HONOURS 2008
GEOCHRONOLOGY MODULE
Dr. Jodie Miller
Practical Exercise 04
Construction of a Concordia diagram and assessment of
ages
Date: 22nd September 2008
Due Date: 10:30, 23rd September, 2008
YOU MUST HAND IN THIS EXERCISE SHOWING ALL WORKING AND WITH A PLOT DRAWN BY
HAND ON GRAPH PAPER.
In this practical you will look at a small set of U-Th-Pb data.
Before you process any data you need to draw a Concordia diagram. To do this you need to calculate
values of e1t-1 and e2t-1 for various values of t. In the lecture notes the table was set out as:
Time
(207Pb/206Pb)*
e1t-1
e2t-1
Where
1 = decay constant 238U = 1.55125 x 10-10 y-1
2 = decay constant 235U = 9.8485 x 10-10 y-1
and (207Pb/206Pb)* = 1/137.88 x (e2t-1/ e1t-1)
Question 1: Construct a table of (207Pb/206Pb)*, e1t-1, and e2t-1 values for values of t between 0 and
3.0Ga in increments of 0.25 Ga.
Once you have calculated your values you then need to construct a Concordia diagram. The
Concordia diagram is constructed by plotting values of 206Pb*/238U versus 207Pb*/235U. Since these are
equal to e1t-1 and e2t-1 you simply need to plot the values in the table that you have already
constructed.
Question 2: Plot a Concordia diagram to show a curve with points equal to 0.25Ga between 0 and
3.0Ga
Now that you have a Concordia curve plotted we need to add some data. The following data has been
determined by SHRIMP analysis on zircon grains.
Sample A
Sample B
Sample C
Sample D
206Pb*/238U
0.3746
0.5256
0.5138
0.4421
207Pb*/235U
9.852
13.664
13.326
11.532
(207Pb/206Pb)*
0.1908
0.1886
0.1881
0.1892
Samples B and C are from a volcanic felsic porphyry while sample A is from a schist.
Question 3: Calculate ages for the above four samples based on their 206Pb*/238U, 207Pb*/235U, and
(207Pb/206Pb)* ratios. Doing this manually you cannot work out the (207Pb/206Pb)* age other than by
interpolation from the Concordia that you have just drawn.
If the samples above are concordant then the dates that you calculate from the above data should be
all the same. If they are not we refer to them as discordant.
Question 4: Are the dates calculated above concordant or discordant and for which samples?
If the samples are discordant it may still be possible to extract information on them by putting a
discordia line through the samples to see where they intersect the Concordia curve. If the samples all
formed at the same time then they should lie on a straight line that intersects the Concordia at the time
of formation which should be where concordant samples plot.
Question 5: Plot the ratios given in the table above on the Concordia diagram that you have drawn.
Can you draw a discordia line through any discordant samples. If so what age does the Concordia
intersect give?
Question 6: Write a short summary of how you interpret this data. This answer should be no more
than ¼ of a page.