Why ?
Common lead is lead of non-radiogenic origin incorporated into a mineral during its initial formation, in subsequent recrystallization processes or by contamination during analysis. As the presence of even small amounts of unsupported lead in a zircon or other datable mineral will increase its apparent U-Th-Pb ages, the presence of uncetected or uncorrected common lead is very detrimental to U-Pb dating. The use of plasma-ionization mass spectrometry with in-situ laser-ablation microsampling (LAM-ICPMS) is a new and promising analytical approach to U-Pb dating of U-enriched minerals (e.g. zircon). The method used to compensate for the presence of common lead in thermal or secondary ionization mass spectrometryuses the minor, non-radiogenic isotope 204Pb as a monitor of common lead, and the signals of the radiogenic isotopes 206Pb, 207Pb and 208Pb are corrected in proportion to their relative abundances in common lead. Unfortunately, this approach cannot generally be applied to LAM-ICPMS analyses, at least not when a quadrupole mass spectrometer is used. This problem arises primarily because the low peak/background ratio of the 204Pb peak is compounded by the ubiquitious presence of Hg in the argon nebulizer gas; 204Hg interferes on 204Pb, while the 202Hg peak is so small that reliable measurement is difficult, if not impossible, and hence an overlap correction of sufficient precision is seldom feasible. Popular methods for common lead correction of such U-Pb analyses make assumptions of ideal concordance of 206Pb/238U and 207Pb/235U or 208Pb/232Th (e.g. Ludwig 2001), which may not always be justified. ComPbCorr uses a different approach to common lead correction, described by Andersen (2002), which neither requires knowledge of the amount of 204Pb present, nor assumes that corrected compositions plot on the concordia.
How is it done ?
In a U-bearing mineral, radiogenic lead isotopes (206Pb, 207Pb and 208Pb) will accumulate with time, due to radioactive decay of uranium and thorium isotopes. For the 238U-206Pb parent-daughter pair, the growth equation is given by:
However, if non-radiogenic lead is incorporated into the mineral at the time of initial crystallization or in some later event, the lead present in the system is no longer exclusively due to in-situ radiogenic accumulation, for example:
,
where 206Pbc is the 206Pb component in the non-radiogienic lead incorporated into the system, known as common lead.
The influence of common lead and lead loss on the present-day composition of a zircon can best be envisaged in terms of a 3D conventional concordia diagram.
Using the notation of Andersen (2002), the radiogenic lead component of in a common lead bearing zircon which crystallized at t1 and lost lead in a subsequent event at t2 can be described by the equations:
The points C1 (concordant lead of age t1), C2 (concordant lead at t2) and B (radiogenic lead component) in the 3D concordia diagram are colinear in three dimensions, and must therefore be related by the expressions:
and
The common lead corrected composition is related to the composition of concordant lead at t1 and t2 and the fraction of lead lost at t2:
The equations above can be combined to an equation which relates the composition of concordant lead at t1 (i.e. point C1 in Fig. 2) to measured composition (A�) and the composition of concordant lead of age t2:
Substituting the relevant expressions for concordant lead compositions at t1 and t2 yields an equation which can be solved numerically for t1.
Once t1 has been determined, the amount of common lead� is given by:
and the amount of lead lost at t2 by:
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GEMOC ARC National Key Centre