has been used as an indicator. The method to measure the water content of melt inclusions by micro-Raman spectroscopy (H. Behrens et al., 2006) has advantages over other mass spectrometry methods in that it is non-destructive and simple to perform, and it has a high spatial resolution of less than 1 µm. This technique has been used for olivine. However, when the host mineral is other than olivine, a peak derived from the host mineral appears at a position overlapping the peak derived from the glass.
Minerals have a crystal structure, and the bonding direction of atoms is regular. On the other hand, glasses have random bonding directions. Raman scattering light, which reflects the state of atomic bonding, is also orientation-dependent for minerals, but not for amorphous materials. In this study, we conducted experiments to develop a method to reduce the Raman spectra of host minerals and to obtain the peak area of glasses with high accuracy by incorporating a polarizer in a micro-Raman spectrometer.
The experimental results showed that the water content of melt inclusions could be determined with an accuracy of up to ±0.2%. In this experiment, the fitting range and peak shape were unified by performing the fitting together. Since this process improved the accuracy, the accuracy of the water content is considered to be due to the fitting accuracy rather than measurement error, and is expected to be further improved by fixing the wavenumber position and other measures not implemented in this study.
On the other hand, the Raman spectrum of the glass changed with the angle of the polarizer, indicating that the glass observed in this study was azimuthal. Since the excitation laser has polarized light, the general micro-Raman spectroscopy without a polarizer may not be able to obtain highly accurate water content values because of the appearance of azimuthality. In fact, the water content in this experiment with polarizers was 2% lower than expected.
Minerals have a crystal structure, and the bonding direction of atoms is regular. On the other hand, glasses have random bonding directions. Raman scattering light, which reflects the state of atomic bonding, is also orientation-dependent for minerals, but not for amorphous materials. In this study, we conducted experiments to develop a method to reduce the Raman spectra of host minerals and to obtain the peak area of glasses with high accuracy by incorporating a polarizer in a micro-Raman spectrometer.
The experimental results showed that the water content of melt inclusions could be determined with an accuracy of up to ±0.2%. In this experiment, the fitting range and peak shape were unified by performing the fitting together. Since this process improved the accuracy, the accuracy of the water content is considered to be due to the fitting accuracy rather than measurement error, and is expected to be further improved by fixing the wavenumber position and other measures not implemented in this study.
On the other hand, the Raman spectrum of the glass changed with the angle of the polarizer, indicating that the glass observed in this study was azimuthal. Since the excitation laser has polarized light, the general micro-Raman spectroscopy without a polarizer may not be able to obtain highly accurate water content values because of the appearance of azimuthality. In fact, the water content in this experiment with polarizers was 2% lower than expected.