Article

Electrical conductivity of basaltic and carbonatite melt-bearing peridotites at high pressures: Implications for melt distribution and melt fraction in the upper mantle

Earth and Planetary Science Letters (Impact Factor: 4.73). 07/2010; 295(3-4):593-602. DOI: 10.1016/j.epsl.2010.04.050

ABSTRACT

Electrical impedance measurements were performed on two types of partial molten samples with basaltic and carbonatitic melts in a Kawai-type multi-anvil apparatus in order to investigate melt fraction–conductivity relationships and melt distribution of the partial molten mantle peridotite under high pressure. The silicate samples were composed of San Carlos olivine with various amounts of mid-ocean ridge basalt (MORB), and the carbonate samples were a mixture of San Carlos olivine with various amounts of carbonatite. High-pressure experiments on the silicate and carbonate systems were performed up to 1600 K at 1.5 GPa and up to at least 1650 K at 3 GPa, respectively. The sample conductivity increased with increasing melt fraction. Carbonatite-bearing samples show approximately one order of magnitude higher conductivity than basalt-bearing ones at the similar melt fraction. A linear relationship between log conductivity (σbulk) and log melt fraction (ϕ) can be expressed well by the Archie's law (Archie, 1942) (σbulk/σmelt=Cϕn) with parameters C=0.68 and 0.97, n=0.87 and 1.13 for silicate and carbonate systems, respectively. Comparison of the electrical conductivity data with theoretical predictions for melt distribution indicates that the model assuming that the grain boundary is completely wetted by melt is the most preferable melt geometry. The gradual change of conductivity with melt fraction suggests no permeability jump due to melt percolation at a certain melt fraction. The melt fraction of the partial molten region in the upper mantle can be estimated to be 1–3% and ∼0.3% for basaltic melt and carbonatite melt, respectively.

