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

(Impact Factor: 4.72). 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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Available from: Mickaël Laumonier, Sep 01, 2015
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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). "
##### Article: Electrical conductivity anisotropy in partially molten peridotite under shear deformation
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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.
Earth and Planetary Science Letters 11/2014; 405:98–109. DOI:10.1016/j.epsl.2014.08.018 · 4.72 Impact Factor
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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. "
##### Article: Three-dimensional electrical structure of the crust and upper mantle in Ordos Block and adjacent area: Evidence of regional lithospheric modification
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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.
Geochemistry Geophysics Geosystems 06/2014; 15(6). DOI:10.1002/2014GC005270 · 3.05 Impact Factor
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• "For basaltic melt with a small amount of impurities, one would need a few % of melt to enhance conductivity to explain geophysical observations (e.g., Shankland et al., 1981). However, recent studies showed the importance of impurities on the electrical conductivity of melts (Gaillard et al., 2008; Ni et al., 2011; Yoshino et al., 2010). These studies showed that when a large amount of highly mobile ions "
##### Article: Does partial melting explain geophysical anomalies?
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ABSTRACT: The existence of partial melt is frequently invoked to explain geophysical anomalies such as low seismic wave velocity and high electrical conductivity. I review various experimental and theoretical studies to evaluate the plausibility of this explanation. In order for a partial melt model to work, not only the presence of melt, but also the presence of appropriate amount of melt needs to be explained. Using the mineral physics observations on the influence of melt on physical properties and the physics and chemistry of melt generation and transport, I conclude that partial melt model for the asthenosphere with homogeneous melt distribution does not work. One needs to invoke inhomogeneous distribution of melt if one wishes to explain observed geophysical anomalies by partial melting. However, most of models with inhomogeneous melt distribution are either inconsistent with some geophysical observations or the assumed structures are geodynamically unstable and/or implausible. Therefore partial melt models for the geophysical anomalies of the asthenosphere are unlikely to be valid, and some solid-state mechanisms must be invoked. The situation is different in the deep upper mantle where melt could completely wet grain-boundaries and continuous production of melt is likely by "dehydration melting" at around 410-km. In the ultralow velocity zone in the D″ layer, where continuous production of melt is unlikely, easy separation of melt from solid precludes the partial melt model for low velocities and high electrical conductivity unless the melt density is extremely close to the density of co-existing solid minerals or if there is a strong convective current to support the topography of the ULVZ region. Compositional variation such as Fe-enrichment is an alternative cause for the anomalies in the D″ layer.
Physics of The Earth and Planetary Interiors 02/2014; 228. DOI:10.1016/j.pepi.2013.08.006 · 2.40 Impact Factor