26Al-26Mg systematics of chondrules in a primitive CO chondrite

We have conducted systematic investigations of formation age, chemical compositions, and mineralogical characteristics of ferromagnesian chondrules in Yamato-81020 (CO3.05), one of the most primitive carbonaceous chondrites, to get better understanding of the origin of chemical groups of chondrites. The Al-26-Mg-26 isotopic system were measured in fourteen FeO-poor (Type I), six FeO-rich (Type II) and two aluminum-rich (Al-rich) chondrules using a secondary ion mass spectrometer. Excesses of Mg-26 in plagioclase (1.0-13.5 parts per thousand) are resolved with sufficient precision (mostly 0.4-6.6 parts per thousand at 2 sigma level) in all the chondrules studied except one. Chemical zoning of Mg and Na in plagioclase were investigated in detail in order to evaluate the applicability of Al-26-Mg-26 chronometer. We conclude that the Al-Mg isotope system of the chondrules in Y-81020 have not been disturbed by parent-body metamorphism and can be used as chronometer assuming homogeneous distribution of Al-26. Assuming an initial Al-26/Al-27 ratio of 5 x 10(-5) in the early solar system, Al-26-Mg-26 ages were found to be 1.7-2.5 Ma after CAI formation for Type I, 2.0-3.0 Ma for Type II and 1.9 and 2.6 Ma for Al-rich chondrules. The formation ages of ferromagnesian chondrules in Y-81020 are in good agreement with those of L and LL (type 3.0-3.1) chondrites in the literature, which indicates that common chondrules in the CO chondrite were formed contemporaneously with those in L and LL chondrites. The concurrent formation of chondrules of CO and L/LL chondrites suggests that the chemical differences between CO and L/LL chondrites might be caused by spatial separation of chondrule formation environments in the protoplanetary disk. (C) 2008 Elsevier Ltd. All rights reserved.