ALLANITE-(Ce) FROM THE EOCENE CASTO GRANITE, IDAHO: RESPONSE TO HYDROTHERMAL ALTERATION

The response of allanite-(Ce) in the Eocene Casto granite of Idaho to hydrothermal activity has been investigated. Unaltered, igneous allanite from the Casto pluton is characterized by a narrow range of compositional variation, with little or no zoning. A representative empirical formula, based on the composition of the least-altered grain, is: (REE0.83Ca1.07Th0.04Mn0.06) (Ti0.11 Mg0.04Fe0.99Fe0.48Al1.38) (Si3.00O12) (OH0.95F0.05), where REE represents (La0.14Ce0.41Pr0.05Nd0.17Sm0.03Y0.03). The small amount of observed compositional variation is consistent with the coupled substitutions Ca2+ + Th4+ ↔ 2REE3+ and REE3+ + Fe2+ + Mg2+ ↔ Ca2+ + Al3+ + Fe3+. Slightly altered crystals exhibit a rim in which Th is enriched, and La and Ce are depleted. Increasing alteration lead to extensive corrosion and replacement of allanite by fluorite, a REE-, Thand P-rich phase (probably monazite), and a Th-rich phase (probably thorianite). In spite of attack by fluoride-rich hydrothermal solutions, the REE and Th originally in allanite were not transported significant distances, but were redeposited locally as secondary phases. The lack of significant REE and Th transport is due to the availability of phosphorus, which allowed the REE to be fixed in a secondary phosphate phase, and the removal of fluoride as fluorite. This work has implications for the behavior of radioactive waste in deeply buried geological repositories upon interaction with heated groundwaters.