October 27, 2021

Team Profile:
Revealing the Magmatic - Hydrothermal Transition
contributor(s)

Isabelle Chambefort

photo credit:

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.

What is the 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.

A conceptual model of continental hydrothermal system: cold waters are heated by magmatic heat sources and rise up to form geothermally-active areas (Modified by Isabelle Chambefort, from Henley & Ellis, 1983, Earth-Sci Rev 19, 1 )

Why is this Interface Important for Supercritical?

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.

This is what scientists think the magmatic crust might look like: different magmatic sources and activity indifferent areas of the TVZ, with specific focus underneath the two active caldera volcanoes.  (Source: Barker et al., 2020, Geology 47, 504)

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).

Geologist preparing cuttings samples to analyse using a scanning electron microscope. To assess the presence of potential magmatic fluids, geologists look at variations in the mineralogy that indicates higher temperature and acidic conditions. (Photo credit: Margaret Low).

In the Geothermal:The Next Generation research programme, our geologists are:

  • analysing fragments of magma that crystallised in the crust (granite) that have been brought up in eruptive events or intersected by drilling, to determine the pressure and temperature conditions at depth and the composition of the fluids (if any) that were present at the time or later of the crystallisation.
  • analysing late stage quartz hosted in open cavities in the granite that crystallised from hydrothermal fluids not the magma and as such can give us an indicative composition of the fluid that were present.
  • modelling the hydrothermal fluid circulation above magma and quantify where supercritical resources can be

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.

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categories

Geology
Geochemistry
Geophysics
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tags

magmatic-hydrothermal transition
geology
learning
geochemistry
supercritical conditions
supercritical fluids
supercritical resources

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