Presentation Information
[MZZ49-02]Development of a mass spectrometric method for identifying high-temperature evaporation products and its application to sulfide minerals
*Shiori Inada1, Atsuki Ishibashi1,2, Nami Sakai2, Tetsuya Hama1 (1.Department of Basic Science, The University of Tokyo, 2.RIKEN Pioneering Research Institute)
Keywords:
evaporation,sulfides,mass spectrometry,cosmochemistry,astronomical observation
Introduction
Evaporation of rocky planetary materials is a key process of elemental volatility fractionation found in Solar System samples. For example, various degrees of sulfur depletion in chondrules are interpreted as results of evaporation of sulfides such as FeS1. Previous laboratory studies in cosmochemistry have investigated evaporation kinetics of minerals and melts modeling meteorite components by analyzing evaporation residues, which has enabled discussions of timescales of evaporation events in the early Solar System2,3. However, chemical processes underlying measured kinetics have remained elusive largely due to the lack of knowledge about gaseous products of evaporation3-5. In addition, evaporation of rocky dust has recently attracted increasing attention in astronomy as a formation pathway of refractory molecules (e.g., SiS and CaS) detected around massive protostars6,7. Identification of evaporation products is important also as a basis for interpreting such observations. The biggest challenge for this has been the experimental difficulty in realizing collision-free conditions to detect nascent products of high-temperature evaporation of materials of interest (typically at >1000 K). To overcome this problem, in this study we developed a new experimental apparatus utilizing mass spectrometry. This apparatus was applied to sulfides contained in primitive chondrites (CaS, MgS, and FeS) to discuss their microscopic evaporation processes and a potential link to astronomical observations.
Methods
The apparatus consists of an ultra-high vacuum chamber (10−6-10−5 Pa during the experiments, corresponding to mean free paths of 103-104 m), a tungsten plate for resistive heating, a quadrupole mass spectrometer (QMS) placed ~10 cm above the heater, and a radiation thermometer. This configuration sufficiently fulfills the collision-free condition (mean free path >> detection distance). Powdered sulfide samples were loaded on the heater and were evaporated at up to ~1700 K. The released gases were detected by QMS after electron impact ionization.
Results and Discussion
CaS (s) and MgS (s) were found to evaporate mainly as Ca or Mg, S, and S2 with different S/S2 ratios. CaS (g) was detected as a minor product from CaS (s), whereas MgS (g) was below the detection limit. FeS (s) evaporated incongruently unlike the other samples as S2, S, and S3. Although the observed evaporation behaviors are consistent with thermodynamics in certain respects (e.g., congruency/incongruency), the products do not necessarily follow thermodynamics. For example, the present results are inconsistent with PeqCaS/PeqCa < PeqMgS/PeqMg (Peqi: equilibrium vapor pressure of species i) above CaS (s) and MgS (s) at the experimental temperatures. The ratios of the sulfur species from FeS (s) are also different from the equilibrium pressure ratios. These results suggest that the evaporation processes are governed by molecular-level dynamics of crystal decomposition and surface reactions between the fragments before release to the gas phase, rather than the Gibbs energy minimization. This should be the reason why the evaporation kinetics cannot be interpreted only in terms of the simple kinetic model based on equilibrium calculations. The present results also help interpretation of observations of sulfur-bearing refractory molecules around protostars, providing hints about astronomical settings of evaporation recorded in Solar System materials. Identification of evaporation products realized by our new method will thus enable multifaceted understanding of evaporation of planetary materials and its roles in the Solar System material evolution.
References
1Tachibana & Huss (2005) GCA 69, 3075.
2Tachibana & Tsuchiyama (1998) GCA 62, 2005.
3Richter et al. (2007) GCA 71, 5544.
4Inada et al. (2024) JCP 160, 154710.
5Inada et al. (2025) GCA 404, 172.
6Tanaka et al. (2020) ApJL 900, L2.
7Tasa-Chaveli et al. (2025) ApJL 993, L42.
Evaporation of rocky planetary materials is a key process of elemental volatility fractionation found in Solar System samples. For example, various degrees of sulfur depletion in chondrules are interpreted as results of evaporation of sulfides such as FeS1. Previous laboratory studies in cosmochemistry have investigated evaporation kinetics of minerals and melts modeling meteorite components by analyzing evaporation residues, which has enabled discussions of timescales of evaporation events in the early Solar System2,3. However, chemical processes underlying measured kinetics have remained elusive largely due to the lack of knowledge about gaseous products of evaporation3-5. In addition, evaporation of rocky dust has recently attracted increasing attention in astronomy as a formation pathway of refractory molecules (e.g., SiS and CaS) detected around massive protostars6,7. Identification of evaporation products is important also as a basis for interpreting such observations. The biggest challenge for this has been the experimental difficulty in realizing collision-free conditions to detect nascent products of high-temperature evaporation of materials of interest (typically at >1000 K). To overcome this problem, in this study we developed a new experimental apparatus utilizing mass spectrometry. This apparatus was applied to sulfides contained in primitive chondrites (CaS, MgS, and FeS) to discuss their microscopic evaporation processes and a potential link to astronomical observations.
Methods
The apparatus consists of an ultra-high vacuum chamber (10−6-10−5 Pa during the experiments, corresponding to mean free paths of 103-104 m), a tungsten plate for resistive heating, a quadrupole mass spectrometer (QMS) placed ~10 cm above the heater, and a radiation thermometer. This configuration sufficiently fulfills the collision-free condition (mean free path >> detection distance). Powdered sulfide samples were loaded on the heater and were evaporated at up to ~1700 K. The released gases were detected by QMS after electron impact ionization.
Results and Discussion
CaS (s) and MgS (s) were found to evaporate mainly as Ca or Mg, S, and S2 with different S/S2 ratios. CaS (g) was detected as a minor product from CaS (s), whereas MgS (g) was below the detection limit. FeS (s) evaporated incongruently unlike the other samples as S2, S, and S3. Although the observed evaporation behaviors are consistent with thermodynamics in certain respects (e.g., congruency/incongruency), the products do not necessarily follow thermodynamics. For example, the present results are inconsistent with PeqCaS/PeqCa < PeqMgS/PeqMg (Peqi: equilibrium vapor pressure of species i) above CaS (s) and MgS (s) at the experimental temperatures. The ratios of the sulfur species from FeS (s) are also different from the equilibrium pressure ratios. These results suggest that the evaporation processes are governed by molecular-level dynamics of crystal decomposition and surface reactions between the fragments before release to the gas phase, rather than the Gibbs energy minimization. This should be the reason why the evaporation kinetics cannot be interpreted only in terms of the simple kinetic model based on equilibrium calculations. The present results also help interpretation of observations of sulfur-bearing refractory molecules around protostars, providing hints about astronomical settings of evaporation recorded in Solar System materials. Identification of evaporation products realized by our new method will thus enable multifaceted understanding of evaporation of planetary materials and its roles in the Solar System material evolution.
References
1Tachibana & Huss (2005) GCA 69, 3075.
2Tachibana & Tsuchiyama (1998) GCA 62, 2005.
3Richter et al. (2007) GCA 71, 5544.
4Inada et al. (2024) JCP 160, 154710.
5Inada et al. (2025) GCA 404, 172.
6Tanaka et al. (2020) ApJL 900, L2.
7Tasa-Chaveli et al. (2025) ApJL 993, L42.