1:45 PM - 2:00 PM
[PPS08-13] An experimental study on the sulfidation of Mg-silicates to form niningerite and oldhamite: Implications for the chondrule formation of EH3 chondrites
Keywords:EH3 chondrite, sulfidation, reproduction experiment, Mg-silicates
Introduction
The sulfidation of magnesian olivine, enstatite, and diopside has been experimentally studied (Imae, 2023), building upon the pioneering work by Fleet and MacRae (1987). The aim was to elucidate the genetic processes of the primary components of type IB chondrules and metal nodules in the EH3 chondrite. In this study, additional experiments were conducted based on thermochemical considerations.
Experiments
Starting materials consisted of magnesian silicates, magnesian olivine (Fe#8-9at%), enstatite (Fe#8-9at%), or diopside. For sulfidation, pyrrhotite or troilite single grains were employed. Each pair of magnesian silicate and sulfide was placed into each graphite crucible, and the setup was sealed in an evacuated silica glass tube. Argon gas was continuously flowed at a rate of 400 cc/min during heating to maintain a reduced condition around the glass, which effectively protected the glass compared to exposure to air. To prevent tube breakage, double-sealed evacuated silica glass tubes were also utilized during the 1420ºC heating runs. Run products were embedded in epoxy resin, then dry polished for observation and analysis. They were observed and analyzed using FE-SEM (JSM-7100F, Jeol) with EDS (AztecEnergy, Oxford) and EPMA (JXA-8200, Jeol) at NIPR. Identification of niningerite and cristobalite was performed using µRaman (inVia Qonter, Renishaw) at NIPR.
Results
Runs using pyrrhotite: Sulfidation occurred from olivine and enstatite at 1200ºC and 1270ºC, and from diopside to form niningerite (Mg~80Fe~20) and cristobalite. Sulfidation also occurred from diopside to form oldhamite, silica, and enstatite at 1200ºC. Sulfidation from olivine also resulted in the formation of secondary enstatite through a reaction between olivine and cristobalite. Niningerite and oldhamite came into incoherent contact with the substrate of Mg-silicates to form an inner layer, while cristobalite formed the outer layer. Iron precipitation from olivine and enstatite due to reduction was observed on the surface, causing olivine and enstatite to change to more magnesian compositions compared to the starting materials. Another starting material pyrrhotite formed a droplet, and the composition changed to more Fe-rich pyrrhotite.
Runs using troilite: Sulfidation to form niningerite from olivine and enstatite did not occur at 1270ºC but did occur at 1420ºC. The product coexisted with niningerite or keilite (Mg~50Fe~50) and iron-sulfide (potentially pyrrhotite) showing eutectic textures. Another starting material troilite formed a droplet, and the iron metals precipitated coexisting with troilite.
Discussion
Oxygen and sulfur gas pressures are inter-related with each other, and those in the pyrrhotite system are regulated by graphite, pyrrhotite, or both during tube heating, while in the troilite system, they are controlled by graphite, troilite, or both. Although the system inside the tube may not reach chemical equilibrium, the gas species and pressures are considered based on thermochemical considerations.
Sulfur gas pressure during runs using pyrrhotite is three orders of magnitude higher than that of troilite, and oxygen gas pressure using pyrrhotite overlaps with the C-CO buffer, whereas that using troilite is significantly lower than the C-CO buffer, reflecting conditions closer to the solar nebula in high-temperature regions. Ebel and Alexander (2011) discussed condensation from an anhydrous C-IDP-enriched system for the origin of Mercury and enstatite chondrites. The experimental conditions are similar to those of an anhydrous C-IDP-enriched system. The main constituents of EH3 chondrites, type IB chondrules, and metal nodules likely formed simultaneously under conditions similar to those discussed by Ebel and Alexander (2011).
Acknowledgement
This study was supported in part by JSPS KAKENHI 21K03652.
References
Imae N. 2023. The 14th Symposium on Polar Science.
Fleet M. E. and MacRae N. D. 1987. Geochim. Cosmochim. Acta 51: 1511-1521.
