L10 - Radiogenic Isotope Geochemistry

Define an Isotope

Same number of protons (Z) and different neutrons (N)

• Example: 12C and 13C

Define an Isobars

Same mass number

• Example: 87Rb and 87Sr or 14C and 14N

Define an isotone

Same number of neutrons (N) and different number of protons (Z)

• Example: 2H and 3He

Define a stable isotope

An isotope that is time invariant

Define a radioactive isotope

An isotope that decays with time

Define a radiogenic Isotope

An isotope that is produced (at least in part) by decay from a radioactive isotope

What is this chart called?

Chart of nuclides

What type of nuclear species represents a vertical change?

Isotones - different number of protons

What type of nuclear species represents a horizontal change?

Isotopes different number of neutrons

What type of nuclear species represents a diagonal change?

Isobars - same mass number (A = Z + N)

What does the black line represent?

The stable isotopes

What do the colored areas represent?

Unstable nuclides

Where is the black line located of the graph?

Where protons(Z) = neutrons(N)

Define alpha decay

When an unstable isotope emits an alpha particle

• Alpha particle = 2 protons and 2 neutrons

When does alpha decay usually occur?

Occurs in heavy nuclei

Define beta decay and its two main types

Beta decay occurs through the emission of a B- or B+ particle

• B+ = add 1 proton, and subtract 1 neutron

• B- = subtract 1 proton, and add 1 neutron

How does the mass change with the occurrence of beta decay?

Atomic mass remains the same

• A = Z + N (they always equate out)

Define Gamma Decay

When a excited nucleus is brought to a relaxed state through the loss of high energy photons

What can stop the different decay particles?

1. Alpha Particle

2. Beta particle

3. Gamma Particle

1. Alpha Particle - paper

2. Beta particle - Aluminium

3. Gamma Particle - Lead

Where do the Radioactive decay schemes plot on the chart of nuclides?

• B- = above the stability line (Z+1, N-1)

• B+ = below the stability line (Z-1, N+1)

• alpha = diagonal down and left (Z-2 , N-2)

How does age terminology change in both absolute and relative age?

1. absolute = Ka, Ma, Ga

2. Relative = kyr, myr, gyr

What equation is this?

The law of radioactive decay

What does the N-value refer to?

The number of particles

What is this equation called?

the familiar decay equation

What do No and N represent?

• No = the number of parent isotopes at time 0

• N = the number of parent isotopes at time t

What is a half-life?

the time it takes for half of the parent nuclides to decay

• defined by this formula

What is this equation called?

The isochron equation and initial ratio

What do the following represent?

1. D

2. D’

3. P

4. t

5. 0

1. D = radiogenic isotope of the daughter element

2. D’ = the stable isotope of the daughter element

3. P = isotope of parent element that decays into daughter

4. t = time elapsed

5. 0 = time 0

What do the following represent?

1. (D/D’)t

2. (D/D’)0

3. P/D’

1. (D/D’)t = the ratio of the system

2. (D/D’)0 = the initial isotope ratio

3. P/D’ = the parent-daughter ratio of the system

Why is the initial isotope ratio important?

Key to understanding the origin and evolution of the system under consideration

What type of plot is this? What is plotted on it?

An Isochron Plot

• plot contains geologic materials of the same initial isotope ratio and age

What part of this equation represents the following on an isochron plot?

1. Y

2. b

3. x

4. m

1. Y = (D/D’)t

2. b = (D/D)0

3. x = (P/D’)t

4. m = e^(lamdat*)-1

How do the sample ratios look on an isochron at t=0 and t = t?

note the flat to diagonal lines

What is Elemental Fractionation

When different elements arrange themselves to achieve the most stable configuration (elements separate)

• different element = separate

What is isotopic homogenization?

Since isotopes have similar properties (same element) the isotopes are arranged randomly among the available sites

• same element and different isotopes = homogenous/mixed

Define the decay constant

The probability that a radioactive nuclide will decay per unit of time

• specific to the radioactive nuclide under consideration