Acfer 217‐A new member of the Rumuruti chondrite group (R)

Abstract

Abatract-F'reviously, three meteorites from Australia and Antarctica were described as a new chondritic "grouplet" ( Acfer 217 is a regolith breccia consistimg of up to cm-sized clasts (=33 ~01%) embedded in a fme-grained, well-limed clastic matrix. The most abundant mineral is olivine (a72 vol%), which has a high Facontent of 37-39 mol%. The major minerals (olivine, Iow-Ca pyroxene, Ca-pyroxene, and plagioclase) show some compositional variability indicating a slightly unequilibrated nature of the meteorite.Considering the mean olivine composition Of Fa37.8 * 5.7, a classification of M e r 217 as a R3.8 chondrite would result; however, Acfer 217 is a regolith breccia consisting of clasts of various petrologic 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 that the R-meteorites are basically chondritic in composition. The pattern of moderately volatile elements is unique in R chon&, Na and Mn are essentially undepleted, similar to ordinary chondrites, while Zn and Se contents are similar to concentrations in CM chondrites. The oxygen isotopic composition in Acfer 217 is similar to that of Rumwuti, Carlisle Lakes, AL.H 85 15 1, and Y-75302. In a 6 170 vs. 61 8O-diagram, the R-meteorites form a group well resolved h m other chondrite groups.Acfer 2 17 was a meteoroid of common size with a radius between 15 -65 cm and with a single stage exposure history. Based on 21Ne, an exposure age of about 35 Ma was calculated.

39Ar‐40Ar chronology of R chondrites

“…Cosmic rays penetrate to depths equivalent to 40 g/cm 2 (Eugster 1988). Bischoff et al (1994) suggested that the cosmogenic nuclide ratios for Acfer 217 and Carlisle Lakes are consistent with exposure in relatively small (15-65 cm or 25 cm, respectively) radius bodies that experienced only a single-stage exposure history. These rocks were apparently well shielded from cosmic rays until they were ejected from the R chondrite parent body.…”

“…However, individual clasts within the meteorites preserve higher shock grades. For example, the matrix and bulk of Rumuruti and Acfer 217 have shock grades of S2, but some fragments have olivine with planar fractures typical of shock stage S3 (Schulze et al 1994;Bischoff et al 1994). The majority of PCA 91002 is of shock stages S3-S4, but a range of shock grades, from S2-S5 (S5 = maskelynite), is preserved in its coarse-grained olivine grains (Rubin and Kallemeyn 1994).…”

Expanding the application of the Eu‐oxybarometer to the lherzolitic shergottites and nakhlites: Implications for the oxidation state heterogeneity of the Martian interior

“…The fO 2 of lunar magmas is low and relatively constant at iron-wüstite (IW−0.5 ± 1.0) as indicated by the fairly ubiquitous presence of Fe-rich metal (Sato et al 1973;Usselman and Lofgren 1976;Haggerty 1978;Sato 1978;Nicholis and Rutherford 2005). With the notable exception of the Martian meteorites, the R chondrites (Nakamura et al 1993;Bischoff et al 1994;Rubin and Kallemeyn 1994;Schulze et al 1994;Kallemeyn et al 1996), and some carbonaceous chondrite groups (e.g., Larimer and Wasson 1988;Brearley and Jones 1998), most meteorite parent bodies also have compositional characteristics (Femetal phases) requiring they crystallized at fO 2 values of IW or below (e.g., eucrites : Stolper 1977, acapulcoites and lodranites: Benedix andLauretta 2006, ureilites: Walker andGrove 1993).…”

Determination of the Specific Activities of the Cosmogenic Nuclides 10Be and 26Al in Meteorites from the Sahara by Means of Accelerator Mass Spectrometry

“…Acfer 217 classified as a member of the Rumurutitype chondrite group (R) is another interesting sample that has been discussed in more detail previously [19]. Therefore here we present only the conclusions which can be made on the basis of model calculations for Acfer 217: The concentrations of all cosmogenic nuclides can be consistently interpreted only if a preatmospheric radius between 15 and 65 cm, a Single stage exposure history and a terrestrial age low compared to the half-life of are assumed.…”

“…Table 3 contains the results of determinations via y-y-coincidence spectrometry of some ordinary chondrites from the Acfer region, which predominantly were performed for a comparison with results of AMS measurements. The Classification of the meteorites indicated in the tables was done by Bischoff et al [17], For some of the meteorites, exposure ages based on ^'Ne determinations [10,18,19] are available. These ages are included in the last column of Tables 1 and 2.…”