2:00 PM - 2:15 PM
[SIT20-02] The stability of CaMgSi2O6 perovskite under the lower mantle condition
Keywords:CM-perovskite, X-ray diffraction, lower mantle, meteorite
The lower mantle, which constitutes approximately 60% of the Earth's volume, consists of bridgmanite, ferropericlase, and davemaoite. Previous experiments have shown that diopside (CaMgSi2O6), a mineral found in the upper mantle, decomposes into bridgmanite (MgSiO3) and davemaoite (CaSiO3) under lower mantle pressure conditions. However, recent observations of CaMgSi2O6 perovskite (CM-perovskite), which contains both magnesium and calcium, in a meteorite discovered in Suizhou, China, have indicated its potential presence in the lower mantle based on the estimated temperature and pressure conditions experienced by the meteorite (Xie and Gu 2023). If CM-perovskite is a thermodynamic equilibrium state under lower mantle conditions, it could challenge current understandings of the mineralogical composition of the lower mantle. Conversely, if CM-perovskite is a metastable phase that decomposes after sustained high-temperature and high-pressure conditions, it could provide new insights into mineral phase transitions in subducting oceanic plates and the pressure-temperature history of meteorite impact events.
To investigate the temperature and pressure under which CM-perovskite appears, we conducted high-temperature and high-pressure experiments using a multi-anvil apparatus. The experiments were performed with the ORANGE 3000 apparatus at GRC under high-temperature and high-pressure conditions, and the recovered samples were analyzed using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The starting materials included two types of glasses and two types of natural polycrystalline samples, which were different pyroxene compositions. The experiments were conducted at pressures of 23 and 25 GPa and a temperature of 2100 K for 3 hours, replicating the pressure-temperature conditions experienced by the meteorite in previous studies.
Furthermore, in situ high-temperature and high-pressure experiments were conducted using the SPEED-Mk.II apparatus at BL04B1 of SPring-8, utilizing synchrotron X-ray radiation. To achieve lower mantle conditions, sintered diamond anvils were employed, with a maximum pressure of 42 GPa and a maximum temperature of 2100 K.
The experimental results showed that in the recovered samples from the 25 GPa experiment, only bridgmanite and davemaoite were identified as stable phases. The 23 GPa experiment also revealed the presence of majorite in addition to these two phases. SEM imaging confirmed the presence of Mg- and Ca-separated regions, and XRD analysis did not detect diffraction peaks considered to be CM-perovskite. On the other hand, in situ synchrotron experiments showed the appearance of CM-perovskite at 40 GPa and 1500–1700 K, while phase decomposition occurred at 1900 K. In situ XRD measurements indicated that CM-perovskite had a lattice parameter similar to cubic perovskite, with a unit cell volume estimated to be intermediate between those of davemaoite and bridgmanite. Furthermore, analysis revealed that CM-perovskite gradually decreased over time under high-temperature conditions.
In this presentation, we will discuss the possibility of CM-perovskite formation under lower mantle conditions and evaluate its stability based on our analytical results.
To investigate the temperature and pressure under which CM-perovskite appears, we conducted high-temperature and high-pressure experiments using a multi-anvil apparatus. The experiments were performed with the ORANGE 3000 apparatus at GRC under high-temperature and high-pressure conditions, and the recovered samples were analyzed using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The starting materials included two types of glasses and two types of natural polycrystalline samples, which were different pyroxene compositions. The experiments were conducted at pressures of 23 and 25 GPa and a temperature of 2100 K for 3 hours, replicating the pressure-temperature conditions experienced by the meteorite in previous studies.
Furthermore, in situ high-temperature and high-pressure experiments were conducted using the SPEED-Mk.II apparatus at BL04B1 of SPring-8, utilizing synchrotron X-ray radiation. To achieve lower mantle conditions, sintered diamond anvils were employed, with a maximum pressure of 42 GPa and a maximum temperature of 2100 K.
The experimental results showed that in the recovered samples from the 25 GPa experiment, only bridgmanite and davemaoite were identified as stable phases. The 23 GPa experiment also revealed the presence of majorite in addition to these two phases. SEM imaging confirmed the presence of Mg- and Ca-separated regions, and XRD analysis did not detect diffraction peaks considered to be CM-perovskite. On the other hand, in situ synchrotron experiments showed the appearance of CM-perovskite at 40 GPa and 1500–1700 K, while phase decomposition occurred at 1900 K. In situ XRD measurements indicated that CM-perovskite had a lattice parameter similar to cubic perovskite, with a unit cell volume estimated to be intermediate between those of davemaoite and bridgmanite. Furthermore, analysis revealed that CM-perovskite gradually decreased over time under high-temperature conditions.
In this presentation, we will discuss the possibility of CM-perovskite formation under lower mantle conditions and evaluate its stability based on our analytical results.