Constraints on fluids in the continental crust from laboratory-based electrical conductivity measurements of plagioclase

Abstract

Aqueous fluids are regarded as the crucial origin of high conductivity and low velocity anomalies observed by magnetotelluric and seismic surveys in the middle to lower continental crust. Integration of field geophysical methods with experimental conductivity data is essential to accurately constrain the fluid fraction and salinity. Studies of fluid content and salinity in the crust from the experimental or theoretically calculated conductivity of fluid–minerals systems and saline-bearing fluid have demonstrated that crustal constituent minerals play a prominent role in the conductivity of fluid–mineral systems. We performed systematic measurements on electrical conductivity of dry and hydrous plagioclase solid solutions at high temperature and pressure. A positive dependence of electrical conductivity on Na contents is observed in dry samples, and discontinuous variation in conductivity is associated with the degree of Al/Si order–disorder. For hydrous samples, trace amounts of water considerably enhance the electrical conductivity. The dominant charge carriers in dry and hydrous plagioclase are Na ions and hydrogen, respectively, both of which migrate via the Frenkel mechanism. If there are no free fluids present in minerals, the crustal major constituent minerals containing structure water cannot account for the high-conductivity anomalies in the middle to lower crust based on our model of the rock conductivity–depth profiles in the continental crust. The calculated bulk conductivities of fluid–rock systems further suggest that at least 1 vol% of fluid with seawater salinity (3.5 wt%) is necessary in the middle to lower crust to interpret the high conductivity anomalies.