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[SMP25-P19] Strain in quartz inclusion probed by Raman spectroscopy
Keywords:strain, inclusion mineral, Raman spectroscopy, quartz, garnet, kyanite
The residual pressure is maintained on the host–inclusion system that reflect the metamorphic conditions when the inclusion was trapped in the host mineral due to the difference in the volume change during their exhumation. In recent years, Raman geothermobarometers have been developed to estimate the metamorphic conditions from the peak shift of Raman spectra of inclusion minerals. The conventional Raman geothermobarometers have assumed that the inclusion minerals are uniformly stressed from the surrounding mineral under the hydrostatic pressure, but in reality, the inclusion minerals are under deviatoric stress due to the anisotropy of the elastic modulus of host and inclusion minerals, and are subjected to the different strains in each axial direction. Murri et al. (2018) proposed a method to estimate the strains from the peak shifts of Raman spectrum using the Grüneisen tensor. In this study, we used a strain calculation program based on this principle (stRAinMAN; Angel et al., 2019) to estimate the strains of quartz inclusions in garnet and kyanite host minerals.
Quartz eclogite from the Gongen area of the Sanbagawa belt, SW Japan, and Lawsonite eclogite from the Motagua Fault Zone, Guatemala, were used for analysis. In addition, we also used garnetite data from the Lago di Cignana unit in the Western Alps reported by van Schrojenstein Lantman et al. (2020). For all samples, the Raman spectra of quartz inclusions in garnet were obtained and the peak shifts from the standard quartz were calculated to estimate the strain. In addition to quartz inclusions in garnet, quartz inclusions in kyanite were also analyzed in quartz eclogite samples from the Sanbagawa belt. The strains were calculated using the values of the peak shifts at 464 cm-1, 205 cm-1, and 128 cm-1 and the Grüneisen tensor of quartz (γ1+ γ2, γ3) reported by Murri et al. (2018), and the strains of ε1+ε2 and ε3 were obtained.
Calculations of the strain of quartz inclusions in garnet show that quartz eclogite from the Sanbagawa belt shows ε1+ε2 ranging from -12×10-3 to -24×10-3 and ε3 from +1×10-3 to -4×10-3, whereas Lawsonite eclogite from Motagua Fault Zone shows ε1+ε2 ranging from -19×10-3 to -26×10-3 and ε3 from -3×10-3 to -8×10-3. In the garnetite of the Lago di Cignana unit reported in the previous study, ε1+ε2 ranges from -12×10-3 to -24×10-3 and ε3 from +1×10-3 to -5×10-3, which is almost the same range as that of quartz eclogite in the Sanbagawa belt. On the other hand, the calculated strains of quartz inclusions in kyanite in the quartz eclogite of the Sanbagawa belt show a range of ε1+ε2 from -5×10-3 to -21×10-3 and ε3 from +4×10-3 to -4×10-3.
The strains estimated from the Raman spectra of quartz inclusions tended to be retained closer to the hydrostatic conditions for the samples with lower peak temperature conditions. Conversely, the sample of the highest temperature condition, the quartz eclogite of the Sanbagawa belt, tended to show a smaller c-axis strain (ε3) than under hydrostatic conditions. This result indicates that the higher the peak metamorphic temperature, the more easily the inclusion strain is relaxed, especially in the c-axis direction. In addition, the ε3 tended to be more relaxed for quartz inclusions in kyanite host. This is due to the anisotropy of the elastic modulus of kyanite, which may lead to resolve the strain, depending on the relationship of the crystal orientation of host and inclusion. Strain analysis of inclusion minerals using Raman spectroscopy is expected to provide new clues to reconstruct the metamorphic history in more detail.
References
Angel, R. J. et. al. (2019). Zeitschrift Für Kristallographie - Crystalline Materials, 234(2), 129–140.
Murri, M., et. al. (2018). American Mineralogist, 103(11), 1869–1872.
van Schrojenstein Lantman, H. W. et. al. (2020). Journal of Metamorphic Geology, 1–18.
Quartz eclogite from the Gongen area of the Sanbagawa belt, SW Japan, and Lawsonite eclogite from the Motagua Fault Zone, Guatemala, were used for analysis. In addition, we also used garnetite data from the Lago di Cignana unit in the Western Alps reported by van Schrojenstein Lantman et al. (2020). For all samples, the Raman spectra of quartz inclusions in garnet were obtained and the peak shifts from the standard quartz were calculated to estimate the strain. In addition to quartz inclusions in garnet, quartz inclusions in kyanite were also analyzed in quartz eclogite samples from the Sanbagawa belt. The strains were calculated using the values of the peak shifts at 464 cm-1, 205 cm-1, and 128 cm-1 and the Grüneisen tensor of quartz (γ1+ γ2, γ3) reported by Murri et al. (2018), and the strains of ε1+ε2 and ε3 were obtained.
Calculations of the strain of quartz inclusions in garnet show that quartz eclogite from the Sanbagawa belt shows ε1+ε2 ranging from -12×10-3 to -24×10-3 and ε3 from +1×10-3 to -4×10-3, whereas Lawsonite eclogite from Motagua Fault Zone shows ε1+ε2 ranging from -19×10-3 to -26×10-3 and ε3 from -3×10-3 to -8×10-3. In the garnetite of the Lago di Cignana unit reported in the previous study, ε1+ε2 ranges from -12×10-3 to -24×10-3 and ε3 from +1×10-3 to -5×10-3, which is almost the same range as that of quartz eclogite in the Sanbagawa belt. On the other hand, the calculated strains of quartz inclusions in kyanite in the quartz eclogite of the Sanbagawa belt show a range of ε1+ε2 from -5×10-3 to -21×10-3 and ε3 from +4×10-3 to -4×10-3.
The strains estimated from the Raman spectra of quartz inclusions tended to be retained closer to the hydrostatic conditions for the samples with lower peak temperature conditions. Conversely, the sample of the highest temperature condition, the quartz eclogite of the Sanbagawa belt, tended to show a smaller c-axis strain (ε3) than under hydrostatic conditions. This result indicates that the higher the peak metamorphic temperature, the more easily the inclusion strain is relaxed, especially in the c-axis direction. In addition, the ε3 tended to be more relaxed for quartz inclusions in kyanite host. This is due to the anisotropy of the elastic modulus of kyanite, which may lead to resolve the strain, depending on the relationship of the crystal orientation of host and inclusion. Strain analysis of inclusion minerals using Raman spectroscopy is expected to provide new clues to reconstruct the metamorphic history in more detail.
References
Angel, R. J. et. al. (2019). Zeitschrift Für Kristallographie - Crystalline Materials, 234(2), 129–140.
Murri, M., et. al. (2018). American Mineralogist, 103(11), 1869–1872.
van Schrojenstein Lantman, H. W. et. al. (2020). Journal of Metamorphic Geology, 1–18.