EPMA analysis for a meteorite or suspected meteorite
From the Microprobe Lab 
To establish a scientifically based reproducible and defensible diagnostic analysis for the classification of a meteorite or suspected meteorite for submission to the Meteoritical Society's Nomenclature Committee and publication in the Meteoritical Bulletin, we need to look beyond SEM based qualitative analysis to the Electron Probe Micro Analyzer (EPMA) using Wavelength Dispersive X-ray Spectroscopy (WDS) for a quantitative geochemical analysis.
For the quantitative geochemical analysis mentioned above, meteoriticists will almost always use an instrument known as an Electron Probe Micro Analyzer (EPMA), AKA "microprobe" to perform the geochemical analysis and it's associated BSE imaging system to perform the *petrographic analysis.
* Some researchers with experience in optical microscopy may use a petrographic microscope to examine a thin section of the sample in transmitted light to perform the petrographic part of the analysis. Those with the requisite skill and experience with optical microscopy will often extol this method as superior to using BSE images. However it should be noted that this technique cannot be used for the quantitative geochemical analysis.
This is the "microprobe" LMAC uses.
Dustin Dickens inspecting the JEOL JXA-8230 equipped with LaB6
electron gun and 5 spectrometers while working on the the classification
of a new lunar meteorite in the EPMA lab at the University of Colorado at Boulder
Why the microprobe?
There are a great many techniques and their associated specialized scientific equipment that meteoriticists use to analyze meteorites. The electron microprobe (EPMA) is only one of them.
When we approach the analysis and subsequent classification of a suspected meteorite, we're looking for combinations of very specific complimentary geochemical and petrologic diagnostic indicators. Endeavoring to either support its placement in, or eliminate it from, an already well established meteorite taxonomy.
While there are many tools with superior resolution, and a greater ability to detect more subtle aspects of a specimen's geochemistry, sometimes less is more. The electron microprobe is a sort of "goldilocks" tool with enough resolution and enough versatility to be just right for this diagnostic analysis required for the initial analysis and characterization. There are certainly exceptions to this, but we can delve into that later.
And so, in performing this initial fundamental work, we find the electron microprobe (EPMA) is indispensable.
The first thing a new user notices when sitting down in front of the control station of the EPMA, is usually a computer screen displaying the realtime backscatter image (BSE) of the sample being studied.
We use this "live" backscatter image to navigate the sample and to see it's individual crystals or grains and their respective boundaries. The different mineral phases of these crystals and grains are depicted in different greyscale tones. This is very useful, though often a true art to decider. The image and different greyscale tones assist the EPMA user to identify different mineral phases and in determining where one mineral grain ends and another begins. The BSE imaging allows the user to be certain that the electron beam is pointing to a clean well-polished spot of the sample being analyze. It also helps the user to identify which particular mineral type (phase) they are looking at. A user typically doesn't just point and analyze a random spot on the sample. Knowing where the electron beam is pointed, and what it's pointed at, is a critical part of the analysis process. This may seem like putting the cart before the horse, but hang in there and we'll dive into this and make some sense of it in just a moment.
Microprobe control station with BSE image at high magnification
BSE image at high magnification showing different shades
indicating different phases
Now that we know an electron microprobe is used to do the analysis, and at least generally why we choose it over other tools, let's look into what we're doing a little more closely.
How can we use BSE imaging and the microprobe to know what we are going to analyze before we actually do the analysis?
This is where terms like analysis or even microprobe are not sufficiently specific enough to explain what is going on. In fact most microprobes have several integrated imaging and detection technologies. It's going to get a little technical moving forward, but nothing you won't be able to follow with a bit of effort.
There are two fundamentally different types of analysis a typical microprobe can perform:
- EDS - Energy Dispersive X-ray Spectroscopy - Faster but less accurate and considered qualitative or semi-quantitative.
- WDS - Wavelength Dispersive X-ray Spectroscopy - Slower but more accurate, and considered quantitative.
*Note that the bulleted descriptions above are simplified to keep the explanations in this article accessible to a general audience.
As you have probably already figured out, the microprobe has an imaging system built into it. This imaging system allows us to visually navigate the sample in the vacuum chamber, as well as take incredibly magnified pictures using the electrons that are reflected or scattered after hitting the sample. The different types of imaging depends on the detector and the depth the electron reaches into the sample before being reflected back to the detector. The electrons used for BSE imaging penetrate the sample to different depths before being reflected, depending on the elemental composition of the area being bombarded with electrons. For geologic samples, this allows for different mineral phases with different elemental compositions to be depicted with different grayscales.
Each different shade of gray represents a different mineral phase in a BSE image. For example: in the illustration above (not a real BSE image) the darker gray might represent plagioclase, the mid-tone gray might represent augite or some other high-Ca pyroxene, and the lighter gray might represent olivine or enstatite. We use the faster quantitative or (semi-qualitative) EDS analysis in real-time to determine what phase each tone represents before proceeding to select the points we want to analyze with the slower and more accurate qualitative WDS analysis.
What are we looking for?
The primary minerals or "phases" we are looking for are usually: olivine, pyroxenes, feldspars, as well as accessory *metals, *oxides and *sulfides. There are of course exceptions to this, but these are the usual suspects.
Geochemically speaking, we are often looking for specific diagnostic geochemical trends in the ratios of Fe and Mg in olivine and pyroxene, and in the Ca in feldspar.
Petrographically speaking, we are often looking for structural characteristics such as brecciation, clast or grain size and shape, overall mineral composition, shock effects, zoning, exsolution, etc.