Experimental determination of electrical conductivity during deformation of melt-bearing olivine aggregates: Implications for electrical anisotropy in the oceanic low velocity zone
Experimental determination of electrical conductivity during deformation of melt-bearing olivine aggregates: Implications for electrical anisotropy in the oceanic low velocity zone
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DOI：
10.1016/j.epsl.2010.11.041
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发表时间：
2011-02-01
影响因子：
5.3
通讯作者：
Trong, Emmanuel Le
中科院分区：
文献类型：
--
作者：
Caricchi, Luca;Gaillard, Fabrice;Trong, Emmanuel Le
采用一种新的实验装置，在300 MPa的围压和873 ~ 1473 K的温度范围内，测量了扭转（简单剪切）变形过程中电导率（EC）的原位变化。该装置旨在测试部分熔融系统的变形是否会产生电各向异性。这项研究的动机来自于观察到，岩石圈软流圈边界（LAB）在洋中脊，特别是在东太平洋隆起是强烈的电各向异性，在最初的一组校准实验，EC随温度（873-1473 K）的变化，确定为卡拉拉大理石，阿海姆纯橄榄岩和玄武岩轴承橄榄石aggregates。然后在1473 K的变形实验期间监测EC，并在6 MHz和1 Hz之间的频率范围内测量。测试的不同材料的电响应作为频率的函数，根据试样中所含熔体的存在、不存在、比例和分布而显著变化。无熔体样品显示一个单一的导电机制，而熔体轴承样品显示两个导电机制串联连接，反映了孤立和连接熔体的贡献。沿沿着样品半径，在平行于扭转实验中固有的剪切梯度的方向上测量阻抗。在测试过程中，测量的阻抗值的增加表明，长范围熔体连通性径向减小，熔体从低剪切应力区域流到高剪切应力区域。根据阻抗测量值计算的电导率很低，与沿着大洋中脊测量的值相当。我们认为，LAB的电各向异性反映了交替的熔体富集和熔体耗尽的通道延长的扩展方向可能引起的扩展速度梯度沿着脊。这意味着，所观察到的电各向异性揭示了更大规模的过程比应变诱导产生的晶体学的优选取向。这种大规模的过程可能影响海山的分布和洋中脊玄武岩的化学变化。(C)2010爱思唯尔有限公司版权所有。
A novel experimental setup was used to measure in-situ variations of electrical conductivity (EC) during deformation in torsion (simple shear) at 300 MPa confining pressure and temperatures between 873 and 1473 K. This setup is designed to test if deformation of partially molten systems can produce electrical anisotropy. The motivation for this study comes from the observation that the Lithosphere-Asthenosphere Boundary (LAB) at mid-ocean ridges and in particular at the East Pacific Rise is strongly electrically anisotropic.In an initial set of calibration experiments, the variation of EC with temperature (873-1473 K) was determined for Carrara marble, Aheim dunite and basalt-bearing olivine aggregates. EC was then monitored during deformation experiments at 1473 K and measured in the frequency range between 6 MHz and 1 Hz. The electrical response of the different materials tested as a function of frequency, changes significantly depending on the presence, absence, proportion and distribution of melt contained in the specimen. Melt-free samples show a single conduction mechanism whereas melt-bearing samples display two conduction mechanisms linked in series, reflecting the contribution of isolated and connected melt. Impedance was measured along the sample radius, in a direction parallel to the shear gradient inherent in torsion experiments. During the tests, increasing values of the impedance measured suggest that the long range melt connectivity decreases radially, and melt drains from low to high shear stress regions. The conductivity, calculated from impedance measurements, is low and comparable to values measured along mid-ocean ridges. We suggest that electrical anisotropy of the LAB reflects an alternation of melt-enriched and melt-depleted channels elongated in the spreading direction possibly induced by spreading velocity gradients along the ridge. This implies that the observed electrical anisotropy reveals larger scale processes than strain-induced generation of crystallographic preferred orientations. Such large-scale processes could influence the distribution of seamounts and chemical variations of mid-ocean ridge basalts. (C) 2010 Elsevier B.V. All rights reserved.