R. Frei et al., STUDIES OF PROGRESSIVE LEACHING IN SINGLE MINERAL PB PB DATING/, Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms, 130(1-4), 1997, pp. 676-681
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 ato
mic scale. It is not clear whether the leaching process is coupled wit
h selective removal of elements from the crystal lattice or whether it
involves major dissolution-redeposition along the reaction front. Fur
ther 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 comparis
on to the main elements in this mineral. (See Fig. 1.) Raster scans of
3 MeV protons were made using the true elemental imaging system (Dyna
mic Analysis) of the NAC Van de Graaff nuclear microprobe. PIXE elemen
tal distribution maps, together with other techniques, revealed that t
he increased Pb isotope spread obtained during PbSL (enabling single m
ineral isochrons to be calculated) is the result of two competing and
interacting processes, namely (i) a very effective surface dependent h
ydrolysis 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.