11:15 〜 11:30
[SIT19-09] Determination of 15N/14N ratios in reduced silicate glasses using Raman spectroscopy.
キーワード：Nitrogen isotopes, Raman, reduced glasses
Although nitrogen is fundamental to the evolution and persistence of Earth’s atmosphere and biosphere, many open questions are still pending about the present-day isotope distribution of its two stable isotopes 14N and 15N (~99.6% and ~0.4%, respectively) between the different deep geochemical reservoirs. Terrestrial reservoirs show a strongly heterogeneous distribution between the 15N-depleted mantle and the 15N-enriched surface. The use of N isotopes to address possible origins of characteristic N isotope signatures in deep Earth reservoirs requires constraints on the magnitude of fractionation between minerals, melts and fluids within the mantle. Because the determination of N isotope fractionation during magmatic processes involving aqueous fluids rely on experiments, we have developed the analytical protocol for measuring N isotopes by Raman spectroscopy, to be subsequently applied to in-situ hydrothermal diamond anvil cell experiments.
We present Raman spectra acquired in compositionally simple silicate glasses (NS4, an anorthite-diopside eutectic glass in CMAS) and in a haplobasalt, a haploandesite and a haplorhyolite, enriched in variable proportions of Si315N4 and Si314N4 (15N/14N: 0.004, 0.08, 0.2, 0.44 and 5.4). We added variable amounts of Si3N4 (a strongly reducing agent) to vary the oxygen fugacity among haplobasaltic glasses. Glasses were synthetized at 1.5 GPa and 1600°C for 4 hours in graphite capsules (to ensure reducing and graphite saturated conditions). Complementary to Raman, glasses were analyzed by secondary ion mass spectrometry (SIMS) to check the N content and 15N/14N molar ratios.
Raman spectra were acquired using the 457.9 nm (blue) excitation source with 13mW on samples, allowing us to detect CN, CO, CH3, CH4, NH3, NH2, OH and H2 species in all glasses. We show that Raman spectroscopy has sufficient resolution to separate the 14N-H vibration peaks from the 15N-H peaks in NH3 and NH2. At this stage, more data are being acquired to discuss the effect of the fO2 and melt composition on the 15N/14N ratio between different N species.
We present Raman spectra acquired in compositionally simple silicate glasses (NS4, an anorthite-diopside eutectic glass in CMAS) and in a haplobasalt, a haploandesite and a haplorhyolite, enriched in variable proportions of Si315N4 and Si314N4 (15N/14N: 0.004, 0.08, 0.2, 0.44 and 5.4). We added variable amounts of Si3N4 (a strongly reducing agent) to vary the oxygen fugacity among haplobasaltic glasses. Glasses were synthetized at 1.5 GPa and 1600°C for 4 hours in graphite capsules (to ensure reducing and graphite saturated conditions). Complementary to Raman, glasses were analyzed by secondary ion mass spectrometry (SIMS) to check the N content and 15N/14N molar ratios.
Raman spectra were acquired using the 457.9 nm (blue) excitation source with 13mW on samples, allowing us to detect CN, CO, CH3, CH4, NH3, NH2, OH and H2 species in all glasses. We show that Raman spectroscopy has sufficient resolution to separate the 14N-H vibration peaks from the 15N-H peaks in NH3 and NH2. At this stage, more data are being acquired to discuss the effect of the fO2 and melt composition on the 15N/14N ratio between different N species.