Electrical conductivity of metasomatized lithology in subcontinental lithosphere
Electrical conductivity of metasomatized lithology in subcontinental lithosphere
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DOI：
10.2138/am-2021-7942
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发表时间：
2022-02
影响因子：
3.1
通讯作者：
Ye Peng;G. Manthilake;M. Mookherjee
中科院分区：
文献类型：
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作者：
Ye Peng;G. Manthilake;M. Mookherjee
摘要地震观测到的次大陆岩石圈中岩石圈不连续（MLD）的一个可能的成因是地幔交代作用。交代地幔可能稳定角闪石等含水相。关于角闪石的现有电导率数据差异很大。角闪石的电导率远高于透闪石。因此，如果角闪石岩真正代表了MLD地区的角闪石品种，那么角闪石很可能会在MLD深度引起高电导率异常。然而，这与MLD深度的大地电磁观测结果不一致。因此，为了更好地理解角闪石电导率数据的这种差异，并评估MLD是否可能由交代作用引起，我们确定了天然交代岩石样品的电导率。交代岩样由~87%的透辉石、~9%的含钠透闪石角闪石和~3%的钠长石组成。我们收集了~3.0 GPa下的电导率数据，即，与MLD相关的深度。我们还跨越了400到1000 K之间的温度范围。我们发现，这种交代岩石样品的电导率随温度而增加。电导率的温度依赖性表现出两个不同的制度。在775 K的低温下，电导率增加，并且由于部分脱水，很可能由含水流体的传导主导。角闪石或含角闪石岩石电导率的研究与前人研究的主要区别在于角闪石组合中钠（Na）的含量。此外，角闪石中较高的Na含量可能导致较高的电导率。榴辉岩和蚀变岩角闪石是交代地幔中最常见的角闪石类型，而我们对含钠透闪石的研究是与这些角闪石最接近的类比。电导率结果与大地电磁观测结果的比较将MLD深度的角闪石丰度限制在<1.5%。这种低模态角闪石的比例只能使地震横波速度降低0.4 - 0.5%，这明显低于观测到的速度降低2 - 6%。因此，同时解释MLD的地震和大地电磁观测可能具有挑战性。
Abstract A plausible origin of the seismically observed mid-lithospheric discontinuity (MLD) in the subcontinental lithosphere is mantle metasomatism. The metasomatized mantle is likely to stabilize hydrous phases such as amphiboles. The existing electrical conductivity data on amphiboles vary significantly. The electrical conductivity of hornblendite is much higher than that of tremolite. Thus, if hornblendite truly represents the amphibole varieties in MLD regions, then it is likely that amphibole will cause high electrical conductivity anomalies at MLD depths. However, this is inconsistent with the magnetotelluric observations across MLD depths. Hence, to better understand this discrepancy in electrical conductivity data of amphiboles and to evaluate whether MLD could be caused by metasomatism, we determined the electrical conductivity of a natural metasomatized rock sample. The metasomatized rock sample consists of ~87% diopside pyroxene, ~9% sodium-bearing tremolite amphibole, and ~3% albite feldspar. We collected the electrical conductivity data at ~3.0 GPa, i.e., the depth relevant to MLD. We also spanned a temperature range between 400 to 1000 K. We found that the electrical conductivity of this metasomatized rock sample increases with temperature. The temperature dependence of the electrical conductivity exhibits two distinct regimes. At low temperatures 775 K, the conductivity increases, and it is likely to be dominated by the conduction of aqueous fluids due to partial dehydration. The main distinction between the current study and the prior studies on the electrical conductivity of amphiboles or amphibole-bearing rocks is the sodium (Na) content in amphiboles of the assemblage. Moreover, it is likely that the higher Na content in amphiboles leads to higher electrical conductivity. Pargasite and edenite amphiboles are the most common amphibole varieties in the metasomatized mantle, and our study on Na-bearing tremolite is the closest analog of these amphiboles. Comparison of the electrical conductivity results with the magnetotelluric observations constrains the amphibole abundance at MLD depths to <1.5%. Such a low-modal proportion of amphiboles could only reduce the seismic shear wave velocity by 0.4–0.5%, which is significantly lower than the observed velocity reduction of 2–6%. Thus, it might be challenging to explain both seismic and magnetotelluric observations at MLD simultaneously.