Members of our GNG team recently co-authored a paper in Economic Geology, with collaborators in New Zealand, Japan and U.S.A.

Massiot,C., McIntosh, I., Deans, J., Milicich, S.D., Tontini, F.C., de Ronde, C.E., Adam, L., Kolandaivelu, K. and Guerin, G., 2022. Petrophysical Facies and Inferences on Permeability at Brothers Volcano, Kermadec Arc, Using Downhole Images and Petrophysical Data. Economic Geology.

The paper focusses on the observations and results from drilling a hole in an underwater volcano. What is exciting for GNG is that one potential target for finding supercritical fluids is being near a magma chamber (but not magma). With magma nearer the surface than in the Taupō Volcanic Zone, results from Brothers Volcano can be used as an example of how hot fluids circulate near magma, and what the rock properties are. This is important to find the hot fluids and their fracture pathways. It will also help drillers anticipate the type of rocks they may drill through so they can select the best-suited equipment.

Brothers volcano is an active submarine volcano located in the Kermadec arc, 400km northeast of the Bay of Plenty coast. The Kermadec arc is the continuation of the Taupō Volcanic Zone offshore New Zealand. Brothers volcano is active, with black and white smokers discharging fluids up to 302°C at the seafloor (seawater nearby is only ~4°C!). These hot fluids carry metals from the deep magma to the seafloor where they form deposits rich in metals such as copper and gold.

However, observations from surface ships and remotely operated vehicles cannot reveal what the rocks are within the volcano, nor how cold seawater and hot magmatic water circulate through and interact with the rocks.

In 2018, an International Ocean Discovery Program (IODP) scientific expedition drilled a series of holes in Brothers volcano more than 1.2 km under sea level with the research vessel Joides Resolution. This was not trivial: the very high temperatures and very low pH were challenging for drilling and damaged equipment. Despite the challenges, the expedition still recovered 225 metres of core and some measurements directly within the volcano. This is the most comprehensive information ever recorded about the inner workings of a submarine arc volcano.

Multiple rock parameters were measured directly within the holes (density, porosity, resistivity, magnetism). Excitingly, there were images from the inside of the holes and temperature profiles were also acquired.

In this paper, the authors present unprecedented views of volcanic rocks and fluid pathways. The drill cores recovered were usually in pieces <10 cm long, cumulating only 10-20% of the drilled interval, so having continuous measurements down the holes is critical to fill the gaps. In addition, some drill cores were so hydrothermally altered that it was very difficult to identify which type of rocks they were. The same can happen in onshore geothermal fields. Using rock parameters and images helped improving the identification of rock types, suggesting more lavas than volcaniclastic rocks.

As expected, the fluids in the active Upper Cone were hot. What was not expected is that the fluids in the NW Caldera hole cooled down very quickly, even though the hole is very close to active black smokers. This showed that down flow of cold seawater that gets heated up and discharged at the seafloor happens within very small distances. The temperature measurements, combined with the rock types and properties, showed that fluids circulate mostly through connected fractures, and sometimes get focused by specific layers.

These results will improve our understanding of submarine volcanism, types of eruptions and associated hazards. Results will also help understanding how metallic deposits form on and near the sea floor. These processes are analogous to those that happened a long time ago and made the mines from which metals are being extracted today.  

Downhole measurements of geothermal rock properties and appearance are still rare. This paper will help interpreting downhole images in geothermal fields worldwide, where drill cores are rarely recovered. Some of the downhole images at Brothers volcano were similar to those at the Wairakei Geothermal Field near Taupō.

This research used samples and data provided by IODP and supported by the New Zealand government via the Te Riu a Maui/Understanding Zealandia research program at GNS Science.

You can also read more about the voyage in this Stuff article.