2:45 PM - 3:00 PM
[PPS08-17] Oxygen isotope exchange kinetics of FeO-bearing amorphous silicate with water vapor
Keywords:amorphous silicate, silicate dust, protoplanetary disks, oxygen isotope, STEM-EELS
Infrared astronomical observations have shown that amorphous silicate is a major dust component in protoplanetary disks (e.g. Henning 2010). The matrices of minimally altered chondrites are composed of FeO-rich amorphous silicates that may have been primordial dust in the solar protoplanetary disk (Abreu and Brearley 2010). These matrices have 16O-poor oxygen isotopic compositions compared to the Sun (e.g. Yurimoto et al. 2008), possibly reflecting oxygen isotope exchange with 16O-poor water vapor in the disk (Yurimoto and Kuramoto 2004). Although the oxygen isotope exchange kinetics of FeO-free amorphous silicates has been determined (Yamamoto et al. 2018, 2020), the conditions required for oxygen isotope exchange between FeO-rich amorphous silicates and water vapor have not yet been investigated. In this study, we investigated the oxygen isotope exchange rate between amorphous silicate grains with major element compositions close to the solar elemental abundances (MgO/SiO2 ~ FeO/SiO2 ~1 in the molar ratio) and water vapor.
Amorphous Mg-Fe silicate powder synthesized by the induction thermal plasma method was used as the starting material (the average grain diameter: ~70 nm). The chemical composition is MgO/SiO2 ~0.96 and FeO/SiO2 ~0.92, which is close to the Fo51 olivine stoichiometry. The starting material was heated in a mixed gas flow of H2 and 18O-rich H2O (18O/(16O+18O) ~0.7) with H2/H2O ~360 at 1.7 Pa and 300–430°C for 3–436 h. Oxygen isotopic compositions were measured by secondary ion mass spectrometry (Cameca ims-1280HR), and Fe3+/ΣFe ratios were determined by electron energy loss spectroscopy (EELS) using scanning transmission electron microscopy (JEOL JEM-ARM200F).
The oxygen isotope exchange reaction of amorphous Mg-Fe silicate proceeded efficiently at lower temperatures than that of amorphous Mg2SiO4 and MgSiO3 (Yamamoto et al. 2018, 2020). The changes in the oxygen isotopic composition of the amorphous grains with heating duration are explained by a diffusion-controlled reaction for short heating durations of <18 h at 330–350°C and <36 h at 380–430°C, during which the Fe3+/ΣFe ratio remained mostly within the range of the starting material (0–9 mol%). On the other hand, the isotope exchange reaction slowed down after these durations and the Fe3+/ΣFe ratio increased with increasing heating duration. This suggests that the oxygen isotope exchange was suppressed with the oxidation of Fe2+ to Fe3+, which could act as a network-forming cation. Thus, we determined the kinetics of the isotope exchange reaction using only samples with little or no Fe2+ oxidation and obtained an activation energy of 77.5 ± 8.4 kJ/mol, which is smaller than those for amorphous Mg2SiO4 and MgSiO3 (Yamamoto et al. 2018, 2020).
Using the prediction formula for the reaction temperature in the accreting protoplanetary disks (Ishizaki et al. 2023), the oxygen isotope exchange temperatures of amorphous Mg-Fe silicate dust with diameters of 0.1 and 1 micron are estimated to be 410–510 K and 500–670 K, respectively, for the disk viscosity parameter α of 10−3 and the mass accretion rate of 10−8–10−6 solar mass yr−1. These temperature ranges are ~300 K lower for 0.1-micron grains and ~150 K lower for 1-micron grains than the crystallization temperature (Sakurai et al. 2023 LPSC abstract). Furthermore, these temperatures are also ~200 K lower than that for oxygen isotope exchange between FeO-free amorphous Mg silicate dust of the same grain diameter and H2O vapor (Yamamoto et al. 2018, 2020). This suggests that FeO-rich amorphous silicate dust effectively changes its oxygen isotopic composition along with disk H2O gas in the inner region of protoplanetary disks.
Amorphous Mg-Fe silicate powder synthesized by the induction thermal plasma method was used as the starting material (the average grain diameter: ~70 nm). The chemical composition is MgO/SiO2 ~0.96 and FeO/SiO2 ~0.92, which is close to the Fo51 olivine stoichiometry. The starting material was heated in a mixed gas flow of H2 and 18O-rich H2O (18O/(16O+18O) ~0.7) with H2/H2O ~360 at 1.7 Pa and 300–430°C for 3–436 h. Oxygen isotopic compositions were measured by secondary ion mass spectrometry (Cameca ims-1280HR), and Fe3+/ΣFe ratios were determined by electron energy loss spectroscopy (EELS) using scanning transmission electron microscopy (JEOL JEM-ARM200F).
The oxygen isotope exchange reaction of amorphous Mg-Fe silicate proceeded efficiently at lower temperatures than that of amorphous Mg2SiO4 and MgSiO3 (Yamamoto et al. 2018, 2020). The changes in the oxygen isotopic composition of the amorphous grains with heating duration are explained by a diffusion-controlled reaction for short heating durations of <18 h at 330–350°C and <36 h at 380–430°C, during which the Fe3+/ΣFe ratio remained mostly within the range of the starting material (0–9 mol%). On the other hand, the isotope exchange reaction slowed down after these durations and the Fe3+/ΣFe ratio increased with increasing heating duration. This suggests that the oxygen isotope exchange was suppressed with the oxidation of Fe2+ to Fe3+, which could act as a network-forming cation. Thus, we determined the kinetics of the isotope exchange reaction using only samples with little or no Fe2+ oxidation and obtained an activation energy of 77.5 ± 8.4 kJ/mol, which is smaller than those for amorphous Mg2SiO4 and MgSiO3 (Yamamoto et al. 2018, 2020).
Using the prediction formula for the reaction temperature in the accreting protoplanetary disks (Ishizaki et al. 2023), the oxygen isotope exchange temperatures of amorphous Mg-Fe silicate dust with diameters of 0.1 and 1 micron are estimated to be 410–510 K and 500–670 K, respectively, for the disk viscosity parameter α of 10−3 and the mass accretion rate of 10−8–10−6 solar mass yr−1. These temperature ranges are ~300 K lower for 0.1-micron grains and ~150 K lower for 1-micron grains than the crystallization temperature (Sakurai et al. 2023 LPSC abstract). Furthermore, these temperatures are also ~200 K lower than that for oxygen isotope exchange between FeO-free amorphous Mg silicate dust of the same grain diameter and H2O vapor (Yamamoto et al. 2018, 2020). This suggests that FeO-rich amorphous silicate dust effectively changes its oxygen isotopic composition along with disk H2O gas in the inner region of protoplanetary disks.