I use X-Ray Fluorescent Spectrometry (XRF) to geochemically analyse volcanic rocks that I collect from my fieldwork in the Lake District. This blog explains what XRF is and takes you through all the stages from collecting my samples to turning solid rock into numbers on an Excel spreadsheet that I can plot on graphs.
What Is XRF Geochemistry?
Geochemistry is a branch of geology that specialises in the chemistry of the rocks – if you couldn’t have guessed – and helps us better understand the processes that formed the rocks and our planet. Depending on the equipment and methods you use, you can determine anything from the age of the rock to whether or not it contains enough gold to make it worth mining.
For my project, the primary reason for using geochemical data is for a technique called ‘finger-printing’. Just like our own finger prints being unique for everyone, the chemistry of different rocks are often unique enough to be able to identify the same deposits across several kilometres.
In other words, if one collection of samples at a locality has the same chemical signature as another collection 10 km to the east, there is a high probability they are the same – not always guaranteed but it makes for a strong argument. Whereas, if the chemistry does not match, they are unlikely to be the same.
The equipment I am using is called an X-Ray Fluorescent Spectrometer – or XRF for short – which is housed at the University of Leicester, where I study. As the name suggests, this machine uses X-rays to calculate the concentration of select elements within the samples.
While it is possible to simply smash a sample into a small enough piece that can fit into the XRF’s analyser, this does not give the most accurate data. For the best results, a series of labour intensive and patience testing steps must be taken to reshape the samples into new forms that the XRF can properly analyse. And so, I shall take you step be step through these preparations.
Collecting Field Samples
Before any samples can be prepared in the lab, they must first be collected from the field. In my case, the field refers to the beautiful mountains of the English Lake District.
For every locality I went to I first had to ask for permission to hammer. The permission had to come from both the land owners and – if it was a SSSI protected area – from Natural England before I could do anything. Thankfully in all cases, all parties were more than happy for me to hammer under the promise of standard sample collecting practices (minimal noise and visual damage etc).
The samples I am collecting are of pyroclastic density currents (PDCs) – avalanches of burning hot gases and molten rock that tear down the slopes of volcanoes at terrifying speeds. The deposit they leave being are called ignimbrites – meaning ‘fire rocks’.
The chemistry can vary – both laterally and stratigraphically – within a single ignimbrite due to changes in lithic-content, magma composition or numerous other reasons. Therefore, a suite of samples must be collected from each unit. Suites comprise at least three to five samples, depending on how thick the ignimbrite is, and gives a representation of the chemical – finger-print – range for the unit.
Not only can the chemistry vary within a single ignimbrite – that can be many kilometres long and hundreds of metres thick – the chemistry can also vary within a single hand sample. To counter this problem and get chemical data that is representative of the unit, I am using a method known as whole-rock analysis. This requires mashing and mixing a sample up, so that all the elements within all the different components of the ignimbrite are equally mixed.
It’s like having a blender fully of different fruits. Each have their own chemical composition. Whole-rock analysis blends them up into a tasty, mixed smoothie!
Cutting The Rocks Open
The first stage of the sample prep is to assess the true quality of the samples. Because ignimbrites can be lithic-rich – containing lots of small, potentially non-volcanic stones – these are problematic for whole-rock analysis and will require an alternative method to extract their chemical composition. The reason for this is because they can be non-volcanic, or not part of the eruption they finally ended up being in, they do not represent the actual chemistry of the volcanic deposit I am trying to analyse.
By using a rock saw – a diamond-coated metal disc – to slice through my rocks I can get a far clearer view of a sample’s internal structure. This is something that’s more difficult on raw, hammered surfaces. If the sample has few lithics that can be easily picked out then I progressed with preparing them.
The freshly cut surface also let me see the extent of surface weathering on each sample. In some cases they had only a thin discoloured rim around the outside, others were much deeper. Deeper weathering alteration is not too much of a problem, it just means I will have less material to work with after I take the samples through the next stage.
