Refractory calcium-aluminum-rich inclusions and aluminum-diopside-rich chondrules in the metal-rich chondrites Hammadah al Hamra 237 and Queen Alexandra Range 94411

The metal-rich chondrites Hammadah al Hamra (HH) 237 and Queen Alexandra Ra nge (QUE) 94411, paired with QUE 94627, contain relatively rare (<1 vol%) c alcium-aluminum-rich inclusions (CAIs) and Al-diopside-rich chondrules. For ty CAIs and CAI fragments and seven Al-diopside-rich chondrules were identi fied in HH 237 and QUE 94411/94627. The CAIs, <similar to>50-400 mum in app arent diameter, include (a) 22 (56%) pyroxene-spinel +/- melilite (+forster ite rim), (b) 11 (28%) forsterite-bearing, pyroxene-spinel +/- melilite +/- anorthite (+forsterite rim) (c) 2 (5%) grossite-rich (+spinel-melilite-pyr oxene rim), (d) 2 (5%) hibonite-melilite (+spinel-pyroxene +/- forsterite r im), (e) 1 (2%) hibonite-bearing, spinel-perovskite (+melilite-pyroxene rim ), (f) 1 (2%) spinel-melilite-pyroxene-anorthite, and (g) 1 (2%) amoeboid o livine aggregate. Each type of CAI is known to exist in other chondrite gro ups, but the high abundance of pyroxene-spinel melilite CAIs with igneous t extures and surrounded by a forsterite rim are unique features of HH 237 an d QUE 94411/94627. Additionally, oxygen isotopes consistently show relative ly heavy compositions with Delta O-17 ranging from -6 parts per thousand to -10 parts per thousand (1 sigma = 1.3 parts per thousand) for all analyzed CAI minerals (grossite, hibonite, melilite, pyroxene, spinel). This sugges ts that the CAIs formed in a reservoir isotopically distinct from the reser voir(s) where "normal", O-16-rich (Delta O-17 < -20<parts per thousand>) CA Is 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-diops ide grains enclosing oriented inclusions of forsterite, and interstitial an orthitic mesostasis and Al-rich, Ca-poor pyroxene, occasionally enclosing s pinel and forsterite. These chondrules are mineralogically similar to the A l-rich barred-olivine chondrules in HH 237 and QUE 94411/94627, but have lo wer 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 c hondrules have O-isotopic compositions (Delta O-17 approximate to -7 +/- 1. 1 parts per thousand) 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 94411/94627 was analyzed and found to have Delta O-17 approximate to -3 +/- 1.4 parts per thousand. The characteristics of the CAIs in HH 237 and QUE 94411/94627 are inconsist ent with an impact origin of these metal-rich meteorites. Instead they sugg est that the components in CB chondrites are pristine products of large-sca le, high-temperature processes in the solar nebula and should be considered bonafide chondrites.