NSFGEO-NERC: Imaging the magma storage region and hydrothermal system of an active arc volcano
Lead Research Organisation:
University of Southampton
Department Name: Sch of Ocean and Earth Science
Abstract
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.
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.
