Petrology, mineralogy, and oxygen isotope compositions of aluminum‐rich chondrules from CV 3 chondrites

Bulk major element composition, petrography, mineralogy, and oxygen isotope compositions of twenty Al‐rich chondrules ( ARC s) from five CV 3 chondrites (Northwest Africa [ NWA ] 989, NWA 2086, NWA 2140, NWA 2697, NWA 3118) and the Ningqiang carbonaceous chondrite were studied and compared with those of ferromagnesian chondrules and refractory inclusions. Most ARC s are marginally Al‐richer than ferromagnesian chondrules with bulk Al 2 O 3 of 10–15 wt%. ARC s are texturally similar to ferromagnesian chondrules, composed primarily of olivine, pyroxene, plagioclase, spinel, Al‐rich glass, and metallic phases. Minerals in ARC s have intermediate compositions. Low‐Ca pyroxene (Fs 0.6–8.8 Wo 0.7–9.3 ) has much higher Al 2 O 3 and TiO 2 contents (up to 12.5 and 2.3 wt%, respectively) than that in ferromagnesian chondrules. High‐Ca pyroxene (Fs 0.3–2.0 Wo 33–54 ) contains less Al 2 O 3 and TiO 2 than that in Ca,Al‐rich inclusions ( CAI s). Plagioclase (An 77–99 Ab 1–23 ) is much more sodic than that in CAI s. Spinel is enriched in moderately volatile element Cr (up to 6.7 wt%) compared to that in CAI s. Al‐rich enstatite coexists with anorthite and spinel in a glass‐free chondrule, implying that the formation of Al‐enstatite was not due to kinetic reasons but is likely due to the high Al 2 O 3 /CaO ratio (7.4) of the bulk chondrule. Three ARC s contain relict CAI s. Oxygen isotope compositions of ARC s are also intermediate between those of ferromagnesian chondrules and CAI s. They vary from −39.4‰ to 13.9‰ in δ 18 O and yield a best fit line (slope = 0.88) close to the carbonaceous chondrite anhydrous mineral ( CCAM ) line. Chondrules with 5–10 wt% bulk Al 2 O 3 have a slightly more narrow range in δ 18 O (−32.5 to 5.9‰) along the CCAM line. Except for the ARC s with relict phases, however, most ARC s have oxygen isotope compositions (>−20‰ in δ 18 O) similar to those of typical ferromagnesian chondrules. ARC s are genetically related to both ferromagnesian chondrules and CAI s, but the relationship between ARC s and ferromagnesian chondrules is closer. Most ARC s were formed during flash heating and rapid cooling processes like normal chondrules, only from chemically evolved precursors. ARC s extremely enriched in Al and those with relict phases could have had a hybrid origin (Krot et al. 2002) which incorporated refractory inclusions as part of the precursors in addition to ferromagnesian materials. The occurrence of melilite in ARC s indicates that melilite‐rich CAI s might be present in the precursor materials of ARC s. The absence of melilite in most ARC s is possibly due to high‐temperature interactions between a chondrule melt and the solar nebula.