The CR (Renazzo-type) carbonaceous chondrite group and its implications

Abstract

A petrologic, geochemical and oxygen isotopic study of eight meteorites, identified as CR (Renazzo-type) chondrites, reveals that they have a set of similar properties that distinguishes them from other chondrite groups and confirms CR as a carbonaceous chondrite group. Characteristics of the CR group include 1. (1) abundant large multilayered, Fe.Ni metal-rich, type I chondrules, 2. (2) abundant matrix and dark inclusions (DI) that contain microchondrules, microfragments and magnetite framboids and platelets, 3. (3) unique assemblages of serpentine- and chlorite-rich phyllosilicates and Ca-carbonates, 4. (4) Ca-carbonate rims on chondrules, 5. (5) abundant Fe,Ni metal with a positive Ni vs. Co trend and a solar Ni:Co ratio, 6. (6) low abundance of refractory-rich inclusions, 7. (7) amoeboid olivine aggregates (AOA) with Mn-rich and Mn-poor forsterite and 8. (8) oxygen isotopic compositions that form a unique mixing line. The CR chondrites are breccias consisting of two major components: the large layered chondrules and the matrix (+ dark inclusions). Al Rais is an anomalous member in having a higher abundance of the dark inclusion component (>70 vol.%) and this accounts for its having a higher volatile content than other members of the group. The conditions that prevailed during aqueous alteration of the CR chondrites differed from those of other hydrated chondrite groups and may be the result of a higher temperature environment than that for the CI and CM chondrites. The CR chondrite components (chondrules, matrix, dark inclusions, etc.) experienced aqueous alteration to differing degrees prior to assembly of the host chondrites and dark inclusions represent a lithology that was altered by water to a greater degree than the rest of the meteorite. The overall degree of hydration in CR chondrites varies among meteorites, with Al Rais being the most hydrated. The oxygen isotopic variation of the whole rocks, chondrules and matrices may be related to degree of hydration, indicating that CR chondrites interacted with a water-rich component having heavy oxygen and that was the same as CI and CM water.