5:15 PM - 6:45 PM
[SCG44-P08] Effect of solute impurities on the grain growth kinetics of labradorite
Keywords: Rheology, plagioclase, grain growth, sintering
Synthesized plagioclase aggregates have been used in previous experiments to study the rheological characteristics of the lower crust [1][2][3]. Grain size is one of the critical parameters controlling the rheology of rocks. Shear zones consisting of fine-grained minerals accommodate strains in the lower crust by grain-size sensitive creep. Therefore, understanding the grain growth can provide a basis for assessing the strength of shear zones. However, in grain growth experiments using olivine + pyroxene polycrystals, the growth rate was much slower than in the single phase of olivine [4]. This is a result of the suppression of grain boundary movement (pinning effect) due to mutual interference between the olivine and pyroxene. On the other hand, several studies have implied that grain growth may be suppressed by only solute impurities at the grain boundary, even if the second phase is not present [5][6]. However, plagioclase was only studied in the pure system [2], and the effect of different phase or solute impurities is unclear.
In this study, we developed a method to prepare dense and homogeneous plagioclase aggregates with intermediate composition following the method proposed by Shigematsu et al. [7] for albite, that are worthy of investigating the rheology of the lower crust of the island arc [8]. We used Madagascar labradorite (An70) as a sample. The crystal was crushed down to 300 nm by planetary ball mills. Then, we synthesized polycrystals and investigated grain growth of labradorite.
Our method uses hot pressing and slip casting to make dense fine aggregates. Both hot pressing [1][3][9] and slip casting [10][11] makes sintering more efficient. We succeeded in producing a polycrystalline aggregate with the same density as plagioclase by the method. However, since the powder was prepared using natural labradorite single crystals, it contained many impurities, and impurities rich in Fe and Ti were detected in the aggregates after sintering.
Then, we conducted grain growth experiments using the synthetized aggregates. Experiments were conducted at 1050 - 1180 ℃ and atmospheric pressure for 5 - 72h. Due to the impurity at the grain boundaries, grain growth rate was slow compared to previously reported anorthite [2]. The present study implies that the grain growth rate is delayed significantly by only solute impurities at the grain boundary. This slow grain growth inhibits the recovery of strength in the lower crust and indicates the long-term weakness of the shear zone.
[1]Rybacki et al., 2006, J. Geophys. Res., 111, B3. [2]Dresen et al., 1996, Tectonophysics, 258, 251-262. [3]Stünitz and Tullis. 2001, Int J Earth Sci, 90, 136-148. [4]Ohuchi and Nakamura. 2007, Phys. Earth Planet. Inter., 160, 1-21. [5]Poirier and Guillopé, 1979, Bull. Minéral., 102, 67-74. [6]Skemer and Karato, 2007, Phys. Earth Planet. Inter., 164, 186-196. [7]Shigematsu et al., 2022, PEPS, 9, 34. [8]Ishikawa et al., 2014, Japanese Mag. Mineral. Petrol. Sci., 43, 100-107. [9]Karato and Jung, 2003, Philos. Mag., 83, 401-414. [10]Tallon et al., 2010, J. Eur. Ceram. Soc., 30, 2819–2826. [11]Zhang et al., 2020, Int. J. Appl. Ceram. Technol., 17, 1811-1821.
In this study, we developed a method to prepare dense and homogeneous plagioclase aggregates with intermediate composition following the method proposed by Shigematsu et al. [7] for albite, that are worthy of investigating the rheology of the lower crust of the island arc [8]. We used Madagascar labradorite (An70) as a sample. The crystal was crushed down to 300 nm by planetary ball mills. Then, we synthesized polycrystals and investigated grain growth of labradorite.
Our method uses hot pressing and slip casting to make dense fine aggregates. Both hot pressing [1][3][9] and slip casting [10][11] makes sintering more efficient. We succeeded in producing a polycrystalline aggregate with the same density as plagioclase by the method. However, since the powder was prepared using natural labradorite single crystals, it contained many impurities, and impurities rich in Fe and Ti were detected in the aggregates after sintering.
Then, we conducted grain growth experiments using the synthetized aggregates. Experiments were conducted at 1050 - 1180 ℃ and atmospheric pressure for 5 - 72h. Due to the impurity at the grain boundaries, grain growth rate was slow compared to previously reported anorthite [2]. The present study implies that the grain growth rate is delayed significantly by only solute impurities at the grain boundary. This slow grain growth inhibits the recovery of strength in the lower crust and indicates the long-term weakness of the shear zone.
[1]Rybacki et al., 2006, J. Geophys. Res., 111, B3. [2]Dresen et al., 1996, Tectonophysics, 258, 251-262. [3]Stünitz and Tullis. 2001, Int J Earth Sci, 90, 136-148. [4]Ohuchi and Nakamura. 2007, Phys. Earth Planet. Inter., 160, 1-21. [5]Poirier and Guillopé, 1979, Bull. Minéral., 102, 67-74. [6]Skemer and Karato, 2007, Phys. Earth Planet. Inter., 164, 186-196. [7]Shigematsu et al., 2022, PEPS, 9, 34. [8]Ishikawa et al., 2014, Japanese Mag. Mineral. Petrol. Sci., 43, 100-107. [9]Karato and Jung, 2003, Philos. Mag., 83, 401-414. [10]Tallon et al., 2010, J. Eur. Ceram. Soc., 30, 2819–2826. [11]Zhang et al., 2020, Int. J. Appl. Ceram. Technol., 17, 1811-1821.