Bulk Compositions and Petrographic Characteristics of Ten Unusual Carbonaceous Chondrites

We used INAA to determine the bulk compositions of 10 carbonaceous chondrites of uncertain classification. Most samples were analyzed in duplicates and the averaged results are given in Table 1 and shown in Figs. 1 and 2. Thin sections were examined microscopically. Mgand Cr-normalized abundance patterns show clear identifications for four specimens (Fig. 1. The others have nonindicative element distributions or plot between established groups (Fig. 2). Dar al Gani 055 (451g) and the probably paired specimen Dar al Gani 056 (703g) contain many mm-size chondrules with coarse-grained rims. Opaques consist mainly of metallic Fe-Ni and sulfide. The rock texturally resembles a reduced CV chondrite (e.g., Efremovka) except that available thin sections of DaG 055 & 056 do not exhibit a petrofabric. The O-isotopic composition of DaG 055 [1] lies within the CO field and near the CV field. The bulk compositions of DaG 055 and 056 are very similar, consistent with pairing. The CIand Cr-normalized refractory lithophile element abundances in these rocks (1.39×CI) are in the CV-CK chondrite range (1.48 & 1.34×CI, resp.). Siderophile and chalcophile abundances are CV like except for Os and Ir which are slightly elevated. The meteorite can be considered as a reduced CV3 chondrite. PCA 91008 (51.7g) appears to be a moderately altered CM2 chondrite with forsteritic olivine (Fa1-2). Refractory lithophile abundances in PCA 91008 (1.22×CI) are consistent with a CM classification (1.21×CI) as are the abundances of Fe, volatile siderophiles and chalcophiles. Refractory (Os, Ir) and some common (Ni, Co) siderophiles are low, probably due to weathering. Tonui et al. [2,3] concluded that this rock was thermally metamorphosed because of the absence of tochilinite and carbonates, the textural integration of chondrules and matrix, and the high analytical totals of matrix serpentines. The loss of some mobile trace elements corresponds to metamorphic temperatures of 500-600 ̊C [2]. LEW 85311 (199.5g) appears to be a CM2 chondrite that experienced moderately low degrees of aqueous alteration. Metallic Fe-Ni constitutes ~2 vol.% of the rock and occurs both in chondrules and the matrix. Its O-rich composition [1] lies outside the main CM field, but is consistent with LEW 85311 being a CM that experienced low degrees of aqueous alteration. The abundances of refractory lithophiles (1.34×CI) are higher than CM; moderately volatile lithophiles, refractory, common and volatile siderophiles, and chalcophiles are in the CM range. Hammadah al Hamra 073 (569g) is a very weathered type-4 carbonaceous chondrite. It contains equilibrated olivine (Fa17.6-18.2) and moderately equilibrated low-Ca pyroxene (Fs5.7-11.3). Weckwerth and Weber [4] suggested that HH073 is the third member of the CoolidgeLoongana 001 (C-L) grouplet, even though the latter meteorites have more magnesian olivines (C: Fa13.4-13.8; L: Fa10.7-12.9 [5]). The O-isotopic composition of HH073 is within the CO field and lies close to a mass-fractionation line through Coolidge [1]; some of the differences in O isotopes may be attributable to the greater degree of weathering of HH073. This is also reflected by the elevated Na and As. Refractory lithophile abundances in HH073 (1.59×CI) are higher than those in C-L (1.40×CI) or CV chondrites; HH073 also has higher abundances of siderophiles and chalcophiles than C-L or CV. We conclude that HH073 is probably related to C-L, but membership in the grouplet cannot yet be established. Sahara 00182 (70 g) has large chondrules with irregular rims; many chondrules have coarse metal grains in their interiors and at their edges. The CIand Cr-norm-