5:15 PM - 6:45 PM
[SMP23-P10] Experimental study on the water solubility of Al-bearing Bridgmanite
Keywords:Bridgmanite, Water solubility, Earth’s mantle, FT-IR, High-P, T experiments
The Earth’s mantle is composed of various polymorphic minerals with different structures. The main minerals constituting the mantle are referred to as nominally anhydrous minerals (NAMs). The minerals can contain trace to moderate amounts of water, ranging from a few wt. ppm to several thousand wt. ppm (Mosenfelder et al., 2006). Bridgmanite (MgSiO3: Bdm), which is the most abundant mineral in the Earth’s mantle, can contain Al2O3. Fu et al. (2019) suggest that Bdm, particularly when containing Al2O3, exhibits a higher water content. Conversely, there are also reports indicating no water incorporation even with Al2O3 accommodation (Liu et al., 2021). However, no study has elucidated the cause of the discrepancy in the saturated water content of Al2O3-bearing Bdm.
Two mechanisms for Al incorporation are considered: (1) the Tschermak mechanism (Mg2+, Si4+→2Al3+) and (2) the oxygen vacancy mechanism. In mechanism (1), Al enters the Mg and Si site by substituting for a cation. In contrast, in mechanism (2), an AlO4 tetrahedron replaces a SiO6 octahedron. However, it has not been considered which mechanism dominates when Al incorporation increases the water content in Bdm. Liu et al. (2019) investigated the temperature dependence of the presence of mechanism (2). The results revealed a rapid increasing in the presence of mechanism (2) with increasing temperature. Therefore, the aim of this study is to examine the temperature dependence of the water solubility Al-bearing Bdm and compare it with the change in the incorporation mechanism of Al to discuss the relationship between the solubility of Bdm and the mechanism of Al incorporation.
Synthesis of hydrous Al-bearing Bdm was conducted using the Kawai-type multi-anvil apparatus ORANGE-3000 (GRC, Ehime Univ.) and SPEED-Mk.II (SPring-8). The starting materials consisted of a mixture of MgO, SiO2, Al2O3, and Mg(OH)2 powders. The mixture was made to cylindrical pellets, enclosed in welded Pt capsules to prevent water loss under high-temperature and high-pressure conditions. The pressure conditions were 25 GPa, and the temperature conditions ranged from 1400 to 1800℃. The synthesized samples were analyzed for chemical composition using the EPMA (JXA8230: Okayama Univ.). The crystal orientation was measured using FE-SEM-EBSD (JSM-IT500HR: GRC, Ehime Univ.), and infrared spectra were obtained using a Fourier transform infrared spectroscopy (FT-IR, IRT-5200EUO) to quantify the water content.
In experiments conducted at 1400 and 1600℃, melt parts and single-crystal Bdm were successfully separated and recovered, achieving the development of well-formed single-crystal Bdm of up to ~200 µm. In experiments conducted at 1650 and 1700℃, not only Bdm but also garnet(Grt) precipitated. The precipitated crystals did not exhibit well-formed morphology but Bdm and Grt were successfully separated and recovered. However, in experiments at 1800℃, the samples were fully melted, and Bdm was not obtained. The synthesized single-crystals of Bdm were measured crystal orientation using SEM+EBSD and chemical composition including water content using EPMA and FT-IR analysis. There were no significant differences in Al2O3 composition observed in the samples at 1400 and 1600℃. The sample at 1650℃ exhibited a significantly higher Al2O3 content compared to the other at two temperature conditions. The sample at 1400℃ denote higher ratio of Mg to Si compared to the other two. However, the solubility of water content of Al-bearing Bdm in each experiments showed similar values (210 – 250 wt.ppm). Additionally, the OH peaks observed in the infrared spectrum (1400 - 1700℃) were at the same wavenumber, and almost no orientation dependence was observed.
Two mechanisms for Al incorporation are considered: (1) the Tschermak mechanism (Mg2+, Si4+→2Al3+) and (2) the oxygen vacancy mechanism. In mechanism (1), Al enters the Mg and Si site by substituting for a cation. In contrast, in mechanism (2), an AlO4 tetrahedron replaces a SiO6 octahedron. However, it has not been considered which mechanism dominates when Al incorporation increases the water content in Bdm. Liu et al. (2019) investigated the temperature dependence of the presence of mechanism (2). The results revealed a rapid increasing in the presence of mechanism (2) with increasing temperature. Therefore, the aim of this study is to examine the temperature dependence of the water solubility Al-bearing Bdm and compare it with the change in the incorporation mechanism of Al to discuss the relationship between the solubility of Bdm and the mechanism of Al incorporation.
Synthesis of hydrous Al-bearing Bdm was conducted using the Kawai-type multi-anvil apparatus ORANGE-3000 (GRC, Ehime Univ.) and SPEED-Mk.II (SPring-8). The starting materials consisted of a mixture of MgO, SiO2, Al2O3, and Mg(OH)2 powders. The mixture was made to cylindrical pellets, enclosed in welded Pt capsules to prevent water loss under high-temperature and high-pressure conditions. The pressure conditions were 25 GPa, and the temperature conditions ranged from 1400 to 1800℃. The synthesized samples were analyzed for chemical composition using the EPMA (JXA8230: Okayama Univ.). The crystal orientation was measured using FE-SEM-EBSD (JSM-IT500HR: GRC, Ehime Univ.), and infrared spectra were obtained using a Fourier transform infrared spectroscopy (FT-IR, IRT-5200EUO) to quantify the water content.
In experiments conducted at 1400 and 1600℃, melt parts and single-crystal Bdm were successfully separated and recovered, achieving the development of well-formed single-crystal Bdm of up to ~200 µm. In experiments conducted at 1650 and 1700℃, not only Bdm but also garnet(Grt) precipitated. The precipitated crystals did not exhibit well-formed morphology but Bdm and Grt were successfully separated and recovered. However, in experiments at 1800℃, the samples were fully melted, and Bdm was not obtained. The synthesized single-crystals of Bdm were measured crystal orientation using SEM+EBSD and chemical composition including water content using EPMA and FT-IR analysis. There were no significant differences in Al2O3 composition observed in the samples at 1400 and 1600℃. The sample at 1650℃ exhibited a significantly higher Al2O3 content compared to the other at two temperature conditions. The sample at 1400℃ denote higher ratio of Mg to Si compared to the other two. However, the solubility of water content of Al-bearing Bdm in each experiments showed similar values (210 – 250 wt.ppm). Additionally, the OH peaks observed in the infrared spectrum (1400 - 1700℃) were at the same wavenumber, and almost no orientation dependence was observed.