Breaking Things Down
The next stage is the Rock Splitter! This ancient looking device is made of two metal wedges. One is fixed in place, while the other is slowly cranked in using a hand operated turning wheel. As the wedges slowly pinch into the sample, the pressure they put on the rock eventually breaks them. Sometimes the sample can be a bit stubborn and requires slotting a lever bar into the turning wheel to help force the sample to crack.
By using the rock splitter enough I could actually use it as a basic way to test the degree of alteration in the sample. Alteration – like the weathering I could see after slicing the rocks – is a bad thing as it forms new minerals that do not represent the original volcanic makeup of the sample. Those that had undergone a lot of alteration, turning a lot of their original material into clay, were very soft and would splinter into pieces. Fresher samples would be much tougher to break, but ultimately could be split into nice small chunks.
During the splitting, the outer surfaces are removed, as well as any chunks that contain large lithics. This is the first part of slowly ‘purifying’ the samples, making sure that by the end I only have good, ‘fresh’ material left over.
With all of the samples broken down into small chunks and bagged up, its time to move onto the Flypress. This equally ancient machine is used to crush the chunks into small granules and later into powder. A metal handle is used to raise and lower the piston to crush the sample with a bit more speed and accuracy. The odd looking balls at the top of the metal bar help to push the piston at the bottom down with extra weight . If the rock splitter was not a workout enough, this does a good job for an arm-day exercise – just make sure to switch arms as best as you can to even your muscles out!
After crushing the sample down to a granular size (a few mm), the metal tray was removed and shook around to separate the granules from the powder – sort of like panning for gold. This powder is mostly made up of the unwanted altered clays and so were tipped into the bin, while the granules were poured into a smaller sample bag. Each sample had this process repeated two more times to end up with plenty of granules to use later on.
Once all the samples were ‘granulised’, they were put under the microscope. With a pair of tweezers in hand, I went through each bag to remove anymore altered pieces or parts of lithics in another stage of ‘sample purification’.
In some cases, garnets were also found within the rocks. Garnets are commonly associated with metamorphic processes, and therefore do not represent the original volcanic material. However, these garnets are special! They are actually volcanic and so can be kept in with the rest of the granules! For anyone interested in learning more about them I recommend reading Oliver, 1956. The paper goes into detail about determining the origin of the garnets.
Now that the samples had been picked clean, leaving only the fresh material behind, it was back to the flypress to crush the granules into a powder.
Milling Rock Flour
Even though the powder created by the flypress may appear to be very fine, it actually needs to be even finer for use in the lab. To achieve this extremely fine powder we need to run the samples through a milling machine. The powder is loaded into metal pots lined with agate and some agate balls. The agate is to minimise any chemical contamination as the sides get scratched. After 20 minutes of being spun round and ground up by the agate balls, the powder is now as fine as flour.
Additionally, this is the ‘blender’ stage of the whole-rock sample prep. The powder is nicely mixed up, resulting in a homogenous sample – meaning all the elements in each mineral have all been mixed together and so a spoonful of the powder will theoretically have the same composition as the original large hand specimen that we had before splitting. Perfect to make some powder pellets and fusion beads with.
Powder Pellets – Gluing Powder Back Into A Rock
Now that we have a perfectly homogenised sample, its time to repurpose the powder into forms that can be put into the XRF and analysed. The first requirement is to make powder pellets. These are analysed to calculate the trace elements in the sample – those that are measured in parts per million (ppm).
To make powder pellets, 8g of the fine powder is weighed out into a pot and mixed with several drops of PVA solution (yes, we are gluing rock powder together). The mixture is then tipped into a metal mould and put into a compressor. This machine – which used to be an old hand cranked device – pushes the piston cylinder in the mould down onto the powder and compresses it with 20 tonnes of pressure. Leaving it to sit at 20T for a short while forces the glue to bind the powder. If done correctly, you will end up with a perfect rocky disk – a power pellet! The mould is then thoroughly cleaned and the process is repeated for all the samples.
