Part 4- Isotope Geochemistry Flashcards
How does radiogenic isotope variation occur?
The radioactive decay of unstable nuclides.
What are the two principle processes that generate isotopic variations?
Radiogenic and stable isotope geochemistry.
How do stable isotopic variations occur?
Variations for physical, chemical or biological processes that change the isotope compositions of elements in a mass dependent manner.
What are isobars?
Elements with the same mass number but different combinations of protons and neutrons.
E.g. 110Cd and 110Pd.
What does amu stand for and what does it represent?
Atomic mass unit.
1/12th of the mass of 12C
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What is Gram Atomic Weight?
Mass of 1 mole of a particular molecule/element.
How does one work out the atomic weight of an element?
The masses of the naturally occurring isotopes multiplied by the molar isotope abundances then added together.
What does the horizontal trend on the chart of nuclides show?
Isotopes.
What does the diagonal trend on the chart of nuclides show?
Isobars.
What are the products of beta/negatron decay?
The new ‘stable’ element.
An electron/beta particle.
An anti-neutrino.
Decay energy (Q).
What happens to the original mass and atomic numbers in beta decay?
The mass number remains the same
The atomic number increases by 1
What are the products of Positron decay?
The new ‘stable’ element.
A positron.
A neutrino (v).
Decay energy.
For which type of nuclides does beta decay occur?
Neutron rich.
For which type of nuclides does positron decay occur?
Proton rich.
What is electron capture decay?
What happens to the atomic/mass numbers?
A reaction between an electron and a proton to form a neutron therefore mass the same atomic down by 1.
What type of nuclides does electron capture decay occur in?
Proton rich.
What is branched decay?
Decay of a nuclide by two or more pathways.
What are the products of alpha decay?
What type of nuclides decay by this?
The new element.
A helium nucleus (alpha particle).
Decay energy.
Heavy nuclides (protons > 58).
What is spontaneous fission?
Which nuclides undergo this?
The break-up of a nuclide into 2 or more fairly heavy daughter nuclides.
Only the heaviest e.g. Thorium and Uranium.
In terms of half-lives, within how many are most radioactive decay systems useful?
Within 5 half-lives.
Give the basic equation of geochronology and name each term.
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
How are the abundances of daughters or parents input to the equation?
The abundance measured relative to a stable non-radiogenic isotope.
How do scientists work out Do?
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.
What conditions are required for the isochron method to be successful?
Isotopic homogenisation at t=0.
Closed system behaviour through time.
Sufficient trace element fractionation.
