The lack of potassium-isotopic fractionation in Bishunpur chondrules

In a search for evidence of evaporation during chondrule formation, the mes ostases of 11 Bishunpur chondrules and melt inclusions in olivine phenocrys ts in 7 of them have been analyzed for their alkali element abundances and K-isotopic compositions. Except for six points, all areas of the chondrules that were analyzed had delta(41)K compositions that were normal within err or (typically +/-3 parts per thousand, 2 sigma). The six "anomalous" points are probably all artifacts. Experiments have shown that free evaporation o f K leads to large K-41 enrichments in the evaporation residues, consistent with Rayleigh fractionation. Under Rayleigh conditions, a 3 parts per thou sand enrichment in delta(41)K is produced by similar to 12% loss of K. The range of L-chondrite-normalized K/Al ratios (a measure of the K-elemental f ractionation) in the areas analyzed vary by almost three orders of magnitud e. If all chondrules started out with L-chondrite-like K abundances and the K loss occurred via Rayleigh fractionation, the most K-depleted chondrules would have had compositions of up to delta(41)K approximate to 200 parts p er thousand. Clearly, K fractionation did not occur by evaporation under Ra yleigh conditions. Yet experiments and modeling indicate that K should have been lost during chondrule formation under currently accepted formation co nditions (peak temperature, cooling rate, etc.). Invoking precursors with v ariable alkali abundances to produce the range of K/Al fractionation in cho ndrules does not explain the K-isotopic data because any K that was present should still have experienced sufficient loss during melting for there to have been a measurable isotopic fractionation. If K loss and isotopic fract ionation was inevitable during chondrule formation, the absence of K-isotop ic fractionation in Bishunpur chondrules requires that they exchanged K wit h an isotopically normal reservoir during or after formation. There is evid ence for alkali exchange between chondrules and rim-matrix in all unequilib rated ordinary chondrites. However, melt inclusions can have alkali abundan ces that are much lower than the mesostases of the host chondrules, which s uggests that they at least remained closed since formation. If it is correc t that some or all melt inclusions remained closed since formation, the abs ence of K-isotopic fractionation in them requires that the K-isotopic excha nge took place during chondrule formation, which would probably require gas -chondrule exchange. Potassium evaporated from fine-grained dust and chondr ules during chondrule formation may have produced sufficient K-vapor pressu re for gas-chondrule isotopic exchange to be complete on the timescales of chondrule formation. Alternatively, our understanding of chondrule formatio n conditions based on synthesis experiments needs some reevaluation.