May 18, 2022

Team Profile:
Analysing Xenoliths

Nick Mortimer

photo credit:
Nick Mortimer

In The Xenolith Hunters we explained what xenoliths were and how they are important for the GNG Programme work. It has now been about 18 months since our xenolith fieldwork on Mt Ruapehu and elsewhere in Taupō Volcanic Zone. This blog post describes what we have done with the xenoliths since we collected them - and why.

As explained previously, the ‘xeno’ or ‘foreign rock’ nature of these xenoliths in the dark lava flows is obvious when you see them in the field. But we need to do more than just recognise and collect them. The amounts of minerals, chemical elements and isotopes inside a rock help us to match the xenoliths to known surface rocks, and thus build up the picture of the underground geology at supercritical geothermal depths.

Making Thin Sections

Usually, the first step after collection is to get a thin section made of the rock. Thin sections are well named. Smalls labs of rock are glued to a 45 x 25 mm glass slide then ground to a thickness of just 30 micron (0.03 mm). At this thinness, light shines through the rock (see first image below) and its minerals, layers and structures can be examined and checked under a microscope. After this basic quality check, decisions are made about what to do next.

Analysing atoms in a rock is neither quick nor easy. For every rock we collected we could, in theory, measure concentrations of every element in the periodic table down to parts per billion. But nobody does this. Certain elements are far more useful than others, and there is also cost and time to consider.

45 x 25 mm thin section of P92622 schist xenolith that was made in the GNS Science Petrology Laboratories. The light shines through the 30 micron thick rock slice. (Photo: Nick Mortimer)

Sample Preparation for Rock Analyses

One of the most time-consuming things in rock analysis is getting the samples ready.  Different methods of analysis (X-ray, electron, laser beam etc) need different sorts of sample preparation.

To analyse rocks for their weight percent and parts per million element concentrations, they need to be smashed and homogenised to a fine smooth powder. In other cases, we want to extract or separate specific minerals from the rock. This mineral separation involves only gentle crushing, along with sieving, to obtain a fine sand.

GNS Science Dunedin technician Louis Whitburn crushes and sieves rocks. (Photo: Nick Mortimer)

Mineral Separation - The Hunt for Zircon

For a dozen xenoliths, we wanted to see if they contained the zirconium silicate mineral zircon. Zircon is very useful to geologists because its age can be determined using uranium and lead atomic clocks. The age of a rock is one of the most basic and useful things a geologist can know.

Mineral separation instructions for getting zircon out of a rock can be written out like a recipe.

Mineral separation for just one sample takes many days of work by three or four people in different places. All these steps require a lot of skill and patience, careful record keeping and accurate tracking of samples between laboratories.

Months later comes the most anticipated step - looking at the heavy, dense grains in the dish under the microscope to see if there are actually any zircons in the sample. There’s no guarantee that there will be: some do, some don’t.

Nick Mortimer checks a sample for zircon under the microscope. (Photo: Andy Tulloch)
P92622 zircon crystals under the microscope. Crystals are about 0.1 mm long. (Photo: Nick Mortimer)

For the Ruapehu and Taupō xenoliths, there was good news and bad news. The bad news was that, of the twelve samples processed, only two contained zircon! The zircons in these two samples were smaller than expected - they showed up as nicely shaped glittery grains up to 100 micron (0.1 mm long) scattered in the dish. The good news was that they were big enough to date. 

Zircon Dating

The dating machine used for this work was an LAICPMS instrument (that’s Laser Ablation Inductively Coupled Plasma Mass Spectrometer) in the Chemistry Department at the University of Otago, Dunedin.  

University of Otago Chemistry Department. (Photo: Nick Mortimer)

The machine is in an air-conditioned cleanroom with double doors to keep the dust out. Street shoes have to be removed.The sample is placed in a vacuum chamber after which everything is controlled via desktop computers. A narrow but powerful laser beam is fired at the zircon grains and vaporises parts of them to plasma, one spot at a time. The plasma is then sniffed by the mass spectrometer part of the instrument and the different isotopes of uranium and lead are measured and an age calculated. Analysing each zircon takes about a minute, so progress is rapid compared to the weeks and months for sample preparation.  

Rose Turnbull dates zircons using the LAICPMS instrument. The sample is in the cabinet on the left. (Photo: Nick Mortimer)

As of May 2022, almost all the analytical work on the xenoliths have been done. Interpretation of results is underway.

The next (and final) xenolith blog will explain how the results of all this chemical analysis and zircon dating are used to improve our knowledge of the deep crustal structure under Taupō Volcanic Zone at critical geothermal depths.

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