In Earth's subduction zones, water is transported into the deep mantle by nominally anhydrous minerals (NAMs) and water-bearing minerals in oceanic plates that react with seawater. Understanding the stability field and water content of these minerals is critical for comprehending water cycle processes in Earth's deep interior. SiO2 minerals are present in the crust that makes up Earth's surface. While quartz is stable at the surface, stishovite is stable in the mantle transition zone and lower mantle.
Recent research has indicated that SiO2 stishovite retains significant water content (>1 wt%) and is considered a potential major water carrier in the lower mantle. Observations have shown that, in water-saturated systems, the unit-cell volume of stishovite expands excessively with water dissolution. However, previous studies reported varying temperature and pressure conditions for this expansion due to challenges in observing well-controlled high-pressure and high-temperature conditions.
Using a newly developed technique for in situ X-ray observation in a water-saturated system at high pressures and temperatures, researchers investigated changes in the unit-cell volume of SiO2 stishovite at 10-30 GPa and up to 1300 C.
"We found that the unit-cell volume of SiO2 stishovite was significantly larger than that of anhydrous stishovite only just after the first crystallization. The experimental results showed that the maximum volume expansion was 3.8%, and the excess volume decreased rapidly with increasing temperature and time, and the unit-cell volume was almost equal to the anhydrous value when above 700 C," explained the researchers.
Furthermore, no excess volume was observed during a subsequent temperature decrease. Thus, the dissolution of water into SiO2 stishovite may be a metastable phenomenon, making it unlikely that hydrous SiO2 stishovite is a stable phase or an important water carrier, at least at the top of the lower mantle.
Research Report:Limited stability of hydrous SiO2 stishovite in the deep mantle
