December 11, 2020

Team Profile:
The Xenolith Hunters
contributor(s)

Nick Mortimer

photo credit:
Hamish Campbell

One of the goals of the EXPLORE project in the Geothermal: The Next Generation research programme is to improve knowledge of the Earth’s deep crust under the Taupō Volcanic Zone (TVZ). This will tell us more about how geothermal fluids move and are controlled by deep geological structures, such as faults. But almost all of the TVZ is blanketed by lavas, ash and ignimbrites, and existing boreholes are mere surface pinpricks.

So, how can we get samples of the deep crust? How do we know what’s going on down there?

Well, instead of having to drill an ultra-deep (and ultra-expensive) borehole or use indirect geophysical techniques, we are fortunate that some volcanoes have coughed up little pieces of crust in rising magma that has erupted as lava flows. These pieces of crust are called xenoliths (from the Greek ‘foreign rock’ or ‘strange stone’ because the rocks are very different from the enclosing lava). In TVZ lava flows, xenoliths typically stand out as being a different colour from the lavas, and range in size from 1-20 cm.

Banded gneiss xenolith from Saddle Cone flow Mt Ruapehu - a piece of the crust under the TVZ (Photo: Andy Tulloch).
Three white quartzite xenoliths in lava near Ohakune (Photo: Nick Mortimer)

For one week in November 2020, a small GNS Science field party [comprising Nick Mortimer - GNS Science Dunedin, Geoff Kilgour - GNS Science Wairakei, and Hamish Campbell - GNS Science Lower Hutt] went searching for xenoliths on the slopes of Mts Ruapehu, Ngauruhoe and Pukeonake.

Hamish Campbell chiselling xenoliths out of a lava boulder (Photo: Nick Mortimer)
The team used GNS Science's Observatory House at Whakapapa as a field base (Photo: Nick Mortimer).

Collecting xenoliths is not easy: most lava flows contain no xenoliths. In ones that do, they are very rare (less than 0.01% by volume), generally small, and they can be difficult to impossible to hammer or chisel out. Despite these problems, 112 separate xenoliths were collected. They show an interesting range in composition, including sugary quartz-rich gneisses, grey fine grained hornfelses and some greywacke and argillite. This variety will be useful to build up a picture of the different sedimentary and metamorphic rocks under the volcanoes, and extrapolate the geology further north to under the geothermal area. 

Fine grained hornfels xenolith near Whakapapa - and a broken hammer (Photo: Nick Mortimer)

This fieldwork was just a start. Now that the xenoliths have been collected, the analytical work begins, and interpretations will follow that. In a year or two, we expect to know a lot more about the 3D geology of the crust under TVZ, and if an ancient crustal terrane boundary helps control the location of deep geothermal fields.

Check back on this website for updates as the mineralogical, radiometric dating and other work proceeds and we ask the rocks to yield up their secrets and tell their story - like this update about GNG researchers using analogues to explore basement rocks, from the surface.

We thank Ngāti Rangi, Ngāti Tūwharetoa and the Department of Conservation for permission to do fieldwork and collect rock samples in Tongariro National Park.

Nick Mortimer back at GNS Science Dunedin office, with cleaned and sorted rock samples (Photo: Andy Tulloch)

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