2:30 PM - 2:45 PM
[PPS08-16] An experimental study on titanium valence state of fassaite crystallized under simulated protosolar disk conditions
Keywords:Calcium-, aluminum-rich inclusions, fassaite (pyroxene), titanium, melt-gas interaction, redox state, protosolar disk
Igneous calcium-, aluminum-rich inclusions (CAIs) experienced high-temperature melting events in the protosolar disk (e.g., Grossman, 1972). Type B igneous CAIs contain fassaite, Ti-rich pyroxene with a quaternary solid solution (e.g., Simon et al., 1991). The high proportion of trivalent Ti in fassaite of Type B CAI (Ti3+/Titot. < ~0.8) is suggestive of crystallization from the melt exposed to a reducing disk gas (e.g., Grossman et al., 2006). Although the valence state of Ti and its distribution in Type B CAI fassaite would be controlled by various factors including the disk gas redox condition, total pressure, and cooling rate, little has been known about the effect of these factors on the chemical composition of fassaite.
In this study, to explore the effect of gas-melt interaction on fassaite composition, crystallization experiments of CAI analog melt were conducted under low-pressure H2 gas and H2-H2O gas mixture (H2/H2O ratio of ~413) with a total pressure (Ptot.) of 2.5 Pa using a vacuum furnace. The starting material named CAIχ-HT in this study is similar in composition to χ in Grossman et al. (2002) which is predicted to be one of the possible precursor compositions for igneous CAIs. The spherical starting materials, pre-melted in air, were heated at a maximum temperature (Tmax) of 1450°C for 1–3 h and then cooled down to 1100°C with the rates (Rc) of 5 and 50°C h–1. Quantitative analysis of fassaite compositions including Ti valence state (Beckett, 1986) was made by FE-EPMA-WDS (JEOL JXA-8530F). In this study, EMPA analyses were also made for the samples of CAIδ (Mg-, Si-rich composition compared to CAIχ) heated at PH2 of 0.1, 1, and 10 Pa and cooled with Rc of 5 and 50°C h–1 (Kamibayashi, 2021) to study the effect of Ptot. and Rc on the valence state of Ti.
For the run products of CAIχ-HT with Rc = 5°C h–1, the good linear anticorrelation between MgO and Al2O3 contents in fassaite established via the reaction 2Al3+ = Mg2+ + Si4+ was observed in the samples heated in H2-H2O gas, whereas a Mg-poor additional trend for fassaite with high Ti content was seen in the samples heated in H2 gas. Because Ti3+ is more compatible in fassaite than Ti4+ (Beckett, 1986) and Ti3+ would substitute with Mg2+ + Si4+, this result suggests that the reduction reaction of Ti4+ into Ti3+ occurred more effectively in H2 gas. This prediction was also supported by the calculated Ti3+/Titot. ratios assuming the stoichiometry of pyroxene (Beckett, 1986). In the samples heated in H2 gas, fassaite with the high Ti content exhibited the highest Ti3+/Titot. values of ~0.6–0.8, which are similar to those in natural Type B CAIs. In contrast, for the samples heated in H2-H2O gas, all values of Ti3+/Titot. was lower than ~0.3, irrespective of the duration at Tmax. If the redox equilibrium of Ti between melt and disk gas persisted at the onset of fassaite crystallization in natural Type B CAIs, the experimental results suggest that disk gas during CAI-forming events was more reductive than that examined in this study, which is consistent with the prediction of Grossman et al. (2008).
