LATE ALTERATION IN TITANITE (CATISIO5) - REDISTRIBUTION AND REMOBILIZATION OF RARE-EARTH ELEMENTS AND IMPLICATIONS FOR U/PB AND TH/PB GEOCHRONOLOGY AND NUCLEAR WASTE-DISPOSAL

Translucent, dark-coloured titanites from a wide variety of geological environments are commonly partly replaced by very fine-grained aggregates of REE-rich minerals (including allanite, monazite, and bastnasite), rutile, quartz, calcite, apatite, and chlorite along grain fractures and boundaries. The aggregates of REE-rich minerals are bordered by an irregular zone of altered titanite, which, by secondary ion mass spectrometry (SIMS), is considerably lower in REEs, U, and Th than the adjacent unaltered matrix. Light REEs and Th are differentially depleted in the altered zone and correspondingly enriched in secondary minerals. Additionally, HREEs and U have been lost to the external system, the loss of HREEs being in proportion to the increase in atomic number. Natural radiation damage was not an important factor in promoting the late alteration of titanite. Titanite which has been subjected to late hydrothermal activity would not remain a closed system in respect to either U or Th decay series and therefore cannot be used for U/Pb and Th/Pb geochronology. The observed leaching and partial dissolution of titanite under hydrothermal conditions are generally consistent with laboratory experimentation, indicating that proper isolation from any communication with hydrothermal fluids is essential in the application of titanite-based ceramics for the immobilization of radioactive waste.