Electrical conductivity anisotropy of dry and hydrous olivine at 8 GPa

Abstract

International audienceThe effects of dissolved HO on the electrical conductivity and its anisotropy in olivine (Fo) at 8GPa were investigated by complex impedance spectroscopy. At nominally anhydrous conditions, conduction along [100] and [001] is slightly higher than along [010] in contrast to observations made at lower pressures in earlier studies. Increasing HO content increases conductivities but activation energies are lower and HO concentration dependent. The use of polarized FTIR spectroscopy to determine HO concentrations reveals a weaker than expected effect that water has on olivine conductivity and distinguishes our results from earlier studies based on analyses using non-polarized infrared spectroscopy. We show that at HO concentrations of a few hundred wt ppm or less, that the dominant conduction mechanism at mantle temperatures continues via small polarons, such as that observed for anhydrous olivine. Our results also suggest that at depths greater than 200km, the presence of HO may not be necessary to explain regions in the upper mantle where both electrical and seismic anisotropy are observed. This can be explained by differences in the pressure dependence of the activation energy for conduction along each of the three crystallographic axes. However, while electrical anisotropy of anhydrous olivine remains weak at 8GPa, it is nevertheless enhanced by elevated concentrations (> several hundred wt ppm) of dissolved HO. At these conditions dominated by proton hopping, conductivity along [010] is highest, approximately an order of magnitude greater than along [100]. Additionally, at 1000wt ppm and 1500°C, an isotropic conductivity derived from the data is about 1 order of magnitude higher than that for nominally anhydrous olivine. Thus, in regions of the mantle characterized by anomalously high conductivities and both electrical and seismic anisotropy, significant amounts of dissolved hydrogen can be expected

Constraints on lithospheric mantle and crustal anisotropy in the NoMelt area from an analysis of long-period seafloor magnetotelluric data

“…Until recently, the understanding has been that dry olivine has a low degree of electrical anisotropy, at most a factor of 2 (e.g., Du Frane et al 2005;Yoshino et al 2006;Poe et al 2010), which is essentially undetectable by magnetotelluric (MT) sounding. Under these conditions, we would not expect significant electrical anisotropy in the lithospheric mantle.…”

Geophysical and petrological constraints on ocean plate dynamics

“…In addition, dissolved H2O can enhance the electrical conductivity of mantle minerals [Wang et al, 2006;Yoshino et al, 2009;Poe et al, 2010;Dai and Karato, 2014], yet there is still considerable disagreement between the various laboratories reporting conductivity measurements on hydrous olivine (see discussion in Evans [2012] and Jones et al [2012]). A recent analysis has attempted to reconcile the published data with a reanalysis of all data sets resulting in a single "unified hydrous olivine" model (UHO) [Gardes et al, 2014].…”

“…Although subsequent published conductivity measurements are less conclusive on whether hydrous olivine conductivity is anisotropic [Yoshino et al, 2006;Poe et al, 2010], recent data do show evidence for higher conductivity along the a-axis [Dai and Karato, 2014], with a factor of …”

Water solubility in orthopyroxene: Dependence on temperature and Al content

“…The presence of water or hydroxyl defects in nominally anhydrous minerals (NAMs) has a dramatical influence on their geophysical properties such as solidus temperature [1], electrical conductivity [2][3][4][5][6] and viscosity [7][8][9], as well as on mantle dynamics such as melt generation [10] and mantle convection [7]. Critical to understanding the role of water in the Earth's mantle, a key step is to estimate the distribution of water inside the mantle and the incorporation mechanism of hydrogen in mantle minerals by combining experimentally determined mineral properties with geophysical observations.…”

“…Two principal approaches can provide significant constraints on the water content in the Earth's interior. One approach is water solubility experiments in mantle minerals [11][12][13], and the other infers the water content based on the electrical conductivity of the NAMs [2][3][4][5][6][14][15][16][17][18][19].…”