The existence of metastable olivine (MO) has been debated as a possible trigger of deep focus earthquakes, but of
equal importance its existence may constrain the amount of hydrogen being brought to Earth’s mantle transition zone (MTZ) via subduction. The rate which olivine (ol) transforms into wadsleyite and ringwoodite (rw) is dependent on hydrogen content, and determines the likelihood that metastable olivine could persist into the MTZ. Previous results indicate that 300 ppm of H_2O is too much to allow for MO [1,2]. How much is too much? We present rw rim growth rates for olivine containing as little as 75 ppm of H_2O