09:00 〜 09:30
[SMP23-01] Re-evaluating the Solubility of CaSiO3 in Bridgmanite in the Lower Mantle
★Invited Papers
キーワード：lower mantle, bridgmanite, davemaoite, CaSiO3
Understanding the mineralogical composition is crucial for deciphering the dynamics of the Earth’s lower mantle. Bridgmanite and davemaoite are widely accepted to coexist in the lower mantle above a depth of 2700 km. A recent study suggested that the solubility of CaSiO3 in bridgmanite significantly increases above 2300 K and 40 GPa, leading to the conclusion that davemaoite may be absent in regions deeper than 1800 km in the mantle. This conclusion, however, contradicts previous research. To address this discrepancy, we investigated the solubility of CaSiO3 in bridgmanite using a multi-anvil press.
Our experiments utilized glasses with various compositions as starting materials. The experiments were conducted under pressure and temperature ranges of 27-50 GPa and 2300-2700 K, respectively, typically lasting for 24 hours. The resulting products were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).
Both bridgmanite and davemaoite were observed in all experimental products. The CaSiO3 content in bridgmanite reached up to 2.2 mol.% under the investigated conditions. Although the CaSiO3 content in bridgmanite increases with temperature, a temperature of 4000 K is necessary for bridgmanite to incorporate all of the Ca in a pyrolite bulk composition. Interestingly, we found that pressure negatively affects CaSiO3 solubility, contrary to the findings of Ref. 1. The CaSiO3 content decreases from 0.5 to 0.1 mol.% as pressure increases from 27 to 40 GPa at 2300 K. The products from the five starting materials showed similarly low solubilities from 1.2 to 2.2 mol.% at 40 GPa and 2600-2700 K, indicating a limited compositional effect.
Given the low elemental diffusivity of bridgmanite, it is possible that bridgmanite may preserve its low-temperature compositions during heating, even after reaching the target temperature. To test this, we conducted two runs at 2600 GPa and 40 GPa for 0 and 40 hours. The grain sizes were 1.2 and 1.5 μm, respectively, suggesting that the 150 nm wide rim portions of the bridgmanite grains from the 24-hour runs formed not during heating but at the target temperature. However, the rim portions adjacent to the davemaoite grains exhibited a low and essentially identical CaSiO3 solubility to the core portions.
Our results suggest that the solubility of CaSiO3 in bridgmanite is limited. Therefore, if the bulk composition of the lower mantle is peridotitic, davemaoite should be present.
Ko, B., Greenberg, E., Prakapenka, V. et al. Calcium dissolution in bridgmanite in the Earth's deep mantle. Nature 611, 88–92 (2022).
Our experiments utilized glasses with various compositions as starting materials. The experiments were conducted under pressure and temperature ranges of 27-50 GPa and 2300-2700 K, respectively, typically lasting for 24 hours. The resulting products were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).
Both bridgmanite and davemaoite were observed in all experimental products. The CaSiO3 content in bridgmanite reached up to 2.2 mol.% under the investigated conditions. Although the CaSiO3 content in bridgmanite increases with temperature, a temperature of 4000 K is necessary for bridgmanite to incorporate all of the Ca in a pyrolite bulk composition. Interestingly, we found that pressure negatively affects CaSiO3 solubility, contrary to the findings of Ref. 1. The CaSiO3 content decreases from 0.5 to 0.1 mol.% as pressure increases from 27 to 40 GPa at 2300 K. The products from the five starting materials showed similarly low solubilities from 1.2 to 2.2 mol.% at 40 GPa and 2600-2700 K, indicating a limited compositional effect.
Given the low elemental diffusivity of bridgmanite, it is possible that bridgmanite may preserve its low-temperature compositions during heating, even after reaching the target temperature. To test this, we conducted two runs at 2600 GPa and 40 GPa for 0 and 40 hours. The grain sizes were 1.2 and 1.5 μm, respectively, suggesting that the 150 nm wide rim portions of the bridgmanite grains from the 24-hour runs formed not during heating but at the target temperature. However, the rim portions adjacent to the davemaoite grains exhibited a low and essentially identical CaSiO3 solubility to the core portions.
Our results suggest that the solubility of CaSiO3 in bridgmanite is limited. Therefore, if the bulk composition of the lower mantle is peridotitic, davemaoite should be present.
Ko, B., Greenberg, E., Prakapenka, V. et al. Calcium dissolution in bridgmanite in the Earth's deep mantle. Nature 611, 88–92 (2022).