Ringwoodite growth rates from olivine with ~ 75 ppmw H_2O: Metastable olivine must be nearly anhydrous to exist in the mantle transition zone

Abstract

It has been previously demonstrated that as little as 300 ppmw H_2O increases wadsleyite and ringwoodite growth rates to magnitudes that are inconsistent with the metastable olivine hypothesis. To further test this hypothesis, we present new ringwoodite growth rate measurements from olivine with ∼75 ppmw H_2O at 18 GPa and 700, 900, and 1100 °C. These growth rates are nearly identical to those from olivine with ∼300 ppmw H_2O, and significantly higher than those from nominally anhydrous olivine. We infer that transformation of olivine with 75–300 ppmw H_2O is primarily enhanced by hydrolytic weakening of reaction rims, which reduces the elastic strain-energy barrier to growth. We present a new method for fitting non-linear nominally anhydrous data, to demonstrate that reduction of growth rates by elastic strain energy is an additional requirement for metastable olivine. Based on previous thermokinetic modeling, these enhanced growth rates are inconsistent with the persistence of metastable olivine wedges into the mantle transition zone. Metastable persistence of olivine into the mantle transition-zone would therefore require <75 ppmw H_2O.

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