Implications for melt in the upper mantle from the electrical conductivity of melt-bearing peridotites at high pressures

Takashi Yoshino^a, Mickael Laumonier^b, Elizabeth McIsaac^c & Tomoo Katsura^a

^aInstitute for Study of the Earth's Interior, Okayama University, Misasa, Tottori 682-0193, Japan, tyoshino@misasa.okayama-u.ac.jp ; Tomo.Katsura@uni-bayreuth.de

^bInstitut des Sciences de la Terre d'Orléans, UMR 6113, Campus Géosciences, 1A, Rue de la Férollerie, 41071 Orléans cedex 2, France, mickael.laumonier@univ-orleans.fr

^cDepartment of Earth Sciences, Dalhousie University, Edzell Castle Circle, Halifax NS, Canada B3H 4J1, EL997655@dal.ca

This webpage is a summary of: Takashi, Y., M. Laumonier, E. McIsaac & T. Katsura, Electrical conductivity of basaltic and carbonatite melt-bearing peridotites at high pressures: Implications for melt distribution and melt fraction in the upper mantle, Earth Planet. Sci. Lett., 295, 593-602, 2010.

Laboratory measurements on anhydrous peridotite and single crystals of dry olivine show that the electrical conductivity of the upper mantle should be ~10^-4 to 10^-2 S/m at mantle temperatures. Electromagnetic observations, however, have demonstrated that the electrical conductivity of some upper mantle regions greatly exceeds these values. In the mantle under the Pacific Ocean, just beneath or close to the mid-ocean ridge, conductivities of >10^-1 S/m have been measured at depths of > 60 km (Evans et al., 2005). On the other hand, the deep electrical conductivity profile, obtained from analysis of data from a submarine cable extending from Hawaii to North America, showed a conductivity peak of 10^-1 S/m in the depth range 200~250 km (Lizaralde et al., 1995).

Highly conductive zones in the upper mantle require the presence of conductive phases, and silicate melts or hydrated olivine crystals are commonly considered. High mantle conductivities have usually been interpreted as indicating trace amounts of hydrogen in olivine. The magnitude of the effect of water on olivine conductivity remains under debate (Wang et al., 2006; Yoshino et al., 2006). However, two recent pieces of work indicate that although the presence of water greatly enhances the electrical conductivity of olivine this effect is not large enough to explain the high conductivity at the top of the asthenosphere as observed at the East Pacific Rise (Yoshino et al., 2009; Poe et al., 2010).

Silicate melt is another candidate to explain conductivity anomalies in the upper mantle. However, conductivity measurements on dry olivine and basaltic melt by Tyburczy & Waff (1983) suggested that a 5-10% melt fraction is required to explain 0.1 S/m based on the Hashin and Shtrikmann upper bound^¶, which assumes an ideal geometry. Such a high melt fraction is not consistent with estimates from seismological studies (e.g., The MELT seismic team, 1998). This discrepancy may be caused by a difference between the ideal geometry and a realistic partial-melt geometry.

Recently Gaillard et al. (2008) measured the conductivity of carbonatite melts at atmospheric pressure, and demonstrated that carbonatite melt has a distinctly higher conductivity than silicate melt. Gaillard et al. (2008) suggested that a very small amount of carbonate melt is enough to explain the conductivity anomaly at the top of the asthenosphere. To assess this suggestion, we thus need to know the effect of carbonatitic melt on the bulk conductivity of partially molten rocks under high pressure.

In our study, we performed electrical impedance measurements on two types of partially molten samples with basaltic and carbonatitic melts in a Kawai-type multi-anvil apparatus in order to investigate melt-fraction vs. conductivity relationships and melt distribution in partially molten peridotite under high pressure. The silicate samples comprised San Carlos olivine with various amounts of mid-ocean ridge basalt (MORB). 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.

Sample conductivity increased with increasing melt fraction. Carbonatite-bearing samples show approximately one order of magnitude higher conductivity than basalt-bearing ones at a similar melt fraction. 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 to ~3% and ~0.3% for basaltic melt and carbonatite melt, respectively.

Just beneath the mid-ocean ridge, the melt fractions required to explain the conductivity of 0.1 S/m are 1 and 3 vol.% at 1600 and 1500 K, respectively. This range of melt fraction is consistent with the minimum melt concentration of 1 to 2 vol.% in the melt production region predicted from seismic shear-wave delays and Rayleigh-wave velocity variations (The MELT seismic team, 1998). Therefore, the conductivity anomaly in this region can be explained by a presence of silicate melt.

Hirschmann (2010) suggested that carbonatite melt can be stable below depths of 130-180 km for mantle peridotite containing typical amounts of volatile components. Melting beneath the mid-ocean ridge occurs at depths up to 330 km, producing 0.03-0.3 wt.% carbonatite melt. Our estimated melt fraction for carbonatite-bearing peridotite falls in this range. The deep electrical conductivity profile, obtained from analysis of data from a submarine cable extending from Hawaii to North America, showed a conductivity peak of 10^-1 S/m in the depth range 200~250 km (Lizaralde et al., 1995). Such a conductive anomaly can be explained by a presence of very small amount of carbonatite melt (Figure 1).

Figure 1: Comparison of laboratory data on electrical conductivity of carbonatite melt, silicate melt and hydrous olivine with geophysically observed electrical conductivity in the upper mantle beneath the Pacific ocean. Red and green lines: electrical conductivity of carbonatite-bearing and silicate-melt-bearing peridotites respectively, blue line: upper bound of the electrical conductivity of hydrous olivine based on the maximum hydroxyl solubility in olivine (Yoshino et al., 2009), thick black dashed line: electrical conductivity of dry olivine (Xu et al., 1998), green and pink areas: geophysically observed conductivity profile in the Pacific ocean (Utada et al., 2003 and Lizzaralde et al., 1995), orange and light-orange regions: conductivity around a depth of 100 km near the East Pacific Rise, light blue area: the range of conductivity above 60 km near the East Pacific Rise (Evans et al., 2005). The conductivity anomaly beneath Hawaii can be explained by a very small amount of carbonatite melt.

^¶Rocks are usually multi-phase. To predict the effective properties as a function of volume fraction of the various phases, the best approach is to predict the upper and lower bounds theoretically. The best bounds, defined as giving the narrowest possible range without specifying the geometries of the constituents, are the Hashin-Shtrikman bounds. The upper bound indicates the case where the conductive phase is the best interconnected.

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