Oxygen isotope ratios of FeO-poor chondrules in CR3 chondrites: Influence of dust enrichment and H2O during chondrule formation

We present detailed electron microprobe analyses and oxygen three-isotope measurements by high precision secondary ion mass spectrometry on 45 type I (FeO-poor) chondrules/fragments and 3 type II (FeO-rich) chondrule fragments from Meteorite Hills 00426 and Queen Alexandra Range 99177, two of the most primitive CR3 chondrites. Type I chondrules/fragments have Mg#'s (defined as the Mg# of constituent olivine and/or low-Ca pyroxene) ranging from 94.2 to 99.2; type II chondrule fragments have Mg#'s of 53-63. Oxygen three-isotope measurements plot on the slope ∼1 primitive chondrule mineral (PCM) line. Within chondrules, Δ17O (=δ17O-0.52 × δ18O) values of coexisting olivine, pyroxene, and plagioclase are homogeneous, with propagated uncertainties of 0.3‰. This indicates each phase crystallized from the final chondrule melt, and that efficient oxygen isotope exchange occurred between ambient gas and chondrule melt. Among type I chondrules there is a well-defined increase in Δ17O, from -5.9‰ to ∼−1‰, as Mg#'s decrease from 99.2 to ∼96; type II chondrule fragments are comparatively 16O-poor (Δ17O: ∼0.2-0.6‰). The relationship between Mg# and Δ17O among type I chondrules confirms that addition of a 16O-poor oxidizing agent to the highest Mg# chondrule precursors resulted in forming lower Mg# CR chondrules. Using aspects of existing equilibrium condensation models and a mass balance we estimate that type I CR chondrules formed at dust enrichments of 100-200×, from dusts with 0-0.8 times the atomic abundance of ice, relative to CI dust. The type II chondrule fragments are predicted to have formed at CI dust enrichments near 2500×.