Found 1975, approx., recognized 1980
35° 21' N., 98° 41' W.
A single stone weighing 3,912 g was recovered by Deon Yearwood from the tines of his cotton cultivator in Washita County, Oklahoma. The rock was considered to be unusual in appearance and so it was saved. Some years later, as communicated by M. Bostick (2005), Mr. Yearwood asked a professor at Oklahoma University if he would look at the stone to verify his suspicions that it was a meteorite. Finding a lack of interest to even look at the rock, he brought it to Southern University at Bedford, Oklahoma, where the suggestion was made to contact Harvey H. Nininger. Upon his initial examination, Nininger immediately recognized the stone as a meteorite, and he and Jim Westcott obtained the meteorite from Mr. Yearwood in return for him receiving two cut slices from the meteorite.
Colony has an unrecrystallized texture containing olivine and low-Ca pyroxene, amoeboid olivine inclusions, and small chondrules with clear glass, in a fine-grained matrix. The matrix has higher FeO and KO, and lower MgO and NaO than normal CO3 matrices. Colony is a highly weathered meteorite and has been depleted in sulfur, selenium, sodium, and nickel due to oxidation. Although terrestrial contamination has likely contributed to the total amino acid content of Colony (660 ppb), studies by H.-S. Chan et al. (2012) based on carbon isotope analyses and racemic enantiomeric ratios indicate that some of the glutamic acid, aspartic acid, valine, and alanine, and possibly some of the β-alanine and γ-ABA, is an indigenous extraterrestrial component.
Since it is among the most unequilibrated of the CO3 meteorites (after ALHA77307 and Y-81020), Colony has retained most of its presolar silicon carbide (~3.7 ppm) and diamond content (~1000 ppm). Just as there exists a metamorphic discontinuity in the SiC content of the ordinary chondrite groups between petrologic grades 3.4 and 3.5, the CO group shows a similar hiatus between petrologic grades 3.0 and 3.1, defining those members which are highly unequilibrated. In support of this, a study by Tomeoka and Itoh (2004) led to the determination that CO chondrites of subtype 3.0 contain no nepheline, a secondary aqueous alteration/dehydration product, while those of higher subtypes contain correspondingly increasing amounts. The composition and texture of CAIs found in Colony is further evidence of its unmetamorphosed natureit contains primary spinel, melilite, anorthite, and hibonite which crystallized from 16O-rich gas. The CAIs also contain radiogenic 26Mg derived from nebular 26Al.
Colony shows evidence of slight hydrothermal alteration at temperatures less than ~400°C, which might have led to the formation of the fine-grained amoeboid olivine aggregates (AOA) and converted some melilite to Ca-pyroxene and other minerals. Alternatively, formation of the AOAs could have occurred as melts in the solar nebula. A likely scenario provides for the initial crystallization of forsterite enclosing a residual anorthitediopside-rich melt. Upon cooling, exsolution of volatiles created an abundance of voids. This model is also consistent with the compact nature of AOAs. Finally, the finding of a previously-formed Ca,Al-rich chondrule within an AOA clearly attests to the molten condition of the AOA as the chondrule was enveloped.
A classification technique utilizing thermoluminescence (TL) sensitivity data, which is based on the feldspar abundance, was employed by Sears et al. (1991) to better resolve the petrologic type of the CO3 chondrites (3.03.9). The TL sensitivity for Colony was found to be an extremely low 3.0, consistent with a very primitive state of metamorphism. However, it has since been argued that this technique is not applicable below subtype 3.2 because feldspar could have been dissolved during the aqueous alteration process. This TL method has been superceded by other more sensitive methods which are able to measure the peak metamorphic temperatures, and thus enable a direct comparison of petrologic types between chemical classes.
To discriminate among subtypes below type 3.2, it has been shown that the Cr content of ferroan olivine is an excellent indicator of metamorphism. Chromite exsolves from olivine in the incipient stages of metamorphism, initially producing heterogeneous Cr2O3 contents, and eventually low-Cr olivine. In a study by Chizmadia and Bendersky (2006), they determined that this sequence progresses from type 3.0, corresponding to high Cr2O3 contents of 0.30.4 wt%, to type 3.2, in which Cr2O3 constitutes less than 0.1 wt%. The gap between these subtypes representing type 3.1 was recently filled by the meteorites A-881632 (0.20.3 wt% Cr2O3), DOM 03238 (0.27 ±0.18 wt% Cr2O3), NWA 2718 (0.26 ±0.10 wt% Cr2O3), NWA 2760 (0.22 ±0.14 wt% Cr2O3), and NWA 2974 (0.110.53 wt% Cr2O3).
Previously, petrologic studies revealed that systematic changes occur in AOAs with increasing subtype, which is directly linked to increasing aqueous and thermal metamorphism (Chizmadia et al., 2002). For example, textures and morphologies of AOAs show changes, olivine in AOAs 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 are the chondrules, which were previously utilized to determine subtype. As a result of their study, Chizmadia et al. (2002) proposed a refinement in the subtypes of the CO3 chondrites; the CO group would span a metamorphic sequence from 3.0, as represented by Colony, to 3.8, as represented by Isna.
