26Al in plagioclase-rich chondrules in carbonaceous chondrites: Evidence for an extended duration of chondrule formation

The Al-26-Mg-26 isotope systematics in 33 petrographically and mineralogically characterized plagioclase-rich chondrules (PRCs) from 13 carbonaceous chondrites (CCs) - one ungrouped (Acfer 094), six CR, five CV, and one CO - reveal large variations in the initial Al-26/Al-27 ratio, (Al-26/Al-27)(0). Well-resolved Mg-26 excesses (delta Mg-26) from the in situ decay of the short-lived nuclide Al-16 (t(1/2) similar to 0.72 Ma) were found in nine chondrules, two from Acfer 094, five from the CV chondrites, Allende and Efremovka, and one each from the paired CR chondrites, EET 92147 and EET 92042, with (Al-26/Al-27)(0) values ranging from similar to 3 x 10(-6) to similar to 1.5 x 10(-5). Data for seven additional chondrules from three CV and two CR chondrites show evidence suggestive of the presence of Al-26 but do not yield well defined values for (Al-26/Al-27)(0) while the remaining chondrules do not contain excess radiogenic 26Mg and yield corresponding upper limits of (11-2) x 10(-6) for (Al-26/Al-27)(0). The observed range of (Al-26/Al-27)(0) in PRCs from CCs is similar to the range seen in chondrules from unequilibrated ordinary chondrites (UOCs) of low metamorphic grade (3.0-3.4). However, unlike the UOC chondrules, there is no clear trend between the (Al-26/Al-27), values in PRCs from CCs and the degree of thermal metamorphism experienced by the host meteorites. High and low values of (Al-26/Al-27)(0), are found equally in PRCs from both CCs lacking evidence for thermal metamorphism (e.g., CRs) and CCs where such evidence is abundant (e.g., CVs). The lower (Al-26/Al-27)(0) values in PRCs from CCs, relative to most CAIs, are consistent with a model in which Al-26 was distributed uniformly in the nebula when chondrule formation began, approximately a million years after the formation of the majority of CAIs. The observed range of (Al-26/Al-27)(0) values in PRCs from CCs is most plausibly explained in terms of an extended duration of similar to 2-3 Ma for the formation of CC chondrules. This interval is in sharp contrast to most CAIs from CCs, whose formation appears to be restricted to a narrow time interval of less than 10(5) years. The active solar nebula appears to have persisted for a period approaching 4 Ma, encompassing the formation of both CAIs and chondrules present in CCs, and raising important issues related to the storage, assimilation and mixing of chondrules and CAIs in the early solar system. (C) 2009 Elsevier Ltd. All rights reserved.