Question 1-3 [10]: You have been asked to confirm an age for a pegmatite that wa
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Question 1-3 [10]: You have been asked to confirm an age for a pegmatite that was obtained using the Rb-Sr method of geochronology. The information provided is: 87Rb (ppm weight) 121.200 87Sr (ppm wt) 3.29679 87Rb Atomic Weight 86.9092 87Sr Atomic Weight 86.9088 No initial 87Sr Decay constant () 1.420 10-11 Student Number: U-Pb geochronology on the same rock unit gave a discordant concordant age of 3.4 Ga and a lower intercept of 1.94Ga (Ga billion years). The Rb-Sr is a wholerock age. a) What is the age of the sample according to the provided Rb-Sr system information? b) Does the Rb-Sr age make sense? Can it be integrated with the U-Pb ages or does it contradict them? c) Can you (briefly) explain a geological situation that might cause the situation? d) If you could not assume that there was no initial daughter product, and you had access to a variety of samples and additional lab time, what might you do?Explanation / Answer
a) The age of the sample according to the provided Rb-Sr system information is 1.88 x 109 years or 1.88 Ga.
b) The relative differences of ages obtained from Rb-Sr and U-Pb suggest that there is an inclusion possibly in the form of zircon which gives an age of 3.4 Ga. The whole rock age suggests that the rock formed much later than the transported inclusion which is now present within the rock. The age thus obtained from Rb-Sr makes sense.
c) The rock unit might have undergone changes through slow rates of cooling which was responsible for late crystallization of the rock constituents while the inclusion of foreign matter took place in the magma.
d) Segregating the samples containing U-Pb possibly having xenocrysts of zircon and determining the ages again using the same isotopic ratios would delineate the differences in ages.
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