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、最近才有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的新见解。我们的目标是位于新西兰以北400公里的太平洋“火环”中的兄弟火山。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，以回收样品并在海底以下近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火山。热液流体在几乎所有火山系统中循环，这种循环是固体地球与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，并在上升的流体中浓缩。熔岩本身是水和气体的额外来源。这些液体可以猛烈地排入海洋。当它们冷却并与海水混合时，浓缩在其中的元素沉淀下来，有时形成含有铜和金等金属的大型矿床。对流体、其在海底的沉积物和周围的蚀变岩石的研究表明：喷发可能是集中的，也可能是扩散的;喷发流体的成分、温度和来源可能多种多样;流体的性质可能随着火山的演变而改变。然而，人们对钻探所能到达的几百米深度范围以外的情况知之甚少。因此，对深部流动路径的形状及其与底层热岩或熔融岩的关系仍然知之甚少。我们的实验涉及使用一个强大的发射器，将电磁波发送到火山中，并使用海底的接收器和发射器后面拖曳的其他接收器测量产生的电磁波动。我们的测量将对海底下至几公里深处的电阻率敏感。固体火山岩具有高导电性，但岩石开始融化时变得更具导电性，从而产生大的对比度。热的热液流体甚至更具导电性，特别是当它们是咸的时。位于或靠近海底的金属矿物沉积物可能再次更具导电性。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功能。三维CSEM成像现在已经可行，我们将首次在海底火山生成这种图像，从而达到前所未有的分辨率，我们将利用我们的电阻率图像估计热液系统下面热源的大小、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;以及由此产生的矿物沉积物的大小和形状。我们将联合收割机将这些图像与国际合作者的结果结合起来，包括通过火山发送声波的重大新实验，以及进一步的钻探和计算机模拟其随时间的演变。我们的研究结果将显示热源下方的形状和内部结构以及热源上方的地壳断层如何驱动热液循环模式，从而驱动地壳的化学蚀变。这种理解可以应用于其他火山，那里的地下结构不太为人所知。我们还将确定循环海水和热源释放的流体对海底附近矿床形成的相对贡献，并与采矿业的合作伙伴合作，评估对目前陆地上此类矿床勘探的影响。

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.