Loss On Ignition
To make the fusion beads there is actually a prior step that needs to be done first. The powder is measured out into a small ceramic crucible. The weight is then recorded by an extremely precise set of weighing scales – it goes to four decimal places, which becomes annoying later on. The crucibles are then places in a furnace for two hours to bake off any volatiles within the powder. These volatiles are associated with alteration in the sample. While we tried to remove as much of this as possible before, it is not possible to remove it all, hence the furnace.
Once baked, the powder – now referred to as ‘ignited powder’ – is reweighed. The missing weight gives the value for the volatiles and is referred to as the ‘Loss on Ignition’ or ‘LOI’ value. If we didn’t go through the earlier stages of removing altered pieces of sample at the rock splitter or flypress, this would be quite a high number and considered not representative of the original chemical composition of the sample to use in future plots. Some high LOI samples can slip through the net, but now they can be easily identified and removed from future plots.
Fusion Beads – Turning Rock Into Glass
With LOIs now calculated, the ignited powder is then used to make the fusion beads. The first step is to weigh out the flux – a white grainy powder that forms the majority of the glassy beads. Around 6g is of flux is used, however, it must be accurately weighed out to within +/- 0.0001g of the given weight based on the flux’s LOI at the time. This is when the super accurate scales become annoying because being 0.0001g out can be the difference between a single grain, and so can become very frustrating when you keep adding/removing in 0.0002g each time.
Once the flux is perfectly measured, 0.8g (+/- 0.0001g) of the ignited powder is then added. The two are then mixed together and loaded into the fusion burner. This new fancy burner automatically swirls four crucibles of sample material over extremely hot Bunsen burners to melt the flux and powdered rock into a molten liquid.
Eventually, the burners under the moulds ignite and warm them up for the molten sample to be poured into (again automatically). If I sound excited about this automated business its because it used to be a single burner that you had to use metal tongues and heat-resistant gloves to swirl the crucible around, switch on the mould burner and pour by hand. While it was very cool to use, it was a slower process.
Once the sample has been poured into the mould, the flames cut off and cool air is then blasted out to cool the liquid into perfect glass disks – fusion beads! The fusion beads can then be used to measure the major elements (SiO2, TiO2, NaO2 etc).
Rocks Go In, Numbers Come Out
Powder pellets and fusion beads now in hand, it is finally time to take them to the XRF. Each disk is loaded into a single metal cup and a computer program logs their location. When the program is set to run an arcade claw machine – that has stronger claws to make sure not to drop the samples – sets into motion, picking up a cup and placing it into a hatch in the middle. The hatch then snaps shut and the sample is then blasted with X-rays.
The XRF measures the X-rays that are re-emitted by the sample at different excited wavelengths. Depending on the wavelengths of the new X-rays and the intensity of them, the XRF can calculate which elements and how much of each of them are in the sample. This is then converted into a spiky graph, where each peak represents an element. The higher the peak, the more of that element is.
The spiky plots can then be converted into digital numbers that are recorded onto an Excel spreadsheet – this step is usually done by the very helpful lab technicians who specialise in running the XRF – and then sent to the sample owners.
Plotting The Data
Excel document in hand – or on a laptop that’s in my hand – the data can now be plotted up. The first stage is to put the chemical concentrations through a series of tests to determine which elements have been mobilised – added to or taken away from the original rock – and so cannot be used in future correlation plots.
I know we have gone through a lot of different stages to ‘purify’ the samples. But over 450 million years of being left outside means these rocks have been through a lot! Not every element can survive in their original concentration since they first saw the light of day.
The immobile elements – those that did stick around, unchanged – can put used for my finger-printing tests to figure out which units are the same and which are different. This is the stage I am at now and am hoping for some exciting results! Wish me luck people.