Colony

CO3.0 (3.05)

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 meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and. 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 olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion and low-Ca pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category., amoeboid olivine inclusions, and small chondrulesRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at  ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine with clear glass, in a fine-grained matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents.. 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 oxidationOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants. Although terrestrial contamination has likely contributed to the total amino acidA small molecule containing and amine group (–NH2) at one end and a carboxylic acid group (–COOH) at the other. They link up with various other amino acids to form proteins. Because they are used to build proteins, most of the human body consists of amino acids with their abundance second content of Colony (660 ppbParts per billion (10).), studies by H.-S. Chan et al. (2012) based on carbonElement commonly found in meteorites, it occurs in several structural forms (polymorphs). All polymorphs are shown to the left with * indicating that it been found in meteorites and impact structures: a. diamond*; b. graphite*; c. lonsdalite*; d. buckminsterfullerene* (C60); e. C540; f. C70; g. amorphous carbon; h. carbon nanotube*. isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: H, H (deuterium), and H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. 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 carbidePresolar interstellar dust grain found in CM and E chondrites; its formula is SiC. (~3.7 ppmParts per million (10).) and diamondOne of the naturally occurring forms of carbon found in meteorites. Each C atom is bonded through covalent sp3 hydrid orbitals to four others. The strength of the C-C bonds makes diamond the hardest naturally occurring substance (according to the Mohs scale) in terms of resistance to scratching. There are content (~1000 ppm). Just as there exists a metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. discontinuity in the SiC content of the ordinary chondriteWork in Progress Ordinary chondrites (OCs) are the largest meteorite clan, comprising approximately 87% of the global collection and 78% of all falls (Meteoritical Society database 2018). Meteorites & the Early Solar System: page 581 section 6.1 OC of type 5 or 6 with an apparent shock stage of S1, 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 chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such 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 CAIsSub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they found in Colony is further evidence of its unmetamorphosed nature—it contains primary spinelMg-Al oxide, MgAl2O4, found in CAIs., meliliteGroup of minerals found in the CAIs of meteorites such as CV chondrites. Melilite consists almost exclusively of the binary solid solution gehlenite (Ca2Al2SiO7) – åkermanite (Ca2MgSi2O7). The melilite in CAIs is closer to gehlenite in composition. The first-formed (highest-temperature) melilite crystallizing from a melt is relatively aluminum-rich and becomes progressively, anorthiteRare compositional variety of plagioclase and the calcium end-member of the plagioclase feldspar mineral series with the formula CaAl2Si2O8. Anorthite is found in mafic igneous rocks such as anorthosite. Anorthite is abundant on the Moon and in lunar meteorites. However, anorthite is very rare on Earth since it weathers rapidly, and hiboniteRefractory mineral, Ca-aluminate (CaAl12O19) that occurs in terrestrial metamorphic rocks and in CAIs of many chondrites. Meteoritic hibonite tends to be blue as seen in the meteorite Isheyevo (Ch/CB). Hibonite is one of the most refractory minerals found in primitive meteorites. 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 aggregatesMillimeter sized, fine-grained inclusions present to a few volume-percent in most carbonaceous chondrites. They can be round but can also be irregularly shaped like an amoeba (thus the name amoeboid). They are forsterite (Mg-rich olivine) and Ca-Al-Ti mineral aggregates. The most characteristic texture of AOAs is an anorthite core (sometimes (AOAMillimeter sized, fine-grained inclusions present to a few volume-percent in most carbonaceous chondrites. They can be round but can also be irregularly shaped like an amoeba (thus the name amoeboid). They are forsterite (Mg-rich olivine) and Ca-Al-Ti mineral aggregates. The most characteristic texture of AOAs is an anorthite core (sometimes) and converted some melilite to Ca-pyroxene and other minerals. Alternatively, formation of the AOAs could have occurred as melts in the solar nebulaThe primitive gas and dust cloud around the Sun from which planetary materials formed.. A likely scenario provides for the initial crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. of forsteritePure* magnesium end-member (Mg2SiO4) of the olivine solid solution series and an important mineral in meteorites. When magnesium (Mg) is completely substituted by iron, it yields the the pure Fe-olivine end member, fayalite (Fe2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo) enclosing a residual anorthite–diopside-rich melt. Upon cooling, exsolutionSegregation, during cooling, of a homogeneous solid solution into two or more different solids. 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 chondruleRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at  ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine within an AOA clearly attests to the molten condition of the AOA as the chondrule was enveloped.

