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

Abstract

We performed a systematic high precision SIMS 26Al-26Mg 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 26Mg-excesses with a maximum δ26Mg of ~1% in two chondrules. The initial 26Al/27Al 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 My and 2.4^(-0.4/+0.7) My. The inferred total span of the chondrule formation ages is at least 1 My, which is too long to form chondrules by the X-wind. The initial 26Al/27Al 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.