carbonaceous chondrite grouplet
standby for coolidge photo
Found 1937
38° 2′ N., 101° 59′ W. A very weathered mass of 4.5 kg was found by Harvey Nininger in Hamilton County, Kansas. Coolidge was originally classified as a metamorphosed member of the reduced subtype of the CV3 group due to certain similarities: 1) an abundance of FeNi-metal and troilite; 2) chondrules and refractory objects that are depleted in Na and K; and 3) a matrix composed of olivine (Noguchi, 1994). Nevertheless, significant differences exist between these chondrites; in contrast to the reduced CV3 chondrites, Coolidge matrix is depleted in Na, chondrule compositions are different, and thermal metamorphism was greater (temperatures to ~900°C for a short duration).

Thermoluminescence (TL) sensitivity analysis was applied to Coolidge, and it was resolved as a petrologic type 4. Kallemeyn and Rubin (1995) found features in Loongana 001 which suggested a genetic relationship exists (i.e., same parent body) with Coolidge. For example, they both have comparable matrix abundances, comparable petrographic types, similar chondrule sizes, similar olivine Fa abundances, high refractory lithophile and refractory siderophile abundances, low abundances of all volatile element groups, and similarities in their opaque assemblages.

Recent petrographic studies of Coolidge, Loongana 001, and HaH 073 (W3, S1), defined a petrologic type range of 3.8–4, implying that these meteorites are not simply metamorphosed samples of CV3 material. Recent studies of HaH 073 show an O-isotopic plot on the CCAM line close to Coolidge, and it has many of the features common to the Coolidge–Loongana grouplet. However, HaH 073 reveals some differences with Coolidge–Loongana; e.g., more ferroan olivine, slightly higher matrix abundance, slightly smaller chondrule size, not all volatiles have low abundances, and slightly different O-isotopic values). Based on these differences and other abundance ratios, Huber et al. (2006) were unable to establish a classification for HaH 073 within the Coolidge–Loongana grouplet. However, in-depth studies of HaH 073, Coolidge–Loongana, and Sah 00182 by Choe et al. (2010) led them to conclude that, given heterogeneous volatile element abundances, these meteorites are likely related. Two other recently found meteorites, NWA 779 and Sah 00177 (photo courtesy of L. Labenne), exhibit low-fayalite olivine compositions and refractory element abundances characteristic of this grouplet, and thus one or both could conceivably be recognized as additional samples constituting the membership of five which is necessary to establish a new carbonaceous chondrite group.

Only the karoondaites (CK) have petrologic type 4 members in common with the three (possibly four) Coolidge-type meteorites, but current oxidation state and bulk composition studies solidly place these meteorites in their own distinct carbonaceous chondrite grouplet. For example, the matrix of CK4 chondrites consists of both olivine and sodic plagioclase, while no plagioclase is present in Coolidge matrix. CK chondrites are not depleted in Na or K. In addition, Coolidge has a finer-grained matrix and exhibits compositional differences in spinel minerals. In contrast to Coolidge, CK chondrites are more highly oxidized as attested by the presence of magnetite with high-Ni sulfides instead of FeNi-metal and troilite. The Coolidge grouplet is also separated by its highly fractionated moderately volatile elements, higher than all other meteorite groups. The finding of Al-rich inclusions unique from other carbonaceous chondrites also supports this classification. Coolidge has a CRE age of 37.7 m.y. The Coolidge specimen shown above is a 1.1 g cut fragment, and the bottom image is an excellent petrographic thin section micrograph of Coolidge, shown courtesy of Peter Marmet. standby for coolidge ts photo
click on image for a magnified view
Photo courtesy of Peter Marmet

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