THE COMPOSITIONAL CLASSIFICATION OF CHONDRITES .7. THE R-CHONDRITE GROUP

Bulk compositional and petrographic data clearly define the new R (Rum uruti) group of chondrites consisting of Rumuruti (the only fall), ALH 85151, Acfer 217, Carlisle Lakes, Dar al Gani 013, PCA91002, PCA91241, Y-75302, Y-793575, and Y-82002. Compositional, petrographic, rare-gas , and O-isotopic data strongly suggest that PCA91002 and PCA98241 are paired. The Yamato specimens are probably not paired. The matrices of the known R chondrites have experienced similar, minor degrees of meta morphism; petrographic types are 3.8-3.9 with the exception of ALH8515 1, 3.6. All except Carlisle Lakes contain equilibrated (R5-R6) clasts. Petrographically, the R chondrites are characterized by a low chondru le/matrix modal abundance ratio, high states of oxidation (reflected b y abundant NiO-bearing olivine with Fa37-40), relatively small chondru les (mean apparent diameters of similar to 400 mu m), abundant (up to similar to 11 wt%) sulfides (mainly pyrrhotite and pentlandite), and n egligible amounts of metallic Fe-Ni. Refractory lithophile abundances are similar to 0.95 x CI, intermediate between those in ordinary chond rites (OC) and CI chondrites. Abundances of the volatile elements Se a nd Zn are greatly enhanced relative to OC. The R chondrites are clearl y distinguished from other chondrite groups on the basis of Al/Mn and Zn/Mn abundance ratios. The oxygen isotopic data plot roughly along a slope-1/2 line, with whole-rock Delta(17)O values higher than for any other chondrite group. Rumuruti, Acfer 217, ALH85151, PCA91002, and PC A91241 have light/dark dark structures and solar-wind-implanted rare g ases indicating that they are regolith breccias. The Yamato specimens also have light/dark structures and are inferred to be regolith brecci as. Carlisle Lakes lacks solar rare gases and is unbrecciated. Based o n similarities in refractory lithophile abundances (less than or equal to 0.95 x CI), oxygen isotope compositions (Delta(17)O greater than o r equal to 0), and refractory inclusion abundances (<0.1 vol%; none ha ve been reported), the R chondrites probably belong to a noncarbonaceo us superclan of chondrites that also includes ordinary and enstatite c hondrites. The high oxidation state, high matrix/chondrule modal abund ance ratio, relatively low abundance of droplet chondrules, and high D elta(17)O composition suggest that the R chondrites formed at greater heliocentric distances than the OC.