CO chondrites

Myr after CAIs (Amelin et al., 2002). With only a few exceptions, their (26A1/27A1)o ratios are below 0.5 x 10 5 (Nagashima et al., 2008). These data imply that most CR2 chondrules formed 1-2 Myr after those in ordinary and CO chondrites (Fig. 9.10). 

As with the ordinary chondrites, the accretion times for CV and CO chondrite parent bodies can be estimated from the times of chondrule formation. The 26Al-26Mg and129I-129Xe data suggest chondrule formation extended to 4565 Ma, 3.2 Myr after CAIs. This is slightly later than, but within the uncertainties of, the time indicated for the accretion of ordinary chondrite parent bodies. 

Cosmic-ray exposure ages of ordinary, CV, and CO chondrites. Modified from Eugster etal. (2006). 

Kurahashi, E., Kita, N. T., Nagahara, H. and Morishita, Y. (2008) 26Al-26Mg systematics of chondrules in primitive CO chondrite. Geochimica et Cosmochimica Acta, 72, 3865-3882. 

Figure 12.17a shows lithophile element abundances, and Figure 12.17b shows sid-erophile and chalcophile element abundances in CM chondrites, normalized to Cl chondrites. Illustrated for comparison are the abundances in CO chondrites, which are the anhydrous carbonaceous chondrite group most closely allied to CM chondrites. As in other chondrites, the greatest differences are in volatile elements. The volatile and moderately volatile elements in CM chondrites are present at 50-60% of the abundances of the refractory elements. The volatile elements are primarily located in the matrix, and the matrix comprises 50-60% of CM chondrites. This implies that the matrix has essentially Cl abundances of all elements, while the chondrules and refractory inclusions have Cl relative abundances of refractory elements but are highly depleted in the volatile elements. The sloping transition in the region of moderately volatile elements indicates... 

Figure 17. TL sensitivity against silicate heterogeneity (a measure of metamorphic intensity since metamorphism homogenizes silicate compositions) expressed as percent mean deviation from the mean for the CO chondrite class.

Figure 9 Combined elemental maps of (a) the C03.1 carbonaceous chondrite Kainsaz and (b) ungrouped CO/CM-like carbonaceous chondrite Acfer 094. Kainsaz contains abundant small chondrules, CAIs, and AOAs. Acfer 094 is texturally and mineralogically similar to CO chondrites, but contains higher abundance of matrix. AOA = amoeboid ohvine aggregate BO = barred olivine chondrule PO(P)i n = type I (II) porphyritic olivine (pyroxene) chondrule.

Another argument that the 0-rich end-member was a ubiquitous component of primitive solids is that it is found in many different chemical forms (different minerals) in many classes of meteorites CAls and amoeboid olivine aggregates (AOAs) from Efremovka (CV3) (Aleon et al., 2002 Fagan et al., 2002), AOA from a CO chondrite, Y 81020 (Itoh et al, 2002), and CAl and AOA from CM and CR chondrites (Krot et al., 2002). This isotopic composition can also serve as an end-member for the chondrule mixing line in Figure 2. 

Since the phyllosilicates in CM chondrites are a few per mil enriched in heavy isotopes relative to the whole rock, material balance requires that the unanalyzed residual anhydrous silicates must be depleted in heavy isotopes by a comparable amount. This puts their composition into the range of CO chondrites. Figure 8 shows the relationship between CO and CM chondrites, which apparently represents different water/rock ratios in the aqueous alteration of a common CO-like precursor. The genetic association of CO and CM... 

The C03 meteorites have been subdivided into metamorphic grades from 3.0 to 3.7, with peak temperatures in the range 450-600 °C (Rubin, 1998). In contrast to the metamorphism of the ordinary chondrites, the CO metamorphism probably occurred in the presence of water, and the system was not closed with respect to oxygen. The least metamorphosed CO meteorites, ALH 77307 and Y 81020, are the most 0-rich Loongana 001 and HH 073, classified as 3.8 or 4, are the most 0-poor. There is not, however, a simple one-to-one correspondence between metamorphic grade and isotopic composition. Since most of the CO chondrites are finds, some may have been altered by terrestrial weathering. 

CR2 chondrite matrices COS chondrite matrices CK3 chondrite matrices CVS chondrite matrices... 

