In situ investigations of allanite hydrothermal alteration: examples from calc-alkaline and anorogenic granites of Corsica (southeast France)

Abstract.

Allanite is, with monazite, the main repository for light rare earth elements (REE) in the continental crust and can be used in U–Th–Pb geochronology. This mineral has been shown to be prone to alteration. The geochemical exchanges occurring between allanite and hydrothermal fluids were explored using backscattered scanning electron microscopy, electron microprobe, and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). The alteration mechanisms found show the restricted role of metamictization and correspond to the allanite to epidote transformation or to the leaching of the allanite A-crystallographic sites. Hence, the REE are mostly removed from this phase during fluid-mineral interactions, although a heavy rare earth-rich fluid may imprint its geochemical signature on the allanite altered zones. It appears that even if this REE holder presents evidence of alteration, the bulk mineral and whole-rock REE and Sm-Nd isotope signatures were not significantly affected in the cases studied. Alteration results mostly in the input of common lead, thus restricting the usefulness of allanite for U–Th–Pb dating when alteration occurs. Finally, the alteration mechanisms found here confirm the much more limited flexibility of silicates crystalline lattice as compared to phosphates.