The Colony Meteorite and Variations in Co3 Chondrite Properties

Abstract

The Colony meteorite is an accretionary breccia containing several millimeter‐to centimeter‐size chondritic clasts embedded in a chondritic host. Colony is one of the least equilibrated CO3 chondrites; it has an unrecrystallized texture and contains compositionally heterogeneous olivine and low‐Ca pyroxene, kamacite with low Ni and Co and high Cr, amoeboid inclusions with low FeO and MnO, a fine‐grained silicate matrix with very high FeO, and numerous small chondrules with clear pink glass. However, Colony differs from normal CO chondrites in several respects: Although Al, Sc, V, Cr, Ir, Fe, Au and Ga abundances are consistent with a CO chondrite classification, certain lithophiles (Mg and Mn), siderophiles (Ni and Co) and chalcophiles (Se and Zn) are depleted by factors of 10–40%. The shape of Colony's thermoluminescence (TL) glow curve is similar to that of Allan Hills A77307 (another unequilibrated chondrite with CO3 petrological characteristics) and different from those of normal CO chondrites. [ALHA77307 also resembles Colony in having low Mg, Mn, Ni and Co, compared to normal CO chondrites, but it possesses CO‐CV levels of Se and Zn and nearly CV levels of Cd.] Colony is badly weathered; it contains 22.7 wt.% Fe2O3 and 5.7 wt.% H2O. Recalculating the analysis on an H2O‐free basis with all Fe2O3, NiO and CoO converted to metal, yields an inferred original metallic Fe, Ni abundance of ∼ 19 wt.%. This is similar to that of Kainsaz (an unweathered CO3 fall), but much higher than that of all other CO3 chondrites (< 6.3 wt.%). Although it is possible that Colony and either ALHA77307 or Kainsaz constitute distinct CO3 chemical subgroups, the weathered nature of Colony and ALHA77307 preclude the drawing of firm conclusions. Nevertheless, it is clear that CO3 chondrites vary more in compositional and petrological properties than was previously recognized.

The onset of metamorphism in ordinary and carbonaceous chondrites

“…The measured area of Chainpur was apparently anomalously rich in matrix, the abundance being more than double that found in most type 3 ordinary chondrites. Conversely, the studied area of ALHA77307 was much lower in matrix than was found by Rubin et al (1985) and closer to those found in other CO chondrites (McSween 1977). Measured metal abundances are highly variable and were clearly affected by weathering in several samples, notably Colony and Rainbow.…”

Correlated petrologic and geochemical characteristics of CO3 chondrites

“…Other features that correlate with increasing whole rock metamorphism include increasing concentrations of FeO and MnO in amoeboid olivine inclusions (AOI), FeO enrichment in spinel in refractory inclusions, MgO enrichment in fine-grained matrices, coarsening of matrix olivine and chondrule-rim sulfide grains, increasing Co and Ni and decreasing Cr and P in kamacite, decreasing abundances of trapped rare gases, decreasing abundances of presolar c6 diamonds, and increasing induced therrnoluminescence (TL) sensitivity of the 130 "C peak of the C 0 3 chondrite glow curves (McSween, 1977a;Keck and Sears, 1987;Scott and Jones, 1990;Huss, 1990;Brearley et al, 1994Brearley et al, , 1995Brearley, 1996a;Russell et al, 1998). Other previously described features of C 0 3 chondrites that correlate with increasing degrees of whole rock metamorphism include the modal abundance of A01 (Rubin et al, 1985) and chondrule size (Rubin, 1989). Discernment of the reasons for the correlations among these features would reveal important insights into the history of the CO parent body.…”

Petrology and origin of amoeboid olivine aggregates in CR chondrites

“…The AOAs in the CR chondrites range in size from about 70 µm to 1.2 mm. Those in CO and CM chondrites are generally <600 µm in size (Rubin et al 1985), while in CV chondrites, they are generally larger, up to 5 mm in size.…”

“…AOAs in CV chondrites have been described by Grossman and Steele (1976), Bar-Matthews et al (1979), Kornacki et al (1983), Kornacki and Wood (1984), Hashimoto and Grossman (1987) and Komatsu et al (2001); in CM chondrites by Cohen et al (1983), Kornacki et al (1983); in CO chondrites by Rubin (1985) and Chizmadia et al (2002); and in CR chondrites by Weisberg et al (1993) and Aléon et al (2002). They are minor (less than 1 vol%) components of many carbonaceous chondrites, but some CV chondrites contain up to 9 vol% (McSween 1977a), and some CO chondrites have up to 16 vol% (McSween 1977b).…”