Formation of magmatic brine lenses via focussed fluid-flow beneath volcanoes
Formation of magmatic brine lenses via focussed fluid-flow beneath volcanoes
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DOI：
10.1016/j.epsl.2018.01.013
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发表时间：
2018-03-15
影响因子：
5.3
通讯作者：
Sparks, Steve
中科院分区：
文献类型：
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作者：
Afanasyev, Andrey;Blundy, Jon;Sparks, Steve
许多活火山或休眠火山在建筑物下面几公里深处显示出高导电性区域。我们探讨的可能性，这些地区代表透镜体的高盐度盐水分离的单相岩浆流体含有H2O和NaCl。由于含氯化物的流体具有高导电性，并具有特殊的运输金属的能力，这些地区可能是火山下活跃热液成矿的指示。为了研究这种可能性，我们进行了流体动力学模拟岩浆脱气到渗透性岩石。在我们的模型中，岩浆源位于7公里的深度和流体盐度接近预期的流体从典型的弧岩浆释放。我们的模型不同于以前类似过程的模型，因为它是（a）轴对称的，（B）包括一个静态的高渗透率的通道，连接岩浆源的表面。该路径模拟了大多数火山系统典型的火山管道和/或补给岩脉丛的存在。导管的存在导致了一些重要的流体动力学后果，没有观察到在以前的模型。重要的是，我们表明，环形卤水透镜结晶岩盐盖可能形成以上积极脱气次火山岩浆体，并可以持续超过250 kyr后脱气停止。参数分析表明，盐水透镜体是更普遍的流体被释放在温度高于湿花岗岩固相线，当岩浆流体盐度高，当高渗透率的途径是狭窄的。计算的深度，形式和电导率我们的模拟系统与出版的火山地下的大地电磁图像共享许多功能。次火山卤水透镜体的形成和持久性对地热系统和热液成矿有影响，尽管在所提出的模型中没有探讨这些特征。(C)2018 Elsevier B. V.版权所有。
Many active or dormant volcanoes show regions of high electrical conductivity at depths of a few kilometres beneath the edifice. We explore the possibility that these regions represent lenses of high salinity brine separated from a single-phase magmatic fluid containing H2O and NaCl. Since chloride bearing fluids are highly conductive and have an exceptional capacity to transport metals, these regions can be an indication of an active hydrothermal ore-formation beneath volcanoes. To investigate this possibility we have performed hydrodynamic simulations of magma degassing into permeable rock. In our models the magma source is located at 7 km depth and the fluid salinity approximates that expected for fluids released from typical arc magmas. Our model differs from previous models of a similar process because it is (a) axisymmetric and (b) includes a static high-permeability pathway that links the magma source to the surface. This pathway simulates the presence of a volcanic conduit and/or plexus of feeder dykes that are typical of most volcanic systems. The presence of the conduit leads to a number of important hydrodynamic consequences, not observed in previous models. Importantly, we show that an annular brine lens capped by crystallised halite is likely to form above an actively degassing sub-volcanic magma body and can persist for more than 250 kyr after degassing ceases. Parametric analysis shows that brine lenses are more prevalent when the fluid is released at temperatures above the wet granite solidus, when magmatic fluid salinity is high, and when the high-permeability pathway is narrow. The calculated depth, form and electrical conductivity of our modelled system shares many features with published magnetotelluric images of volcano subsurfaces. The formation and persistence of sub-volcanic brine lenses has implications for geothermal systems and hydrothermal ore formation, although these features are not explored in the presented model. (C) 2018 Elsevier B.V. All rights reserved.