The Rumuruti chondrite group

Since 1994, the Rumuruti (R) chondrites have been recognized as a new, well-established chondrite group differing from carbonaceous, ordinary, and enstatite chondrites. The first R chondrite, Carlisle Lakes, was found in Australia in 1977. Meanwhile, the number has increased to 107 (December, 2010). This group is named after the Rumuruti meteorite, the first and so far the only R chondrite fall. Most of the R chondrites are breccias containing a variety of different clasts embedded in a clastic matrix. Some textural and mineralogical characteristics can be summarized as follows: (a) the chondrule abundance in large fragments and in unbrecciated rocks is similar to 35-50 vol%; (b) Ca,Al-rich inclusions are rare; (c) the olivine abundance is typically 65-78 vol%; (d) the mean chondrule diameter is similar to 400 mu m; (e) in unequilibrated R chondrites, low-Ca pyroxene is dominating, whereas in equilibrated R chondrites it is Ca-rich pyroxene; (f) the typical olivine in a metamorphosed lithology is similar to Fa(38-40) (g) matrix olivine in unequilibrated, type 3 fragments and rocks has much higher Fa (similar to 45-60 mol%) compared to matrix olivines in type 4-6 lithologies (similar to Fa(38-41)); (h) spinels have a high TiO2 of similar to 5 wt%; (i) abundant different noble metal-bearing phases (metals, sulfides, tellurides, arsenides) occur. The exception is the metamorphosed, type 5/6 R chondrite La Paz Icefield 04840 which contains hornblende, phlogopite, and Ca-poor pyroxene, the latter phase typically occurring in low-grade metamorphosed R chondrites only. In bulk composition, R chondrites have some affinity to ordinary chondrites: (a) the absence of significant depletions in Mn and Na in R chondrites and ordinary chondrites is an important feature to distinguish these groups from carbonaceous chondrites; (b) total Fe (similar to 24 wt%) of R chondrites is between those of H and L chondrites (27.1 and 21.6 wt%, respectively); (c) the average Cl/Mg-normalized lithophile element abundances are similar to 0.95 x CI, which is lower than those for carbonaceous chondrites (>= 1.0 x CI) and slightly higher than those for ordinary chondrites (similar to 0.9 x CI); (d) trace element concentrations such as Zn (similar to 150 ppm) and Se (similar to 15 ppm) are much higher than in ordinary chondrites; (e) the whole rock Delta O-17 of similar to 2.7 for R chondrites is the highest among all meteorite groups, and the mean oxygen isotope composition is delta O-17 = 5.36 +/- 0.43, delta O-18 = 5.07 +/- 0.86, Delta O-17 = +2.72 +/- 0.31; (f) noble gas cosmic ray exposure ages of R chondrites range between similar to 0.1 and similar to 70 Ma. More than half of the R chondrites analyzed for noble gases contain implanted solar wind and, thus, are regolith breccias. The 43 R chondrites from Northern Africa analyzed so far for noble gases seem to represent at least 16 falls. Although the data base is still scarce, the data hint at a major collision event on the R chondrite parent body between 15 and 25 Ma ago. (C) 2011 Elsevier GmbH. All rights reserved.