The metal-rich chondrites Hammadah al Hamra (HH) 237 and Queen Alexandra Range (QUE) 94411, paired with QUE 94627, contain relatively rare (<1 vol%) calcium-aluminum-rich inclusions (CAIs) and Al-diopside-rich chondrules. Forty CAIs and CAI fragments and seven Al-diopside-rich chondrules were identified in HH 237 and QUE 94411/94627. The CAIs, ∼50-400 μm in apparent diameter, include (a) 22 (56%) pyroxene-spinel ± melilite (+forsterite rim), (b) 11 (28%) forsterite-bearing, pyroxene-spinel ± melilite ± anorthite (+forsterite rim) (c) 2 (5%) grossite-rich (+spinel-melilite-pyroxene rim), (d) 2 (5%) hibonite-melilite (+spinel-pyroxene ± forsterite rim), (e) 1 (2%) hibonite-bearing, spinel-perovskite (+melilite-pyroxene rim), (f) 1 (2%) spinel-melilite-pyroxene-anorthite, and (g) 1 (2%) amoeboid olivine aggregate. Each type of CAI is known to exist in other chondrite groups, but the high abundance of pyroxene-spinel ± melilite CAIs with igneous textures and surrounded by a forsterite rim are unique features of HH 237 and QUE 94411/94627. Additionally, oxygen isotopes consistently show relatively heavy compositions with △ 17O ranging from-6‰ to-10‰ (1σ = 1.3‰) for all analyzed CAI minerals (grossite, hibonite, melilite, pyroxene, spinel). This suggests that the CAIs formed in a reservoir isotopically distinct from the reservoir(s) where "normal", 16O-rich (△17O < -20‰) CAIs in most other chondritic meteorites formed. The A1-diopside-rich chondrules, which have previously been observed in CH chondrites and the unique carbonaceous chondrite Adelaide, contain A1-diopside grains enclosing oriented inclusions of forsterite, and interstitial anorthitic mesostasis and A1-rich, Ca-poor pyroxene, occasionally enclosing spinel and forsterite. These chondrules are mineralogically similar to the A1-rich barred-olivine chondrules in HH 237 and QUE 94411/94627, but have lower Cr concentrations than the latter, indicating that they may have formed during the same chondrule-forming event, but at slightly different ambient nebular temperatures. Aluminum-diopside grains from two A1-diopside-rich chondrules have O-isotopic compositions (△17O ≈ -7 ± 1.1‰) similar to CAI minerals, suggesting that they formed from an isotopically similar reservoir. The oxygen-isotopic composition of one Ca, A1-poor cryptocrystalline chondrule in QUE 94411/94627 was analyzed and found to have △17O ≈ -3 ± 1.4‰. The characteristics of the CAIs in HH 237 and QUE 94411/94627 are inconsistent with an impact origin of these metal-rich meteorites. Instead they suggest that the components in CB chondrites are pristine products of large-scale, high-temperature processes in the solar nebula and should be considered bona fide chondrites.
|ジャーナル||Meteoritics and Planetary Science|
|出版物ステータス||Published - 2001 1 1|
ASJC Scopus subject areas
- Space and Planetary Science