GEOCHRONOLOGY 2006 Lecture 04 U-Th-Pb Dating

U-Th-Pb Systematics During partial melting and fractional crystallisation of magma, U and Th are concentrated in the liquid phase and become incorporated into the more silica-rich products. 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.

Estimated U-Th-Pb Contents of the Present Day Crust and Mantle

Decay of U to Pb Uranium has three naturally occuring isotopes all of which are radioactive 238U, 235U and 234U Thorium exists primarily as one radioactive isotope 232Th However, there are five short lived intermediate daughters of 238U, 235U and 232Th 238U, 235U and 232Th are each the parent of a chain of radioactive daughters ending with stable isotopes of Pb.

Decay of U to Pb The three principal decay schemes are: 92U238 -> 82Pb206 + 82He4 + 6b- + Q 92U235 -> 82Pb207 + 72He4 + 4b- + Q 90Th232 -> 82Pb208 + 62He4 + 4b- + Q Although 43 isotopes of 12 elements are formed as intermediate daughters in these decay systems, none is a member of more than one series Each decay chain leads to the formation of a specific isotope of Pb

The decay of 238U to 206Pb

U-Th-Pb Decay Equations The standard decay equation for each of the U-Th-Pb decay systems is referenced to 204Pb which is the only non radiogenic isotope of Pb. 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.

U-Th-Pb Decay Equations

Accepted Values The decay constants of the naturally occurring long-lived isotopes of U and Th and the atomic 238U/235U ratio were fixed by the IUGS Subcommission on Geochronology during 1977 (Steiger and Jäger, 1977) as follows

Dating using U-Th-Pb The dates obtained from the three decay systems should be concordant (give the same age) and represent the age of the sample, provided the following conditions are met: 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 238U, 235U and 232Th 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 235U. All analytical results are accurate and free of systematic errors.

Discordant Ages In many instances dates calculated for minerals containing U and Th are not concordant (different ages depending on the decay scheme used). Pb, U or Th loss Intermediate daughter product loss The effect of Pb loss on U-Pb dates can be minimised by calculating a date based on the 207Pb/206Pb ratio. 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. The relationship between the 207Pb/206Pb ratio and time results from the differences in the half lives of their respective parents.

206/207Pb Ages The relationship expressing the relationship between 206Pb and 207Pb ages can be found by combining the decay equations for 238U/206Pb and 235U/207Pb

206/207Pb Ages The above relationship contains the ratio 235U/238U whose value is a constant = 1/137.88. Also the relationship (207Pb/206Pb)* is the ratio of radiogenic 207Pb* to radiogenic 206Pb* where the * indicates that it is radiogenic. The radiogenic (207Pb/206Pb)* ratio is calculated by subtracting the assumed initial 207Pb/204Pb and initial 206Pb/204Pb values from the measured values of these ratios. What this means is that the calculation of 207-206 dates does not require the concentration of Pb or U to be known, just the isotope ratios of Pb

206/207Pb Ages Using the previous information, 207-206 dates can be calculated from the following equation But now we have a problem because the above equation can’t be solved algebraically To remedy this, tables of values for (207/206Pb)* for different values of t can be compiled and an interpolation made

206/207Pb Ages

Concordia Diagrams Data from the previous table can also be used to plot a concordia diagram. Concordia diagrams are the most commonly used diagram for plotting radiogenic U-Pb data This is because isochron diagrams utilising only one decay scheme cannot indicate if the ages are concordant with respect to one of the other two decay schemes Concordia diagrams plot 206Pb*/238U ratios versus 207Pb*/235U ratios which are equal to the el1t-1 and el2t-1 values in the previous table

U-Pb Concordia Diagram

U-Pb Concordia Diagram

U-Pb Concordia Diagram

Behaviour of Accessory Phases To be useful for dating by the U-Th-Pb method, a mineral must be retentive with respect to U, Th and Pb and the intermediate daughters as well as being wide distributed in a variety of rock types This criteria is best satisfied by the mineral Zircon, but other suitable minerals include monazite, titanite, allanite, apatite and xenotime. Zircon is the most useful though because its structure excludes Pb2+ because of the large ionic radius and lower charge compared to Zr4+, U4+ and Th4+ Therefore zircon when it forms contains very little Pb and hence has very high U/Pb and Th/Pb ratios which is good for geochronology

Behaviour of Accessory Phases Let’s look at an example of discordant U-Th-Pb dates Zircon in the Boulder Creek Batholith of Colorado (Stern et al., 1971) 206Pb/204Pb = 1404 Ma 207Pb/204Pb = 1523 Ma 208Pb/204Pb = 1284 Ma (207Pb/206Pb)* = 1682 Ma Dates are very discordant and increase in the order 208 < 206 < 207 < 207-206* This pattern is typical of discordant zircon dates Most likely explanation for discordia is that the zircon has not been a closed system and has lost radiogenic Pb If the reason for discordia is loss of radiogenic Pb then the 207-206 age is probably the closest estimate of the real age

U-Pb Concordia Diagram – Discordia Lines

U-Pb Discordia Lines

Discordia Data plotted on a Concordia Diagram will invariably plot either on the concordia or below the concordia A straight line fitted through the discordant zircons back to the concordia curve gives the age of formation of the zircon The lower intercept is more ambiguous. 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

A Real Example

An Alternative to Concordia Diagrams – The Terra-Wasserburg Plot In the previous example, we could see a variety of different points. Those indicated by the light blue (the points between roughly 900 – 700 Ma) are discordant and plot on a discordia line. 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 1973 and 1974.

Terra-Wasserburg Diagrams The Terra-Wasserburg diagram uses 238U/206Pb* vs (207Pb/206Pb)* and has a different curvature to the traditional concordia

Terra-Wasserburg Diagrams

Real Data

Interpreting U-Th-Pb Dating Results As you have seen we often end up with a big spread in data points on either a Concordia diagram or a Terra-Wasserburg diagram How then do we decide what is what? Two main clues Zircon CL imaging Th/U ratios

Intepreting Zircon Cl Images Zircons have lots of different shapes and different internal zoning structures General Observations Fine oscillatory zoning – igneous Broad unzoned zircon – metamorphic Embayed and eroded clearly preserved cores – usually igneous Overgrowths on 3 – usually Always exceptions to the rule Look at Th/U ratios as well

Interpreting Zircon CL Images

Interpreting Zircon Th/U Ratios