NSFGEO-NERC: Imaging the magma storage region and hydrothermal system of an active arc volcano

我们的项目将使用一种强大的、最近才可用的地球物理技术来探测活火山热液系统的内部，从而获得对这种系统如何工作的独特新见解。我们的目标是位于新西兰北部400公里的太平洋“火环”上的兄弟火山。这座火山拥有世界上最活跃的海底热液系统之一。在过去的二十年里，它一直是国际研究的焦点，最终在2018年进行了科学钻探，以恢复样本并在海底以下近450米的深度进行测量。因此，它可以说是世界上同类火山中研究得最好的。热液流体在几乎所有的火山系统中循环，这种循环是固体地球和海洋之间化学和热交换的主要机制。沉入地壳的水被地表以下几公里处的热岩石或熔融岩石加热，然后返回地表。化学物质与岩石交换，并在上升的液体中浓缩。熔岩本身是水和气体的额外来源。这些液体会大量排放到海洋中。当它们冷却并与海水混合时，集中在其中的元素会沉淀，有时会形成含有铜和金等金属的大型沉积物。对这些流体、它们在海床上的沉积和周围蚀变岩石的研究表明：喷口可能是集中的，也可能是扩散的；喷口流体可以具有各种成分、温度和来源；随着火山的演变，液体的性质也会发生变化。然而，人们对可以通过钻探进入的几百米深度以外的区域知之甚少。因此，人们对深层流动路径的形状以及它们与底层热或熔融岩石的关系仍然知之甚少。我们的实验包括使用一个强大的发射器向火山发射电磁波，并使用海底接收器和拖在发射器后面的其他接收器测量由此产生的电磁波动。我们的测量将对海床下几公里深处的电阻率很敏感。固体火山岩具有很高的电阻率，但岩石在开始融化时变得更导电，从而形成了巨大的反差。热液的导电性更强，特别是当它们是咸的时候。在海床上或靠近海床的金属矿床的导电性会更强。因此，我们提出的可控源电磁（CSEM）技术可以用来成像所有这些特征。三维CSEM成像现在是可行的，我们将首次在海底火山上生成这样的图像，从而实现前所未有的分辨率。我们将利用我们的电阻率图像估计热液系统下热源的大小、形状、温度和熔体含量；热液流体的温度和盐度以及它们在地壳内的路径形状；以及由此形成的矿藏的大小和形状。我们将把这些图像与国际合作者的结果结合起来，包括向火山发射声波的重大新实验，以及进一步的钻探和计算机模拟火山随时间的演变。我们的研究结果将表明，热源下方的形状和内部结构以及热源上方的地壳断裂如何驱动热液循环模式，从而驱动地壳的化学变化。这种认识可以应用于其他地下结构不太为人所知的火山。我们还将确定循环海水和热源释放的流体对海底附近矿藏形成的相对贡献，并与采矿业的伙伴合作，评估目前在陆地上勘探这类矿藏的影响。

Our project will use a powerful and only recently available geophysical technique to probe the interior of the hydrothermal system of an active volcano and thus gain unique new insights into how such systems work. Our target is Brothers volcano in the Pacific "Ring of Fire" about 400 km north of New Zealand. This volcano hosts one of the most active submarine hydrothermal systems in the world. It has been a focus for international study over the past two decades, culminating in scientific drilling in 2018 to recover samples and make measurements up to depths of nearly 450 m below the seafloor. Consequently it arguably the best-studied volcano of its type in the world. Hydrothermal fluids circulate in almost all volcanic systems, and this circulation is the main mechanism of chemical and heat exchange between the solid Earth and the oceans. Water sinking into the crust is heated by hot or molten rocks a few kilometres below the surface, then returns to the surface. Chemicals are exchanged with the rocks and can become concentrated in the rising fluids. Molten rock itself is an additional source of water and also gases. These fluids can vent vigorously into the ocean. As they cool and mix with seawater, the elements concentrated within them precipitate, sometimes forming large deposits containing metals such as copper and gold. Studies of the fluids, their deposits on the seabed and the surrounding altered rock have shown that: venting may either be focused or diffuse; vent fluids can have a variety of compositions, temperatures and origins; and the nature of the fluids can change as the volcano evolves. However, little is known about what lies beyond the few hundred metres depth range that can be accessed by drilling. Thus the shape of flow paths at depth and their relationship to the underlying hot or molten rock remain poorly understood. Our experiment involves using a powerful transmitter that sends electromagnetic waves into the volcano and measuring the resulting electromagnetic fluctuations using receivers on the seafloor and others towed behind the transmitter. Our measurements will be sensitive to the electrical resistivity beneath the seabed down to depths of several kilometres. Solid volcanic rocks have high resistivities, but rocks become much more conductive when they start to melt, resulting in a large contrast. Hot hydrothermal fluids are even more conductive, particularly when they are salty. Metallic mineral deposits at or close to the seabed can be more conductive again. Thus our proposed controlled source electromagnetic (CSEM) techniques can be used to image all of these features. Three-dimensional CSEM imaging is now feasible, and we will generate such images for the first time at a submarine volcano, thus achieving unprecedented resolution.We will use our resistivity image to estimate the size, shape, temperature and melt content of the heat source beneath the hydrothermal system; the temperature and salinity of the hydrothermal fluids and the shape of the pathways that they take within the crust; and the size and shape of the resulting mineral deposits. We will combine these images with results from international collaborators, including major new experiments involving sending sound waves through the volcano and further drilling and computer modelling of its evolution over time. Our results will show how the shape and internal structure of the heat source below and the faulting of the crust above that heat source drive patterns of hydrothermal circulation and thus the chemical alteration of the crust. This understanding can then be applied to other volcanoes where the subsurface structure is less well known. We will also determine the relative contributions of circulating seawater and fluids released from the heat source to the formation of mineral deposits near the seafloor, and work with partners in the mining industry to assess implications for exploration for such deposits now on land.