Pb-Pb dating of chondrules from CV chondrites by progressive dissolution

To refine absolute Pb-Pb ages of chondrules, we have experimented with progressive dissolution procedures of chondrules in the CV chondrite Allende in an attempt to effectively remove terrestrial contamination and generate Pb fractions of sufficient size and spread in 207Pb-206Pb space to adequately define meaningful isochrons. Samples are extricated from the matrix, ultrasonicated, abraded and repeatedly pre-cleaned by ultrasonicating in distilled ethanol, water and acetone. Subsequent steps of weak HCl and HBr acids combined with ultrasonic agitation and modest heating effectively removes terrestrial contamination in most samples we analyzed. Subsequent exposure to warm 4 M HNO3 typically isolates the least-radiogenic component of the stepwise dissolution series. Continuing the dissolution steps with HCl of increasing molarity with longer and higher temperature heating steps returns more radiogenic Pb fractions. The most radiogenic Pb fraction typically corresponds to the first warm 1 M HF step with subsequent steps in stronger HF yielding progressively less radiogenic signatures. Importantly, the Pb components extracted in the dissolution of the final residues with 28 M HF + 14 M HNO3 typically lie slightly below the isochron defined by previous steps, corresponding to a slightly younger 207Pb/206Pb model age than the inferred real age. The wide range of total Pb contents (20-> 2000 pg) in different fractions returned by this method requires analyses by TIMS using a high-efficiency Pb emitter, 202Pb/205Pb double spike and diligent monitoring of laboratory Pb blank. As expected, U and Pb are highly and variably fractionated during the procedure such that no U/Pb age information is recovered. A multi-chondrule fraction from the CV chondrite Allende yields an age of 4565.32 ± 0.81 Ma, which is statistically younger than calcium-aluminum inclusions from the CV chondrite Efremovka.