Electrical conductivity of orthopyroxene: Implications for the water content of the asthenosphere

Abstract

Electrical conductivity of minerals is sensitive to water content and hence can be used to infer the water content in the mantle. However, previous studies to infer the water content in the upper mantle were based on pure olivine model of the upper mantle. Influence of other minerals particularly that of orthopyroxene needs to be included to obtain a better estimate of water content in view of the high water solubility in this mineral. Here we report new results of electrical conductivity measurements on orthopyroxene, and apply these results to estimate the water content of the upper mantle of Earth. We found that the electrical conductivity of orthopyroxene is enhanced by the addition of water in a similar way as other minerals such as olivine and pyrope garnet. Using these new results, we calculate the electrical conductivity of pyrolite mantle as a function of water content and temperature incorporating the temperature and water fugacity-dependent hydrogen partitioning. Reported values of asthenosphere conductivity of 4 × 10−2−10−1 S/m corresponds to the water content of 0.01–0.04 wt%, a result in good agreement with the petrological model of the upper mantle.

Solubility of water in pyrope‐rich garnet at high pressures and temperature

“…Substantially higher water solubility in pyrope-rich garnet might affect the relative strength of garnet with respect to other mineral phases such as olivine [Katayama and Karato, 2008]. In addition, hydrogen defects in pyrope-rich garnet also affect electrical conductivity [Dai and Karato, 2009b].…”

“…Based on these studies, the water storage capacity in the upper mantle and transition zone has been estimated [Hirschmann et al, 2005]. Constraints on the water content in these regions were also inferred based on the results of electrical conductivity measurements on olivine [Wang et al, 2006], orthopyroxene [Dai and Karato, 2009b], wadsleyite and ringwoodite [Dai and Karato, 2009c;Huang et al, 2005]. However, these studies are incomplete since these minerals occupy only $40-80% of these regions, and the robustness of results of these studies hinges upon the degree to which the remaining mineral phases influence the hydrogen solubility and distribution.…”

Water solubility in orthopyroxene: Dependence on temperature and Al content

“…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].…”

“…Thus, the water content in the aluminous enstatite reaches values close to 1 wt.% at low pressures and temperatures. Therefore, aluminous enstatite could be a more important host than olivine for water in the Earth's upper mantle, and it may be related to abnormal geophysical observations at the top of the asthenosphere [1,3,7,13,16]. The solubility of water as a function of temperature and pressure has been investigated to determine the mechanism and thermodynamics of the incorporation of water in orthopyroxene [13,21,24,25,[27][28][29].…”

Origin of High Electrical Conductivity in the Lower Continental Crust: A Review

“…Data for dry synthetic and natural diopside (FeO total \ 1%, CaMgSi 2 O 6 ) are from Hinze et al (1981) and Huebner and Voigt (1988), respectively, with the slight difference probably being caused by experimental methods and uncertainty (Yang et al 2011b). Data for dry mantle orthopyroxene (Wo 1 En 82 Fs 17 ) are from Dai and Karato (2009), corrected by a factor of 5 for the effects of pressure and oxygen fugacity (Yang et al 2011a). Data for dry albite (An 3 Ab 97 ) are from Khitarov and Slutskii (1965); the higher electrical conductivity of albite relative to lower crustal plagioclase is probably related to the higher Na content in the former (Yang et al 2011a).…”