Revised 1-D mantle electrical conductivity structure beneath the north Pacific

• Published in: Geophysical journal international Vol. 180; no. 3; pp. 1030 - 1048
• Main Authors: Shimizu, Hisayoshi, Koyama, Takao, Baba, Kiyoshi, Utada, Hisashi
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.03.2010
• Subjects: Composition of the mantle; Electrical properties; Geomagnetic induction
• ISSN: 0956-540X; 1365-246X
• DOI: 10.1111/j.1365-246X.2009.04466.x

The 1-D electrical conductivity model in the mid-mantle beneath the northern Pacific was revised in order to discuss the mean state of the mantle and to obtain a credible starting model for 3-D inversions. Semi-global geomagnetic depth sounding (GDS) responses obtained at 13 stations and submarine cable magnetotelluric (MT) responses for eight cables in a period range of 1.7–113 d were used to obtain the revised structure. It is well known that surface conductivity heterogeneity due to the ocean–land conductivity contrast has a large influence on the responses up to a period of 20 d. The effect of surface ocean–land distribution was removed by performing 3-D electromagnetic induction modelling including a surface conductivity heterogeneity distribution laid above a 1-D mantle structure. The corrected responses were averaged in the logarithmic space, and the resulting responses (quasi-1-D response) were inverted to obtain 1-D conductivity models. Correction of the surface heterogeneity effect requires a 1-D conductivity model, and the 1-D model must be similar to the resultant 1-D model for a consistent correction. We attained this by making iterations of the surface layer correction and 1-D inversion. Synthetic tests revealed that the iterative scheme could recover the supposed structure even for a 3-D heterogeneous mantle. Since electromagnetic sounding is not sensitive to the presence of a sharp discontinuity, we examined three different cases of conductivity variation with depth: (1) a model with no discontinuity, (2) a model with two jumps at 400 and 650 km depths and (3) a model with three jumps at 400, 500 and 650 km depths. The conductivity of the two-jump model in the transition zone is higher than the experimentally determined conductivity of dry wadsleyite and ringwoodite. If the difference is entirely due to the effect of water in the transition zone, then the conductivity is consistent with a water content in the region of 0.5 wt per cent. However, if an additional discontinuity of electrical conductivity is allowed at the 500 km depth, the obtained conductivity for the upper 100 km of the transition zone is lower, and can be explained by a less water content, 0.1 wt per cent, in wadsleyite.