A classification technique utilizing thermoluminescence (TL)Emission of light caused by the heating of certain minerals. sensitivity data, which are based on the feldsparAn alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars and due to their abundance, feldspars are used in the classification of igneous rocks. A more complete explanation can be found on the feldspar group page. abundance, was employed by Sears et al. (1991) to better resolve the petrologic typeMeasure of the degree of aqueous alteration (Types 1 and 2) and thermal metamorphism (Types 3-6) experienced by a chondritic meteorite. Type 3 chondrites are further subdivided into 3.0 through 3.9 subtypes. of the CO3 chondrites (3.0–3.9). 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. Based on Raman spectroscopyTechnique of splitting electromagnetic radiation (light) into its constituent wavelengths (a spectrum), in much the same way as a prism splits light into a rainbow of colors. Spectra are not smooth but punctuated by 'lines' of absorption or emission caused by interaction with matter. The energy levels of electrons in results along with other temperature indicators, Busemann et al. (2007) estimated the metamorphic sequence and peak metamorphic temperature for various CO meteorite samples as follows: ALHA77307 (3.0; 220°C)
Colony (3.0; 250–260°C)
Kainsaz (3.1; 390°C)
Y-791717 (3.3; 540°C)
Lancé (3.4; 480°C)
Ornans (3.4; 350–560°C)
ALHA77003 (3.5; 500°C)
ALH 83108 (3.5; 570°C)
Isna (3.7; 580–600°C) 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. ChromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. 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 0.3–0.4 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 (0.2–0.3 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 (0.11–0.53 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, troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. 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 substitutionReplacement of one ion or ionic group for another in the same structural site in a mineral yielding a solid solution. Most substitution in minerals is of cations which are smaller and essentially sit in a lattice of oxygen anions. Anionic substitution does occur in halides. Substitutions are classified based 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 chondrites—the 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 3.1–3.2. In their studies, Grossman and Rubin determined that the CO3 chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such 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 organicPertaining to C-containing compounds. Organic compounds can be formed by both biological and non-biological (abiotic) processes. 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 metal–sulfide associations (e.g., metal–sulfide separation and angularity increases with higher metamorphic grades). In addition, the abundances of presolar grainsMineral grains that formed before our solar system. These tiny crystalline grains are typically found in the fine-grained matrix of chondritic (primitive) meteorites. Most grains probably formed in supernovae or the stellar outflows of red giant (AGB) stars before being incorporated in the molecular cloud from which the solar system (diminished in higher metamorphic grades), noble gasesElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. (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: 3.03: ALHA77307
3.1: Colony
3.6: Kainsaz
>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 chondriteOrdinary chondrites ("low Fe" / "low metal") with only 1 to 3% free metal. Their olivine is more Fe-rich than in the other ordinary chondrites (Fa27-32), implying that the LL types must have formed under more oxidizing conditions than their H or L cousins. Orthopyroxene compositions are also Fe-the rich Semarkona, the L chondriteOrdinary chondrites low in free Ni-Fe metal (4 to 10 vol. %), containing olivine (Fa22-26) and the orthopyroxene hypersthene (Fs19-22). Average chondrule diameters (0.7 mm) are larger than those in H chondrites. The asteroid 433 Eros is suspected as a parent body, based on reflectance spectra, but most L chondrites NWA 7731, and the ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). (probably CO-related; Simon and Grossman, 2015) carbonaceous chondriteCarbonaceous chondrites represent the most primitive rock samples of our solar system. This rare (less than 5% of all meteorite falls) class of meteorites are a time capsule from the earliest days in the formation of our solar system. They are divided into the following compositional groups that, other than 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 chondriteMeteorite class named after the Ornans meteorite that fell in France in 1868, are related in chemistry and composition to the CV chondrites and may, with them, represent a distinct clan of carbonaceous chondrites that formed in the same region of the early solar system. However, COs are usually blacker, with an assigned petrologic type of 3.03. A detailed petrographic study of CO3 chondrites was conducted by Davidson et al. (2014, #1384) 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 with the highest H/C, D/H, and 15N/14N ratios among CO3 chondrites, which reflects minimal alteration (Davidson et al., 2014; Alexander et al., 2014, 2017); DOM 08006 also contains the highest abundance of presolar silicates (240 [±25] ppm) as determined by Nittler et al. (2013).
