Contrasted monazite hydrothermal alteration mechanisms and their geochemical implications

Abstract

In spite of the major importance of monazite as a repository for the rare earths and Th in the continental crust, for U-Th-Pb geochronology, and as a possible form for high-level nuclear waste, very little work has been carried out so far on the behaviour of this mineral during fluid-rock events. This contribution describes two contrasting examples of the hydrothermal alteration of monazite. The first case comes from a sample of the Carnmenellis granite (Cornwall, Southwest England), chloritized at 284 ± 16°C, whereas the other occurs in the Skiddaw granite (Lake District, Northwest England), which underwent greisenization at 200 ± 30°C. An integrated study involving backscattered scanning electron microscopy, electron microprobe analyses, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) reveals that the chloritization event was characterized by the coupled substitution 2REE 3+ ⇌ Th 4+ + Ca 2+ in the altered parts of the monazite, thus leaving the P-O framework of the crystal untouched. In contrast, greisenization led to the coupled substitution REE 3+ + P 5+ ⇌ Th 4+ + Si 4+, and therefore involved a partial destruction of the phosphate framework. The resulting rare earth element patterns are quite different for these two examples, with a maximum depletion for Dy and Er in the altered parts of the Carnmenellis monazite, whereas the Skiddaw monazite shows a light rare earth depletion but an Yb and Er enrichment during alteration. This latter enrichment, accompanied by an increase in U but roughly unchanged Pb concentrations, probably resulted from a decrease in the size of the 9-coordinated site in monazite, thereby favouring the smaller rare earths. These contrasted styles of monazite alteration show that the conditions of fluid-rock interaction will not only affect the aqueous geochemistry of the lanthanides, actinides and lead, and the relative stability of the different minerals holding these elements. Variations in these conditions will also lead to various possible chemical exchanges between the crystalline phases and the hydrothermal fluids. The occurrence of common lead along penetrative cracks in the Carnmenellis monazite shows that only a leaching, prior to the U-Pb analyses of the whole-grain, will permit an accurate determination of the magmatic crystallization age. In contrast, for the Skiddaw case it may be possible to date the fluid-rock event by in situ 207Pb/ 206Pb geochronology. The observation that the altered parts of both monazite examples display Nd leaching and no significant Sm/Nd fractionation indicates that they should not affect the host whole-rock Nd isotopic signatures. Finally, it appears that monazite-like ceramics designed for the containment of high-level nuclear wastes will retain Th and the geochemically equivalent transuranic elements during fluid-rock events similar to those documented in this study but may release Nd, U and the corresponding radionuclides to the environment.