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首页> 外文期刊>Contributions to Mineralogy and Petrology >Electrical conductivity of hydrous basaltic melts: implications for partial melting in the upper mantle
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Electrical conductivity of hydrous basaltic melts: implications for partial melting in the upper mantle

机译：含水玄武质熔体的电导率：对上地幔部分熔融的影响

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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 w 0.5)/(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.
机译：地球的最高流软层通常与低地震波速度和高电导率有关。从大地电磁研究中观察到的电导率异常归因于地幔矿物的水化，碳酸盐熔体或硅酸盐熔体的痕迹。我们报告了H 2 O含量（0–6 wt％H 2 O）和CO 2 含量（0.5 wt％CO 2 ）玄武质熔体在2 GPa和1,473–1,923 K的条件下使用活塞缸设备中的阻抗谱进行了测量。在这样的浓度水平下，CO 2 几乎不影响电导率。水对玄武岩熔体电导率的影响明显大于先前对不同组成的硅酸盐熔体进行测量得出的结论。水含量超过6 wt％的玄武岩熔体的电导率接近碳酸盐的值。我们的数据通过等式logσ= 2.172-（860.82-204.46 w 0.5 ）/（T-1146.8）在1.1的范围内重现，其中σ是电导率，单位为S / m，T是温度，单位为K，w是H 2 O含量，以重量％计。我们表明，在地幔中含125 ppm水的情况下，对于矿物与熔体之间的总水分配系数为0.006，熔体的2 vol％将占地震低速区中观测到的电导率。但是，对于合理的较高水含量，更强的水分配到熔体中或在管状结构中熔体偏析（甚至少于1 vol％的含水熔体）可能足以产生观察到的电导率。我们还表明，如果地幔水含量，水分配系数以及水对橄榄岩熔点的影响的不确定性适当，则低速区中约1 vol％的含水熔体可能是稳定的。考虑过的。

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