Isotopic composition of uranium and thorium in crystalline rocks

Abstract

The ²³⁸U‐²³⁴U‐²³⁰Th‐²³²Th 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 ²³⁸U, ²³⁴U, and ²³⁰Th indicate five predominant geochemical processes that affected uranium in the rock: (1) bulk uranium leaching where ²³⁸U and ²³⁴U were removed with little or no fractionation; (2) preferential ²³⁴U leaching by alpha recoil displacement (²³⁴U recoil loss) with lesser ²³⁸U loss; (3) ²³⁴U recoil loss with little or no ²³⁸U loss; (4) uranium assimilation where both ²³⁸U and ²³⁴U were added with present‐day ²³⁴U/²³⁸U activity ratios varying from 0.8 to 1.2; and (5) addition of ²³⁴U and ²³⁰Th by daughter emplacement processes (²³⁴U + ²³⁰Th recoil gain). Evidence for the existence of ²³⁴U and ²³⁰Th recoil gain in rocks is the most important finding of this investigation. Radioactive disequilibrium occurs in the majority of rocks analyzed where ²³⁴U recoil loss is the predominant process associated with incipient weathering; U assimilation and ²³⁴U+²³⁰Th 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 ²³⁸U‐²³⁴U‐²³⁰Th 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.