Iron isotopes in chondrules: Implications for the role of evaporation during chondrule formation

We have measured the delta Fe-57 of olivines in nine Chainpur chondrules. A ll are within error of normal (typically 2 sigma less than or equal to 1-2% o). Most of the chondules could not have lost more than similar to 20% of t heir FeO by Rayleigh evaporation and none can have lost more than similar t o 61%. Yet, the range of Fo contents in these chondrules is Fo(78-99.9) The isotopic compositions of the chondrules clearly demonstrate that, for inst ance, type I chondrules cannot form from type II chondrules by evaporation of FeO under Rayleigh conditions. The isotopic compositions also place cons traints on the minimum cooling rates these chondrules could have experience d. These cooling rates must also be equal to or slower than those required to produce the chondrule textures. Assuming flash heating and evaporation r ates like those measured in vacuum, the minimum cooling rates necessary to prevent detectable Fe isotopic fractionation via Rayleigh evaporation appro ach those needed to produce barred and porphyritic textures. The presence o f hydrogen in the nebula, non-linear cooling and other effects will all ten d to increase the cooling rates required to prevent delta Fe-57 > 1-2 parts per thousand, perhaps by as much as 1-2 orders of magnitude. The two most likely ways that the cooling rates required to prevent delta Fe-57 > 1-2%o, can be kept below those needed to produce barred and porphyritic textures are (1)the pH(2) in the nebula was low enough to keep evaporation rates clo se to those in vacuum, or (2) back reaction of chondrules with Fe in the ga s suppressed isotopic fractionation.