instrumental techniques applied to mineralogy and geochemistry

Radiogenic isotopes and their applications within a range of scientific fields 119
FIGURE 4. Obtained ion microprobe ages (in billion years, Ga) from a zircon of the Amitsoq
gneiss, Greenland (upper part shows a SEM image with traces of the craters produced by the
incident primary ion beam, whilst the lower is a CL image showing oscillatory zoning and overgrowths).
This is obviously important when for instance a zircon contains inclusions
(crystallized prior to the main zircon-forming event) that may contain non-negligable
amounts of either U or Pb, or secondary over-growths. If such zircons are analyzed in a
conventional manner using a small number of crystals, a geologically meaningless,
mixed age would result. The quality of ion probe data (ca. ±1 % uncertainty in individual
U-Pb ages) is comparable with that of laser results, but the latter is a destructive method
creating ablation craters with a depth penetration of some 10-20 micrometers compared
to only a few micrometer deep pits induced by the ion microprobe. Lasers produce very
short light pulses, with a uniform wavelength, that can be focused onto the surface of a
sample (see Kosler & Sylvester, 2003 for applications in geochronology). When the
sample is hit, material sputters away (ablates) and can be transported by a carrier gas to a
ICP-MS system where small, solid sample particles are ionized and subsequently