ByNaiyar Imam

Zircon Geo-Chronology and its application in Provenance Analysis

Why dating is important?The age of a sediment layer can establish its historical and/or environmental context.

A time spam between two dates gives a rate of change and may indicate the process that caused the change.

Correlation with other records. Uraninite and monazite were the first minerals used in U-Pb geochronology.

Why Zircon Geo-chronology?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

zircon is a U-rich mineral with a much wider distribution, which is present in most intermediate to acid rocks.

Introduction It’s very difficult task to determine the time of deposition of unfossiliferous sedimentary rocks of the Precambrian.

To date inter bedded volcanic horizons we can use radiometric dating methods such as 40Ar-40Ar of high K2O minerals by U-Pb daing of high U-minerals such as Zircon.

Why 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.

Uranium has three naturally occurring 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 PbThe three principal decay schemes are:92U238 -> 82Pb206 + 82He4 + 6b- + Q92U235 -> 82Pb207 + 72He4 + 4b- + Q90Th232 -> 82Pb208 + 62He4 + 4b- + Q

Each decay chain leads to the formation of a specific isotope of Pb

U-Th-Pb Decay EquationsThe 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.

The decay equations can be solved if the concentrations of U, Th and Pb are determined along with the isotopic composition of Pb.

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)

Dating using U-Th-PbThe 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… 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 AgesIn 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 AgesThe relationship expressing the relationship between 206Pb and 207Pb ages can be found by combining the decay equations for 238U/206Pb and 235U/207Pb

Using the previous information, (206/207Pb)* dates can be calculated from the 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

Concordia PlotThe named ‘Concordia’ was given by G. W. Wetherill in1956, defined as a single curve.

Concordia diagrams are the most commonly used diagram for plotting radiogenic U-Pb data.

This is because isochron diagrams utilizing 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 e1t-1 and e2t-1 values in the previous table

Discordia PlotIf the ratios plotted for samples fall below the concordant line, this produces a discordant plot pattern.

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 uncertain. It can indicate the time of Pb-loss.

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.

Wetherill Plot In1956 Wetherill introduced the Concordia diagram which plots 206Pb*/238U versus 207Pb*/235U.

The parametric Concordia will be non linear because 238U and 235U have different half lives.

The parametric concordia curve can be drawn as the set of solution for equal values of t which is non linear because 238U and 235U have different half lives.

In other words point on concordia curve are where 207Pb*/235U and 206Pb*/238U both corresponds to the same date.

Terra-Wasserburg PlotTera-Wasserburg concordia diagram was given by Tera and Wasserburg in 1972.

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

Terra-Wasserburg diagram is concave upward diagram.

If a cogenetic suite of samples fall off of concordia from variable contamination by a single initial Pb composition then a line drawn through the dataset will intercept concordia at the true age.

Interpreting U-Th-Pb Dating Results Two main clues

1.Zircon CL imaging2.Th/U ratios

Zircons have lots of different shapes and different internal zoning structures

General Observations1.Fine oscillatory zoning – igneous2.Broad unzoned zircon – metamorphic3.Embayed and eroded clearly preserved cores – usually igneous

4.Overgrowths

Provenance Setting U-Pb geo-chronology is the most powerful and common technique for extracting source information from detrital zircon grains.

This approach has been revolutionized with the development and use of SIMS and ICPMS about 40 years back.

Zircon has been used extensively as markers of petrologic identity and provenance in sediments and sedimentary rocks because it is a heavy mineral.

An alternative approach to the dating of deposition of sedimentary rocks is by the U-Pb dating of detrital zircon using an ion microprobe.

The ion nicroprobe SHRIMP can date more than 50 zircon in one session of 24 h.

U-Pb dating of detrital zircons can make important contribution to sediment provenance studies, history of uplift, tectonism etc.

Analysis of detrital zircons from a series of samples will gives clue to formulate the stratigraphic relationship and this will enable comparison of maximum depositional age.