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[SCG47-09] Development of a method for quantifying the water content of olivine in Peridotite using FT-IR method
Keywords:Fourier Transform Infrared Spectroscopy (FT-IR), Olivine, Kimberlite , Water content, Double-sided polished thin section, Spectral analysis algorithm
Olivine is a major constituent mineral in the upper mantle. Although olivine is basically an anhydrous mineral that does not contain water, a small amount of OH in the order of ppm can exist in defects in the crystal structure, which is called "water" in olivine. Since the amount of "water" (water content) is a factor that affects the physical properties of the upper mantle and the lower crust of the craton, studies on the determination of water content in olivine have been conducted for a long time. The water content in olivine is mainly measured by Fourier transform infrared spectroscopy (FT-IR), but the relationship between rock microstructure and water content has not been clarified because most studies are crashed rocks and picked up olivine grains one by one for analysis. In this study, we prepared double-sided polished thin sections of peridotite and performed FT-IR analysis to verify the distribution of "water" in the rocks.
Peridotite xenoliths of the kimberlite from the Kaapvaal Kraton (South Africa) were used for analysis. The sample contains many coarse-grained olivines larger than 1 mm, while serpentine is present at grain boundaries and in grain fractures. The rock samples were prepared as double-sided polished thin sections, and the olivine grains with relatively few fractures and high transparency were selected for FT-IR mapping analysis. Based on the water content formula reported by Mateev & Stachel (2007), a spectral analysis algorithm using MATLAB was developed to map the water content in the olivine grains.
As a result of the analysis, we found that to quantify the water content of grains with a thickness of less than 100 μm is difficult because the OH peak intensity of olivine was insufficient. In addition, the clear OH peak of olivine could not be obtained for the thickness of 200 μm or more, because other minerals such as serpentine were often mixed in the optical path. For the above reasons, a thickness of about 100–200 μm is considered appropriate for the analysis of double-sided polished thin sections that retain the microstructure of the rock. In addition, the OH peak of serpentine (around 3700 cm-1) is mixed near the grain boundary of olivine, and the water content of is overestimated. Therefore, an analysis step was incorporated to extract the water content of only the olivine grains by filtering out the serpentine peak.
As a result of the analysis using the above procedure, the water content of olivine grains in kimberlite from the Kaapvaal Kraton is about 200 ppm. The distribution of water content within a single grain is almost constant at 150–250 ppm, although there is a slight gradient in concentration.
This study established a method for mapping the water content in olivine by FT-IR measurement using double-sided polished thin sections. This method is expected to improve our understanding of the behavior and properties of water in the upper mantle by enabling us to compare the distribution of "water" in the mantle with information on microstructure and chemical composition.
[Reference] Matveev S. & Stachel T. (2007) Geochim. Cosmochim. Acta 71, 5528–5543.
Peridotite xenoliths of the kimberlite from the Kaapvaal Kraton (South Africa) were used for analysis. The sample contains many coarse-grained olivines larger than 1 mm, while serpentine is present at grain boundaries and in grain fractures. The rock samples were prepared as double-sided polished thin sections, and the olivine grains with relatively few fractures and high transparency were selected for FT-IR mapping analysis. Based on the water content formula reported by Mateev & Stachel (2007), a spectral analysis algorithm using MATLAB was developed to map the water content in the olivine grains.
As a result of the analysis, we found that to quantify the water content of grains with a thickness of less than 100 μm is difficult because the OH peak intensity of olivine was insufficient. In addition, the clear OH peak of olivine could not be obtained for the thickness of 200 μm or more, because other minerals such as serpentine were often mixed in the optical path. For the above reasons, a thickness of about 100–200 μm is considered appropriate for the analysis of double-sided polished thin sections that retain the microstructure of the rock. In addition, the OH peak of serpentine (around 3700 cm-1) is mixed near the grain boundary of olivine, and the water content of is overestimated. Therefore, an analysis step was incorporated to extract the water content of only the olivine grains by filtering out the serpentine peak.
As a result of the analysis using the above procedure, the water content of olivine grains in kimberlite from the Kaapvaal Kraton is about 200 ppm. The distribution of water content within a single grain is almost constant at 150–250 ppm, although there is a slight gradient in concentration.
This study established a method for mapping the water content in olivine by FT-IR measurement using double-sided polished thin sections. This method is expected to improve our understanding of the behavior and properties of water in the upper mantle by enabling us to compare the distribution of "water" in the mantle with information on microstructure and chemical composition.
[Reference] Matveev S. & Stachel T. (2007) Geochim. Cosmochim. Acta 71, 5528–5543.