Radioactive secular equilibrium in 238 U and 232 Th series in granitoids from Greece A. Papadopoulos a,n , G. Christofides a , A. Koroneos a , S. Stoulos b , C. Papastefanou b a Department of Mineralogy, Petrology and Economic Geology, School of Geology, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece b Laboratory of Atomic and Nuclear Physics, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece HIGHLIGHTS c Generally, a strong correlation between 238 U and 226 Ra is obvious. c Post-magmatic processes caused radioactive disequilibrium in the 238 U isotopic system. c All samples are in radioactive secular equilibrium in the 232 Th isotopic system. c There is no correlation between mineralogy, texture, colour, age and the isotopic ratios studied. article info Article history: Received 1 September 2012 Accepted 7 February 2013 Available online 13 February 2013 Keywords: Post-magmatic processes Rock–water interaction Secular equilibrium Granitoid 238 U radioactive series 232 Th radioactive series abstract 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 238 U and 226 Ra from 238 U radioactive series and 228 Ra and 228 Th from 232 Th radioactive series by using gamma-ray spectroscopy. Results on the radioactive secular equilibrium of both 238 U and 232 Th radioactive series are presented by studying the 226 Ra/ 238 U and 228 Ra/ 228 Th ratios. The majority of the samples are in radioactive secular equilibrium for 226 Ra/ 238 U 71s. However, several samples exhibit 226 Ra/ 238 U ratios significantly different from 1. The distortion of the secular equilibrium in the 226 Ra– 238 U isotopic system of those samples has occurred over the last 1 Ma, and can be associated with post-magmatic processes, mainly rock–water interac- tions. All studied samples have 228 Ra/ 228 Th ratios equal to unity 71s. Consequently, they can be considered to be in secular equilibrium for the last 40 a. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction The natural radioactive series, headed by 238 U, 235 U, and 232 Th, are composed of nuclides with different chemical and physical properties. Their half-lives vary from less than 1 s up to trillions of years. The measurement of the specific activities of the 238 U and 232 Th decay chain members and the correlation between them in igneous plutonic rocks, allows detecting the presence of rock– water interaction and generally posting magmatic processes (Bonotto et al., 2001; Gascoyne and Schwarcz, 1986; Guthrie, 1991). The use of uranium series disequilibrium in the geological sciences also includes applications such as determining the age of deposits younger than 1 Ma (Schwarcz and Gascoyne, 1984), mantle evolution (Condomines et al., 1988), hydrological tracing (Guttman and Kronfeld, 1982) and nuclear waste disposal (Schwarcz et al., 1982, Smellie, 1982). In the latter application, the 238 U series technique examines the state of radioactive equilibrium, existing between the radionuclides of the 238 U- decay series (Schwarcz et al., 1982; Smellie et al., 1986; Gascoyne and Schwarcz, 1986; Chapman and Smellie, 1986). When an isotopic system is undisturbed, it can be considered as closed. In this case the activity of the parent radionuclide is similar to that of its intermediate or final decay product, suggesting that the decay rate of the radionuclides of a particular isotopic series is nearly the same. This state of a radioactive series is defined as permanent radioactive equilibrium (radioactive secular equilibrium). The radioactive secular equilibrium can be disturbed when the parent radionuclide or its decay products enter or depart from the isotopic system, during a period comparable to the half-life period of the daughter radionuclide and at distance comparable to the size of the system. The main processes which produce disequili- brium between the natural radionuclides were first defined by Rosholt (1983), and have since been recognised in crystalline rock Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/apradiso Applied Radiation and Isotopes 0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.02.006 n Corresponding author. Tel.: þ30 693 727 5072. E-mail addresses: argpapad@geo.auth.gr, argpapad@hotmail.com (A. Papadopoulos). Applied Radiation and Isotopes 75 (2013) 95–104