2:00 PM - 2:15 PM
[PCG19-08] Oxygen isotope exchange kinetics between amorphous silicate dust and CO gas: Implication for oxygen isotope evolution in the early Solar System
Keywords:amorphous silicate, oxygen, isotope exchange, kinetics, protosolar disk
The oxygen isotopic compositions of primitive Solar System materials exhibit mass-independent variation as a result of O isotope exchange between different O isotope reservoirs, such as primitive solid materials and disk gas in the early Solar System (e.g., Yurimoto and Kuramoto, 2004; McKeegan et al., 2011). Oxygen-16-rich CO and -poor H2O gas would be produced through self-shielding of CO gas in the Sun’s parent molecular cloud or in the early Solar System (Yurimoto and Kuramoto, 2004; Lyons and Young, 2005). The O isotope exchange between three major oxygen isotope reservoirs (primitive solid materials, CO, and C H2O) would thus be responsible for the O isotope variation within extraterrestrial materials.
Amorphous silicate dust is recognized as primitive solid materials in the early Solar System (e.g., Nuth et al., 2005). Oxygen isotopic exchange kinetics between amorphous silicates and H2O gas has been experimentally investigated in our previous studies (Yamamoto et al., 2018, 2020), and the gas-phase O isotope exchange kinetics between CO and H2O was discussed by Alexander (2004). However, O isotope exchange between amorphous silicates and CO gas has not yet been examined. In this study, we performed O isotope exchange experiments between amorphous silicate with forsterite (Mg2SiO4) stoichiometry (hereafter amorphous forsterite) and low-pressure CO gas.
The O isotope exchange experiments between amorphous forsterite dust (80 nm in average diameter) (Yamamoto et al., 2018) and 18O-enriched CO gas (>95 atom% 18O) were performed at 803, 853, and 883 K under low-pressure CO gas condition (PCO = 0.3 Pa) for 3–177 hours using a gold-mirror vacuum furnace (Yamamoto et al., 2018, 2020) equipped with a gas flow system. The gas species in the furnace were monitored by a quadrupole mass spectrometer (QMS). Oxygen isotope measurements were conducted by secondary ion mass spectrometry (Cameca ims-1280HR) using the analytical procedure described in Yamamoto et al. (2020).
The heated samples are expected to remain amorphous under the heating conditions (Yamamoto and Tachibana, 2018), and their oxygen isotopic compositions [18O/(18O + 16O)] (f18O) increased almost linearly with time. The QMS analysis showed that the partial pressure of H218O was more than three orders of magnitude lower than PCO (< ~1 × 10–4 Pa). The f18O of heated samples was much larger than that expected for the O isotope exchange between amorphous forsterite and H218O in the furnace (< ~1 × 10–4 Pa) (Yamamoto et al., 2018). We thus conclude that O isotope exchange occurred between amorphous forsterite and C18O. The linear increase of f18O with time cannot be explained by a three-dimensional spherical diffusion equation (Yamamoto et al., 2018; 2020), suggesting that O isotope exchange reaction is governed by gas dissolution into the amorphous structure and isotope exchange process between dissolved CO and structural oxygen atoms. The reaction timescale between amorphous forsterite and CO is about 1–2 order of magnitude smaller than that with H2O at the same T-P condition (Yamamoto et al., 2018). Despite the slow O isotope exchange rate of amorphous forsterite with CO, the O isotope exchange timescale for ~0.1 µm-sized amorphous forsterite dust with CO is 4–5 orders of magnitude shorter than the timescale of gas-phase CO-H2O isotopic equilibrium at 803–883 K and PCO = PH2O ~ 0.1 Pa in the protosolar disk (Alexander, 2004). This implies that amorphous forsterite dust would accelerate CO-H2O isotopic equilibrium through isotope exchange of amorphous forsterite with both CO and H2O, and the gas-phase isotope equilibrium timescale would be determined by the timescale of O isotope exchange between amorphous forsterite dust and CO (e.g., ~1 yr at 800–900 K). In other words, the oxygen isotope equilibrium in the protosolar disk is governed by the rate of oxygen isotope exchange between amorphous dust and CO in the mid- to low-temperature region (~800–1200 K).
Amorphous silicate dust is recognized as primitive solid materials in the early Solar System (e.g., Nuth et al., 2005). Oxygen isotopic exchange kinetics between amorphous silicates and H2O gas has been experimentally investigated in our previous studies (Yamamoto et al., 2018, 2020), and the gas-phase O isotope exchange kinetics between CO and H2O was discussed by Alexander (2004). However, O isotope exchange between amorphous silicates and CO gas has not yet been examined. In this study, we performed O isotope exchange experiments between amorphous silicate with forsterite (Mg2SiO4) stoichiometry (hereafter amorphous forsterite) and low-pressure CO gas.
The O isotope exchange experiments between amorphous forsterite dust (80 nm in average diameter) (Yamamoto et al., 2018) and 18O-enriched CO gas (>95 atom% 18O) were performed at 803, 853, and 883 K under low-pressure CO gas condition (PCO = 0.3 Pa) for 3–177 hours using a gold-mirror vacuum furnace (Yamamoto et al., 2018, 2020) equipped with a gas flow system. The gas species in the furnace were monitored by a quadrupole mass spectrometer (QMS). Oxygen isotope measurements were conducted by secondary ion mass spectrometry (Cameca ims-1280HR) using the analytical procedure described in Yamamoto et al. (2020).
The heated samples are expected to remain amorphous under the heating conditions (Yamamoto and Tachibana, 2018), and their oxygen isotopic compositions [18O/(18O + 16O)] (f18O) increased almost linearly with time. The QMS analysis showed that the partial pressure of H218O was more than three orders of magnitude lower than PCO (< ~1 × 10–4 Pa). The f18O of heated samples was much larger than that expected for the O isotope exchange between amorphous forsterite and H218O in the furnace (< ~1 × 10–4 Pa) (Yamamoto et al., 2018). We thus conclude that O isotope exchange occurred between amorphous forsterite and C18O. The linear increase of f18O with time cannot be explained by a three-dimensional spherical diffusion equation (Yamamoto et al., 2018; 2020), suggesting that O isotope exchange reaction is governed by gas dissolution into the amorphous structure and isotope exchange process between dissolved CO and structural oxygen atoms. The reaction timescale between amorphous forsterite and CO is about 1–2 order of magnitude smaller than that with H2O at the same T-P condition (Yamamoto et al., 2018). Despite the slow O isotope exchange rate of amorphous forsterite with CO, the O isotope exchange timescale for ~0.1 µm-sized amorphous forsterite dust with CO is 4–5 orders of magnitude shorter than the timescale of gas-phase CO-H2O isotopic equilibrium at 803–883 K and PCO = PH2O ~ 0.1 Pa in the protosolar disk (Alexander, 2004). This implies that amorphous forsterite dust would accelerate CO-H2O isotopic equilibrium through isotope exchange of amorphous forsterite with both CO and H2O, and the gas-phase isotope equilibrium timescale would be determined by the timescale of O isotope exchange between amorphous forsterite dust and CO (e.g., ~1 yr at 800–900 K). In other words, the oxygen isotope equilibrium in the protosolar disk is governed by the rate of oxygen isotope exchange between amorphous dust and CO in the mid- to low-temperature region (~800–1200 K).