Mobilities of volatiles (H, F and C) in apatite at high temperatures

Apatite serves as a notable host for diverse volatiles, but experimental data concerning the mobilities of volatiles out of its lattice remains scarce. Despite several investigations elucidating the diffusivities of F, Cl, and OH, their interplay mechanism remains poorly understood. Herein, to unveil the mobilities of hydrogen, fluorine, and carbon out of the apatite lattice and their intricate interaction mechanisms during high temperature processes, we carry out annealing experiments at 900, 1000, and 1100 °C on two apatite samples with distinct fluorine contents. Our findings indicate that along the c-axis, hydrogen diffusion coefficients surpass those of fluorine by 2–3 orders of magnitude, with lower activation energies observed. Hydrogen diffusion shows strong anisotropy, with lager diffusivities and lower activation energies along the c-axis than the a-axis. For both hydrogen and fluorine, the Durango apatite shows smaller diffusivities along the c-axis than the Imilchil apatite. Contrarily, carbon diffusion is absent in both samples at these temperatures. These differences in diffusivity may suggest that hydrogen diffuses as OH− along the a-axis and as H+ along the c-axis, while fluorine diffusion relies on hydrogen diffusion along the c-axis. Based on these new data, the capacity of apatite to record volatiles can be evaluated. All volatiles, including hydrogen, is likely preserved within the core of apatite during thermal events from several days to years, such as rapid volcanic eruptions. However, for prolonged thermal events lasting hundreds of millions of years, apatite preserves volatiles for F, Cl and C, but not hydrogen.