Porosity and serpentinization inferred from laboratory experiments and geophysical data of incoming oceanic plate at the outer-rise region

Abstract:

Recent geophysical surveys in incoming oceanic plate have identified that seismic velocity is markedly decreased in the outer-rise region close to trench, indicating water infiltration into uppermost mantle along bending faults (e.g., Fujie et al., 2018; Grevemeyer et al., 2018). This influences the water budget carried into the Earth’s interior. In this study, we estimate the degree of hydration from seismic velocity structure at outer-rise region and reevaluate the global subducting water flux. Assuming that velocity reduction in oceanic crust at outer-rise region is caused by cracks filled with water, the fraction of pore water (i.e. porosity) is calculated applying the effective medium theory (e.g., O'Connell and Budiansky, 1974; Kuster and Toksös, 1974).Velocity reduction in uppermost mantle at outer-rise region is likely caused by not only porosity but also serpentinization. Hence, the serpentinization and water content in uppermost mantle is estimated based on the recent laboratory experiments showing the relationship between seismic velocity and serpentinization considering porosity (Hatakeyama and Katayama, 2020). Our calculations indicate that the water content in uppermost mantle is comparable or even higher than that of hydrated oceanic crust. These results suggest that the global subducting water flux is estimated to ~3 times larger than previous estimate not including water trapped by the outer-rise hydration.