If I want to plot detrital zircon ages of Precambrian age (600 to 2500 Ma) on a probability density plot, which age (i.e. 206Pb/238U or 207Pb/206Pb) should I use? In the literature, one often encounters the recommendation that for Precambrian ages the 207Pb/206Pb age should be used. Why is this so? Does it have something to do with the common lead correction? If I do not apply such a correction to my laser-data, is it nevertheless an advantage to use the 207/206 age?
Hi Dominik,
You will often find that for ages over 1000Ma that the 207/206 ages are quoted while those younger than 1000 Ma and particularly those below 500 Ma the 206/238 ages are used. This is because and to paraphrase the Ludwig paper I upload "206/238 ratios for Phanerozoic samples carries all of the chronometric information and the 207/206 ratios are used for evalution purposed only". More detail in the attached paper. Hope it helps.
Dear Chao and Robin,
thanks a lot for your useful answers and for the Ludwig-Paper.
I think there is not straight forward answer to your question. If your data is highly discordant, the results may be misleading, whatever ration you use. A short example from a Precambrian paleosoil on the Archaean basement which was metamorphosed in 1900 Ma in granulite-grade. Some zircons are concordant with either 2.5-2.6 Ga or 1.9 Ga, the others are discordant and all ratios plot between 1.9 - 2.5 Ga. The older concordant grains clearly give the age basement tonalite and the younger the age of granulite metamorphism. The discordant grains simply show the various lead loss between 2.5 - 1.9 (maybe during weathering and /or following metamorphic event). Unfortunately I have not yet published the data. But, think if these palesoil zircons are subject to erosion, transport and deposition. You don´t have any or at least lesser amount of geological control on what happened. The histogram, whatever ratio you use could be quite misleading, considering the other possible sediment sources coming in and the fact that there could be zircons from the metamorphosed paleosoil at leas from 2.5 to 2.4 and 2.0 to 1.9 seemingly statistically concordant.
If your zircons are younger than 1,0 Ga, then only 206Pb/238U age should be used. But in case of concordabce you may better use a Concordia age.
Dear All,
thanks a lot for the helpful and interesting answers to my question. The example form the metamorphosed paleosol is quite instructive and it is also a warning not to forget the many factors which might seriously bias detrital zircon ages.
Best regards
Dominik
See this paper: "Martin, A. J., Burgy, K. D., Kaufman, A. J., & Gehrels, G. E. (2011). Stratigraphic and tectonic implications of field and isotopic constraints on depositional ages of Proterozoic Lesser Himalayan rocks in central Nepal. Precambrian Research, 185(1), 1-17". Under method section, you will find why 07/06Pb ages are best to use when grains are >1400 Ma.
Dear Subhadip,
thanks a lot for your answer and for directing my attention to the Martin et al. paper.
Best regards
Dominik
Dear Mr. Dominik,
According to Gehrels (2014), the decision is based on analytical precision.
Below I copy the sentece from Gehrels (2014):
"Once data have been acquired, an important decision is whether the interpreted age should be based on the 206Pb*/238U, 207Pb*/235U, or 206Pb*/207Pb* date. For a concordant analysis, the decision is based on analytical precision, which varies such that 206Pb*/238U is most precise for 1.2 Ga analyses. The 207Pb*/235U date always has greater uncertainty than either the 206Pb*/238U or 206Pb*/207Pb* date and is rarely reported as the preferred age. The only exception is for very young zircons, where 206Pb*/238U dates are compromised by the decay of Th (Schärer 1984)."
I hope this help you.
Best wishes,
Felipe.
DOI: 10.1146/annurev-earth-050212-124012
http://www.annualreviews.org/doi/abs/10.1146/annurev-earth-050212-124012
Dear Felipe,
many thanks for your answer and for the very concise sentence from the Gehrels paper.
Best regards
Dominik
you should use 207Pb/206Pb for zircon age >= 1000 Ma because old zircon is normally affected by post crystalisation event leading to lost of Pb* in zircon as Pb* does not stay in the crystal lattice as Uranium. Lead lost can be see on concordia diagram that analytical points lie below the concordia curve. Therefore 206Pb/238U gives younger age than its real age. In order to avoid more or less this problem, 207Pb/206Pb should be used because 207Pb and 206Pb are the same chemical symbol then have nearly the same chemical behavior leading nearly unchange ratio value of 207Pb/206Pb
Hi Dominik!
I've only just found this question and I am sure you know the answer by now but I thought I would take this opportunity to advocate the use of single analysis concordia ages (Ludwig, 1998; see Zimmermann et al., 2017 for an explanation)! This removes the need for a cutoff age and uses the MSWD as a statistical measure for assessing the validity of the age. See also https://www.researchgate.net/post/Why_not_report_and_use_single_analysis_concordia_ages_Ludwig_1998_for_detrital_zircon_data
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