Hedging the Approval of the CT Group

The search is on, but no need for drones or metal detectors to join this adventure. We’re on the hunt for a handful of already classified carbonaceous chondrites that may soon be part of the new carbonaceous Telakoast (CT) group. So, digital sleuths and number nerds gear-up! We’re searching through collections, eBay listings and bags of stashed stones on this meteorite hunt. Fair warning to our Business members, double check your carbonaceous inventory!

A grouplet of 16 different carbonaceous chondrites which are currently classified as CM2, CM-an, or C-ung, might soon be getting an official group of their own. A stellar team of meteoriticists, including lead author A. Irving, and coauthors J. Gattacceca, K. Ziegler, C. Sonzogni, P. K. Carpenter, and L. A. J. Garvie, have recently proposed this new group in an abstract presented to the 53rd Lunar and Planetary Science Conference titled, CT Chondrites: A Newly Recognized Carbonaceous Chondrite Group with Multiple Members, Including Telakoast 001, Chwichiya 002 and Cimarron. The abstract shows that the stable oxygen isotope ratios of the grouplet plot to roughly the same point against the terrestrial fractionation line, indicating a common parent body. The authors report that all specimens have relatively small chondrules with a mean apparent diameter of 280 μm. They also note that olivine shows Fayalite peaks at Fa3 and Fa40. Perhaps most interestingly, they mention that none of the specimens contain any CAIs. This lack of CAIs is another indicator of a common parent body which was apparently depleted of refractory minerals.

We should keep in mind that this newly proposed group has not been approved by the Nomenclature Committee, and there are no guarantees it will be. Furthermore, oxygen ratios are only one indicator used to determine if meteorites belong to the same group. However, the sheer number and diversity of current classifications that fit into the proposed CT group, all now arguably misclassified, is quite compelling with regards to an expedited approval.

All of this is important because carbonaceous chondrites are uniquely important to planetary science and represent a rich sampling of primitive materials from early in the Solar System’s evolution. They’re rich in water and other volatiles and contain pre-solar grains and refractory minerals (CAIs) that help cosmochemists to determine and further refine the age of our Solar System.

At present the currently recognized taxonomy for carbonaceous chondrites, as reflected in the official Meteoritical Bulletin database, includes: Ivuna-like (CI), Mighei-like (CM), Ornans-like (CO), Vigarano-like (CV), Karoonda-like (CK), Renazzo-like (CR), Bencubbin-like (CB), High-metal (CH), and Loongana-like (CL). The C and C-ung designations without a petrologic type are essentially “old” placeholders for carbonaceous chondrites and should not be considered as distinct parts of the taxonomy as much as holdovers from a less specific time in the evolution of carbonaceous chondrite taxonomy. This point is an important consideration for type collectors. Meteoriticists have increasingly adopted more stringent taxonomy guidelines for ungrouped carbonaceous chondrites, with best practices requiring the -ung designation and an associated petrologic type, e.g., 1,2,3, etc. A great example of this new guideline is the classification of the 25 August 2020 fall in Morocco called Tarda, a C2-ung.

So, what are we looking for in this new CT group? Irving, et al have identified 16 classified meteorites that fall into two petrologic types, CT2 and CT3, including the following meteorites:

  1. CT3: Telakoast 001, Cimarron, Chwichiya 002, NWA 6862, NWA 8781, NWA 12416, NWA 12957, NWA 13671, NWA 13984, NWA 14051, NWA 14139, NWA 14179, NWA 14200.
  2. CT2: NWA 11699, Sarir Tazirbu 001, Qued Mya 002.

While all 16 of the meteorites listed above will be representative in some way of the CT parent body, the crème-de-le-crème is the type specimen, Telakoast 001. Regardless of if you stumble upon a piece of Telakoast 001 or Chwichiya 002, or NWA 11699 they will all be poised to be reclassified as CT. Certain collectors may prefer an aqueously altered CT, in which case one of the CT2 meteorites in the list would be sought out. What’s interesting to note, is that while oxygen plots can reveal parentage and change the primary classification of a chondrite, for example, from CV to CR, it will never affect the petrologic grade. This is because this part of the classification is determined entirely by the geochemical equilibration and aqueous alteration of the meteorite’s primitive minerology. This means the CM2 and C2-ung in the list should map to CT2, and that the C3-ung should map to CT3.

This author’s enthusiasm for the search for these secrets hidden in plain view comes from having already discovered several of the listed meteorites in my own collection. I also enjoy a good scavenger hunt from time to time, and this has a bit of that flavor to it. Though speculation on my part, it seems likely that if the CT group is approved, then the value of these 16 meteorites will increase. There are no guarantees that either the CT group will be approved, or that if it is, it will increase the value of these meteorites, however, seems like a pretty good hedge to me.