On the possibility of determining rates of removal of uranium from crystalline igneous rocks using U-series disequilibria—1: a U-leach model, and its applicability to whole-rock data

Abstract

The removal of U from granitic rocks is characterised by 234U/ 238U≤1 and 230Th/ 234U> 1. It is notable, from weathered Eye-Dashwa lakes (Canada) granite samples and from the published data, that 230Th/ 234U ratios seldom exceed a limiting value of about 2. A U-leach model is discussed in detail which may account for these observations. The model assumes that 230Th is practically immobile and that the removal of U can then be measured against the decay rate of the unsupported 230Th. A property of the model is that the isotope ratios reach constant values reflecting a kind of “secular equilibrium”, and this implies that U is seldom removed faster than the decay of 230Th. If the limited 230Th/ 234U ratios from the data correspond to “secular equilibrium” ratios then this allows simple calculation of U-leach rates from any given subsurface sample. A test for the model using 235U/ 231Pa ratios is suggested. A comparison of leach rates derivable from U-Th-Pb systematics, of Colorado and Wyoming granite samples, with the corresponding estimates from the model shows that the latter are high by a factor of 10–50. Data from rapidly weathered surface rinds suggests that the presence of labile U restricts the application of the model to subsurface, low-weathered zones.