Image courtesy of Schrader and Davidson, GCA, vol. 214, p. 164 (2017)
‘CM and CO chondrites: A common parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. or asteroidal neighbors? Insights from chondrule silicates’ (https://doi.org/10.1016/j.gca.2017.07.031) 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 diffusionMovement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be represented by a change in concentration). Diffusion, the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomena. Because diffusion is thermally activated, coefficients for diffusion processes (olivine composition and heterogeneity, Ni, Co, and Cr content in kamaciteMore common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% Ni, and forms large body-centered cubic crystals that appear like broad bands or beam-like structures on the etched surface of a meteorite; its name is derived from the Greek word); 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. ALHA77307: 3.0
Colony: 3.0
MIL 07099 pairing group: 3.2
DOM 08004 pairing group (excludes DOM 08006): 3.2
Kainsaz: 3.2
Felix, Lancé, and Ornans: 3.4
Warrenton: 3.6
Isna: 3.7 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 material—initially similar to the primitive CO3.00 DOM 08006 and CO3.03 ALHA77307 chondrites—which 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 AUThe astronomical unit for length is described as the "mean" distance (average of aphelion and perihelion distances) between the Earth and the Sun. Though most references state the value for 1 AU to be approximately 150 million kilometers, the currently accepted precise value for the AU is 149,597,870.66 km. The (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 heliocentricCentered around a sun. Our own Solar System is centered around the Sun so that all planets such as Earth orbit around the Sun. Note that 25% of Americans incorrectly believe the Sun revolves around the Earth. distance) based on early research on refractory lithophile abundances, chondrule size and composition, anhydrous mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous 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 elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of compositions (Greenwood et al., 2014 reference therein).

The petrographic and chemical similarities that exist between the CM and CO groups indicate they likely formed from a common reservoir under similar conditions. Further evidence for a common CO–CM 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 also demonstrates significant overlap which is consistent with a common CO–CM parent body. Nevertheless, Schrader and Davidson (2017) recognize multiple lines of evidence which indicate these two groups derive from separate parent bodies including the following:

1. difference in Δ17O and ε54Cr values (Sanborn et al., 2014 [see diagram])
2. difference in matrix abundance (CM: 70 wt% vs. CO: 34 wt%; Weisberg et al., 2006)
3. difference in average chondrule diameter (CM: 0.3 mm vs. CO: 0.15 mm; Weisberg et al., 2006)
4. difference in group average 21Ne-based CRE ages (CM: 2.8 [±3.1] m.y. vs. CO: 22 [±18] m.y.; Mazor et al., 1970 [see diagram])
5. difference in FeO-poor relict grains (CM: ~12% vs. CO: ~48%; Schrader and Davidson, 2017)
6. no known CM/CO meteorite breccias (Schrader and Davidson, 2017)

In their comprehensive oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only 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 silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the 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) Although there is a hiatus 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 accretionAccumulation of smaller objects into progressively larger bodies in the solar nebula leading to the eventual formation of asteroids, planetesimals and planets. The earliest accretion of the smallest particles was due to Van der Waals and electromagnetic forces. Further accretion continued by relatively low-velocity collisions of smaller bodies in the (within ~2 m.y. of CAISub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they formation) of a planetesimal of significant size (>120 km in diameter), composed of low-density material akin to the CM chondrites, could experience internal differentiationA process by which a generally homogeneous chondritic body containing mostly metal, silicates and sulfides will melt and form distinct (differentiated) layers of different densities.   When the melting process continues for a long enough period of time, the once chondritic body will re-partition into layers of different composition including without eruption of magmaMolten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called lava. to the surface, thereby retaining a primitive hydrated crustal region.
