Results of a wide-ranging isotopic investigation of the unique Antarctican angrite LEW-86010 (LEW) are presented, together with a reassessment of the type angrite Angra dos Reis (ADOR). The principal objectives of this study are to obtain precise radiometric ages, initial Sr isotopic compositions, and to search for the erstwhile presence of the short-lived nuclei Sm-146 and Al-26 via their daughter products. The isotopic compositions of Sm, U, Ca, and Ti were also measured. This allows a detailed appraisal to be made of the relations between, and the geneology of, these two angrites. LEW proves to be severely contaminated with modern terrestrial Pb, which is shown to result from terrestrial weathering. Nevertheless, concordant Pb-Pb model ages of pyroxene separates were obtained (2-sigma): 4.55784 +/- 52 Ga for LEW and 4.55780 +/- 42 Ga for ADOR. Uranium isotopic compositions are normal within error. The inferred initial Pb isotopic composition is within error of primordial Pb, as defined by Canon Diablo troilite. Because of the extreme U/Pb ratios of both angrites, this places an upper limit of approximately 2 Ma on the time between volatile element-loss of the angrite parent body or its precursor planetesimals and the final crystallization of the angrites as differentiates. Initial Sr-87/ Sr-86 ratios were found to be indistinguishable in LEW, ADOR, and the cumulate eucrite Moore County. The derived initial Sr-87/Sr-86 for all three meteorites is 0.698970 +/- 15 (2-sigma). A Sm-147-Nd-143 isochron for LEW was obtained, yielding an age of 4.553 +/- 34 Ga with an initial Nd-143/Nd-144 of 0.506682 +/- 49 (2-sigma). Sm-146-Nd-142 isotope systematics were also measured. Anomalies on Nd-142 arising from extinct Sm-146 were clearly resolved, resulting in an initial Sm-146/Sm-144 of 0.0071 +/- 17 with epsilon(Nd)(142) = -2.57 +/- 0.62 at the time of isotopic closure. Both of these Sm-Nd methods imply derivation of LEW from a reservoir with chondritic Sm/Nd ratio; they are also quite consistent with the previously reported systematics for ADOR. Overall, the age and isotopic similarities between LEW and ADOR are striking; it suggests almost simultaneous production on the same asteroid, even though recent experimental studies imply that the two are not comagmatic. Calcium and titanium do not exhibit enrichments in the n-rich isotopes, in contrast to CAIs, making it unlikely that the angrite parent body is inherently rich in early condensates from the solar nebula. No evidence was found for live Al-26 in LEW. When combined with the Pb-Pb age and initial Sr-87/Sr-86 data, this firmly excludes Al-26 being an important heat source in the early solar system; heat supplied for, for example, (1) the differentiation of the angrite and eucrite parent bodies, and (2) metamorphism of the chondrite parent bodies must come from some other source. The debate over live vs. extinct Al-26 in the earliest solar system remains unresolved by this new data from LEW, however. Both the angrite and eucrite parent bodies have extremely low Rb/Sr and high U/Pb ratios compared to the solar nebula. Therefore, the common initial Sr-87/Sr-86 of 0.698970 can be firmly associated with the U-Pb angrite age above, and the formation of both bodies. This enables an absolute chronology of the early solar system to be established. Based upon Sr-87/Sr-86 differences, the oldest CAIs are a minimum of 11 +/- 4 Ma older than the angrites, i.e., 4.569 +/- 5 Ga. The age and isotopic constraints are discussed with respect to current collapse, condensation, and accretion timescales calculated for the solar nebula.
