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Geochemisches Zentrallabor, Universität Tübingen, Germany. Sr was loaded with a Ta-
HF activator on pre-conditioned W filaments and measured in single-filament mode. Nd
was loaded as phosphate on pre-conditioned Re filaments and measurements were
performed in a Re double filament configuration. The 87 Sr/ 86 Sr isotope ratios were
normalised to 86 Sr/ 88 Sr = 0.1194 and the 143 Nd/ 144 Nd isotope ratios to 146 Nd/ 144 Nd =
0.7219. Analyses of 24 separate loads of Ames metal (Geological Survey of Canada,
Roddick et al. 1992) gave a 143 Nd/ 144 Nd of 0.512125 ± 0.000010 (errors are 2� of the
mean) and within the same period the NBS 987 Sr standard yielded a 87 Sr/ 86 Sr of
0.710259 ± 0.000012 (n = 28). Total procedural blanks (chemistry and loading) were <
200 pg for Sr and < 50 pg for Nd. Sm and Nd isotopic ratios of 17 samples are given in
Appendix A, Tab. A6.
2.5 U/Pb isotope dilution and Pb/Pb evaporation age determination methods on
zircons
For conventional U/Pb analyses zircon populations consisting of a few
morphologically identical grains were washed in hot 6 N HCl and hot 7 N HNO3 prior to
dissolution to remove surface contamination. A mixed 205 Pb/ 235 U tracer solution was
added to the grains. Dissolution was performed in PTFE vessels in a 1900 psig (130
bar) PARR acid digestion bomb (Parrish 1987) according to the vapour digestion
method (Wendt & Todt 1991). The bomb was placed in a temperature-controlled oven
at 200°C for one week in 22N HF and for one day in 6N HCl to assure re-dissolution of
the fluorides into chloride salts and avoid U/Pb fractionation (Mattinson 1994).
Separation and purification of U and Pb were carried out on Teflon columns with a 40 �l
bed of AG1-X8 (100-200 mesh) anion exchange resin following the procedure described
in Poller et al. (1997). Further details on U/Pb analytical techniques are given in Chen et
al. (2000).
Thermal Ionization Mass Spectrometry was applied for isotopic measurements of 11
samples on a Finnigan MAT 262 mass spectrometer at the Geochemisches Zentrallabor,
Universität Tübingen, Germany. The measurements were performed by Fukun
Chen, Wolfgang Siebel, and myself. Four samples were measured at the Isotope
Geochemistry Lab of the University of Washington, USA, by Bruce K. Nelson. Pb was
loaded onto a pre-conditioned Re filament according to the Si-gel method (Cameron et
al. 1969) and measured at ~1300°C in single-filament mode. U was loaded with 1N
HNO3 onto a pre-conditioned Re filament and was measured in double-filament
configuration. Total procedural blanks were < 10 pg for Pb and < 2 pg for U. A factor of
1‰ per mass unit for instrumental mass fractionation was applied to all Pb analyses,
using NBS SRM 981 as reference material. Initial common Pb remaining after correction
for tracer and blank was corrected using values from the Stacey & Kramers (1975)
model. The decay constants for U are those given in Jaffey et al. (1971). U/Pb discordia
intercepts were calculated using regression treatment (Wendt 1986).
The single zircon Pb-evaporation analyses were made by Fukun Chen following the
procedures of Kober (1986, 1987), Körner & Todt (1988), Cocherie et al. (1992), Klötzli
(1997) and Kröner & Hegner (1998). Untreated whole zircon grains or fragments were
analysed using a double Re filament configuration. The motif behind this method is the
assumption that the Pb with the highest activation energy, i.e. the highest evaporation