20) isotopes and radioactive decay

stable vs unstable isotopes, and applications

stable: don’t change over time, isotope thermometry, tracers

unstable: change into diff elemetn over time through radioactive decay, geochronology, tracer

elemental fractionation

controlled by size and charge
controlled by partitioning into diff minerals, meaning controlled by compatibility
compatibility is controlled by size and charge

isotopic fractionation

controlled by difference in mass

ionic radius is the same, same number of electrons, diff neutrons.

much smaller influence than elemental fractionation. during natural process, there is sometimes a slight preference for a material to incorporate a heavier isotope than a lighter one

relative mass difference between 16-17 and 16-18, twice difference
fractionates the isotopes by twice the amount
if we change 18/16 ratio by 2 units, the 17/16 ratio will change by 1 unit
slope is 0.5 = mass dependent fractionation line

relative mass difference of heavy elements is smaller, so isotopic fractionation will be smaller to (1/87 vs 1/86)

temperature influence of isotope fracitonation

increasing temperature means mass difference matter less due to flexibility of sites
at a certain temp they will have the same isotopic composition

low temp = larger fractionation

3 important points about isotope fractionation

mass dependent
effects are larger for bigger relative mass differences between isotopes
effects are stronger at lower temperatures

why does radioactive decay occur? extra info

unstable nuclides are in a higher energy state
band of stability is at the bottom of an energy valley
its an attempt of unstable nuclide to become more stable by getting rid of extra energy

radiogenic: generated by a radioactive isotope, daughter of parent isotope
decay rate is dependent on the energy state of the nuclide
independent of pressure, temp, and chem composition

types of radioactive decay

beta, positron, electron capture, alpha, nuclear fission

details on beta decay

neutron converted to a proton and beta- particle (negatron)
1 less neutron, 1 more proton
diagonally up-left

details on positron decay

proton converted to neutron and beta+ particle (positron)
1 less proton, 1 more neutron
diagonally down-right

details on electron capture decay

proton capturing an electron from orbital changes into neutron
1 less proton, 1 more neutron
diagonally down-right

details on alpha decay

alpha particle ejected from nucleus
mass number decreases by 4, number of protons decreases by 2
diagonally down-left skipping a box
Q is total alpha decay energy

details on nuclear fission

heavy nuclide breaking into smaller nuclides and energy

what’s important about K40

can decay to 2 different daughters
using positron decay (Ca40), or electron capture decay (Ar40)

know this a little bit

blue is beta+ or electron capture
red is beta–
yellow is alpha decay
green is spontaneous fission

details on gamma ray

gamma rays aren’t a radioactive decay mechanism, but they are released when decay occurs in any mechanism

what is a decay chain

when a parent decays to a stable daughter product through intermediate daughter nuclides that are also radioactive
U238 all the way to Pb206: first is alpha, then beta-, then again beta-, then a bunch of alpha decays to Po or Pb206