ACFER 217-A NEW MEMBER OF THE RUMURUTI CHONDRITE GROUP (R)

Previously, three meteorites from Australia and Antarctica were descri bed as a new chondritic ''grouplet'' (Carlisle Lakes, Allan Hills (ALH ) 85151, Yamato (Y)-75302; Rubin and Kallemeyn, 1989). This grouplet w as classified as the ''Carlisle Lakes-type'' chondrites (Weisberg et a l., 1991). Recently, one Saharan sample and four more Antarctic meteor ites were identified to belong to this group (Acfer 217, Y-793575, Y-8 2002, PCA91002, PCA91241). The latter two are probably paired. With th e meteorite Rumuruti, the first fall of this type of chondrite is know n (Schulze et al., 1994). We report here on the Saharan meteorite Acfe r 217 which has chemical and mineralogical properties very similar to Rumuruti and Carlisle Lakes. All eight members of this group, Rumuruti , Carlisle Lakes, ALH85151, Y-75302, Y-793575, Y-82002, Acfer 217, and the paired samples PCA91002 and PCA91241 justify the introduction of a new group of chondritic meteorites, the Rumuruti meteorites (R). Acf er 217 is a regolith breccia consisting of up to cm-sized clasts (almo st-equal-to 33 vol%) embedded in a fine-grained, well-lithified clasti c matrix. The most abundant mineral is olivine (almost-equal-to 72 vol %), which has a high Fa-content of 37-39 mol%. The major minerals (oli vine, low-Ca pyroxene, Ca-pyroxene, and plagioclase) show some composi tional variability indicating a slightly unequilibrated nature of the meteorite. Considering the mean olivine composition of Fa37.8 +/- 5.7, a classification of Acfer 217 as a R3.8 chondrite would result; howev er, Acfer 217 is a regolith breccia consisting of clasts of various pe trologic types. Therefore, we suggest to classify Acfer 217 as a R3-5 chondrite regolith breccia. The bulk meteorite is very weakly shocked (S2). The bulk composition of Acfer 217 and other R-meteorites show th at the R-meteorites are basically chondritic in composition. The patte rn of moderately volatile elements is unique in R chondrites; Na and M n are essentially undepleted, similar to ordinary chondrites, while Zn and Se contents are similar to concentrations in CM chondrites. The o xygen isotopic composition in Acfer 217 is similar to that of Rumuruti , Carlisle Lakes, ALH 8515 1, and Y-75302. In a deltaO-17 vs. deltaO-1 8-diagram, the R-meteorites form a group well resolved from other chon drite groups. Acfer 217 was a meteoroid of common size with a radius b etween 15-65 cm and with a single stage exposure history. Based on Ne- 21, an exposure age of about 35 Ma was calculated.