Water controls the fields of metastable olivine in cold subducting slabs

We have measured the rate of the olivine‐wadsleyite transformation in Mg2SiO4 under hydrous condition by in‐situ X‐ray diffraction method. The concentration of hydroxyl of samples was estimated using infrared spectroscopy. Combined with previous kinetic data on this transformation, a total of 15 growth rate data obtained at 13.4–15.8 GPa, 1003–1373 K, and 660–5000 ppm weight H2O have been used to establish the pressure, temperature and OH content dependences. Results demonstrate that the growth rate in the olivine‐wadsleyite transformation is proportional to OH content to the power of about 3. The depth of the olivine transformation significantly changes with the water content in cold slabs, which must be considered to estimate fields of the metastable olivine.