TY - JOUR
T1 - Refractory calcium-aluminum-rich inclusions and aluminum-diopside-rich chondrules in the metal-rich chondrites Hammadah al Hamra 237 and Queen Alexandra Range 94411
AU - Krot, A. N.
AU - McKeegan, K. D.
AU - Russell, S. S.
AU - Meibom, A.
AU - Weisberg, M. K.
AU - Zipfel, J.
AU - Krot, T. V.
AU - Fagan, T. J.
AU - Keil, K.
PY - 2001
Y1 - 2001
N2 - 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.
AB - 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.
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U2 - 10.1111/j.1945-5100.2001.tb01954.x
DO - 10.1111/j.1945-5100.2001.tb01954.x
M3 - Article
AN - SCOPUS:0034782724
VL - 36
SP - 1189
EP - 1216
JO - Meteoritics and Planetary Science
JF - Meteoritics and Planetary Science
SN - 1086-9379
IS - 9
ER -