Five stones with black, bubbly crust were found by a member of the Geological Survey of Egypt in a farm field on the Nile River near Luxor. Today only a single 23 kg specimen remains preserved.
Recent studies have determined that systematic changes occur in amoeboid olivine aggregates (AOAs) with increasing subtype, which is directly linked to increasing aqueous and thermal metamorphism (Chizmadia et al., 2002; Grossman and Rubin, 2006). For example, textures and morphologies of AOAs show changes, olivine in AOIs becomes progressively FeO-rich, troilite becomes more prevalent, and trace elements become more equilibrated. Because of their smaller grain size, olivines in AOAs are better indicators of alteration processes (such as the substitution of Fe for Mg) than the chondrules previously utilized to determine subtype. As a result of this study, a refinement in the subtypes of the CO3 chondrites was proposed; the CO carbonaceous chondrite group would span a petrographic sequence from type 3.0, represented by Colony, to type 3.8, represented by Isna.
Chizmadia and Bravo-Ruiz (2013) employed a similar method to that of Grossman and Rubin (2006) to classify CO3 chondrites by the degree of aqueous alteration; they utilized the Fe-Mg composition and distribution in olivines in AOAs. However, Chizmadia and Bravo-Ruiz extended their study to comprise the entire metamorphic range. Based on their study, they proposed that Isna should be assigned to petrologic type 3.75. They also better resolved Colony as type 3.05, and assigned the MET 00694 pairing group to the highest CO3 petrologic type of 3.8.
In a different analysis of CO3 petrologic types conducted by Bonal et al. (2005), they found that an accurate comparison could be made between the metamorphic grades of the CO and the ordinary chondrites using Raman spectrometry combined with petrographic analysis. Their method is based on the structural order of the chondritic organic matter, which was initially accreted in the same proportions in both CO and ordinary chondrites. Based on their data, the CO group would span a petrographic sequence from 3.03 as represented by ALHA77307, to 3.7 as represented by both Warrenton and Isna.
D.W.G. Sears (2016) conducted an in-depth petrographic study of CO chondrites in an effort to bring a measure of consistency to the wide diversity of classification schemes that now exist for this meteorite group. He studied a significant number of the "MIL" and "DOM" CO chondrites that were found in Antarctica (representing low petrographic types), and updated the petrologic classification of five of the six CO meteorites that were recovered as fresh falls around the world (representing the higher petrographic types). Computer software was used to ascertain which of the many metamorphic properties can best serve as accurate indicators of petrologic type. The results of this "Principle Component Analysis" revealed that 83% of the correlation between metamorphic alteration and petrologic type can be explained by three component types, none of which are decisive when considered alone: 30% is explained by bulk properties (bulk composition, bulk C content, trapped inert gas content, reflectance spectra at 0.8 µm), 28% is explained by metamorphism-induced phase changes (TL sensitivity, matrix composition, graphitization), and 25% is explained by Fe diffusion processes (olivine composition and heterogeneity, Ni, Co, and Cr content in kamacite); the remainder (17%) of the correlation to metamorphic grade can be explained by several less accurate properties such as O- and C-isotopic values and AOA textures. After assessing each of these parameters for the CO chondrites in his study as well as for the known falls (except Moss), a petrologic grade was assigned to each sample (see the following). Based on this CO chondrite study, D.W.G. Sears argues that it is ill-advised to construct a petrologic classification scheme for a common application among different chondrite groups, and he contends that resolution of the metamorphic grade to an accuracy greater than a single decimal place is not warranted for this group given the current techniques.
MIL CO3 chondrites: 3.2
DOM CO3 chondrites: 3.2
Felix, Lancé, and Ornans: 3.4
Isna represents one of the most highly metamorphosed CO meteorites, both thermally and aqueously. The peak metamorphic temperature of Isna was ~530°C, reflected by its higher Ni and Co content, lower Cr in kamacite concentration, very coarse texture, decreased abundance of presolar diamonds, and increased TL sensitivity due to feldspar crystallization from chondrule glass. Compared to other CO chondrites, Isna has an exceptionally low cosmic ray exposure age of ~0.15 m.y. A high solar noble gas content attests to a regolith origin.
A high hydrothermal alteration is reflected in the increased size and abundance of rimmed AOAs, an apparant increase in chondrule size, and melilite-rich refractory inclusions altered to fine-grained, spinelpyroxene inclusions. In addition, 16O compositions are lowest in the highest petrographic subtypes, reflecting an increased oxygen exchange in an 16O-poor water reservoir during hydrothermal metamorphism.
Some theories have linked the CO3 group to the K-class asteroids of the Eos family, an asteroid group that possesses S-type spectra in visible wavelengths and C-type spectra in the near-infrared. Based on reflectance spectra and albedo, the asteroids 221 Eos (104 km-diameter) and 653 Berenike (39 km-diameter) were found to be good analogs for the olivine-rich CO3 meteorites. These asteroids are located near 3 AU at the 9:4 resonance, where meteorites are expected to become Earth-crossers on timescales greater than tens of millions of years (see diagram below). At the same time, utilizing more advanced spectrographic techniques comparing a broader spectrum, it was determined that a better fit to the Eos family was the ungrouped achondrite Divnoe (Mothé-Diniz and Carvano, 2005). Then too, using improved techniques to compare the reflectance spectra of meteorites and asteroids, Eschrig et al. (2019) determined that the CK group is most closely matched to the K-type asteroids of the Eos family, while the CO/CV chondrites are a good match to both K-class and L-class asteroids. In another study compiling reflectance spectra for a large number of rare meteorites, Burbine et al. (2020, 2235) demonstrated that the K class is the best asteroid analog for CK chondrites of types 4-6, CV3 chondrites, and a CM12 chondrite.
Diagram credit: M. M. M. Meier et al., Earth and Planetary Science Letters, vol. 490 (2018)
'Cosmic history and a candidate parent asteroid for the quasicrystal-bearing meteorite Khatyrka'
Isna has an average porosity of 14.5% (Macke et al., 2011). For additional information on the formation of the CO chondrites and their metamorphic heirarchy, visit the Colony page. The specimen of Isna shown above is a 4.5 g partial slice.