Elsevier

Chemical Geology

Volume 192, Issues 1–2, 16 December 2002, Pages 59-79
Chemical Geology

Correction of common lead in U–Pb analyses that do not report 204Pb

https://doi.org/10.1016/S0009-2541(02)00195-XGet rights and content

Abstract

The presence of common lead contamination in zircons used for U–Pb geochronology is a potentially serious source of error. Traditionally, common lead is measured by analysis of 204Pb, and the isotopic composition of lead corrected accordingly. Some analytical methods (e.g. LAM-ICPMS) do not report 204Pb. Correction methods are available for such analyses, but these assume that the only source of discordance in a zircon is the presence of common lead. Using such a correction on a lead analysis that contains a discordance component caused by lead loss will invariably lead to overcorrection, and hence to a meaningless, young age. By assuming that the observed 206Pb/238U, 207Pb/235U and 208Pb/232Th ratios of a discordant zircon can be accounted for by a combination of lead loss at a defined time, and the presence of common lead of known composition, a correction method can be designed that neither uses 204Pb nor assumes concordance. The method proposed here involves a numeric solution to a set of equations relating the content of radiogenic lead in a zircon or other U/Th-enriched mineral to its total lead content, the amount of common lead present, the age of initial crystallization, the age of lead loss and the amount of lead lost in that process. An estimate for the age of lead loss is needed, but in the absence of prior knowledge of this age, the recalculation procedure can be set up in such a way that the bias in initial age caused by a systematic error in the age of lead loss is minimized. Despite this limitation, the method will give less bias in the corrected ages than alternative correction methods.

Introduction

The U–Th–Pb system of high U–Pb minerals such as zircon provides us with some of the most versatile, precise and robust geochronometers currently available. Common lead is lead of nonradiogenic 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 undetected or uncorrected common lead is very detrimental to U–Pb dating. In U–Pb geochronology using thermal or secondary ionization mass spectrometers, the minor, nonradiogenic isotope 204Pb is analysed 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. The use of this correction is critically dependent on precise measurement of 204Pb, which is routine in a thermal or secondary ionization mass spectrometry.

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 combines the lateral spatial resolution of the ion microprobe with greater speed of analysis and considerably less capital investment. Unfortunately, the method used to compensate for the presence of common lead in thermal or secondary ionization mass spectrometry cannot generally be applied to LAM-ICPMS analyses. This problem arises primarily because the low peak/background ratio of the 204Pb peak is compounded by the ubiquitous 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.

Current 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. In this paper, an alternative approach to common lead correction of U–Pb data is presented, which neither requires knowledge of the amount of 204Pb present, nor assumes that corrected compositions plot on the concordia. This method is thus applicable to U–Pb analyses which do not report 204Pb, and to grains which have suffered lead loss in addition to contamination by common lead.

Section snippets

Theoretical background

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:206Pbr=238U(eλ238t−1)where λ238 is the decay constant for 238U, and subscript r denotes radiogenic lead. Similar equations apply to the 235U–207Pb and 232Th–208Pb decay series; decay constants and other relevant data can be found in standard introductory texts (e.g.

The correction algorithm

To avoid unnecessarily cluttered equations, a shorthand notation defined in Table 1 is introduced for the derivation of the expressions used to determine the amount of common lead and the error propagation. Let fc be the atomic fraction of common 206Pb in an analysed lead, defined by: fc=206Pbc/206Pb=206Pbc/(206Pbc+206Pbr) where 206Pbr is the radiogenic lead and 206Pbc is the common lead. The radiogenic lead component of a common lead-bearing zircon is then given by:yr=y(1−fc)xr=x−yc7kfczr=z−yc8

Comparison with 207Pb and 208Pb corrections

In Fig. 2, the line AB, and hence the path followed by a point during correction for common lead is defined by the position of the analysed, common lead-bearing zircon (at A′) and the composition of common lead (at infinity). Hence, this line can only intersect the 3D concordia if fl=0, i.e. for zircons which have not lost any lead (and for fl=1, i.e. for zircons which retain no memory of t1). The alternative correction methods (i.e. the 207Pb and 208Pb corrections, Ludwig, 2001) work with 2D

Conclusions

LAM-ICPMS U–Pb analyses of zircons and other U-enriched minerals which do not include data on 204Pb can be corrected for common lead. The method developed in this paper assumes that the discordance of Pb/U and Pb/Th isotopic ratios observed in a zircon is a sum of the effects of common lead contamination and lead loss. The radiogenic lead component in the zircon and its initial age can be determined by the simultaneous solution of six equations in the six variables describing the mass balance

Acknowledgements

Thanks are due to S.Y. O'Reilly and W.L. Griffin for invitations to two extended visits to GEMOC, Maquarie University, in 2000 and 2001, during which this work was done. Travel costs were covered by grants from the Faculty of Mathematics and Natural Sciences, University of Oslo, and from the Norwegian Research Council. The study has benefited greatly from discussions with Bill Griffin, Simon Jackson, Norman Pearson, Elena Belousova and Chris Ryan, and from reviews by Kenneth R. Ludwig and

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