The movement of uranium during higher grade metamorphic processes

The movement of uranium in rocks is largely governed by movement of water and by partial melting. The movement of uranium at the surface and during diagenetic processes is not considered here. The physical conditions under which higher grade metamorphic processes in the sense of this paper take place are such that rock strength does not contribute significantly to the maintenance of an open pore system. A separate fluid phase can then only exist if its pressure is at least equal to the load pressure. Such conditions exist at a minimum depth corresponding to rock pressures of about 2 kbar (200 MPa) ( ca . 6 km) and temperatures above 200 °C. These are the conditions of rocks crystallized under greenschist, amphibolite and granulite facies conditions of metamorphism, and it is the behaviour of uranium under such conditions which is discussed here. Gamma ray spectrometry analyses during the last 15 years have contributed significantly to the quantity of data available on uranium concentration in metamorphic rocks. Nuclear emulsion and fission track studies combined with leaching experiments have given significant information on how uranium is distributed in rocks and in which it occurs. Isotope studies of the U-Pb systems and also studies of homogenization of Sr isotopes in metamorphic processes can give valuable information about the movement of uranium under these conditions.