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The Double-Edged Sword of High-Precision U-Pb Geochronology

We think you have liked this presentation. If you wish to download it, please recommend it to your friends in any social system. Share buttons are a little bit lower. Published by Austen Alexander Modified over 3 years ago. Therefore igneous rocks of granitic composition are more enriched in U and Th than basaltic or ultramafic rocks Consequently, the continental crust has more U and Th than the upper mantle. Similar to previous systems, the decay equations can be solved if the concentrations of U, Th and Pb are determined along with the isotopic composition of Pb.

Therefore in theory, you end up with three independent dates based on three separate decay systems. The mineral has remained closed to U, Th and Pb and all intermediate daughters throughout its history Correct values are used for the initial Pb isotope ratios The decay constants of U, U and Th are known accurately The isotopic composition of U is normal and has not been modified by isotope fractionation or by the occurrence of a natural chain reaction based on fission of U.

All analytical results are accurate and free of systematic errors. This ratio is insensitive to recent lead loss, ie when the Pb that was lost had the same isotope composition as the Pb that remained in the mineral being analysed.

What this means is that the calculation of dates does not require the concentration of Pb or U to be known, just the isotope ratios of Pb. It can indicate the time of Pb-loss if Pb loss is episodic, ie occurs as a single event If however Pb-loss occurs via continuous diffusion then the lower intercept has less meaning Monazite sometimes displays U-loss which causes it to plot above the concordia curve on an extrapolation of the discordia line.

Those indicated by the light blue the points between roughly — Ma are discordant and plot on a discordia line.

The Double-Edged Sword of High-Precision U-Pb Geochronology

The interpretation of this line is difficult because of the small different between the curvature of the concordia and the discordia line. To overcome this, an alternative presentation called the Terra-Wasserburg diagram was developed based on the work of Terra and Wasserburg in and What problems can K-Ar dating address?

What are the main limitations of. The U-Pb method is most-often applied to U-bearing accessory minerals such as zircon found in igneous rocks and the most common way to graphically represent the data is the Concordia diagram. U-Pb dating is performed on a mineral that accepts U in the crystal lattice during mineral formation. In zircon, Pb is not readily incorporated into the crystal lattice during mineral formation.

Therefore any measurable lead in the mineral is that which was generated by radiometric decay. When several analyses with different proportions of common- and radiogenic-Pb is plotted on a Tera-Wasserburg concordia diagram a linear regression can be plotted through these analyses anchored at the common-Pb composition. Where this regression or discordia crosses the concordia at its lower-age intercept is interpreted as the age at which the mineral began to accumulate radiogenic-Pb or became a closed system.

The different techniques types discussed in more detail below. Two separate dates for a zircon based on each individual decay scheme may be calculated and plotted on a concordia diagram Fig.

It has been known for several decades that zircons often show evidence of post-crystallization Pb loss. The dating method is usually performed on the mineral zircon. The mineral incorporates uranium and thorium atoms into its crystal structure , but strongly rejects lead.

Therefore, one can assume that the entire lead content of the zircon is radiogenic , i. Thus the current ratio of lead to uranium in the mineral can be used to determine its age.

The method relies on two separate decay chains , the uranium series from U to Pb, with a half-life of 4. The above uranium to lead decay routes occur via a series of alpha and beta decays, in which U with daughter nuclides undergo total eight alpha and six beta decays whereas U with daughters only experience seven alpha and four beta decays.

The existence of two 'parallel' uranium—lead decay routes U to Pb and U to Pb leads to multiple dating techniques within the overall U—Pb system.

The term U—Pb dating normally implies the coupled use of both decay schemes in the 'concordia diagram' see below. However, use of a single decay scheme usually U to Pb leads to the U—Pb isochron dating method, analogous to the rubidium—strontium dating method.

GEOCHRONOLOGY 2006 Lecture 04 U-Th-Pb Dating

Finally, ages can also be determined from the U—Pb system by analysis of Pb isotope ratios alone. This is termed the lead—lead dating method. Clair Cameron Patterson , an American geochemist who pioneered studies of uranium—lead radiometric dating methods, is famous for having used it to obtain one of the earliest estimates of the age of the Earth. Although zircon ZrSiO 4 is most commonly used, other minerals such as monazite see: Where crystals such as zircon with uranium and thorium inclusions do not occur, a better, more inclusive, model of the data must be applied.

U-pb dating concordia

These types of minerals often produce lower precision ages than igneous and metamorphic minerals traditionally used for age dating, but are more common in the geologic record. During the alpha decay steps, the zircon crystal experiences radiation damage, associated with each alpha decay. This damage is most concentrated around the parent isotope U and Th , expelling the daughter isotope Pb from its original position in the zircon lattice.

In areas with a high concentration of the parent isotope, damage to the crystal lattice is quite extensive, and will often interconnect to form a network of radiation damaged areas.

2 comments

  1. Kajit

    I can not take part now in discussion - it is very occupied. But I will soon necessarily write that I think.

    Reply

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