Mineralogy, petrography, bulk chemical, iodine-xenon, and oxygen-isotopic compositions of dark inclusions in the reduced CV3 chondrite Efremovka

We studied the petrography, mineralogy, bulk chemical, I-Xe, and O-isotopic compositions of three dark inclusions (E39, E53, and E80) in the reduced C V3 chondrite Efremovka. They consist of chondrules, calcium-aluminum-rich i nclusions (CAIs), and fine-grained matrix. Primary minerals in chondrules a nd CAIs are pseudomorphed to various degrees by a mixture largely composed of abundant (>95%), fine-grained (>0.2 mu m) fayalitic olivine (Fa(35-42)) and minor amounts of chlorite, poorly-crystalline Si-Al-rich material, and chromite; chondrule and CAI shapes and textures are well-preserved. Seconda ry Ca-rich minerals (Ti-andradite, kirschsteinite, Fe-diopside) are common in chondrule pseudomorphs and matrices in E39 and E80. The degree of replac ement increases from E53 to E39 to E80. Fayalitic olivines are heavily stra ined and contain abundant voids similar to those in incompletely dehydrated phyllosilicates in metamorphosed GM and CI chondrites. Opaque nodules in c hondrules consist of Ni- and Co-rich taenite, Go-rich kamacite, and wairaui te; sulfides are rare; magnetite is absent. Bulk O-isotopic compositions of E39 and E53 plot in the field of aqueously altered CM chondrites, close to the terrestrial fractionation line; the more heavily altered E39 is isotop ically heavier than the less altered E53. The apparent I-Xe age of E53 is 5 .4 Ma earlier than Bjurbole and 5.7 +/- 2.0 Ma earlier than E39. The I-Xe d ata are consistent with the most heavily altered dark inclusion, E39 having experienced either longer or later alteration than E53. Bulk lithophile el ements in E39 and E53 most closely match those of CO chondrites, except tha t Ca is depleted and K and As are enriched. Both inclusions are depleted in Se by factors of 3-5 compared to mean CO, CV, CR, or CK chondrites. Zinc i n E39 is lower than the mean of any carbonaceous chondrite groups, but in E 53 Zn is similar to the means in CO, CV, and CK chondrites. The Efremovka dark inclusions experienced various degrees of aqueous altera tion, followed by low degree thermal metamorphism in an asteroidal environm ent. These processes resulted in preferential oxidation of Fe from opaque n odules and formation of Ni- and Go-rich metal, metasomatic alteration of pr imary minerals in chondrules and CAIs, and the formation of fayalitic olivi ne and secondary Ga-Fe-rich minerals. Based on the observed similarities of the alteration mineralization in the Efremovka and Allende dark inclusions , we infer that the latter may have experienced similar alteration processe s.