Temperature Dependence of H₂O Solubility in Al-Free Stishovite

The role of stishovite in transporting water in subducting slabs has been a subject of debate for several decades. Here we investigated stishovite ' s water solubility and its potential role in water transportation as a function of temperature from 1300 to 2100 C-circle at 22 GPa. Under water-saturated conditions, high-quality, Al-free stishovite single crystals were synthesized with multi-anvil press, and their water contents were measured using infrared spectroscopy. The H2O solubility in stishovite increases from 128(20) to 521(47) wt. ppm with increasing temperature from 1300 to 1700 C-circle and decreased to 145(26) wt. ppm at 2100 C-o. The maximum H2O content was about seven times larger than earlier high-temperature multi-anvil studies but significantly lower than recent laser-heated diamond anvil cell and low-temperature multi-anvil studies. We suggest that Al-free stishovite may not be a significant H2O carrier in subducting slabs, at least at the topmost lower mantle corresponding to our experimental conditions. Plain Language Summary Stishovite is a high-pressure phase of silicon dioxide (SiO2). It was recently reported that stishovite could contain a substantial amount of water based on diamond anvil cell experiments and, consequently, transport water into the lower mantle through subducting slabs. However, the water contents in their samples lack precise constrain due to the limited size of the recovered crystals, making them inaccessible for measurements using reliable FTIR spectroscopy or secondary-ion mass spectroscopy. To settle this debate, we produced high-quality single crystals of stishovite under water-saturated conditions at high pressures and temperatures and measured their water contents using infrared spectroscopy. We discovered that stishovite contained only several hundred ppm of water under conditions at the top of the lower mantle. Therefore, it is likely that stishovite does not contain much water in the upper region of the lower mantle and does not transport water into the deep lower mantle.