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Abstract

Ar-Ar isochron ages of EL chondrites suggest closure of the K-Ar system at 4.49 +/- 0.01Ga for EL5 and 6 chondrites, and 4.45 +/- 0.01Ga for EL3 MAC 88136. The high-temperature release regimes contain a mixture of radiogenic Ar-40* and trapped primordial argon (solar or Q-type) with Ar-40/Ar-36(TR)~0, which does not affect the Ar-40 budget. The low-temperature extractions show evidence of an excess Ar-40 component. The Ar-40/Ar-36 is 180-270; it is defined by intercept values of isochron regression. Excess Ar-40 is only detectable in petrologic types >4/5. These lost most of their primordial Ar-36 from low-temperature phases during metamorphism and retrapped excess Ar-40. The origin of this excess Ar-40 component is probably related to metamorphic Ar mobilization, homogenization of primordial and in situ radiogenic Ar, and trapping of Ar by distinct low-temperature phases. Ar-Ar ages of EH chondrites are more variable and show clear evidence of a major impact-induced partial resetting at about 2.2Ga ago or alternatively, prolonged metamorphic decomposition of major K carrier phases. EH impact melt LAP 02225 displayed the highest Ar-Ar isochron age of 4.53 +/- 0.01Ga. This age sets a limit of about 25-45Ma for the age bias between the K-Ar and U-Pb decay systems.