1:45 PM - 2:00 PM
[PPS08-01] Oxygen Isotope Exchange between FeO-bearing Amorphous Silicates and Water Vapor in Protoplanetary Disks
Keywords:amorphous silicate, silicate dust, protoplanetary disks, oxygen isotope
Earth and extraterrestrial materials originating from asteroids, comets, moons, and Mars show 16O-poor compositions compared to the Sun, suggesting that O-isotope exchange reactions occurred between 16O-poor water vapor and silicate dust which is the dominant solid component in the Sun's protoplanetary disk (e.g., Yurimoto and Kuramoto, 2004). Infrared observations have shown that amorphous silicates are the primary component of dust in protoplanetary disks (e.g., Henning et al., 2010), and the matrices of pristine chondrites contain FeO-bearing amorphous silicates (e.g., Scott and Krot, 2005). If this FeO-rich dust was the primordial dust in the disk, O-isotope exchange reactions could have progressed between FeO-rich amorphous silicate dust and disk H2O gas. The kinetics of the O-isotope exchange reaction between amorphous Mg silicate and low-pressure H2O gas has been experimentally determined (Yamamoto et al., 2018, 2020), but the kinetics between FeO-rich amorphous silicate and H2O gas has not. In this study, we conducted O-isotope exchange experiments between amorphous silicate particles with an olivine-like composition with the Mg/(Mg+Fe) ratio of ~0.51 and low-pressure water vapor to determine the O-isotope exchange rate and the effect of FeO on the reaction rate.
The starting material (2–50 mg) synthesized with an induction thermal plasma system was heated in a vacuum furnace with a mixed gas of hydrogen and 18O-rich water vapor at 480°C and 0.48–2.44 Pa for 12–132 h. The mixing ratio of the gases was controlled within the range of H2/H2O ~100–500 to keep almost all of the FeO in the amorphous silicate (i.e., no significant oxidation/reduction of FeO to form iron oxides or metallic iron). The run products were analyzed with X-ray diffraction (PANalytical X’Pert Pro MPD) and Fourier transform infrared spectrometry (JASCO FT/IR-4200). Samples were formed into pellets (3 µm in diameter), and sintered in vacuum at ~1150°C for 20 h, and their O-isotopic compositions were measured with secondary ion mass spectrometry (CAMECA IMS 1280-HR) at Hokkaido University.
In the samples heated at 480°C, 0.48 Pa and PH2O ~5×10−3 Pa (H2/H2O ~100) for 12 h, the higher the amount of starting material, the closer the 18O/(16O+18O) ratio (f18O) was to that of the starting material and the larger its variance. This is probably due to insufficient H218O supply to the interior of the cluster of starting material powder, which resulted in the f18O of each amorphous particle not being constant. In this case, O diffusion in the amorphous silicate grains is suggested to be sufficiently fast compared to the water vapor supply. In the present experiments, f18O increased from 0.002 (starting material) to 0.43 ± 0.03 for the sample heated at 480°C, 1.72 Pa and PH2O ~6×10−3 Pa (H2/H2O ~300) for 12 h with an initial amount of 5 mg. In contrast, O-isotope exchange between water vapor and amorphous silicates with forsterite/enstatite compositions (amorphous forsterite/enstatite) is diffusion-controlled (Yamamoto et al., 2018, 2020) and amorphous forsterite requires ~700 h to reach this f18O value at the same P-T conditions (Yamamoto et al., 2018). Thus, at 480°C, the diffusion-controlled O-isotope exchange rate of FeO-bearing amorphous silicates is predicted to be >2–3 orders of magnitude faster than that of amorphous forsterite. The crystallization rate of this FeO-bearing amorphous silicate is 3–4 orders of magnitude faster than that of amorphous forsterite (Sakurai et al., 2023 LPSC abstract), suggesting more efficient breakage of the Si-O-Si bond. Since this breakage is also necessary for oxygen isotope exchange, it is consistent with the present results. From the present study, we can predict that the temperature conditions required for O-isotope exchange in the disk shift to lower temperatures when FeO is present in amorphous silicates, but quantitative discussion requires experiments under lower temperature conditions where O-isotope exchange is controlled by O diffusion in the grains.
The starting material (2–50 mg) synthesized with an induction thermal plasma system was heated in a vacuum furnace with a mixed gas of hydrogen and 18O-rich water vapor at 480°C and 0.48–2.44 Pa for 12–132 h. The mixing ratio of the gases was controlled within the range of H2/H2O ~100–500 to keep almost all of the FeO in the amorphous silicate (i.e., no significant oxidation/reduction of FeO to form iron oxides or metallic iron). The run products were analyzed with X-ray diffraction (PANalytical X’Pert Pro MPD) and Fourier transform infrared spectrometry (JASCO FT/IR-4200). Samples were formed into pellets (3 µm in diameter), and sintered in vacuum at ~1150°C for 20 h, and their O-isotopic compositions were measured with secondary ion mass spectrometry (CAMECA IMS 1280-HR) at Hokkaido University.
In the samples heated at 480°C, 0.48 Pa and PH2O ~5×10−3 Pa (H2/H2O ~100) for 12 h, the higher the amount of starting material, the closer the 18O/(16O+18O) ratio (f18O) was to that of the starting material and the larger its variance. This is probably due to insufficient H218O supply to the interior of the cluster of starting material powder, which resulted in the f18O of each amorphous particle not being constant. In this case, O diffusion in the amorphous silicate grains is suggested to be sufficiently fast compared to the water vapor supply. In the present experiments, f18O increased from 0.002 (starting material) to 0.43 ± 0.03 for the sample heated at 480°C, 1.72 Pa and PH2O ~6×10−3 Pa (H2/H2O ~300) for 12 h with an initial amount of 5 mg. In contrast, O-isotope exchange between water vapor and amorphous silicates with forsterite/enstatite compositions (amorphous forsterite/enstatite) is diffusion-controlled (Yamamoto et al., 2018, 2020) and amorphous forsterite requires ~700 h to reach this f18O value at the same P-T conditions (Yamamoto et al., 2018). Thus, at 480°C, the diffusion-controlled O-isotope exchange rate of FeO-bearing amorphous silicates is predicted to be >2–3 orders of magnitude faster than that of amorphous forsterite. The crystallization rate of this FeO-bearing amorphous silicate is 3–4 orders of magnitude faster than that of amorphous forsterite (Sakurai et al., 2023 LPSC abstract), suggesting more efficient breakage of the Si-O-Si bond. Since this breakage is also necessary for oxygen isotope exchange, it is consistent with the present results. From the present study, we can predict that the temperature conditions required for O-isotope exchange in the disk shift to lower temperatures when FeO is present in amorphous silicates, but quantitative discussion requires experiments under lower temperature conditions where O-isotope exchange is controlled by O diffusion in the grains.