5:15 PM - 6:45 PM
[SIT14-P09] Nitrogen solubility and speciation in oxidized magmas of planetesimals
Keywords:Nitrogen solubility, Magma ocean, Carbonaceous chondrites
To constrain the origin of life and atmospheres on terrestrial planets, it is important to clarify how life-essential and atmospheric volatiles were acquired during their formation. Carbonaceous chondrites (CCs) are thought to be the major building blocks of terrestrial planets and have played an important role in the volatile delivery to the planet. Previous studies have reported that nitrogen in carbon-rich achondrites, such as ureilites is more depleted than carbon and argon compared to primitive CCs (e.g., Marty et al., 2020). There are two possible causes of the depletion of nitrogen: An incorporation of nitrogen into the core and the selective loss of nitrogen to the space by degassing. In this study, we experimentally investigated the latter possibility.
Previous experimental studies have shown that nitrogen solubility in silicate melt is controlled by pressure, silicate-melt composition, and oxygen fugacity (e.g., Dasgupta et al., 2022; Gao et al., 2022). However, it is not still poorly constrained which factor is the most significant for controlling the solubility of nitrogen in the silicate melt, especially at conditions of differentiated planetesimals composed of CCs. Here we report the solubility of nitrogen in silicate melts under relatively oxidizing conditions similar to CCs.
High-pressure experiments were conducted at 1500-1600 ℃ and 3-5 GPa using the 1000-ton multi-anvil press at Geodynamics Research Center (GRC), Ehime University. The silicate part of the starting material was prepared from the mixture of oxides to be mid-ocean ridge basalt composition. Then, ammonium nitrate (NH4NO3) was added to the mixture as a nitrogen source with the mixing ratio of 20 wt% to the starting material. The starting material was enclosed in a graphite capsule and surrounded by a welded Pt capsule to prevent the loss of volatiles during heating experiments. To investigate the effect of carbon on the solubility of nitrogen in silicate melts, a SiO2 glass-Pt double capsule was also used in some experiments. For a SiO2 glass-Pt double capsule, the oxygen fugacity of the samples was controlled by Mo-MoO2 buffer or Re-ReO2 buffer.
Nitrogen speciation in the quenched glass of the recovered samples was identified using Raman spectroscopy at GRC and electron probe micro-analyzer (EPMA) equipped with a soft X-ray emission spectrometer (SXES) at the Institute for Planetary Materials, Okayama University. Nitrogen content in the quenched glass was determined by EPMA at GRC.
Raman spectra of the recovered samples indicate the presence of N2 as the dominant nitrogen specie in our experimental conditions. Nitrogen contents in the quenched glass of the recovered samples were estimated from 198 to 1699 ppm and well consistent with the area of N2 peak of the Raman spectrum. The nitrogen content in the quenched glass was also well correlated with NBO/T (non-bridging oxygen per tetrahedral cations) which describes the polymerization degree of silicate melts. Because NBO/T of the silicate melt is related to its structural vacancy (Ionic porosity) (e.g., Marrocchi and Toplis, 2005), the relationship between N2 solubility in the silicate melt and NBO/T suggests that the presence of such a structural vacancy in silicate melts is the main controlling factor for N2 solubility. Because N2 is chemically inert, noble gases, such as argon are also expected to have a similar solubility. At a given pressure and NBO/T conditions, N2 solubility in the silicate melt is nearly one order of magnitude lower than argon probably due to the difference in the size between N2 and Ar. This suggests a possible occurrence of solubility-controlled fractionation of N/Ar ratios during the magma-ocean stage of planetesimals under N2-stable conditions. The lower solubility of N2 in silicate melt relative to Ar may naturally explain the lower N/Ar ratios of partially differentiated parent bodies of thermally altered CCs (i.e., CV and CO) and ureilites than primitive ones (i.e., CM and CI).
Previous experimental studies have shown that nitrogen solubility in silicate melt is controlled by pressure, silicate-melt composition, and oxygen fugacity (e.g., Dasgupta et al., 2022; Gao et al., 2022). However, it is not still poorly constrained which factor is the most significant for controlling the solubility of nitrogen in the silicate melt, especially at conditions of differentiated planetesimals composed of CCs. Here we report the solubility of nitrogen in silicate melts under relatively oxidizing conditions similar to CCs.
High-pressure experiments were conducted at 1500-1600 ℃ and 3-5 GPa using the 1000-ton multi-anvil press at Geodynamics Research Center (GRC), Ehime University. The silicate part of the starting material was prepared from the mixture of oxides to be mid-ocean ridge basalt composition. Then, ammonium nitrate (NH4NO3) was added to the mixture as a nitrogen source with the mixing ratio of 20 wt% to the starting material. The starting material was enclosed in a graphite capsule and surrounded by a welded Pt capsule to prevent the loss of volatiles during heating experiments. To investigate the effect of carbon on the solubility of nitrogen in silicate melts, a SiO2 glass-Pt double capsule was also used in some experiments. For a SiO2 glass-Pt double capsule, the oxygen fugacity of the samples was controlled by Mo-MoO2 buffer or Re-ReO2 buffer.
Nitrogen speciation in the quenched glass of the recovered samples was identified using Raman spectroscopy at GRC and electron probe micro-analyzer (EPMA) equipped with a soft X-ray emission spectrometer (SXES) at the Institute for Planetary Materials, Okayama University. Nitrogen content in the quenched glass was determined by EPMA at GRC.
Raman spectra of the recovered samples indicate the presence of N2 as the dominant nitrogen specie in our experimental conditions. Nitrogen contents in the quenched glass of the recovered samples were estimated from 198 to 1699 ppm and well consistent with the area of N2 peak of the Raman spectrum. The nitrogen content in the quenched glass was also well correlated with NBO/T (non-bridging oxygen per tetrahedral cations) which describes the polymerization degree of silicate melts. Because NBO/T of the silicate melt is related to its structural vacancy (Ionic porosity) (e.g., Marrocchi and Toplis, 2005), the relationship between N2 solubility in the silicate melt and NBO/T suggests that the presence of such a structural vacancy in silicate melts is the main controlling factor for N2 solubility. Because N2 is chemically inert, noble gases, such as argon are also expected to have a similar solubility. At a given pressure and NBO/T conditions, N2 solubility in the silicate melt is nearly one order of magnitude lower than argon probably due to the difference in the size between N2 and Ar. This suggests a possible occurrence of solubility-controlled fractionation of N/Ar ratios during the magma-ocean stage of planetesimals under N2-stable conditions. The lower solubility of N2 in silicate melt relative to Ar may naturally explain the lower N/Ar ratios of partially differentiated parent bodies of thermally altered CCs (i.e., CV and CO) and ureilites than primitive ones (i.e., CM and CI).