The effect of total pressure and cooling rate on the redox state of Ti was also studied from the CAIδ samples. For the samples heated at PH2 = 0.1 Pa, almost all Ti is tetravalent. However, the samples heated at PH2 = 10 Pa exhibit the highest Ti3+/Titot. values of ~0.6–0.8. At intermediate PH2 of 1 Pa, values of Ti3+/Titot. are smaller than ~0.2 for Rc = 50°C h–1, whereas Ti in fassaite was more reductive (Ti3+/Titot. <~0.7) when Rc is 5°C h–1. The constant Ti3+/Titot. ratios during fassaite crystallization observed in some natural Type B CAIs (e.g., Simon et al., 1991) suggest that the reduction rate of Ti4+ into Ti3+ in the melt by disk gas is much faster than the fassaite crystallization rate. The present results imply that these Type B CAIs crystallized at high Ptot. of reducing disk gas with slower cooling rates while the melt was in contact with disk gas.
In this study, to explore the effect of gas-melt interaction on fassaite composition, crystallization experiments of CAI analog melt were conducted under low-pressure H2 gas and H2-H2O gas mixture (H2/H2O ratio of ~413) with a total pressure (Ptot.) of 2.5 Pa using a vacuum furnace. The starting material named CAIχ-HT in this study is similar in composition to χ in Grossman et al. (2002) which is predicted to be one of the possible precursor compositions for igneous CAIs. The spherical starting materials, pre-melted in air, were heated at a maximum temperature (Tmax) of 1450°C for 1–3 h and then cooled down to 1100°C with the rates (Rc) of 5 and 50°C h–1. Quantitative analysis of fassaite compositions including Ti valence state (Beckett, 1986) was made by FE-EPMA-WDS (JEOL JXA-8530F). In this study, EMPA analyses were also made for the samples of CAIδ (Mg-, Si-rich composition compared to CAIχ) heated at PH2 of 0.1, 1, and 10 Pa and cooled with Rc of 5 and 50°C h–1 (Kamibayashi, 2021) to study the effect of Ptot. and Rc on the valence state of Ti.
For the run products of CAIχ-HT with Rc = 5°C h–1, the good linear anticorrelation between MgO and Al2O3 contents in fassaite established via the reaction 2Al3+ = Mg2+ + Si4+ was observed in the samples heated in H2-H2O gas, whereas a Mg-poor additional trend for fassaite with high Ti content was seen in the samples heated in H2 gas. Because Ti3+ is more compatible in fassaite than Ti4+ (Beckett, 1986) and Ti3+ would substitute with Mg2+ + Si4+, this result suggests that the reduction reaction of Ti4+ into Ti3+ occurred more effectively in H2 gas. This prediction was also supported by the calculated Ti3+/Titot. ratios assuming the stoichiometry of pyroxene (Beckett, 1986). In the samples heated in H2 gas, fassaite with the high Ti content exhibited the highest Ti3+/Titot. values of ~0.6–0.8, which are similar to those in natural Type B CAIs. In contrast, for the samples heated in H2-H2O gas, all values of Ti3+/Titot. was lower than ~0.3, irrespective of the duration at Tmax. If the redox equilibrium of Ti between melt and disk gas persisted at the onset of fassaite crystallization in natural Type B CAIs, the experimental results suggest that disk gas during CAI-forming events was more reductive than that examined in this study, which is consistent with the prediction of Grossman et al. (2008).
The effect of total pressure and cooling rate on the redox state of Ti was also studied from the CAIδ samples. For the samples heated at PH2 = 0.1 Pa, almost all Ti is tetravalent. However, the samples heated at PH2 = 10 Pa exhibit the highest Ti3+/Titot. values of ~0.6–0.8. At intermediate PH2 of 1 Pa, values of Ti3+/Titot. are smaller than ~0.2 for Rc = 50°C h–1, whereas Ti in fassaite was more reductive (Ti3+/Titot. <~0.7) when Rc is 5°C h–1. The constant Ti3+/Titot. ratios during fassaite crystallization observed in some natural Type B CAIs (e.g., Simon et al., 1991) suggest that the reduction rate of Ti4+ into Ti3+ in the melt by disk gas is much faster than the fassaite crystallization rate. The present results imply that these Type B CAIs crystallized at high Ptot. of reducing disk gas with slower cooling rates while the melt was in contact with disk gas.