Acfer 182 and paired samples, an iron-rich carbonaceous chondrite: Similarities with ALH85085 and relationship to CR chondrites

Abstract

Three samples of a new, Fe-rich chondrite were found in the Sahara in 1990 and 1991 (Acfer 182, Acfer 207, Acfer 214). The samples are paired and the meteorite will be designated as Acfer 182. The chondrite is chemically, texturally, and mineralogically similar to the Allan Hills meteorite ALH85085. One important difference between the two meteorites is the smaller average chondrule size in ALH85085. The major components of Acfer 182 (in decreasing abundance) are 1. (1) highly altered (by terrestrial weathering) matrix 2. (2) mineral and polymineralic silicate fragments and aggregates 3. (3) chondrule fragments 4. (4) chondrules 5. (5) metal 6. (6) fine-grained, dark inclusions. The abundance of chondrules is lower and the average chondrule size ( $ ̃ 90 μm) smaller than in most other chondrites. Chondrule fragments are often so large that they cannot be derived from the present chondrule population. Apparently, size sorting has prevented accumulation of the larger parent chondrules. Several spectacular Ca,Al-rich inclusions were found, rich in Ca-dialuminate, hibonite, or Zr-, Y-, Sc-bearing phases. The chemical composition of Acfer 182 and of ALH85085 are almost indistinguishable. Major chemical signatures are 1. (1) uniform enrichment of Fe and other nonvolatile metals relative to CI-chondrites by about 70% 2. (2) absence of enrichment in refractory lithophiles, characteristic of most type 2 and 3 carbonaceous chondrites 3. (3) strong depletion of volatile and moderately volatile elements. Based on the oxygen isotopic composition, the chemical composition, and the abundances of chondrules and matrix, Acfer 182 should be classified as a carbonaceous chondrite. Considering their affinity to carbonaceous chondrites and their high bulk iron content the two meteorites, Acfer 182 and ALH85085, are designated as CH-chondrites. There are mineralogical and chemical similarities among Acfer 182, ALH85085, and CR chondrites which distinguish these meteorites from other types of carbonaceous chondrites: 1. (1) low FeO contents of olivine and pyroxene and correspondingly high metal contents 2. (2) high Cr-content in olivine 3. (3) abundant fine-grained dark inclusions 4. (4) abundant Ca-dialuminate (CaAl 4O 7) in CAIs 5. (5) similarities in oxygen isotopic composition 6. (6) low contents of moderately volatile elements 7. (7) low refractory element contents 8. (8) presence of a unique component of subsolar rare gases. These observations suggest similar conditions of formation for the components of these meteorites. A single common parent body is unlikely in view of the differences in chemical composition and in the size distribution of individual components.