Radioactive secular equilibrium in 238U and 232Th series in granitoids from Greece

Granitoid rocks belonging to plutons of Greece, covering a wide range of compositions and rock-types, have been studied for their specific activity (Bq/kg) of 238U and 226Ra from 238U radioactive series and 228Ra and 228Th from 232Th radioactive series by using gamma-ray spectroscopy. Results on the radioactive secular equilibrium of both 238U and 232Th radioactive series are presented by studying the 226Ra/238U and 228Ra/228Th ratios. The majority of the samples are in radioactive secular equilibrium for 226Ra/238U±1σ. However, several samples exhibit 226Ra/238U ratios significantly different from 1. The distortion of the secular equilibrium in the 226Ra–238U isotopic system of those samples has occurred over the last 1 Ma, and can be associated with post-magmatic processes, mainly rock–water interactions. All studied samples have 228Ra/228Th ratios equal to unity ±1σ. Consequently, they can be considered to be in secular equilibrium for the last 40 a.

► Generally, a strong correlation between 238U and 226Ra is obvious.
► Post-magmatic processes caused radioactive disequilibrium in the 238U isotopic system.
► All samples are in radioactive secular equilibrium in the 232Τh isotopic system.
► There is no correlation between mineralogy, texture, colour, age and the isotopic ratios studied.