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mean UO+/U+ and Pb+/U+ value. Since the age of the standard and its actual Pb/U is
known, unknown parameters can be calibrated against the standard. Data reduction
was carried out through versions of PRAWN and Lead, data reduction programs based
on routines developed at ANU. PRAWN (Playing Retrospectively Around With
Numbers) produces isotope ratios from the observed intensities, while lead carries out
the renormalisation against the standard.
The Stanford SHRIMP-RG incorporates a different mass analyser in comparison with
that used on the SHRIMP II models. Double-focusing mass spectrometers consist of an
electrostatic analyser (ESA) and a magnetic sector. The purpose of the ESA is to
remove velocity dispersion from the mass filtered beam by producing an equal and
opposite dispersion to that produced in the magnet. The double focusing refers to the
refocus of the ion beams of a single mass without any dispersion from the angular
trajectory of the ion beams or the velocity of the ions. When the ESA precedes the
magnet, it is referred to as having a forward geometry. By contrast, a magnet
positioned before the ESA is reverse geometry. In a reverse geometry mass
spectrometer, mass separation occurs relatively early in the beam path and so only one
mass is passed through to the collector. Because the analyser is only transmitting a
single mass ion beam, the abundance sensitivity (the degree to which scattered ions
interfere with the peak of interest) is much increased because any neighbouring
intense ion beam is rejected well out of sight of the collector. Perhaps the biggest
advantage of the SHRIMP-RG is the larger magnet dispersion afforded by the reverse
geometry. Given similar source and collector slits, SHRIMP-RG yield four times the
mass resolution of the SHRIMP-FG designs with the same sensitivity.
2.7 Cathodoluminescence of zircons
The cathodoluminenscence (CL) technique is a high resolution method to determine
the internal crystal growth structure of zircons. This method has been often employed
in recent years to support the interpretation of U/Pb zircon ages (e.g. Chen 1999a,
Poller et al. 1997, Vavra et al. 1996). Studies of Marshall (1988) and Sommerauer (1976)
showed the correlation between colour and brightness of the luminescence and
variations in the concentration of certain trace elements. The variation of the chemical
composition in minerals will be therefore simply reflected in CL zoning. As shown by
Frank et al. (1982), a boundary between two successive zones reveals the shape of the
crystal corresponding to a particular time in its growth history. Furthermore, irregular
boundaries record stages of growth interrupted by some degree of solution. For the CL
analyses zircons were attached to the bottom of a vessel and fixed with Epofix � resin
for polishing. The luminescence was determined by an energetic electron beam on a
JEOL JXA Superprobe at the Institut für Geowissenschaften, Universität Tübingen,
Germany. The Superprobe is equipped with a cathodoluminescence (CL)-detector at an
acceleration voltage of 15 kV. Note that the zircons used for the Pb-evaporation and
conventional U/Pb analyses are not identical with those taken for the CL studies,
however they are supposed to be representative. Cathodoluminescence images of
zircons are presented in Appendix B, plates B1-B5.