Effect Of Water On The Electrical Conductivity Of Minerals In Eclogite And Peridotite And Geophysical Implications

A prime goal of modern Earth Sciences is to explore the structure,composition,nature and evolution of the deep Earth and the way it works.Two basic approaches have been commonly adopted for these studies,one by analyzing natural samples and the other by geophysical depth sensing.Natural samples,in particular those entrained by volcano-related eruptions such as deep xenoliths and xenocrysts,provide the most straightforward information for the Earth’s deep interior.Unfortunately,the originated depths of natural samples are usually restricted to the shallowest～200 km,mostly within the lithosphere,and their spatial distribution is also very limited.Consequently,geophysical depth sensing is of key importance for probing/deciphering the deep Earth.Among the various geophysical parameters including velocity,gravity,density and electrical conductivity,electrical conductivity is a very special one,because it is very sensitive to the minor variation of Earth materials（e.g.,water and other volatiles）and the presence and connectivity of melts/fluids along grain boundaries.However,the explanation of geophysically-yielded electromagnetic data relies critically on accurate measurements of the electrical conductivity of deep Earth materials under high-pressure and high-temperature conditions.The conductivity of a given rock matrix is controlled by the main constitutive minerals,e.g.,the conductivity of eclogite is dominated by omphacite and garnet and the conductivity of peridotite by olivine.Therefore,the conductivity of a rock can be assessed by measuring the conductivity of the main constituents.On the other hand,water（or more commonly H）strongly affects the conductivity of nominally anhydrous minerals,e.g.,olivine,omphacite and garnet.The main purpose of this work was to carefully evaluate the effect of water on the electrical behaviors of omphacite,garnet and olivine,so that a solid constraint is placed on the electrical structure of subducting slabs and the upper mantle.Electrical conductivity measurements were carried out by using an end-loaded piston cylinder press and an impedance spectroscopy.Temperature,pressure,redox state,and chemical composition（mainly water and Fe content）were well controlled in the experiments.The main discoveries are summarized as following:（1）the effects of Fe and H on the electrical conductivity of omphacite and garnet in eclogites were systematically investigated,providing novel constraints on the deep water cycling.Starting materials were fresh terrain eclogites from China and Germany,and high-quality omphacite and garnet grains with different Fe contents were carefully separated.OH-bearing omphacite and garnet were pre-prepared by H-annealings at high-pressure and high-temperature conditions.The conductivities were determined at1-2.5 GPa,200-850°C and Ni-Ni O buffered conditions.The conductivity of both minerals increases with increasing temperature,and with increasing both Fe and water content under otherwise identical conditions.The bulk conductivities of eclogites with different modal mineral composition and Fe and water contents were modeled with the obtained mineral conductivity data.The bulk conductivity of OH-bearing eclogites is similar to that of deep crustal granulites,but is greater than that reported for mantle peridotites.As such,the regional enrichments of eclogites may produce high electrical conductivities in the upper mantle,but is unable to influence significantly the electrical structure of the deep crust.By combining with geophysically-based electrical structure of the subducting oceanic crusts,the water contents of omphacite and garnet in the subducting oceanic crust at～70-120 km depth,are strikingly small,even smaller than100 ppm H₂O.This suggests that the ability of subducting oceanic crusts in bringing water to the deep mantle is very limited.However,more geophysical sounding are needed to examine this model.（2）The effects of temperature,redox state and water content on the conductivity of olivine were systematically evaluated,providing a novel explanation for the origin of the conductive asthenosphere and the electrical structure of the whole upper mantle.Starting materials were gem-quality natural olivine single crystals,and OH-bearing olivine samples were pre-prepared by H-annealings under high-pressure and high-temperature conditions.The conductivities were determined at 1 GPa,600-1350°C and Ni-Ni O（NNO）and Mo-Mo O₂（MMO）buffered conditions,note that the effect of pressure itself is minor on the conductivity of silicate minerals.The conductivity of dry olivine is greater by about 0.5 log units at NNO than at MMO,and reaches～0.01-0.1S/m at 1000-1350°C and NNO,which is broadly comparable to the geophysically-resolved high conductivity of the asthenosphere.For typical water contents in the upper mantle（<500 ppm H₂O）,the conductivities of dry and wet olivine are very similar at temperatures in the upper mantle（e.g.,>1000°C）.This suggests that the high conductivity in the asthenosphere may have been related to the prevailing high temperature and oxidized state close to NNO,but not water as often considered in the community.