PbPb dating of apatite by a stepwise dissolution technique

Apatites from different geologic settings have been dated by a new sequential dissolution method. The procedure is based on stepwise exposure of apatite to weak-intermediate acids. A pegmatite sample from the Kerala Khondalite Belt in southern India yielded an age of 509 ± 25 Ma. Apatites from an enderbite sample from the Northern Marginal Zone of the Limpopo Belt in southern Africa yielded an age of 1970 ± 26 Ma which is in agreement with the high-grade metamorphism at around 2.0 Ga, reported from the Triangle Shear Zone and amphibolite-facies metamorphism in the Transition Zone. Apatites from an amphibolite-facies gneiss from the Transition Zone south of the NMZ yielded an age of 1878 ± 18 Ma. This age is somewhat younger than a sphene age on the same rock of 1971 ± 17 Ma and is believed to reflect the time of retrograde metamorphism in the presence of a fluid phase. The sequential dissolution method is a new tool that allows dating of apatites which have relatively unradiogenic Pb ratios because the spread can be significantly increased and no correction for common lead is needed. This study further demonstrates that apatites from magmatic and high-grade metamorphic rocks yield ages very close to those of monazite, zircon or garnet, unless the rock subsequently interacted with a fluid phase.