One of the GNG workstreams in the EXPLORE project is to examine the magmatic – hydrothermal transition. Read on to find out more about what this is, why it is important for supercritical research, and how we aim to reveal more about the nature of the TVZ’s magmatic-hydrothermal transition.
The magmatic-hydrothermal transition is the interface where magma transfers heat to surrounding circulating water. These fluids (i.e. water in liquid and/or vapour or supercritical form) then transport the heat energy. Magmatic-hydrothermal fluid circulation systems provide heat and mass transfer for geothermal systems, as well as the formation of ore deposits and volcanic hazards.
Hydrothermal is a subset of geothermal. Geothermal refers to any system that hosts heat from within the Earth to the surface. When heat and mass transfer are associated with aqueous based fluids then scientists refer to a hydrothermal system.
Most supercritical fluids at accessible levels in the crust globally are closely associated with magma. But supercritical is not magma. In developing future supercritical energy projects, the target for deeper drilling in New Zealand is supercritical fluids. It is not magmatic fluids, and definitely not magma.
Magmatism is associated with all convergent plate boundaries, such as Aotearoa New Zealand’s location on a subduction zone. Here, magma is the deep heat source for the major geothermal and volcanic areas.
At the magma – hydrothermal interface, we expect there to be a gradation going deeper: from solid rock to a mush zone of increasing melt content. We don’t expect a sudden change from solid rock and liquidus magma, but we currently don’t know what this interface looks like under Aotearoa New Zealand’s geothermal areas.
To reach supercritical resources, Aotearoa New Zealand will need to drill deep (>4km) wells. We need to work out where the resources are and then, by knowing the geology, geochemistry and rock properties at this interface, this information will tell us when we are getting close to target depths and conditions…and when we are getting close to magma. Drilling will also be strongly supported by field scale geophysical imaging of the crust beneath the geothermal system.
We needs to benefit from the proximity of magmatic heat source(s), but not to drill into the magma. This has happened in Iceland, Kenya and Hawaii, and handling super-heated fluids is much more favourable than handling magma. Read more about why supercritical is not magma.
But we can’t expect to make reliable deep drilling targets without knowing more about magma, and in particular the present-day magmatic crust in Aotearoa New Zealand. Our geographical area of focus is the Taupō Volcanic Zone (TVZ), in the central North Island, where there are areas of intense hydrothermal activity, shallow magma bodies, frequent small eruptions, and common (for geologists in geological time!) large-scale caldera forming eruptions. We have working conceptual models of geothermal systems in the central TVZ, but many of the details are sketchy or speculative, especially at depth.
Investigating the Magmatic - Hydrothermal Transition
We are working to reveal more about the nature of the TVZ’s magmatic-hydrothermal transition. Our team are combining a range of geophysical, geochemical, geological and modelling techniques to determine how close to the surface magma exists, and how closely connected magma is to the hydrothermal system.
We are combining studies of the host rock (magma) with hydrothermal features (fluids). From a geological and geochemical perspective, we are looking at the hydrothermal envelope over and around the magma – looking for complex minerals and volatiles (i.e. gases).
In the Geothermal:The Next Generation research programme, our geologists are:
Our target depths are between ~4 and >5 km. Check back in to find out more about how heat and magmatic fluids reach the depths currently reached by drilling, and the permeability of the deep crust.