The evolution of electrical property of subducted oceanic sediment and its implications
The electromagnetic measurements of the Earth's crust reveal that there are high conductivity anomalies in the forearc region(＜40 km)of the oceanic subduction zone.The anomalies are commonly attributed to the released fluids by the metamorphic dehydration reaction of the subducted slab.Oceanic sediments are important carriers of water and carbon of the subducted slab.The released fluid from these sediments may serve as a significant source for the deep-seated forearc fluids.The conductivity is highly sensitive to the dehydration,decarbonation,phase transition,and microstructural change of mineral and rocks.It can be used to monitor the evolution of physicochemical properties of oceanic sediments in the plate subduction process,and to constrain the electrical structure of the subduction zone and the influence of devolatilization on the source of mantle fluid.In this study,by utilizing the YJ-3000t multi-anvil press and using the AC impedance spectroscopy method,we have experimentally investigated the electrical conductivities of oceanic sediments at temperatures ranging from 350 ℃ to 900 ℃ and pressures of 1.0 GPa.The results show that the conductivities of the sediments are gradually increased with the increase of temperatures at 1.0 GPa.The noticeable change of the increasing trend of electrical conductivity occurred at 550 ℃ due to the dehydration of clay minerals.The metamorphic dehydration reactions of the subducted oceanic sediments resulted in the fluid escape and the formation of high-resistance mineral phases after dehydration.Then the partial melting of dehydrated oceanic sediments occurred at 900 ℃ caused a significant decrease of conductivity of the high-resistance mineral phases.Based on petrological features and geothermal structures,aqueous fluids produced by the dehydration of oceanic sediments may be one of the main causes of the high conductivity anomalies in the forearc region(＜40 km).
oceanic sedimentssubduction zonehigh temperature and high pressuredevolatilizationhigh conductivity anomalies
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