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Practical 10C

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					Practical 10C

Objectives:

   1) to provide practice in using isotopic data to establish the geological age of
      formation or discrete igneous body
   2) To assist students to gain an appreciation of the application of relative time and
      absolute time estimates to resolving a geological problem.

Preamble:

The Western Australian Craton or Yilgarn is an important host to gold and nickel
deposits largely hosted within greenstone belts (altered and metamorphosed volcano-
sedimentary complexes). With the approaching exhaustion of the great open pit at
Kalgoorlie, and the search for new gold deposits, it became important to know more
about the timing of gold deposition and its controls.
The White Lake Conglomerate which is located near the base of the greenstone belt in
this region is a likely target to establish the onset of sedimentation and tectonics of the
greenstone belt by dating its contained sodic granite pebbles or clasts. It unconformably
overlies yet older beds that have been intruded by the Mungari potassic granite that has
contact metamorphosed the older beds. Geological interpretation has the Mungari Granite
intruding at a later time than the White Lake conglomerate; however this needs to be
confirmed.

Data:

You are provided with two data sets one for the sodic pebbles in the White Lake
conglomerate, the other for the Mungari Granite. Analyse these data and establish
whether the current geological interpretation is correct.

Table 1: Data for granite pebbles from the White Lake metaconglomerate.
                                                              87
Rock type      Sample #        Rb ppm         Sr ppm            Rb/86Sr      87
                                                                               Sr/86Sr
granite        58213           24.1           234.5           0.297          0.7093
granite        58217           15.8           455             0.100          0.7062
gneiss
granite        58216           12.8           399             0.109          0.7065
granite        58209           15.9           342             0.134          0.7066
granite        58210           26.5           385             0.199          0.7098
granophyre     58219           6.1            230             0.077          0.7053
granite        58211           16.3           133.5           0.352          0.7170
granite        #12             11.3           253             0.129          0.7070
gneiss
porphyritic      #14               18.5              318               0.168             0.7086
granite
granite          #16               8.5               325               0.075             0.7052
gneiss
granite          #17               14.8              207               0.206             0.7101
gneiss
porphyry         #02               16.9              266               0.183             0.7101
porphyry         #04               9.9               306               0.094             0.7062
porphyry         #06               50.6              257               0.570             0.7231
porphyry         #07               33.0              246               0.387             0.7163
porphyry         #09               13.1              285               0.132             0.7082
porphyry         #10               31.7              374               0.245             0.7118

The coefficient of variation for 87 Rb/86 Sr is estimated as 1% and the standard error for 87 Sr/86 Sr
is 0.0002.

Questions:

    1)   Would you include the data point for #58213 in your age assessment?
    2)   Are there any other data point values that you might exclude from your treatment?
    3)   What is the age of the pebbles?
    4)   Do you consider that the porphyry pebbles are of the same age as the granite
         pebbles (i.e. are they co-magmatic)?



Table 1: Isotope data for Mungari Granite.
                                                                       87
Rock type        Sample #          Rb ppm            Sr ppm              Rb/86Sr         87
                                                                                           Sr/86Sr
Total rock       G32 (B)           400.3             84.9              13.62             1.2041
Total rock       G32 (D)           422.2             95.4              12.79             1.1745
K-felspar        G32 (B)           635.8             130.5             14.08             1.2246
Xenolith         G33               127.4             86.3              4.268             0.8693
Xenolith         G33               123.2             82.0              4.342             0.8700
Biotite          G33               1998.0            8.06              716.7             27.286

The coefficient of variation for 87 Rb/86 Sr is 0.5%, and the standard error for 87 Sr/86 Sr is 0.0004.

Both data sets have been modified and adapted from Compston and Turek, J of the Geol Soc of
Australia, vol 20, Pt 2, pp217-222, 1973 for this exercise.

Questions:

    1) What is the age of the Mungari Granite?
    2) Is this younger or older than the White Lake Conglomerate pebbles?
   3) What is your best estimate of the age of deposition of the Conglomerate?

Approach:

This short exercise serves to familiarise you with the use and application of one of the
isotopic systems (the Rubidium-Strontium decay scheme) used to resolve timing issues
within the geosciences. If you have not yet read pages 8.14 to 8.15 of the study book
please read them before commencing this procedure.
Ensure that you work carefully and precisely, otherwise error magnification can affect
your outcomes.
Note that the decay constant has been recently revised after more precise measurement to
λ =1.402 x 10-11 yr-1 and that the value for the slope “m” needs to be determined with
precision.

Age equation: t = 1 / λ . ln (1 + m); where m= the slope of the regression.

Solution:

You should determine geological ages that lie within 2300-2600 Ma age range.

JMW/10/10/07

				
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posted:10/12/2011
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