PECORA-ESCARPMENT-91002 - A MEMBER OF THE NEW RUMURUTI (R) CHONDRITE GROUP

Pecora Escarpment (PCA)91002 is a light/dark-structured chondrite brec cia related to Carlisle Lakes and Rumuruti; the meteorite contains app roximately 10-20 vol% equilibrated (type-5 and -6) clasts within a cla stic groundmass, much of which was metamorphosed to type-3.8 levels. T he olivine compositional distribution forms a tight cluster that peaks at Fa38-40; by contrast, low-Ca pyroxene compositions are highly vari able. Opaque phases identified in PCA91002 and its paired specimen, PC A91241, include pyrrhotite, pentlandite, pyrite, chromite, ilmenite, m etallic Cu and magnetite. The majority of the rock is of shock stage S 3-S4; there are numerous sulfide-rich shock veins and 50-mum plagiocla se melt pockets. Instrumental neutron activation analysis shows that, unlike Carlisle Lakes and ALH85151, PCA91002 exhibits no Ca enrichment or Au depletion; because PCA91002 is relatively unweathered, it seems probable that the Ca and Au fractionations in Carlisle Lakes and ALH8 5151 were caused by terrestrial alteration. The Rumuruti-like (formerl y Carlisle-Lakes-like) chondrites now include eight separate meteorite s. Their geochemical and petrographic similarities suggest that they c onstitute a distinct chondrite group characterized by unfractionated r efractory lithophile abundances (0.95 +/-0.05x CI), high bulk DELTAO-1 7, a low chondrule/groundmass modal abundance ratio, mean chondrule di ameters in the 400 +/- 100 mum range, abundant NiO-bearing ferroan oli vine, sodic plagioclase, titanian chromite, abundant pyrrhotite and pe ntlandite and negligible metallic Fe-Ni. We propose that this group be called R chondrites after Rumuruti, the only fall. The abundant NiO-b earing ferroan olivine grains, the occurrence of Cu-bearing sulfide, a nd the paucity of metallic Fe-Ni indicate that R chondrites are highly oxidized. It is unlikely that appreciable oxidation took place on the parent body because of the essential lack of plausible oxidizing agen ts (e.g., magnetite or hydrated silicates). Therefore, oxidation of R chondrite material must have occurred in the nebula. A few type-I porp hyritic olivine chondrules containing olivine grains with cores of Fa3 -4 composition occur in PCA91002; these chondrules probably formed ini tially as metallic-Fe-Ni-bearing objects at high nebular temperatures. As temperatures decreased and more metallic Fe was oxidized, these ch ondrules accreted small amounts of oxidized material and were remelted . The ferroan compositions of the >5-mum olivine grains in the R chond rites reflect equilibration with fine-grained FeO-rich matrix material during parent body metamorphism.