Effect of Water on Olivine Metastability in Subducting Lithosphere

The roles of metastably persisting olivine in subducting lithosphere has been investigated in light of mineralogical and thermal structures, plate velocities, and deep seismogenesis. Many different variables have been used in these examinations: latent heat feedback, kinetic models, and thermal conductivity models, to name a few. In all these models, however, the olivine has been anhydrous, though the presence of H2O in the (Mg,Fe)2SiO4 system is well-known to enhance the rates of phase transformation. Diedrich et al. [2005] recently determined growth rates for olivine with low (300 wt ppm) water content and found that, compared with dry samples, hydrous olivine has growth rates 1.7 orders faster at 1100°C. Using these kinetic data, we modeled 25 subducting slabs with thermal parameters (φ) ranging from 3400-17000 km to see the effect of water on olivine metastability. Except within the coldest, fastest slabs, (φ > 10000 km) all olivine had transformed to wadsleyite or ringwoodite within 1-2 km of the equilibrium phase boundary, resulting in regions of metastability only 5-10% in size as compared with slabs with dry olivine. Wedges of metastable olivine did form in slabs moving at velocities ≥14 cm/yr, yet these were only 4-5% in size with respect to their anhydrous counterparts. Metastable olivine only reached maximum depths of ~360 km, compared with 500-550 km for dry slabs, 5-15 km down-dip of the equilibrium phase boundary. These results suggest that, given small amounts of water, subducting slabs will exhibit near-equilibrium mineralogical structures. Therefore, in consequence, explanations of deep seismogenesis that rely on the persistence of metastable olivine would seem unlikely for hydrous lithosphere. In addition, any decrease in subduction velocity due to the existence of a low-density wedge of metastable olivine (the parachute effect) in dry slabs would not be evident in hydrous ones.