Incorporation mechanisms of Ta and Nb in zircon and implications for pegmatitic systems

Abstract

To investigate the processes that control Ta incorporation in zircon, two types of synthesis experiments were performed: (1) crystallization of zircon from an Li-Mo flux at 1 atm, and (2) crystallization of zircon (with or without coexisting tantalite) from a highly fluxed pegmatitic melt at 200 MPa and nearly water-saturated conditions. The first type of experiment is used to identify the influence of various doping elements (Hf, P, Al, and Mn) on Ta incorporation in zircon. These experiments reveal that P hinders the incorporation of Ta, whereas Al enhances Ta incorporation via charge balancing, and that Ta can be incorporated in the absence of any other doping element via the creation of vacancies in the zircon structure. Hafnium does not affect significantly Ta incorporation. Manganese and lithium do not enter the structure of zircon, except in the presence of P. Experiments with Nb show that the concentration of this element in zircon is nearly one order of magnitude lower than Ta (for similar Ta and Nb concentrations in the flux).