Jun
03
2010

SEM scan no. 2 of K-Pg boundary material
SEM scan no. 2 of K-Pg boundary materialCourtesy ASPEX Corporation
Chemical analysis of sample number 2: The sample shows high concentrations of aluminum, silicon, oxygen, and sulfur, traces of iron, but no iridium.
Chemical analysis of sample number 2: The sample shows high concentrations of aluminum, silicon, oxygen, and sulfur, traces of iron, but no iridium.Courtesy ASPEX Corporation
Last December Joe made a post about a company that offered to provide free scanning electron microscope images (in this case a backscatter scanning electron microscope or BSEM) of whatever people sent in (within reason I suppose). I took the ASPEX Corporation up on its offer and mailed in some of the clay I had collected from the K-Pg boundary I visited last spring in southern Colorado. The K-Pg boundary, as most of you should know by now, marks the end of the Cretaceous period and the beginning of the Paleogene period. It used to be called the K-T boundary for Cretaceous-Tertiary but the term “Tertiary” has fallen into disfavor. Anyway, some 65 million years ago something wiped out all the non-avian dinosaurs, and left a tidy 1-inch layer of iridium-rich clay in several locations worldwide for geologists to puzzle over. Iridium is a rare-earth element (atomic number 77) but is fairly common in asteroids, meteorites, and other such extra-terrestrial space objects, so eventually, scientists came to the conclusion that such high amounts of iridium had to be from an extraterrestrial impact, and sure enough, an impact crater just the right size and age was eventually uncovered off the Yucatan Peninsula near Chicxulub, Mexico. (There’s been many years of discussion about the cause of the dinosaurs’ demise but just last month a committee of highly scientific mucky-mucks officially declared that the Chicxulub asteroid was the guilty culprit.)

SEM scan no. 7 of K-Pg boundary material
SEM scan no. 7 of K-Pg boundary materialCourtesy ASPEX Corporation
Chemical analysis of sample number 7: also contains traces of titanium and potassium.
Chemical analysis of sample number 7: also contains traces of titanium and potassium.Courtesy ASPEX Corporation
In Colorado, I found evidence of the Chicxulub impact about 10 miles west of Trinidad, Colorado. The outcrop at the Madrid East site on the way to Long Canyon is probably one of the better-defined exposures of the K-Pg boundary and is easily accessible off the main highway. A distinctive 1-inch layer of ashen gray clay can be seen sandwiched between two layers of coal – a 2-inch layer above and a 16 inch layer below - and the sequence is capped by massive Paleogene sandstone. While there, I scraped out a sample of the whitish clay to bring home.

Anyway, when Joe mentioned the free SEM scan I sent ASPEX some of the clay I’d collected hoping the scan might reveal some evidence of high iridium content, and of shocked quartz and glass spherules, telltale signs of an impact. Once sent, I promptly forgot about it until recently when a reply showed up in my mailbox.

The K-Pg boundary layer in the Raton Basin of Colorado
The K-Pg boundary layer in the Raton Basin of ColoradoCourtesy Mark Ryan
The results weren’t quite what I was hoping to see, but I did learn something. The images sent back don’t show much – at least not in the way of iridium, shocked quartz, etc. But the chemical analysis shows the clay layer (yellow arrow in photo) is indeed mostly just that - clay, or more specifically aluminum silicate hydroxide better known as kaolinite (it also contained some titanium, potassium, and iron). Kaolinitic clay is thought to result from the altering of volcanic ash beds in acidic coal swamps, but in this case it’s the result of a doomsday shroud of impact material interacting with a coal swamp. The kaolinite is the one-inch white stripe in the photo and is the layer I sampled. Unfortunately, the analysis from ASPEX shows no signs of an iridium anomaly, and here’s why:

The iridium I’ve since learned isn’t actually concentrated in the clay layer itself but in the 2 layers directly above it (red arrows in photo): that is the impact layer (smectite - blue arrow in photo), and the 2-inch coal layer directly above that. I was under the incorrect assumption the iridium was in the clay layer itself since that layer is what seems to mark the K-Pg boundary at least in the Raton Basin in southern Colorado.

My problem is I’m only an amateur geologist so I tend to operate on limited knowledge. Once the results of the scan showed up, I did do some further research and discovered much more information than I had when I visited the site last year. I was at the right place and was able to identify the boundary layer, I just didn’t have all the facts. But at least I’ve learned something from my mistake, so it turns out not to be such a bad thing. And now you’ve learned something, too.

LINKS

ASPEX Send Us Your Sample page (I’m not sure they’re still doing it)
More about the K-Pg extinction impact
USGS K-Pg in the Raton Basin info site
Universe Today K-Pg boundary info
Buzz post on dinosaur extinction

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