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

doi:10.1016/0016-7037(93)90422-S

Geochimica et Cosmochimica Acta

Volume 57, Issue 11, June 1993, Pages 2631-2648

https://doi.org/10.1016/0016-7037(93)90422-S Get rights and content

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 (CaAl₄O₇) 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.

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The carbonaceous Bencubbin-like (CB), high-metal (CH), and Renazzo-like (CR) chondrites are metal-rich chondrites that have been suggested to be genetically linked and are sometimes grouped together as the CR chondrite clan. Of these, the CB and CH chondrites are thought to have formed in an impact-generated vapor-melt plume from material that may be isotopically akin to CR chondrites. We report Mo, Ti, Cr, and Hf-W isotopic data for CB and CH chondrites in order to determine their formation time, to assess whether these chondrites are genetically related, and to evaluate their potential link to CR chondrites. An internal Hf-W isochron for the CH chondrite Acfer 182 yields an age of 3.8 ± 1.2 Ma after the formation of Ca-Al-rich inclusions (CAIs), which is indistinguishable from the mean Hf-W model age for CB metal of 3.8 ± 1.3 Ma. The Mo isotopic data for CB and CH chondrites indicate that both contain some of the same metal and silicate components, which themselves are isotopically distinct. As such, the different Mo isotopic compositions of bulk CB and CH chondrites reflect their distinct metal-to-silicate ratios. CR metal exhibits the same Mo isotopic composition as CB and CH metal, but CR silicates have distinct Mo and Ti isotopic compositions compared to CB and CH silicates, indicating that CB/CH chondrites may be genetically related to CR metal, but not to CR silicates. Together, the new isotopic data are consistent with formation of CB and CH chondrites in different regions of a common impact-generated vapor-melt plume and suggest that the CB and CH metal may derive from a metal-rich precursor genetically linked to CR chondrites. The Hf-W systematics of CH and CB chondrites indicate that the impact occurred at 3.8 ± 0.8 Ma after the formation of Ca-Al-rich inclusions and, hence, up to ∼1 Ma earlier than previously inferred based on Pb-Pb chronology.
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