On this cruise, the goal is to select the ideal sample set out of the rocks we pull in from dredges. In past expeditions, we relied on the expertise of PIs and students to visually determine general characteristics of the rock samples from each dredge in order to help select samples that best benefit the kind of analyses we’re pursuing.
While this is generally a well tested, tried-and-true method of sample selection, there’s always some uncertainty in what the rock is really made out of, what its composition is, and sometimes, visual characteristics alone can be misleading. Two rocks that might look like basalts could turn out to be different rock types that may (or may not!) end up being useful. For this reason, we decided to run an onboard experiment on this expedition and bring a portable laser spectrometer system (Porta-LIBS) on board. This allows us to get a better idea of the rocks we are working with.
How does the laser spectrometer work?
The explanation starts at the atomic level of the rocks. Atoms of each element on the periodic table, specifically the electrons encircling these atoms, can be energized by heat, x-rays, and other energy sources. When the source of that energy is removed, the electrons lose this energy and give it off in the form of light. This light has different frequencies, or colors, that are specific to what element they are (see Figure 1 for an example). For example, sodium atoms will give off a characteristic yellow color, which is why certain types of streetlights look yellowish. But how does this tie into helping us pick out the best samples for our research?
In this case, the laser part of the laser spectrometer is a great energy source for exciting atoms and 
figuring out what elements, and how much of each, are in a rock. The laser is powerful, so not only does it excite atoms of the rock sample being tested, it blasts a very small crater in the surface of the rock (Figure 2). The area sampled is very small, so only a small amount of light is emitted from the atoms. This means that the laser and sample have to sit in a completely dark chamber in order for the light to be seen, measured, and recorded.
When the atoms give off these small amounts of light, fiber optic detectors aimed at the laser spot on the rock transmit the light to several spectrometers, which can see the different frequencies of light (a spectrum) and how much (how intense) the different frequencies are (for schematic, see Figure 3). Since we know what elements give off what frequencies, we can identify what is in a sample using the laser (Figure 4).
But how can you figure out how much is there?
This is the tricky part. We don’t necessarily know just from the spectrum how much of each element is there. In order to figure this out, we have a set of glass chips made out of powdered, well-mixed rock samples whose compositions are well-known in the geological community. To make a glass, powdered rock is heated past its melting point and is allowed to completely melt. This melt is then cooled quickly, so no crystals start forming. These are what we call rock standards, having been measured many times by many different scientists around the world. By shooting the laser at these standards many times, we can generate an array of spectra used to calibrate the laser and then use these to estimate the compositions of unknown rocks, such as the ones we dredge out of the ocean.
So how does this help scientists?
There are several reasons for doing this instead of just taking all of the rocks we dredge:
1. Rocks are heavy and therefore expensive to ship. We can only ship so many rocks, so we have to be careful that we don’t take too many. If we take everything, we have a lot of rocks to deal with later that ultimately prove unuseful or redundant, or are not necessary to keep in the sample archive from this expedition.
2. Other scientists, over the next few
years, will be seeing the research we’re working on and may have a project idea of their own, and they might want some pieces of our samples. We can use this compositional data to help figure out which samples may be most useful to them, and quickly pick them out of the long-term sample repository where the archive will be kept.
3. This is an experiment to learn the best ways to use portable/small analyzers in a field environment, and how useful such data is as a preliminary step in sample selection. Some of this remains to be seen, once we get our samples back to our labs on the mainland and can get to work!
4. Once we are back on mainland, we will obtain chemical data on some of the samples using more precise measurement methods (which can’t be taken out to sea) than the laser spectrometer. We can compare the data we gathered as a first estimate onboard to this more precise data and determine how well our methods as well as the portable equipment work for fast geochemical analyses.
How many samples were analyzed using the laser spectrometer?
559 samples (2672 total data points) were analyzed over the course of approximately three weeks of dredging. No sharks, real or fake, were harmed during this exercise.
by Val Finlayson
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