To make the metamorphic sequence of the CO3 chondrites equivalent to that recognized for the ordinary chondrites, Grossman and Rubin (2006) calibrated the petrologic scale for CO3 chondrites based on the same factors used for ordinary chondritesthe Fa and Cr contents of olivine, and the S content of the matrix (see the Dhofar 015 page for further details). They were able to establish a petrologic sequence consistent with the one utilized for ordinary chondrites, following a progression of metamorphic subtypes in the order 3.00, 3.05, 3.1, 3.15, 3.2, 3.3, etc. They determined that Colony best fits into the 3.05 metamorphic interval. They also demonstrated that the method which utilizes the FeO content of olivine in AOAs (as described above), was only useful in the subtype range of 3.13.2. In their studies, Grossman and Rubin determined that the CO3 chondrite Dominion Range 03238 fits the requirements for a petrologic type 3.1.
Chizmadia and Bravo-Ruiz (2013) employed a similar method to that of Grossman and Rubin (2006) above to classify CO3 chondrites through the entire range by degree of aqueous alteration, utilizing the Fe-Mg composition and distribution in olivines in AOAs. Based on their analyses, they proposed that Colony should be assigned to petrologic type 3.05. They also better resolved Isna as type 3.75, previously designated 3.8, with the MET 00694 pairing group being assigned to the highest CO3 subtype of 3.8.
In a different study of CO3 petrologic types conducted by Bonal et al. (2005, 2007), they found that an accurate comparison could be made between the metamorphic grades of the CO and ordinary chondrites using Raman spectroscopy. This methodology is based on various spectral parameters associated with the structural order of insoluble polyaromatic organic matter, which was initially accreted in the same proportions in both CO and ordinary chondrites. This structural order is irreversibly transformed by thermal metamorphism (from carbonization to graphitization) to a commensurate degree across chemical classes. A correlation has now been made between this maturation grade of organic matter and the peak metamorphic temperature of the meteorite, and this can then be directly associated with the petrologic grade. To increase accuracy, the Raman shift method was combined with other parameters such as petrographic analyses of phenocrysts in type-I chondrules, including FeO zoning measurements in olivine phenocrysts and Fs compositions in pyroxene phenocrysts (both showing enrichments in higher metamorphic grades), along with petrographic analyses of textures of metalsulfide associations (e.g., metalsulfide separation and angularity increases with higher metamorphic grades). In addition, the abundances of presolar grains (diminished in higher metamorphic grades), noble gases (P3; diminished in higher grades), and siderophile elements (imprecise indicators) were utilized in their study. Each of these classification parameters were correlated with the degree of thermal metamorphism. From their data, Bonal et al. concluded that the CO group members they studied should span a petrologic sequence as follows:
>3.6: Felix, Lancé, and Ornans (in order of increasing grade)
≥ 3.7: Warrenton and Isna
In a further expansion of the Raman spectroscopy method, Quirico et al. (2006) determined that LL3.0 Semarkona has experienced thermal metamorphism beyond the onset stage, and they proposed a new petrologic scale to provide consistency in the range as follows: Semarkona would become petrologic type (PT) 1, with PT 0 being reserved for the stage of true onset of thermal metamorphism. All other meteorites analyzed to date would have a PT greater than 1.
Following the scheme of J. Grossman and A. Brearley (2005), the LL chondrite Semarkona, the L chondrite NWA 7731, and the ungrouped (probably CO-related; Simon and Grossman, 2015) carbonaceous chondrite Acfer 094 (Kimura et al., 2006) were each assigned to the least equilibrated subtype 3.00; however, Semarkona has more recently been determined to represent a petrologic subtype 3.01. For some time ALHA77307 had been considered to be the least metamorphosed CO chondrite, with an assigned petrologic type of 3.03. A detailed petrographic study of CO3 chondrites was conducted by Davidson et al. in 2014 in order to better define a metamorphic trend for this group. They analyzed the Cr content in olivine of type-II chondrules and identified DOM 08006 as the least metamorphosed CO3 chondrite, assigning it a petrologic type of 3.00. In further support of this low petrologic type assignment, DOM 08006 was determined to have the highest insoluble organic matter (IOM) and C content and the highest H/C, D/H, and 15N/14N ratios among CO3 chondrites, which reflects minimal alteration; DOM 08006 also contains the highest abundance of presolar silicates (Davidson et al., 2014; Alexander et al., 2014).
