GEMOC ARC National Key Centreρt: The correlation coefficient between errors in 206Pb/238U and 207Pb/235U in the uncorrected data. If measured, values should be given in the appropriate input column in the worksheet. Otherwise, the parameter can be controlled from the parameter setting dialog box.
ρρ: The correlation coefficient between errors in common-lead corrected 206Pb/238U and 207Pb/235U ratios. The correlation coefficient is modified (reduced) by the discordance correction (see equation). No modification is made in case of 207Pb or 208Pb corrections.
Observed discordance: The conventional discordance in the input data, in percent. A negative value is
Uncorrected ages: 207Pb/206Pb, 206Pb/238U, 207Pb/235U and 208Pb/232Th ages are calculated from the raw data, with errors.
Disc: Discordance correction
Pb-207: 207Pb corrections is applied, see comment for reason
Pb-208: 208Pb correction has been applied (only if allowed by user)
None: No correction is applied, either because zircon is concordant within chosen error limit for concordance test, calculated amount of common lead is less than analytical error, or because correction has failed. See comments column.
Comment: Explains what has 'gone wrong' if discorance correction does
Common lead: Percentage of common lead of chosen compositon in analysis (fc), with 1 s error.
Corrected ratios: Ratios after common lead correction.
Intercept age: The best estimate of the crysallization age of a zircon which has been corrected by the discordance correction method (t1)
Corrected ages: 207Pb/206Pb, 206Pb/238U, 207Pb/235U and 208Pb/232Th ages are calculated from the corrected data. If the 207Pb correction has been used, 208Pb/232Th may become negative.
The composition of common lead can either be estimated from the global lead model of Stacey & Kramers (1975) at any given age (default is 0 Ma), or given explicitly by entering ratios in the first dialog box.
The age of lead loss is an input parameter which must be constrained by other information, or assumed. If t2 is not known in advance, the parameters must be set up so that the resulting bias in t1 is minimized
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Concordance limit To decide which points are concordant within error and which are not, ComPbCorr must know at what level of confidence the user wants to test for concordance. Default is 1σ, this can be set to any multiple of the analytical error.
Error correlation: The errors in 206Pb/238U and 207Pb/238U are highly positively correlated. The correlation coefficient must be taken into account when data are used for geochronology.