Figure 6 Oxygen-isotopic compositions of individual minerals in CAIs from the CO chondrites Y-81020, Colony, Kainsaz and Ornans. Primary minerals in CAIs from the least metamoprhosed CO chondrites Y-81020 (type 3.0) and Colony (3.0) are uniformly 0-enriched, whereas CAIs from Kainsaz (3.2) and Omans (3.3) tend to show oxygen isotopic heterogeneity with spinel and high-calcium pyroxene enriched in 0 and melilite and secondary nepheline depleted in 0. Based on these observations, Wasson et al (2001) inferred that oxygen isotope exchange took place during thermal metamorphism and alteration in an asteroid (data from Itoh et al, 2000 Wasson et al, 2001).

Itoh S., Kojima H., and Yurimoto H. (2000) Petrography and oxygen isotope chemistry of calcium-aluminum-rich inclusions in CO chondrites. In Lunar Planet. Sci. XXXI, 1323. The Lunar and Planetary Institute, Houston (CD-ROM). 

Figure 25 Oxygen isotopes in CM and CO chondrite CAIs. (sources Fahey etai, 1987b Wasson a/., 2001).

CO chondrites have clearly experienced limited aqueous alteration during a relatively low-temperature event. However, the CO chondrites may have also experienced aqueous alteration contemporaneously with metamorphism (Rubin, 1998). This latter type of alteration is best described as fluid-assisted metamorphism, rather than aqueous alteration and shares some similarities with alteration in the oxidized CV chondrites. Hence, CO chondrites may have experienced two periods of aqueous alteration. 

Evidence to constrain the location of alteration for the CO chondrites is limited. However, most data suggest a late-stage event that postdated metamorphism, implying that it was probably a parent-body process. Development of alteration phases in CO matrices has occurred interstitially to matrix phases and there is no evidence of distinct hydrous phases intermixed with unaltered phases that would support pre-accretionary alteration. However, formation of phyllosilicates in chondmles in ALH77307 may have occurred prior to accretion, based on the fact that the ALH77307 matrix shows essentially no evidence of aqueous alteration (Brearley, 1993). 

Evidence of oxidation that may be related to metasomatic processes appears to be essentially restricted to the oxidized CV chondrites, some of the CO chondrites, and a few unequilibrated ordinary chondrites. This section will examine the evidence for these processes and will include a brief mineralogical overview of the evidence followed by a discussion of the available chemical and isotopic evidence for oxidation and metasomatism. 

Although not as extensively developed, there is evidence that the components of some CO chondrites were affected by iron-alkali metasomatism and oxidation processes similar to those experienced by CV chondrites. 

Figure 9 Backscattered electron image of a region of a metasomatized type I chondnile from the CO chondrite Kainsaz (3.1). The chondnile contains clinoenstatite (cen), olivine (ol) metal, and sulfide grains (bright) and fine-grained mesostasis (mes). Primary anorthite has been extensively replaced by nephehne, which occurs as dark lamellae within the plagioclase. In addition the mesostasis has been partially replaced by salitic pyroxene which is also considered to be a metasomatic alteration product (reproduced by permission of Miner-alogical Society of America from Planetary Materials, 1998, 36, chap. 3, 3-1-3-398).

Oxidation effects in CO chondrites are variably developed. Many CO chondrites contain unaltered kamacite, whereas in ALH77307, Ornans, and several Antarctic CO chondrites, magnetite is common, replacing metal (McSween, 1977b Scott and Jones, 1990 Shibata, 1996). Pentlandite appears to have replaced troilite in ALH77307, whereas troilite still coexists with magnetite in Ornans. 

Secondary alteration of CAls in CO chondrites has been noted by a number of workers (Ikeda, 1982 Holmberg and Hashimoto, 1992 Tomeoka et al., 1992 Kojima et al., 1995 Russell et al., 1998). Nepheline is the most common replacement phase, with minor amounts of sodalite, ilmenite, and monticellite (Brearley and Jones, 1998). Russell et al. (1998) studied CAls in a suite of C03 chondrites ranging from petrologic type 3.0-3.7. They observed that CAls showed significant effects of alteration, such that, in some cases, all the primary components of the CAls have either been replaced or had their compositions modified. The key evidence of alteration includes increases in the FeO content of spinels in spinel-pyroxene inclusions, increased replacement of melilite by nepheline, sodalite, and diopside, and disturbed aluminum-magnesium systematics in some inclusions. 

---