The effect of small amounts of H₂O on olivine to ringwoodite transformation growth rates and implications for subduction of metastable olivine

It has been suggested that in relatively cold subduction zones, the interior of down-going lithospheric plates may be cool enough to inhibit the transformation of olivine to its high-pressure polymorphs, wadsleyite and ringwoodite, resulting in the persistence of metastable olivine in the mantle transition zone. The rapid transformation of this metastable olivine has been identified as a possible cause of deep-focus earthquakes. However, it is known that olivine can incorporate hydrogen at high pressure and this hydrogen may enhance transformation rates. We conducted a series of high-pressure experiments to hydrate and then partially transform olivine to ringwoodite. We measured transformed rim thickness as a function of run duration and fit a linear function to our results to determine transformation growth rates. The addition of H₂O to olivine appears to promote olivine to ringwoodite transformation by increasing the growth rate, decreasing the activation enthalpy for growth, and weakening the ringwoodite and/or olivine to allow for dissipation of elastic strain sufficient to maintain constant or near-constant growth rates throughout transformation. The addition of approximately 300 ppmw ²H₂O to olivine increases the growth rate of olivine to ringwoodite transformation (at 1100 °C and 18 GPa) by almost a factor of 40 from 4.0 (±0.8) ×10^-9 ms^-1 to 1.5 (±0.3) × 10^-7 ms^-1. There is a corresponding decrease in the activation enthalpy of growth from 392 kJ/mol to 237 kJ/mol (assuming activation volume of 0 cm³/mole). Analyses of hydrogen distributions in partially transformed samples suggest that the hydrogen concentration in the bulk of the olivine crystal may be less important than hydrogen at the olivine and ringwoodite interface.