THE BEHAVIOR OF RARE-EARTH ELEMENTS IN HIGH-TEMPERATURE CL-BEARING AQUEOUS FLUIDS - RESULTS FROM THE ODEGARDENS-VERK NATURAL LABORATORY

The apatite from the ancient mines at Odegarden, northeast of Kragero, South Norway, is known to be chlorapatite. Alteration during metamorp hism involved the replacement of chlorapatite by hydroxyapatite and ra re-earth phosphates, while hornblende developed elsewhere in the veins and in the country rocks. Ti-in-hornblende thermometry indicates ca. 700-degrees-C for this assemblage. The apatite provides a natural labo ratory for the study of the behavior of the REE in high-temperature Cl -bearing aqueous solutions. La, Ce, Dy, Y, Na and Fe are leached from apatite, together with Cl. Coleaching of REE and Na indicates their co upled substitution in apatite. A different behavior is indicated for h eavy and light REE. the latter being less depleted in Cl-poor apatite. Relatively more LREE precipitated as monazite than HREE as xenotime. The HREE-bearing complexes with Cl seem to have been more stable under the conditions described.