Fluorine and fossils
I checked the literature, it has been reported that fluorine ions replace hydroxyl ions in osseous tissue (bone) so by making a measurement of the flourine content of bone it is possible to estimate the age of the bones.

One early paper on the subject is Cook, S. F.; Ezra-cohn, H. C. An Evaluation of the Fluorine Dating Method. Southwestern Journal of Anthropology 1959, 15 (3), 276–290. DOI: 10.1086/soutjanth.15.3.3628977.

One method would be to use neutron activation anaylsis on bones to measure the flourine-19, using the n,gamma reaction. This method does have a problem as sodium-23 can form flourine-20 by a n,alpha reaction. As a result a correction will have to be made for the sodium content of the bones.

This is discussed in Cheng, T. P.; Anderson, H. D.; Mills, D. S.; Spate, V. L.; Baskett, C. K.; Morris, J. S. Determination of the fluoride distribution in rabbit bone using instrumental neutron activation analysis. J. Radioanal. Nucl. Chem. 1997, 217 (2), 171–174. DOI: 10.1007/BF02034437.