U [sbnd]Th [sbnd]Pb in chondrites—evidence of elemental mobilities and the singularity of primordial Pb

Abstract

A unified graphical approach that emphasizes the strict corellativity in the U-Th-Pb systematics and projects subtle diagnostic deviations from it has been adopted for the evaluation of the existing data base. It has been revealed that: (1)Excess radiogenic Pb in chondrites is largely an artifact stemming from Pb contamination of some samples and apparent recent U loss from other samples (problems with U analysis?). (2)Taken at face value, the data indicate that recent U-Th mobility on the chondrule-scale is pervasive in Allende. In contrast, apparent recent U-Th mobility in Barwell is generally consistent with gain of terrestrial Pb. (3)Allende carries the isotopic imprints of recent multiple disturbances in which elemental mobilities were effected but not isotopic homogenization of Pb. (4)For Allende, the Pb isotope pattern of the matrix samples suggests terrestrial contamination. If this is the case, the systematics indicates for the matrix recent U gain as well. The U-rich chondrules and inclusions, whose discordance is often consistent with U loss, are the probable source. In contrast to the contaminated matrix, the uncontaminated matrix should then be discordant by Pb loss (i.e., U gain). (5)Canyon Diablo primitive Pb appears to represent the primordial composition from which chondritic Pb evolved. Allegations to the contrary appear ill founded. (6)Identification of variable ancient initial leads by the Pb-Pb method in correlation with as much as 100 m.y. difference in the ages of formations, is not feasible, apparently because of the low U/Pb in the nebula (μ ⋍ 0.25).