|Title:||Refractory calcium‐aluminum‐rich inclusions and Al‐diopside‐rich chondrules in the metal‐rich chondrites Hammadah al Hamra 237 and Queen Alexandra Range 94411|
|Authors:||A.N. Krot, K.D. McKeegan, S.S.Russell, A. Meibom, M. K. Weisberg, J. Zipfel, T. V. Krot, T. J. Fagan, and K. Keil|
|Publication:||Meteorit. Planet. Sci., v. 36, p. 1189‐1216.|
|Publish Date:||Sep 2001|
The metal‐rich chondrites Hammadah al Hamra 237 (HH 237), and QUE 94411 (QUE), 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. 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. Additionally, oxygen isotopes consistently show relatively heavy compositions with Δ17O 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 (Δ17O < −20‰) CAIs in most other chondritic meteorites formed. The Al‐diopside‐rich chondrules, which have previously been observed in CH chondrites and the unique carbonaceous chondrite Adelaide, contain Al‐diopside grains enclosing oriented inclusions of forsterite, and interstitial anorthitic mesostasis and Al‐rich, Ca‐poor pyroxene, occasionally enclosing spinel and forsterite. These chondrules are mineralogically similar to the Al‐rich barred‐olivine chondrules in HH 237 and QUE, 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 Al‐diopside‐rich chondrules have O‐isotopic compositions (Δ17O ≈ −7± 1.1‰) similar to CAI minerals, suggesting that they formed from an isotopically similar reservoir. The oxygen isotopic composition of one Ca, Al−poor cryptocrystalline chondrule in QUE was analyzed and found to have Δ17O ≈ −3 ± 1.4‰. The characteristics of the CAIs in HH 237 and QUE 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.