TY - JOUR

T1 - Electrical conductivity of hydrous basaltic melts

T2 - Implications for partial melting in the upper mantle

AU - Ni, Huaiwei

AU - Keppler, Hans

AU - Behrens, Harald

PY - 2011/2/24

Y1 - 2011/2/24

N2 - The Earth's uppermost asthenosphere is generally associated with low seismic wave velocity and high electrical conductivity. The electrical conductivity anomalies observed from magnetotelluric studies have been attributed to the hydration of mantle minerals, traces of carbonatite melt, or silicate melts. We report the electrical conductivity of both H2O-bearing (0-6 wt% H2O) and CO2-bearing (0.5 wt% CO2) basaltic melts at 2 GPa and 1,473-1,923 K measured using impedance spectroscopy in a piston-cylinder apparatus. CO2 hardly affects conductivity at such a concentration level. The effect of water on the conductivity of basaltic melt is markedly larger than inferred from previous measurements on silicate melts of different composition. The conductivity of basaltic melts with more than 6 wt% of water approaches the values for carbonatites. Our data are reproduced within a factor of 1.1 by the equation log σ = 2.172 - (860.82 - 204.46 w0.5)/(T - 1146.8), where σ is the electrical conductivity in S/m, T is the temperature in K, and w is the H2O content in wt%. We show that in a mantle with 125 ppm water and for a bulk water partition coefficient of 0.006 between minerals and melt, 2 vol% of melt will account for the observed electrical conductivity in the seismic low-velocity zone. However, for plausible higher water contents, stronger water partitioning into the melt or melt segregation in tube-like structures, even less than 1 vol% of hydrous melt, may be sufficient to produce the observed conductivity. We also show that ~1 vol% of hydrous melts are likely to be stable in the low-velocity zone, if the uncertainties in mantle water contents, in water partition coefficients, and in the effect of water on the melting point of peridotite are properly considered.

AB - The Earth's uppermost asthenosphere is generally associated with low seismic wave velocity and high electrical conductivity. The electrical conductivity anomalies observed from magnetotelluric studies have been attributed to the hydration of mantle minerals, traces of carbonatite melt, or silicate melts. We report the electrical conductivity of both H2O-bearing (0-6 wt% H2O) and CO2-bearing (0.5 wt% CO2) basaltic melts at 2 GPa and 1,473-1,923 K measured using impedance spectroscopy in a piston-cylinder apparatus. CO2 hardly affects conductivity at such a concentration level. The effect of water on the conductivity of basaltic melt is markedly larger than inferred from previous measurements on silicate melts of different composition. The conductivity of basaltic melts with more than 6 wt% of water approaches the values for carbonatites. Our data are reproduced within a factor of 1.1 by the equation log σ = 2.172 - (860.82 - 204.46 w0.5)/(T - 1146.8), where σ is the electrical conductivity in S/m, T is the temperature in K, and w is the H2O content in wt%. We show that in a mantle with 125 ppm water and for a bulk water partition coefficient of 0.006 between minerals and melt, 2 vol% of melt will account for the observed electrical conductivity in the seismic low-velocity zone. However, for plausible higher water contents, stronger water partitioning into the melt or melt segregation in tube-like structures, even less than 1 vol% of hydrous melt, may be sufficient to produce the observed conductivity. We also show that ~1 vol% of hydrous melts are likely to be stable in the low-velocity zone, if the uncertainties in mantle water contents, in water partition coefficients, and in the effect of water on the melting point of peridotite are properly considered.

KW - Basaltic melt

KW - Electrical conductivity

KW - Hydrous melt

KW - Partial melting

UR - http://www.scopus.com/inward/record.url?scp=80052037039&partnerID=8YFLogxK

U2 - 10.1007/s00410-011-0617-4

DO - 10.1007/s00410-011-0617-4

M3 - Article

AN - SCOPUS:80052037039

VL - 162

SP - 637

EP - 650

JO - Contributions to Mineralogy and Petrology

JF - Contributions to Mineralogy and Petrology

SN - 0010-7999

IS - 3

ER -