Studies of progressive leaching in single mineral Pb/Pb dating

Silicate mineral Pb/Pb dating (PbSL) using progressive leaching steps was recently developed. Although the method represents a breakthrough in dating many silicate minerals, it is not fully understood on an atomic scale. It is not clear whether the leaching process is coupled with selective removal of elements from the crystal lattice or whether it involves major dissolution-redeposition along the reaction front. Further investigation was carried out on a single crystal of gem quality titanite from Otter Lake, Canada. PIXE mapping was performed to study the behaviour of U, Th and Pb during the stepwise leaching in comparison to the main elements in this mineral. (See Fig. 1.) Raster scans of 3 MeV protons were made using the true elemental imaging system (Dynamic Analysis) of the NAC Van de Graaff nuclear microprobe. PIXE elemental distribution maps, together with other techniques, revealed that the increased Pb isotope spread obtained during PbSL (enabling single mineral isochrons to be calculated) is the result of two competing and interacting processes, namely (i) a very effective surface dependent hydrolysis of metal cations, and (ii) a release rate delimiting, volume controlled adsorption of HFS-elements within the leached layer, which gets more effective as leaching progresses. (C) 1997 Elsevier Science B.V.