Partial silicate melting in the asthenospheric LVZ: Evidence from electrical conductivity of hydrous basaltic melts

Huaiwei Ni¹, Hans Keppler¹ & Harald Behrens²

¹Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany
huaiwei.ni@uni-bayreuth.de, hans.keppler@uni-bayreuth.de

²Institut für Mineralogie, Leibniz Universität Hannover, Callinstr. 3, 30167 Hannover, Germany
h.behrens@mineralogie.uni-hannover.de

This webpage is a summary of: Ni, Huaiwei, Hans Keppler & Harald Behrens, Electrical conductivity of hydrous basaltic melts: implications for partial melting in the upper mantle, Cont. Min. Pet., 162, 637-650, DOI: 10.1007/s00410-011-0617-4

Introduction

Partial silicate melting in the asthenospheric LVZ is not a new idea, as the presence of silicate melt is known to reduce seismic velocity. Recent magnetotelluric studies (e.g., Evans et al., 2005) reported electrical conductivity as high as 0.1 S/m and strong conductivity anisotropy in the uppermost asthenosphere. Available electrical conductivity data of dry basaltic melt requires at least 5% mantle melting to match the observed conductivity anomaly, but it would be difficult for so much silicate melt to stay stable in the LVZ for a long period of time. Furthermore, such a degree of melt is not consistent with geochemical constraints. Some authors (e.g., Wang et al., 2006) resorted to hydrated mantle minerals, but later studies (Yoshino et al., 2006) showed that hydrated olivine cannot account for the observed anomaly or anisotropy. Carbonate melt is highly conductive and has also been proposed to be the conductive phase in the LVZ (Gaillard et al., 2008), but carbon concentration in the mantle is simply too low (several tens of ppm) to form enough carbonate melt, which contains at least 30 wt% CO₂. Our study assesses whether hydrous silicate melt can reconcile the observations, as the transport properties of silicate melts are known to be significantly affected by a few percent dissolved water.

Conductivity measurements

We investigated the electrical conductivity of both H₂O-bearing (0-6 wt% H₂O) and CO₂-bearing (0.5 wt% CO₂) Fe-free basaltic melts at 2 GPa and 1473-1923 K in a piston cylinder apparatus (Figure 1). The H₂O contents of starting glasses were analyzed by both Karl Fischer Titration and FTIR. A glass tube 4 mm long was bracketed by a Pt rod 0.5 mm in diameter (the inner electrode) and a Pt₉₅Rh₅ tube with an inner diameter of 3.2 mm (the outer electrode). The two electrodes were connected to a Solartron 1260 impedance analyzer using Pt wires 0.35 mm in diameter. Temperature was monitored using a Pt-Pt₉₀Rh₁₀ thermocouple, which was not involved in the circuit of conductivity measurements. The impedance contribution from the Pt wires was evaluated by a short-circuit experiment. Samples maintained their shape after experiments but showed some H₂O loss. Conductivities acquired during the first heating cycle, which were least affected by H₂O loss, are presented here.

Figure 1. Sketch of sample assembly for electrical conductivity measurements in a piston-cylinder apparatus (drawn to scale).

The electrical conductivity of basaltic melt depends on temperature and H₂O content (Figure 2) according to the formula logs = 2.172 – (860.82 – 204.46 w^0.5)/(T – 1146.8), where s is the electrical conductivity in S/m, T is the temperature in K, and w is the H₂O content in wt%. The effect of small amounts of CO₂ of the order of 0.5 wt% on electrical conductivity is negligible compared to the effect of H₂O. The positive effect of water on conductivity is attributed to enhanced Na mobility and is more pronounced than that of felsic silicate melts. The conductivity of basaltic melt with > 6 wt% H₂O approaches that of carbonatite melt.

Figure 2. Electrical conductivity of volatile-bearing basaltic melts at 2 GPa.

Implications for partial melting in the LVZ

Our results can be used to calculate the electrical conductivity of a partial molten mantle using the upper (HS+) and lower (HS-) bounds of Hashin & Shtrikman (1962) or the parallel model. In a depleted mantle source initially with 125 ppm water, assuming a conductivity of 0.01 S/m for the solid phases and a bulk water partition coefficient of 0.006 between minerals and melt, 2% of melt will result in a bulk mantle conductivity of 0.1 S/m based on the HS+ model (Figure 3). However, for plausible higher water contents, stronger water partitioning into the melt or a more efficient structure (the parallel model), even less than 1% of hydrous melt is sufficient to produce the observed conductivity. Taking this into consideration, our data suggest that between 0.3 and 2% of hydrous basaltic melt can account for the observed electrical conductivity in the LVZ. The melt should be segregated into tube-like structures, and elongated in the direction of plate spreading, which would be consistent with both the observed electric and seismic anisotropy.

Figure 3. Evolution of the electrical conductivity of the mantle (with 600 ppm or 125 ppm H₂O) with increasing degree of partial melting, based on the lower bound (HS-) and upper bound (HS+) of Hashin & Shtrikman (1962), and the parallel model.

References

• Evans R.L., Hirth G., Baba K., Forsyth D., Chave A., Mackie R., (2005) Geophysical evidence from the MELT area for compositional controls on oceanic plates. Nature, 437, 249-252.
• Gaillard F., (2004) Laboratory measurements of electrical conductivity of hydrous and dry silicic melts under pressure. Earth Planet. Sci. Lett., 218, 215-228.
• Hashin Z., Shtrikman S., (1962) A variational approach to the theory of the effective magnetic permeability of multiphase materials. J. Appl. Phys., 33, 3125-3131.
• Ni, Huaiwei, Hans Keppler & Harald Behrens, Electrical conductivity of hydrous basaltic melts: implications for partial melting in the upper mantle, Cont. Min. Pet., 162, 637-650, DOI: 10.1007/s00410-011-0617-4
• Wang D., Mookherjee M., Xu Y., Karato S.-I., (2006) The effect of water on the electrical conductivity of olivine. Nature, 443, 977-980.
• Yoshino T., Matsuzaki T., Yamashita S., Katsura T., (2006) Hydrous olivine unable to account for conductivity anomaly at the top of the asthenosphere. Nature, 443, 973-976.