Petrography and isotopic studies of refractory inclusions and Al‐rich chondrules in Semarkona, <scp>ALHA81251</scp> , and Chainpur unequilibrated ordinary chondrites

Abstract Ordinary, enstatite, and Rumuruti type have the lowest abundance of refractory inclusions amongst chondritic meteorites. Calcium‐aluminum‐rich inclusions (CAIs) within these are hallmarked by a relatively small average diameter of ~45 μm (size range 4–382 μm). One CAI, one amoeboid olivine aggregate (AOA), one spinel‐bearing chondrule, and two aluminum‐rich chondrules from Semarkona (LL3.00) along with one CAI each from Allan Hills (ALHA) 81251 (LL3.2) and Chainpur (LL3.4) were identified following an extensive search. These objects were studied for their petrography, mineral chemistry, relative ( 26 Al) chronology, and three oxygen isotopic compositions. The initial 26 Al/ 27 Al ratio of (4.96 ± 0.14) × 10 −5 (2 σ ) in a type A CAI in Chainpur, the largest size (1500 × 1200 μm) found so far in the noncarbonaceous (ordinary) chondrites, forming in an 16 O‐rich early solar system reservoir (Δ 17 O = −24‰) is consistent with previous studies. The Chainpur CAI 1 has a Wark–Lovering rim, the first reported case within the noncarbonaceous chondrites. The hibonite–pyroxene spherule in ALHA81251 (CAI 1) is the first reported case of a hibonite–pyroxene spherule in the ordinary chondrites of these rare objects (~12 known so far) within meteorites. The hibonite–pyroxene spherule in ALHA81251 has a low abundance of 26 Al/ 27 Al ratio of (1.2 ± 0.6) × 10 −5 with Δ 17 O of ~ −14.5‰ ± 2.0‰. An olivine‐phyric Al‐rich chondrule in Semarkona (Ch 54) formed at ~0.9 Ma with Δ 17 O of ~0‰, while Semarkona (Ch 44) formed in a relatively 16 O‐rich reservoir with Δ 17 O of ~ −2.0‰. The spinel‐bearing chondrule in Semarkona (Ch 205) shows no resolved excess in Δ 26 Mg and has a planetary‐like oxygen isotopic composition. Oxygen isotope composition and 26 Al‐ 26 Mg relative chronology of these objects confirm their origin and evolution under cosmochemical conditions similar to their “typical” carbonaceous kindred and extend the knowledge of the cosmochemical environment in the early solar system.