Oxygen isotopic constraints on the origin of magnesian chondrules and on the gaseous reservoirs in the early Solar System

Abstract: We report in situ ion microprobe analyses of the oxygen isotopic composition of the major silicate phases (olivine, low-Ca pyroxene, silica, and mesostasis) of 37 magnesian porphyritic (type I) chondrules from CV (Vigarano USNM 477-2, Vigarano UH5, Mokoia, and Efremovka) and CR (EET 92042, EET 92147, EET 87770, El Djouf 001, MAC 87320, and GRA 95229) carbonaceous chondrites. In spite of significant variations of the modal proportions of major mineral phases in CR and CV chondrules, the same isotopic characteristics are observed: (i) olivines are isotopically homogeneous at the ‰ level within a chondrule although they may vary significantly from one chondrule to another, (ii) low-Ca pyroxenes are also isotopically homogeneous but systematically 16O-depleted relative to olivines of the same chondrule, and (iii) all chondrule minerals analyzed show 16O-enrichments relative to the terrestrial mass fractionation line, enrichments that decrease from olivine (±spinel) to low-Ca pyroxene and to silica and mesostasis. The observation that, in most of the type I chondrules studied, the coexisting olivine and pyroxene crystals and glassy mesostasis have different oxygen isotopic compositions implies that the olivine and pyroxene grains are not co-magmatic and that the glassy mesostasis is not the parent liquid of the olivine. The δ18O and δ17O values of pyroxene and olivine appear to be strongly correlated for all the studied CR and CV chondrules according to:These correlations are consistent with mass balance of oxygen isotopes calculated in the case of chondrule formation according to the following gas–melt–mineral reactions:These equations imply that 2/3 of the oxygen incorporated by low-Ca pyroxene during its crystallization in the melt is derived from the dissolution of precursor olivine and 1/3 from the addition of chemical components such as SiO from the nebular gas. The composition of the nebular gaseous reservoir in which type I CR and CV chondrules formed is determined from the present data set to be δ18O(gas) ∼3.6±1‰ and δ17O(gas) ∼1.8±1‰. This isotopic composition is in excellent agreement with that of the gaseous reservoir previously inferred from the oxygen isotope systematics of bulk chondrules from various chondrite groups (i.e., CV, CR, unequilibrated ordinary, and EH). Thus, the oxygen isotopic systematic strongly supports the idea that most of the olivines in type I chondrules are relict grains and that gas–melt interaction played a major role in the formation of type I chondrules. [Copyright &y& Elsevier]