Image courtesy of Davidson et al., 45th LPSC, #1384 (2014)
(see also Grossman & Brearley, MAPS, vol. 40 (2005)
'The onset of metamorphism in ordinary and carbonaceous chondrites' (http://dx.doi.org/10.1111/j.1945-5100.2005.tb00366.x)
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
The most unequilibrated CO3 chondrites have isotopic compositions that are similar to anhydrous silicates in the CM group, a group with which it also shares many chemical and petrographic similarities. In fact, the CO and CM groups may represent common precursor materialinitially similar to the primitive CO3.00 DOM 08006 and CO3.03 ALHA77307 chondriteswhich subsequently experienced different degrees of low-temperature aqueous alteration (Clayton and Mayeda, 1999). Both of these groups probably formed in the same nebular region located beyond 3 AU (Wasson, 1988; Rubin, 2010). Beyond that, new O-isotopic analyses conducted by Greenwood et al. (2014) on a large sampling of CM chondrites led them to suggest that a possible group relationship (same parent body) may exist between the CM and CO chondrites, previously considered to constitute a clan (groups formed at a similar heliocentric distance) based on early research on refractory lithophile abundances, chondrule size and composition, anhydrous mineral compositions, and O-isotopic composition of high-temperature phases (Kallemeyn and Wasson, 1979, 1981). In addition, it was found that the matrix component in meteorites of both groups have nearly identical minor element compositions (Greenwood et al., 2014 reference therein).
Further evidence for a common COCM parent body was presented by Schrader and Davidson (2016; #1288). They analyzed the Cr content in olivine grain cores of type-II (FeO-rich) chondrules for a number of CM chondrites spanning the full range of petrologic types (e.g., Sutter's Mill [2.0/2.1]... QUE 97990 [2.6]). Utilizing a coupled diagram comparing the mean Cr2O3 content to the standard deviation (σ) of Cr2O3 content, they demonstrated that both the CO and CM thermal metamorphism curves overlap. Their study also shows that thermal metamorphism and aqueous alteration are not coupled. Another coupled diagram presented by Schrader and Davidson (2016) comparing the Fe and Mn contents of the type-II chondrules among the CM samples is also consistent with a common COCM parent body.
Despite the hiatus that occurs between the CM and CO groups on an oxygen three-isotope diagram, the additional data plots calculated by Greenwood et al. (2014) clearly show that the CM O-isotopic trend line intersects the CO field, and they have posited a new theory based on the premise that both groups formed on a common parent body. They suggest that the CO group might represent an anhydrous inner zone, in which the initial hydrous component was rapidly liberated through endogenous (radiogenic) heating and vented to the surface and into space. Conversely, the outer zone represented by the CM group experienced a high degree of aqueous alteration over an extended duration. A compatable scenario was presented by Fu and Elkins-Tanton (2013) in which early accretion (within ~2 m.y. of CAI formation) of a planetesimal of significant size (>120 km in diameter), composed of low-density material akin to the CM chondrites, could experience internal differentiation without eruption of magma to the surface, thereby retaining a primitive hydrated crustal region.
In their comprehensive oxygen isotope study of carbonaceous chondrite groups, Clayton and Mayeda (1999) showed that many ungrouped members plot along the same mixing line and fill the hiatus between the CO and CM fields (see diagram below). They suggest that both CO and CM groups consist of a common anhydrous silicate precursor, while the CM group represents the interaction of this anhydrous precursor with an aqueous reservoir. The ungrouped members are transitional, with variable water:rock ratios as indicated by the tick marks along the mixing line.
Image courtesy of Clayton and Mayeda, GCA, vol. 63, p. 2094 (1999)
'Oxygen isotope studies of carbonaceous chondrites'
See also this oxygen three-isotope diagram presented by Jacquet et al., MAPS, vol. 51, #5, p. 862 (2016)
'Northwest Africa 5958: A weakly altered CM-related ungrouped chondrite, not a CI3' (http://dx.doi.org/10.1111/maps.12628)
Kimura et al. (2008) argue for the additional inclusion of CR, CH, CB, and CM carbonaceous chondrites as petrologic type 3.00 examples, notwithstanding the general designation of some meteorites in these groups as type 2 due to aqueous alteration features. In light of this petrologic typing paradox, they propose that a separate scale be adopted to describe aqueous alteration distinct from that which describes thermal metamorphism. Separate classification schemes for aqueous alteration have since been proposed (see the Murchison page for details.
Studies show that the highly unequilibrated (3.03.05) CO chondrite NWA 4530 is devoid of FeNi-metal, contains chondrules of type GF, and is unusually strongly oxidized, all of which suggest that the regolith of the CO parent body is heterogeneous (Bunch et al., 2010). A study of Colony magnetite IXe systematics indicates that this isotopic system closed 6.1 (±3.1) m.y. after Shallowater, and ~8 m.y. after Orgueil (Pravdivtseva et al., 2007). The photo of Colony shown above is a 0.2 g specimen, and the bottom image is an excellent petrographic thin section micrograph of Colony, shown courtesy of Peter Marmet.
click on photo for a magnified view
Photo courtesy of Peter Marmet