Ebel D. S. and Alexander C. M. O’D. 2011. Planet. Space Sci. 59: 1888-1894.
The sulfidation of magnesian olivine, enstatite, and diopside has been experimentally studied (Imae, 2023), building upon the pioneering work by Fleet and MacRae (1987). The aim was to elucidate the genetic processes of the primary components of type IB chondrules and metal nodules in the EH3 chondrite. In this study, additional experiments were conducted based on thermochemical considerations.
Experiments
Starting materials consisted of magnesian silicates, magnesian olivine (Fe#8-9at%), enstatite (Fe#8-9at%), or diopside. For sulfidation, pyrrhotite or troilite single grains were employed. Each pair of magnesian silicate and sulfide was placed into each graphite crucible, and the setup was sealed in an evacuated silica glass tube. Argon gas was continuously flowed at a rate of 400 cc/min during heating to maintain a reduced condition around the glass, which effectively protected the glass compared to exposure to air. To prevent tube breakage, double-sealed evacuated silica glass tubes were also utilized during the 1420ºC heating runs. Run products were embedded in epoxy resin, then dry polished for observation and analysis. They were observed and analyzed using FE-SEM (JSM-7100F, Jeol) with EDS (AztecEnergy, Oxford) and EPMA (JXA-8200, Jeol) at NIPR. Identification of niningerite and cristobalite was performed using µRaman (inVia Qonter, Renishaw) at NIPR.
Results
Runs using pyrrhotite: Sulfidation occurred from olivine and enstatite at 1200ºC and 1270ºC, and from diopside to form niningerite (Mg~80Fe~20) and cristobalite. Sulfidation also occurred from diopside to form oldhamite, silica, and enstatite at 1200ºC. Sulfidation from olivine also resulted in the formation of secondary enstatite through a reaction between olivine and cristobalite. Niningerite and oldhamite came into incoherent contact with the substrate of Mg-silicates to form an inner layer, while cristobalite formed the outer layer. Iron precipitation from olivine and enstatite due to reduction was observed on the surface, causing olivine and enstatite to change to more magnesian compositions compared to the starting materials. Another starting material pyrrhotite formed a droplet, and the composition changed to more Fe-rich pyrrhotite.
Runs using troilite: Sulfidation to form niningerite from olivine and enstatite did not occur at 1270ºC but did occur at 1420ºC. The product coexisted with niningerite or keilite (Mg~50Fe~50) and iron-sulfide (potentially pyrrhotite) showing eutectic textures. Another starting material troilite formed a droplet, and the iron metals precipitated coexisting with troilite.
Discussion
Oxygen and sulfur gas pressures are inter-related with each other, and those in the pyrrhotite system are regulated by graphite, pyrrhotite, or both during tube heating, while in the troilite system, they are controlled by graphite, troilite, or both. Although the system inside the tube may not reach chemical equilibrium, the gas species and pressures are considered based on thermochemical considerations.
Sulfur gas pressure during runs using pyrrhotite is three orders of magnitude higher than that of troilite, and oxygen gas pressure using pyrrhotite overlaps with the C-CO buffer, whereas that using troilite is significantly lower than the C-CO buffer, reflecting conditions closer to the solar nebula in high-temperature regions. Ebel and Alexander (2011) discussed condensation from an anhydrous C-IDP-enriched system for the origin of Mercury and enstatite chondrites. The experimental conditions are similar to those of an anhydrous C-IDP-enriched system. The main constituents of EH3 chondrites, type IB chondrules, and metal nodules likely formed simultaneously under conditions similar to those discussed by Ebel and Alexander (2011).
Acknowledgement
This study was supported in part by JSPS KAKENHI 21K03652.
References
Imae N. 2023. The 14th Symposium on Polar Science.
Fleet M. E. and MacRae N. D. 1987. Geochim. Cosmochim. Acta 51: 1511-1521.
Ebel D. S. and Alexander C. M. O’D. 2011. Planet. Space Sci. 59: 1888-1894.