Part 4- Isotope Geochemistry

Question 0

How does radiogenic isotope variation occur?

Answer 0

The radioactive decay of unstable nuclides.

Question 1

What are the two principle processes that generate isotopic variations?

Answer 1

Radiogenic and stable isotope geochemistry.

Question 2

How do stable isotopic variations occur?

Answer 2

Variations for physical, chemical or biological processes that change the isotope compositions of elements in a mass dependent manner.

Question 3

What are isobars?

Answer 3

Elements with the same mass number but different combinations of protons and neutrons.
E.g. 110Cd and 110Pd.

Question 4

What does amu stand for and what does it represent?

Answer 4

Atomic mass unit.
1/12th of the mass of 12C
6

Question 5

What is Gram Atomic Weight?

Answer 5

Mass of 1 mole of a particular molecule/element.

Question 6

How does one work out the atomic weight of an element?

Answer 6

The masses of the naturally occurring isotopes multiplied by the molar isotope abundances then added together.

Question 7

What does the horizontal trend on the chart of nuclides show?

Answer 7

Isotopes.

Question 8

What does the diagonal trend on the chart of nuclides show?

Answer 8

Isobars.

Question 9

What are the products of beta/negatron decay?

Answer 9

The new ‘stable’ element.
An electron/beta particle.
An anti-neutrino.
Decay energy (Q).

Question 10

What happens to the original mass and atomic numbers in beta decay?

Answer 10

The mass number remains the same

The atomic number increases by 1

Question 11

What are the products of Positron decay?

Answer 11

The new ‘stable’ element.
A positron.
A neutrino (v).
Decay energy.

Question 12

For which type of nuclides does beta decay occur?

Answer 12

Neutron rich.

Question 13

For which type of nuclides does positron decay occur?

Answer 13

Proton rich.

Question 14

What is electron capture decay?

What happens to the atomic/mass numbers?

Answer 14

A reaction between an electron and a proton to form a neutron therefore mass the same atomic down by 1.

Question 15

What type of nuclides does electron capture decay occur in?

Answer 15

Proton rich.

Question 16

What is branched decay?

Answer 16

Decay of a nuclide by two or more pathways.

Question 17

What are the products of alpha decay?

What type of nuclides decay by this?

Answer 17

The new element.
A helium nucleus (alpha particle).
Decay energy.

Heavy nuclides (protons > 58).

Question 18

What is spontaneous fission?

Which nuclides undergo this?

Answer 18

The break-up of a nuclide into 2 or more fairly heavy daughter nuclides.

Only the heaviest e.g. Thorium and Uranium.

Question 19

In terms of half-lives, within how many are most radioactive decay systems useful?

Answer 19

Within 5 half-lives.

Question 20

Give the basic equation of geochronology and name each term.

Answer 20

D = Do + N(e^lt - 1)

D: total number of daughter atoms

Do: number of daughter atoms present at time of formation (original no.)

N: no of parent atoms in sample

l: lambda, decay constant
t: time

Question 21

How are the abundances of daughters or parents input to the equation?

Answer 21

The abundance measured relative to a stable non-radiogenic isotope.

Question 22

How do scientists work out Do?

Answer 22

Analyse 2 or more samples from the same unit.

Plot the D values- will create a sloping line: isochron.

Draw line across at t = 0.

Where they intercept is value of Do.

Question 23

What conditions are required for the isochron method to be successful?

Answer 23

Isotopic homogenisation at t=0.

Closed system behaviour through time.

Sufficient trace element fractionation.