Diagram courtesy of Greenwood et al., 45th LPSC #2610 (2014)
A81:ALHA81002; A83:ALH 83100; CB:Cold Bokkeveld; E:Essebi; Ma:Maribo; MET:MET 01070; MI:Mighei; Mo:Moapa; M:Murchison; Mu:Murray; N:Nogoya; P:Paris (mean); PA:Paris-altered; PL:Paris-less altered; S:SCO06043; Q93:QUE93005; Q97:QUE97990; Y:Y791198; W:WIS91600; CO3 falls:Moss A comparitive analysis of CM and CO chondrites led Chaumard et al. (2018) to the conclusion that both of these groups formed in a common isotopic reservoir and accreted identical anhydrous precursor material comprised of the same two type I chondrule populations: 1) Δ17O ~ –2.5‰, Mg# <96, and 2) Δ17O ~ –5‰, Mg# >98.5. Both CM and CO chondrites also accreted identical type II chondrule populations. However, they recognized the many other characteristics that indicate a formation for these two groups on separate parent bodies, including differences in chondrule size (0.15 and 0.30 mm for CO and CM, respectively), matrix abundance (30–35 and 70 vol% for CO and CM, respectively), abundance of type II chondrules that contain relict olivine grains (~48% and 12–25% for CO and CM, respectively), average CRE age (22 [±18] and 2.8 [±3.1] m.y. for CO and CM, respectively), accretion age (~2.1–2.7 and ~3.5–5.0 m.y. after CAIs for CO and CM, respectively), and in abundance of hydrous phases (ice:rock ratio of ~0.1–0.2 and ~0.3–0.6 for CO and CM, respectively). They propose a scenario in which the snow line moved inward during the time interval between the accretion at nearly the same location (~2–3 AU) of these two distinct planetesimalsHypothetical solid celestial body that accumulated during the last stages of accretion. These bodies, from ~1-100 km in size, formed in the early solar system by accretion of dust (rock) and ice (if present) in the central plane of the solar nebula. Most planetesimals accreted to planets, but many –.
Diagram credit: Chaumard et al., GCA, vol. 228, p. 220–242 (1 May 2018)
‘Oxygen isotope systematics of chondrules in the Murchison CM2 chondrite and implications for the CO-CM relationship’
(https://doi.org/10.1016/j.gca.2018.02.040) Kimura et al. (2008) argue for the additional inclusionFragment of foreign (xeno-) material enclosed within the primary matrix of a rock or meteorite. 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 (3.0–3.05) CO chondrite NWA 4530 is devoid of FeNi-metal, contains chondrules of type GF, and is unusually strongly oxidizedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants, all of which suggest that the regolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles).   of the CO parent body is heterogeneous (Bunch et al., 2010). A study of Colony magnetiteFe oxide, FeFe2O4, containing oxidized iron (Fe) found in the matrix of carbonaceous chondrites and as diagnostic component in CK chondrites. In CK chondrites, magnetite is typically chromian, containing several wt. % Cr2O3. I–Xe systematics indicates that this isotopic systemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with closed 6.1 (±3.1) m.y. after Shallowater, and ~8 m.y. after OrgueilA large carbonaceous Ivuna-like (CI1) chondrite that disintegrated and fell in fragments near the French town of Orgueil on May 14, 1864. About 20 pieces, totaling ~12 kg in mass, were subsequently recovered from an area of several square km, some head-sized but most were smaller than a fist. Specimens (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 sectionThin slice or rock, usually 30 µm thick. Thin sections are used to study rocks with a petrographic microscope. micrograph of Colony, shown courtesy of Peter Marmet.
click on image for a magnified view
Photo courtesy of Peter Marmet