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    • "Though experimentally assessing the bulk electrical conductivity of olivine aggregates is necessary for accurately interpreting EM data, empirical relationships alone do not provide much textural information about the partially molten rock. Most studies (Roberts and Tyburczy, 1999; ten Grotenhuis et al., 2005; Yoshino et al., 2010) find that the data on partially molten samples overlap the upper Hashin–Shtrikman bound, which is intended to represent a loose pack of uniformly wetted spheres. We argue that this interpretation is inconsistent with microstructural observations of texturally equilibrated rocks. "
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    ABSTRACT: Estimates of melt content beneath fast-spreading mid-ocean ridges inferred from magnetotelluric tomography (MT) vary between 0.01 and 0.10. Much of this variation may stem from a lack of understanding of how the grain-scale melt geometry influences the bulk electrical conductivity of a partially molten rock, especially at low melt fraction. We compute bulk electrical conductivity of olivine-basalt aggregates over 0.02 to 0.20 melt fraction by simulating electric current in experimentally obtained partially molten geometries. Olivine-basalt aggregates were synthesized by hot-pressing San Carlos olivine and high-alumina basalt in a solid-medium piston-cylinder apparatus. Run conditions for experimental charges were 1.5 GPa and 1350 °C. Upon completion, charges were quenched and cored. Samples were imaged using synchrotron X-ray micro-computed tomography (μ-CT). The resulting high-resolution, 3-dimensional (3-D) image of the melt distribution constitutes a digital rock sample, on which numerical simulations were conducted to estimate material properties. To compute bulk electrical conductivity, we simulated a direct current measurement by solving the current continuity equation, assuming electrical conductivities for olivine and melt. An application of Ohm's Law yields the bulk electrical conductivity of the partially molten region. The bulk electrical conductivity values for nominally dry materials follow a power-law relationship σbulk=Cσmeltϕm with fit parameters m=1.3±0.3 and C=0.66±0.06. Laminar fluid flow simulations were conducted on the same partially molten geometries to obtain permeability, and the respective pathways for electrical current and fluid flow over the same melt geometry were compared. Our results indicate that the pathways for flow fluid are different from those for electric current. Electrical tortuosity is lower than fluid flow tortuosity. The simulation results are compared to existing experimental data, and the potential influence of volatiles and melt films on electrical conductivity of partially molten rocks is discussed.
    No preview · Article · Dec 2015 · Earth and Planetary Science Letters
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    • "Previous experimental studies on the electrical properties of partially molten systems demonstrated that partial melting enhances bulk conductivity due to the much higher conductivity of the melt phase (Roberts and Tyburczy, 1999; ten Grotenhuis et al., 2005; Yoshino et al., 2010) and its high connectivity (Waff and Bulau, 1979; Faul et al., 1994; Yoshino et al., 2009). On the other hand, many laboratory deformation studies have shown that pronounced anisotropic redistribution of melt in partially molten rocks does occur during shear deformation (Holtzman et al., 2003a, 2003b; Kohlstedt and Holtzman, 2009). "
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    ABSTRACT: The electrical conductivity of partially molten peridotite was measured during deformation in simple shear at 1 GPa in a DIA type apparatus with a uniaxial deformation facility. To detect development of electrical anisotropy during deformation of partially molten system, the electrical conductivity was measured simultaneously in two directions of three principal axes: parallel and normal to the shear direction on the shear plane, and perpendicular to the shear plane. Impedance spectroscopy measurement was performed at temperatures of 1523 K for Fe-bearing and 1723 K for Fe-free samples, respectively, in a frequency range from 0.1 Hz to 1 MHz. The electrical conductivity of partially molten peridotite parallel to shear direction increased to more than one order of magnitude higher than those normal to shear direction on the shear plane. This conductivity difference is consistent with the magnitude of the conductivity anisotropy observed in the oceanic asthenosphere near the Eastern Pacific Rise. On the other hand, conductivity perpendicular to the shear plane decreased gradually after the initiation of shear and finally achieved a value close to that of olivine. The magnitude and development style of conductivity anisotropy was almost the same for both Fe-bearing and Fe-free melt-bearing systems, and also independent of shear strain. However, such conductivity anisotropy was not developed in melt-free samples during shear deformation, suggesting that the conductivity anisotropy requires a presence of partial melting under shear stress. Microstructural observations of deformed partially molten peridotite samples demonstrated that conductivity anisotropy was attributed to the elongation of melt pockets parallel to the shear direction. Horizontal electrical conductivity anisotropy revealed by magnetotelluric surveys in the oceanic asthenosphere can be well explained by the realignment of partial melt induced by shear stress.
    Full-text · Article · Nov 2014 · Earth and Planetary Science Letters
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    • "For the deeper conductive structures in the mantle, C2 and C3, interpretation is far more complicated due to the lack of thermal and compositional constraints from mantle xenoliths within the OB. For anomaly C2, which lies mostly below the inferred thermal lithosphere boundary [Zang et al., 2005], a low resistivity of 10 Xm could be produced with a 1–2 wt% basaltic melt in the olivine system [Yoshino et al., 2010]. However , such a melt proportion would cause a >6% Vs reduction [Hammond and Humphreys, 2000], which is not allowed by the seismic data. "
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    ABSTRACT: Long period magnetotelluric (MT) data from project SINOPROBE were acquired and modeled, using three-dimensional (3D) MT inversion, to study the electrical structure of Ordos Block, a component of the North China Craton. For the first time a high resolution 3D resistivity model of the lithosphere is defined for the region. Contrary to what would be expected for a stable cratonic block, a prominent lithospheric conductive complex is revealed extending from the upper mantle to the mid-to-lower crust beneath the northern part of Ordos. Correlating well with results of seismic studies, the evidence from our independent magnetotelluric data supports regional modification of the lithosphere under the north Ordos and lithosphere thinning beneath Hetao Graben. The abnormally conductive structure may result from upwelling of mantle material in mid-to-late Mesozoic beneath the northern margin of the Ordos block.
    Full-text · Article · Jun 2014 · Geochemistry Geophysics Geosystems
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