July 26, 2023

Team Profile:
Using oxygen isotopes to track movement of water through the crust in geothermal systems

Shane Rooyakkers

photo credit:
Shane Rooyakkers

In geothermal systems, hot rocks and water meet and interact. During these interactions, oxygen — the most abundant element in crustal rocks and a key constituent of water (H2O) — is exchanged between water and rock, as the rock reacts to form new minerals (a process called “hydrothermal alteration”). This exchange causes a lowering of rock oxygen isotope (18O/16O) ratios, which can be used to fingerprint and track the movement of water through the crust in geothermal systems.

The heat that drives geothermal systems ultimately comes from cooling magma deep in the crust. If this cooling magma meets hydrothermally altered rock, it can incorporate the rock, thereby inheriting its low 18O/16O signature.

Low 18O/16O ratios, diagnosing this process, have been found in magmas erupted in a few large volcanic and geothermal areas, such as Iceland and the Yellowstone super volcano in the USA, but in general they are rare. This rarity long been a puzzle for Earth scientists – if magmas provide the heat that drives geothermal systems, then shouldn’t interactions between magmas and hydrothermally altered rocks be commonplace?

Schematic cross section through a large silicic TVZ magmatic system, showing the key processes of magma generation and magma-crust interaction. (Rooyakers et al, 2023)

As one of the most active volcanic and geothermal areas on the planet, the Taupō Volcanic Zone (TVZ) is a prime candidate for low 18O/16O magmas, but they have not been found. Resolving this conundrum is important to understand the deep roots of our geothermal systems, where supercritical resources may be located.

Are TVZ magmas stored too deep to interact with their overlying geothermal systems?

Does this mean that supercritical conditions close to magma bodies are not expected in the TVZ?

Alternatively, are interactions between magmas and altered rock somehow obscured by other processes?

A team of GNG researchers have spent the last 2.5 years analysing the 18O/16O ratios of TVZ magmas and crustal basement terranes to answer these questions.

In a new study just published in Geochemica et Cosmochimica Acta, the team present their answers.

By comparing oxygen with other isotope ratios (strontium and neodymium), they found that although the 18O/16O ratios of TVZ magmas are relatively high, they are lower than expected if the magmas had only interacted with deep unaltered basement rocks. Some incorporation of shallower hydrothermally altered rocks is needed to lower the magma 18O/16O ratios.

The team suggest that the absence of low 18O/16O ratios in TVZ magmas primarily reflects the relatively high 18O/16O ratios of North Island rainwater, which limits the extent to which the oxygen isotopic ratios of altered rocks can be lowered in this setting. They found that when differences in the 18O/16O ratios of local waters and altered rocks are accounted for, the importance of interactions between altered rocks and magmas are similar in the TVZ and in regions with low18O/16O magmas, like Iceland and Yellowstone.

They conclude that the interface between TVZ magmatic and geothermal systems is a dynamic zone, where proximity between deep-circulating water and shallow magma bodies leads to large-scale interactions between magmas and altered materials. These are ideal conditions in which to search for supercritical fluids!

Laser fluorination line at the National Isotope Centre at GNS Science where the oxygen was extracted. (Shane Rooyakkers)

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