H2O solubilities in Al-free and Al-bearing high-pressure silica polymorphs as a function of pressure and temperature: H2O storage in the lower mantle

• Applicant: Professor Dr. Tomoo Katsura
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 518398967

To understand the deep H2O cycle, it is necessary to know the H2O solubility in the mantle min-erals. Some upper-mantle minerals have high H2O storage capacity, but the H2O solubility in lower-mantle peridotite minerals is negligible. Hence, we speculate that silica minerals in basaltic fragments of subducted slabs host H2O in the lower mantle. Laser-heated diamond-anvil-cell (LH-DAC) experiments reported large H2O solubility in high-pressure silica minerals, namely, stishovite and CaCl2-structured SiO2, under conditions of the upper part of the lower mantle, which is referred to as the mid-mantle. However, the results of the LH-DAC studies were contra-dictory. Therefore, we will investigate the H2O solubility in stishovite and CaCl2-structured SiO2 under mid-mantle conditions using our advanced multianvil technology. Since Al is known to increase H2O solubility of stishovite and is considered a major secondary cation for stishovite, both Al-bearing and Al-free systems will be investigated in this project. This project consists of two sub-projects. Sub-project I is an in situ X-ray diffraction study using the multi-anvil press on beamline P61B of PETRA-III at DESY. First, we will investigate whether H2O loss occurs from high-pressure silica during and after decompression. Next, we will cali-brate the relationship between volume and H2O-content at ambient temperature and high pres-sure. We will then measure the volumes of high-pressure silica at pressures of 10, 20, 30, 40 and 50 GPa and increasing temperatures to 2000 K in 100 K steps. At each temperature step, the sample will be once quenched to ambient temperature, and the H2O content is determined from the volume – H2O-content relationship determined above. At the same time, splits of (120)-(210), (211)-(121), and (130)-(310), and (031)-(301) peaks will carefully be observed to determine the phase transition between stishovite and CaCl2-structured SiO2. In the Sub-project II, single crystals of Al-free and Al-bearing high-pressure silica will be synthe-sized at pressures of 14 - 35 GPa and temperatures of 1300 - 2300 K. The synthesized single crystals will be used to determine the H2O content using Fourier transform infrared spectrosco-py and secondary ion mass spectrometry. Prior to the single-crystal growth, the maximum Al2O3 solubility will be determined as a function of the pressure and temperature in the Al-bearing system.

• DFG Programme: Research Grants