GEMOC ARC National Key Centre

The first dialog box: Setting parameters

In this dialog box, the parameters controlling the correction algoritm can be set

The default composition of common lead is present-day average global Stacey & Kramers (1975) lead. This can be changed by chosing another age on the Stacey-Kramers curve.

or by ticking the Specify lead composition box.

and enter the appropriate lead isotope ratios.

The age of lead loss is an input parameter - default is zero. It can be changed at will, but chosing an inappropriate value can cause complications. Se Andersen (2002) for details.

The errors in ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U are positively correlated.

You may chose the following options:

Read observed error correlation coefficients from the input columns

Use a constant error correlation coefficient (default is 0.90).

Or infer an approximate error correlation coefficient from the reduced isotope ratios. This is not really an appropriate method, and is likely to underestimate the real correlation coefficient.

Use discordance correction. If this option is unchecked: Analyses which will correct to inversely discordant compositions by the discordance method will be corrected by the ²⁰⁷Pb method. If checked, the discordance correction will always be used.

Sigma level.Sets the limit for concordance to within a multiple of the analytical error. Error correlation is observed.

Checking the Enable 208Pb correction box allows the ²⁰⁸Pb correction for grains which will not correct by other methods. Note that this is a cosmetic rather than scientifically sound correction method (see Andersen 2002).

The composition of common lead        The age of lead loss      Error correlation     Optional ²⁰⁷Pb and ²⁰⁸Pb corrections    The concordance limit