Chapter 5 : Geochronology

Question 1

What are some radioactive isotopes in the crust and mantle?

Answer 1

U238, Th232,K40

Question 2

Geochronology

Answer 2

Using radioactive decay to date rocks

Question 3

Half-life

Answer 3

Time for halve the original atoms to decay

Question 4

What does measuring D/P by mass spectrometer assume?

Answer 4

Initially no daughter atoms, system is closed (no exchanges of parent/daughter w/ outside), T1/2 comparable to t so D and P of same order of magnitude

Question 5

Isochron

Answer 5

Plot measurements of daughter and parent normalized by a non-radioactive isotope of the daughter for various minerals of a rock/collection of rocks of constant origin to obtain a straight line to give the age and initial ratio of the rock

Question 6

What does the initial ratio of an isochron indicate?

Answer 6

Source of the rock (ancient mantle, remelting of crust, etc.)

Question 7

What was wrong with the Kelvin model of the age of the earth?

Answer 7

No budget for heat generated by radioactivity, no knowledge of earth convection

Question 8

What is the evolution of Pb in the earth?

Answer 8

Homogenous earth at formation of U, Pb, Th, soon after differentiated into subsystems with different U/Pb ratios, subsystems now closed and U/Pb ratio changes via radioactive decay