Metastable olivine provides regional constraints on hydrogen content based on transformation kinetics

The existence of metastable olivine has been debated as a possible trigger of deep focus earthquakes. Of equal importance, its existence may constrain the amount of hydrogen being subducted into the Earth’s transition zone. Olivine transformation rates into wadsleyite and ringwoodite have been demonstrated to be dependent on hydrogen content, and determine whether metastable olivine persists into the Earth’s mantle transition zone as downwelling material. For nominally anhydrous olivine, in this and previous studies, transformation rates decrease over time as a rigid reaction rim is formed. If viscoelastic relaxation of the rim occurs at a relatively slow rate, strain energy resulting from the negative volume change of the reaction counteracts the chemical free energy driving force for growth. It has been proposed that hydrogen enhances olivine transformation rates through hydrolytic weakening of the growth rim, which promotes relaxation of transformation stress, but it is not clear how much hydrogen is required for this mechanism to occur. We present ringwoodite growth rate measurements for interface-controlled growth, using olivine spheres hydrated in a piston cylinder with 75 +/- 15 ppm H2O and nominally anhydrous San Carlos olivine spheres with ≤ 10 ppm H2O. Hydrogen contents have been determined using new FTIR and SIMS data that indicate that hydrogen partitions into the growth rim as it forms. As the rim continues to grow and the core becomes depleted, the rim will eventually exhaust itself of hydrogen and may later become rigid if there is not enough hydrogen available to weaken the growth rim throughout transformation. However, we did not see evidence of non-linear rim growth in samples with 75 ppm H2O, with up to 76% growth fraction at 1100 °C. For olivine initially containing 75 ppm H2O, the ringwoodite growth rates at 18 GPa are 5.1(+/-0.4)x10-11 at 700 °C, 1.8(+/- 0.6)x10-9 m/s at 900 °C, 3.8(+/-1.5)x10-8 at 1100 °C with activation enthalpy of 187 +/- 40 kJ/mol. These rates are similar to rates for olivine with 300 ppm D2O: 2.1x10-9 m/s at 900 °C, 18 GPa with activation enthalpy of 232 kJ/mol [Diedrich, et al., 2009]. The similarity of growth rates for olivine containing a range of H2O (75-300 ppm) suggests that even a small amount of hydrogen can significantly enhance olivine growth rates as long as enough hydrogen partitions to the rim to hydrolytically weaken it, and prevent the buildup of elastic strain energy. Nominally anhydrous olivine may also transform at a similar rate if the growth rims have sufficiently high viscoelastic relaxation rates in Earth. Without the effects of elastic strain energy, growth rates of this magnitude have been demonstrated by thermo-kinetic models to be incompatible with formation of a metastable olivine wedge [Marton, et al., 2006]. Therefore if metastable olivine exists within subducting slabs, olivine must contain less than 75 ppm H2O and transformation rates must be reduced by elastic strain energy. Evidence of metastable olivine in subducting slabs would thus produce regional constraints on hydrogen content in earth’s transition zone.