Evidence for the stability of the potential nuclear waste host, sphene, over geological time, from uranium-lead ages and uranium-series measurements

Abstract

A sphene-based, glass-ceramic material is being investigated as a potential host for Canadian nuclear fuel recycle waste. The ability of sphene to retain certain radionuclides over geological time is examined using observed properties and age measurements of natural sphenes. The long-term stability of sphene is determined by comparing U Th Pb isotopic age data for sphene, from the literature, with ages determined for zircon and other minerals. Better internal agreement is found between U Th Pb sphene ages than for zircon ages indicating the resistance of sphene to daughter radionuclide loss over periods as long as 3000 Ma. Excellent agreement was observed between sphene ages and the estimated age of crystallization of the host rocks or vein infillings, except where subsequent metamorphim had reset the sphene age. Complete lead loss was generally found to accompany high-grade metamorphic conditions when temperatures exceeded ∼ 500°C. Stability over shorter periods of geological time was found for vein infilling and mineral separates of sphene using uranium-series disequilibrium analysis. Conditions of secular equilibrium in the decay chain 238U- 234U- 230Th- 226Ra were observed for most of the 21 sphenes studied indicating that no radionuclide migration had occurred over periods as long as the last 1 Ma. Exceptions to this were four sphenes that had 234U deficiencies between 6 and 24% and two sphenes that showed 226Ra/ 230Rh disequilibrium. No correlation was found between the extent of radiation damage suffered by the sphenes and the presence of uranium-series disequilibrium. The presence of alteration products in three sphenes did correlate, however, with measured disequilibrium, indicating that some 234U and 226Ra had been lost or gained by these samples in the last 1 Ma and 8 ka, respectively.