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

摘要

Bulk compositional and petrographic data clearly define the new R (Rumuruti) group of chondrites consisting of Rumuruti (the only fall), ALH85151, 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 PCA91241 are paired. The Yamato specimens are probably not paired. The matrices of the known R chondrites have experienced similar, minor degrees of metamorphism; petrographic types are 3.8-3.9 with the exception of ALH85151, 3.6. All except Carlisle Lakes contain equilibrated (R5-R6) clasts. Petrographically, the R chondrites are characterized by a low chondrule/matrix modal abundance ratio, high states of oxidation (reflected by abundant NiO-bearing olivine with Fa37-40), relatively small chondrules (mean apparent diameters of similar to 400 mu m), abundant (up to similar to 11 wt%) sulfides (mainly pyrrhotite and pentlandite), and negligible amounts of metallic Fe-Ni. Refractory lithophile abundances are similar to 0.95 x CI, intermediate between those in ordinary chondrites (OC) and CI chondrites. Abundances of the volatile elements Se and Zn are greatly enhanced relative to OC. The R chondrites are clearly 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 PCA91241 have light/dark dark structures and solar-wind-implanted rare gases indicating that they are regolith breccias. The Yamato specimens also have light/dark structures and are inferred to be regolith breccias. Carlisle Lakes lacks solar rare gases and is unbrecciated. Based on similarities in refractory lithophile abundances (less than or equal to 0.95 x CI), oxygen isotope compositions (Delta(17)O greater than or equal to 0), and refractory inclusion abundances (<0.1 vol%; none have been reported), the R chondrites probably belong to a noncarbonaceous superclan of chondrites that also includes ordinary and enstatite chondrites. The high oxidation state, high matrix/chondrule modal abundance ratio, relatively low abundance of droplet chondrules, and high Delta(17)O composition suggest that the R chondrites formed at greater heliocentric distances than the OC. [References: 76]