The relative formation ages of ferromagnesian chondrules inferred from their initial aluminum-26/aluminum-27 ratios

We performed a systematic high-precision secondary ion mass spectrometry Al-26-Mg-26 isotopic study for 11 ferromagnesian chondrules from the highly unequilibrated ordinary chondrite Bishunpur (LL3.1). The chondrules are porphyritic and contain various amounts of olivine and pyroxene and interstitial plagioclase and/or glass. The chemical compositions of the chondrules vary from FeO-poor to FeO-rich. Eight chondrules show resolvable Mg-26 excesses with a maximum delta(26)Mg of similar to1% in two chondrules. The initial Al-26/Al-27 ratios inferred for these chondrules range between (2.28 +/- 0.73) x 10(-5) to (0.45 +/- 0.21) x 10(-5). Assuming a homogeneous distribution of Al isotopes in the early solar system, this range corresponds to ages relative to CAIs between 0.7 +/- 0.2 Ma and 2.4(+0.7)(-0.4) Ma. The inferred total span of the chondrule formation ages is at least I Ma, which is too long to form chondrules by the X-wind. The initial Al-26/Al-27 ratios of the chondrules are found to correlate with the proportion of olivine to pyroxene suggesting that olivine-rich chondrules formed earlier than pyroxene-rich chondrules. Though we do not have a completely satisfactory explanation of this correlation we tentatively interpret it as a result of evaporative loss of Si from earlier generations of chondrules followed by addition of Si to the precursors of later generation chondrules.