Isotopic composition of uranium and thorium in crystalline rocks

The 238 U‐ 234 U‐ 230 Th‐ 232 Th system has been investigated in 84 silicic crystalline rocks obtained from drill cores, surface, and near‐surface samples in California, Wyoming, Colorado, and Illinois. Results of these analyses displayed on ternary diagrams with apexes for 238 U, 234 U, and 230 Th indicate five predominant geochemical processes that affected uranium in the rock: (1) bulk uranium leaching where 238 U and 234 U were removed with little or no fractionation; (2) preferential 234 U leaching by alpha recoil displacement ( 234 U recoil loss) with lesser 238 U loss; (3) 234 U recoil loss with little or no 238 U loss; (4) uranium assimilation where both 238 U and 234 U were added with present‐day 234 U/ 238 U activity ratios varying from 0.8 to 1.2; and (5) addition of 234 U and 230 Th by daughter emplacement processes ( 234 U + 230 Th recoil gain). Evidence for the existence of 234 U and 230 Th recoil gain in rocks is the most important finding of this investigation. Radioactive disequilibrium occurs in the majority of rocks analyzed where 234 U recoil loss is the predominant process associated with incipient weathering; U assimilation and 234 U+ 230 Th recoil gain occur under conditions of substantial water penetration along fractures and into weathered zones in the rocks. Relatively unfractured and petrographically fresh rocks from the UPH‐3 drill hole in northern Illinois are closest to being in radioactive equilibrium for any suite of rocks included in this study, and they demonstrate that equilibrium during the last 0.5 m.y. can be maintained over a substantial vertical distance where there has been little or no movement of water in the basement rock. The 238 U‐ 234 U‐ 230 Th system can be a sensitive indicator of geologically recent U mobility and rock/water interaction both in petrographically fresh core samples and in ‘sealed’ fracture zones.