[MTT51-11] SIMSによる火山ガラス含水量測定におけるマトリックス効果補正法の確立
キーワード：火山ガラス、SIMS、含水量測定
In the analyses of concentrations and isotope ratios by SIMS (Secondary Ion Mass Spectrometry), matrix-matched standard samples are required due to a matrix effect on correction coefficients. Regarding the water content of volcanic glasses, Miyagi and Yurimoto (Bull. Volcanol. Soc. Japan. 1995, v40, p349) reported that there is a matrix effect, but Hauri et al., (Chem. Geol. 2006, v235, p352) and Shimizu et al. (Geochem. J. 2017, v51, p299) reported that rhylolitic to basaltic glasses share a single linear calibration curve, and that the matrix effect was considered to be small.
In this study, we prepared ~50 volcanic glasses with foiditic, basaltic, andesitic, and rhyolitic compositions, that are collected from deep seafloor, or experimental products. Their H2O contents were determined by the manometry method (rhyolitic glasses; Yamashita, J. Petrol. 1999, v40, p1497) or FTIR (Fourier transform infrared spectrometer), and were ranging from 0.02-4.8 wt%. The analyses were performed using CAMECA ims-1280HR at JAMSTEC, under the same conditions as Shimizu et al. (2017). The primary ion of Cs+ of 20 keV 0.5 nA with 10 keV of eletron gun is used to analyze the 5 micron area on the surface of the volcanic glasses, and the 16OH/30Si ratios were compared with H2O contents.
We observed that the calibration curvesof H2O vs. 16OH/30Si differed significantly depending on the composition of the volcanic glasses. At the same 16OH/30Si ratio, the water content differed up to five times depending on the composition of volcanic glasses, and we verified that there was a matrix effect. Although the calibration curve slopes of water [H2O/(16OH/30Si)] weakly correlated with their SiO2 content, they are more likely to be correlated with their molar weights (g/mol, a one oxygen mole basis) as King et al. (Am. Mineral. 2002, v87, p1077) reported. We suggest that maritx effect on H2O content of volcanic glasses could be corrected with their molar weights, being applicable to volcanic glasses with any chemical composition.
In this study, we prepared ~50 volcanic glasses with foiditic, basaltic, andesitic, and rhyolitic compositions, that are collected from deep seafloor, or experimental products. Their H2O contents were determined by the manometry method (rhyolitic glasses; Yamashita, J. Petrol. 1999, v40, p1497) or FTIR (Fourier transform infrared spectrometer), and were ranging from 0.02-4.8 wt%. The analyses were performed using CAMECA ims-1280HR at JAMSTEC, under the same conditions as Shimizu et al. (2017). The primary ion of Cs+ of 20 keV 0.5 nA with 10 keV of eletron gun is used to analyze the 5 micron area on the surface of the volcanic glasses, and the 16OH/30Si ratios were compared with H2O contents.
We observed that the calibration curvesof H2O vs. 16OH/30Si differed significantly depending on the composition of the volcanic glasses. At the same 16OH/30Si ratio, the water content differed up to five times depending on the composition of volcanic glasses, and we verified that there was a matrix effect. Although the calibration curve slopes of water [H2O/(16OH/30Si)] weakly correlated with their SiO2 content, they are more likely to be correlated with their molar weights (g/mol, a one oxygen mole basis) as King et al. (Am. Mineral. 2002, v87, p1077) reported. We suggest that maritx effect on H2O content of volcanic glasses could be corrected with their molar weights, being applicable to volcanic glasses with any chemical composition.