Uranium-lead age of the Bruderheim L6 chondrite and the 500-Ma shock event in the L-group parent body

Measured Pb-Pb whole meteorite data for the Bruderheim L6 chondrite scatter slightly about a line passing above Can?on Diablo lead and yielding an age of 4.482 + 0.017 Ga, using the terrestrial²³⁸U/²³⁵U ratio of 137.88. The measured U isotopic composition for Bruderheim, using the dissolution procedures employed for these U-Pb studies, is near the terrestrial composition. In the concordia diagram the U-Pb data chiefly plot above the concordia curve and define a line which intersects the concordia curve at 4.536 + 0.006 Ga and 0.495 Ga, but the data for Bruderheim cannot be understood at all in terms of the more usual two- or three-stage episodic U-Pb models involving a fixed μ-value in the first stage. Most samples show an apparent excess of radiogenic lead for single-stage (closed system) evolution when Can?on Diablo troilite is used for the initial lead composition. Evidence is presented to show that the apparent excess radiogenic lead cannot be explained by terrestrial contamination alone.A different U-Pb model is presented which describes qualitatively and quantitatively most features of the U-Pb data for Bruderheim. If this model correctly describes the U-Pb evolution of Bruderheim then the “formation” age is given as 4.536 Ga by both U-Pb and Pb-Pb data, the meteorite U-Pb system was disturbed by a later (shock?) event at about 500 Ma ago, and the data are consistent with (though do not require) a Can?on Diablo initial lead composition. This interpretation suggests that the classical phenomenon of apparent excess radiogenic lead reflects the application of a single-stage model to a meteorite that has evidently experienced at least a two-stage history. The explanation of the observation that in the concordia diagram most meteorite samples (corrected for Can?on Diablo lead) plot in the lead excess region remains obscure, though this may be due to the wrong choice of initial lead for Bruderheim.