COLONY

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.

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 that are highly unequilibrated. In support of this, a study has found 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 (Tomeoka and Itoh, 2004). The composition and texture of CAIs found in Colony is further evidence of its unmetamorphosed nature—it 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 may have formed 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 calls for the crystallization of forsterite first, enclosing a residual anorthite–diopside-rich melt. Upon cooling, the exsolution of volatiles created an abundance of voids. This model is also consistent with their compact nature. 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.

The classification technique utilizing TL sensitivity, which is based on the feldspar abundance, is not applicable below subtype 3.2 since feldspar may be dissolved in the aqueous alteration process. This TL method has been superceded by more sensitive methods which are presently able to measure the peak metamorphic temperatures, and thus enable the direct comparison of petrologic types between chemical classes.

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. This metamorphic sequence previously had no representative meteorite between 3.0 and 3.2, but Chizmadia and Bendersky (2006) have now found that the CO3 chondrite Asuka 881632 has features consistent with petrologic type 3.1 (e.g., the Fa and Cr contents of olivine).

In order 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. 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 used above, which utilizes the FeO content of olivine in AOAs, was only useful in the subtype range of 3.1–3.2. In their studies, Grossman and Rubin determined that the probable CO3 chondrite Dominion Range 03238 fits the requirements for a petrologic type 3.1.

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 is then directly associated with the petrologic grade. This Raman shift method was also combined with 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)—and analyses of the textures of metal–sulfide associations (e.g., metal–sulfide separation and angularity increases with higher metamorphic grades). In addition, abundance data 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, which were each 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 this 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), only the LL chondrite Semarkona and the ungrouped carbonaceous chondrite Acfer 094 (Kimura et al., 2006) were assigned to the least equilibrated subtype 3.00. However, Semarkona has more recently been determined to represent a petrologic subtype 3.01. This specific metamorphic type for Semarkona is also consistent with findings based on the FeNi-metal component, the features of which provide one of the most sensitive indicators for the onset of thermal metamorphism. The technique reveals that primary martensite decomposes to fine-grained plessite during very low degrees of thermal metamorphism in Semarkona, but which did not occurred in Acfer 094 (Kimura et al., 2008). Furthermore, they found that metal in and around Semarkona chondrules does not show a solar ratio of Co/Ni like that in Acfer 094, reflecting the greater degree of metamorphism that affected Semarkona. Moreover, low temperature aqueous alteration has occurred in Semarkona as attested to in the presence of secondary alteration products such as smectite.

Kimura et al. (2008) also argue for the inclusion of the carbonaceous chondrites of groups CR, CH, CB, and CM as 3.00 type specimens, notwithstanding their general designation 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.

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 oxidized, all of which suggest that the regolith of the CO parent body is heterogeneous (Bunch et al., 2010). The least metamorphosed CO chondrite is ALHA77307 with a petrologic type of 3.03. It has isotopic compositions which are similar to anhydrous silicates in the CM group, a group with which it also shares similar chemical compositions. In fact, the CO and CM groups may represent different degrees of low-temperature aqueous alteration of common precursor material that was similar to ALHA77307 (Clayton and Mayeda, 1999).

A study of Colony magnetite I–Xe 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 shown above is a 0.2 g specimen of Colony.

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