It has long been proposed that water incorporation in olivine has dramatic effects on the upper mantle properties, affecting large-scale geodynamics, and triggering high electrical conductivity. But the laboratory-based laws of olivine electrical conductivity predict contrasting effects of water, precluding the interpretation of geophysical data in term of mantle hydration. We review the experimental measurements of hydrous olivine conductivity and conclude that most of data are consistent when errors in samples water contents are considered. We report a new law calibrated on the largest database of measurements on hydrous olivine oriented single crystals and polycrystals. It fits most of measurements within uncertainties, and is compatible with most of geophysical data within petrological constraints on mantle olivine hydration. The conductivity anisotropy of hydrous olivine might be higher than dry olivine, but preferential orientation should produce moderate anisotropy ($0-0.8 log unit). In the oceanic mantle, the enhancement of olivine conductivity is limited to $1 log unit in the maximum range of mantle olivine water concentrations (0-500 wt ppm). Strongest enhancements are expected in colder regions, like cratonic lithospheres and subduction settings. High conductivities in melt-free mantle require great depths and high water concentrations in olivine (>0.1 S/m at >250 km and >200 wt ppm). Thus, the hydration of olivine appears unlikely to produce the highest conductivities of the upper mantle.

The magnetotelluric component of the Mantle Electromagnetic and Tomography (MELT) Experiment measured the electrical resistivity structure of the mantle beneath the fast-spreading southern East Pacific Rise (EPR). The data reveal an asymmetric resistivity structure, with lower resistivity to the west of the ridge. The uppermost 100 kilometers of mantle immediately to the east of the ridge is consistent with a dry olivine resistivity structure indicating a mantle depleted of melt and volatiles. Mantle resistivities to the west of the ridge are consistent with a low-melt fraction (about 1 to 2 percent interconnected melt) distributed over a broad region and extending to depths of about 150 kilometers. The asymmetry in resistivity structure may be the result of asymmetric spreading rates and a westward migration of the ridge axis and suggests distinct styles of melt formation and delivery in the mantle beneath the two plates.

Abstract. In April-June 1989, seafloor magnetotelluric data across and along the leading edge of the Tahiti hotspot were obtained. The magnetotelluric response functions were found to be strongly influenced by bathymetric and island effects, and a new procedure for modeling and removing this distortion using a thin sheet approach combined with the measured water depths is introduced. The corrected response functions are consistent with a two-dimensional structure. Inversion of the data shows a slightly higher conductivity (relative to a reference site located away from the hotspot) down to 130 km depth beneath the active area southeast of Tahiti underlain by a more resistive structure. There is a suggestion for a change in conductivity in the 400-450 kln depth range, which is consistent with elevated temperatures. This result is consistent with a mantle plume of limited extent (less than 150 km radius) located near the leading edge of the Tahiti hotspot. The rnagnetotelluric data provide no evidence for lithospheric thinning or for a strong thermal influence over a large area, as would be required by a superswell model.

No abstract

No abstract

No abstract

To cite this version:Pascal Tarits, Mioara Mandéa. The heterogeneous electrical conductivity structure of the lower mantle.Physics This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.Page 1 A c c e p t e d M a n u s c r i p t that temperature and compositional effects known as predominant on both conductivity and velocity could be associated with other processes affecting mainly the conductivity, probably in relation with minor phases.

A deep magnetotelluric sounding in the French Alps provided a vertical electrical conductivity profile between ∼200–1000 km. Two prominent features are observed. First, the conductivity in the depth range 400–800 km is smaller than the conductivity of a pyrolite mantle obtained from laboratory results for a normal geotherm. Second, the data do not require the conductivity to change throughout the transition zone (410–660 km). In this part of the mantle, a temperature of 350–450 C less than normal explains the magnetotelluric conductivity profile. At 200–400 km, our model favors a cold mantle with 1000–1500 ppm of water dissolved in olivine. If correct, this model suggests that the subducted slab is dehydrated before reaching the transition zone.