Reflections on the 230Th/U dating of dirty material

High correlation coefficients between the activity ratios of the uranium and thorium isotopes common for the "isochron" approach of 230Th/U dating of dirty material mask the requirement to check for closed-system conditions of uranium in addition to the detrital correction of 230Th/U ages. The difficulty to calculate standard deviations of 230Th/U dates derived by the "isochron" approach complicates the problem. The methodical reasons are discussed. Closed-system conditions are checked by the 234U/ 238U - 230Th/ 238U AR plot which can also be used to carry out least-squares fitted detrital correction of 230Th/U ages of dirty material.