Part 8- U-Th-Pb Decay System

Question 0

Which classification does Pb belong to?

Answer 0

Lithophile, siderophile and chalcophile.

Question 1

Which of Goldschmidt’s classifications do U and Th belong to?

Answer 1

Lithophile.

Question 2

What compatibility do U, Th and Pb have in the mantle?

Answer 2

All incompatible. Th most incompatible, Pb least incompatible.

Question 3

Give the equations for the 3 decay systems within the U-Th-Pb system.

Answer 3

238U —> 206Pb + 8a

235U —> 207Pb + 7a

232Th —> 208Pb + 6a

Question 4

What is the fluid mobility of U, Th and Pb?

Answer 4

All are fluid-mobile even at low grade met and during weathering.

Question 5

What is so useful about this decay system?

Answer 5

2 different U isotopes decay to two different Pb isotopes in chemically identically reactions. Due to this, just by measuring the Pb compositions for a suite of samples and using constants we can can calculate an age. From the age parent daughter ratios can be worked out.

Question 6

Give the equation for the slope of a line in a plot of 207Pb/204Pb vs. 206Pb/204Pb and define the terms.

Answer 6

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