Analysis and Classification of a Suspected Meteorite
Narrative from the microprobe lab...
With meteorites, rules of thumb are useful if you're doing home-based process-of-elimination with limited resources and a lighthearted nature. Most importantly is that no money or reputations are on the line. In these cases, many of the so called 'rules of thumb' can be quite useful at eliminating many of the well known artifacts, minerals and rocks that many people initially mistake for meteorites.
On the other-hand, if establishing a scientifically based defensible diagnostic classification is the high bar one has set out to meet, well then, one must immediately and with the fullness of one's heart discard these rules of thumb and the many other fantastic ferric fallacies that seem to permeate the atmosphere in the lay world of meteorites.
But if we can't use a magnet, a ceramic tile, or an optical microscope, and our own two eyes and keen observational skills cannot be relied on to expertly and conclusively classify a meteorite, well then I say again..."how on earth are we supposed to do it?".
I'm glad you asked! I'm also quite impressed you have stayed with it this far. Exceptional stick-to-it'iveness! This is where it starts to get good!
To approach the initial analysis of a suspected meteorite most often meteoriticists, yes that's what we're called, will use an instrument known as an electron microprobe, AKA "microprobe", AKA "probe", and often referred to in textbooks and other technical literature as an "Electron Probe Micro Analyzer" or "EPMA".
This is the one we use...
The JEOL JXA-8230 equipped with LaB6 electron gun located at
the University of Colorado at Boulder. Dustin (me) grabbing a selfie
while working on the the classification of a new lunar meteorite in
the microprobe lab at the University of Colorado at Boulder
Hear what the lab has to say about the this microprobe...
"5-spectrometer instrument (compared to 4 on the old one) offers higher spatial resolution (electron beam size ca. 0.2-0.7 µm compared to ≥1 µm) and greater analytical capabilities, notably in terms of minor and trace element analysis. A major advantage is the presence of many large-area monochromators, which offers 2 to 3-fold higher count rates, and thus higher sensitivity and lower detection limits. The software capabilities have also immensely improved; as if you were switching from DOS 3.0 to Windows 10! The fully automated instrument offers more accurate and precise results in far less time. Major elements in silicates are analyzed in less than a minute, X-ray element maps can now be quantified, trace element analysis down to 1-10 ppm range of detection limit is reached in just a few minutes, complex minerals with over 25 elements (e.g., REE-bearing minerals) are analyzed in less than 10 minutes, and accurate analysis of beam sensitive materials is easily done without cutting too much on the precision. These updates will certainly foster new collaborations and discoveries for researchers and private customers. For instance, the new instrument now allows us to analyze rare and precious elements (Au, Ag, Te...) in sulfides, trace element analysis in beam sensitive materials, including carbonate, titanium analysis in quartz for thermometry, U-Th-Pb dating of monazite, quick yet accurate and precise homogeneity tests in synthetic materials, etc." quote credit the University of Colorado at Boulder's microprobe lab webpage.
Okay, so why the microprobe?
There are a great many technologies that meteoriticists use to analyze meteorites, the electron microprobe is only one of them. There are even times when a traditional petrographic polarizing optical microscope is the preferred tool. However, these days most meteoriticists I know don't spend much time scrutinizing minerals optically.
When we approach the analysis of a suspected meteorite, we're looking for very specific petrologic and geochemical diagnostic indicators. Endeavoring to either support its placement in, or eliminate it from, an established meteorite taxonomy.
While there are many tools with superior resolution, detection ability, and which are able to detect more subtle things, sometimes less is more. The electron microprobe is a great "goldilocks" tool with enough resolution and enough versatility to be just right for this diagnostic analysis we require for the initial characterization. There are certainly exceptions to this, but we can delve into that later.
And so in doing this initial fundamental work, we find the electron microprobe is indispensable. The first thing a new user notices when sitting down in front of the control station of the microprobe, other than the many monitors and the 1980's looking control boxes with old school nobs and buttons (mouse replaces all of them, but nice to have sometimes), is the realtime backscatter image (BSE) of the specimen being studied.
We use this "live" backscatter image to navigate the sample with different mineral phases being depicted in different greyscale tones. This is very useful (though a true art to decider sometimes!) to determine where one mineral grain ends and another begins, and ensuring that the electron beam is pointing to a clean spot on the area of the sample you want to analyze.
Microprobe control station with BSE image at high magnification
BSE image at high magnification showing different shades
indicating different phases
Now we know that an electron microprobe is used to do the classification analysis, and at least generally why we choose it over other tools. So, let's look into what we are doing a little more closely.
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.