COSMOGENIC NUCLIDES

1
COSMOGENIC NUCLIDES

• Examples 3H, 10Be, 14C, 26Al, 32Si, 35Mn, 36Cl,
  39Ar.
• These radionuclides are produced by nuclear
  reactions between cosmic rays and stable atoms in
  the atmosphere and at the Earths surface
  (SPALLATION ? turns a larger ion into several
  smaller ones)
• The radionuclides are removed from the atmosphere
  by precipitation, and if prevented from contact
  with cosmic rays, radioactive decay will be the
  dominant process controlling their concentration.
• Once removed from contact with cosmic rays, the
  concentration of these nuclides decreases with
  time, i.e., a clock starts ticking.

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CARBON-14

• 14N neutron ? 14C 1H
• The 14C produced in the atmosphere is
  incorporated into CO2 and is rapidly mixed
  throughout the atmosphere.
• The CO2 is then absorbed by plants during
  photosynthesis. Animals take up 14C by eating
  plants, etc. As long as the plant or animal is
  alive, a steady state exists. That is, the rate
  of production of 14C by cosmic rays is just
  balanced by the rate of radioactive decay.
• When the organism dies, uptake of 14C ceases and
  the activity of 14C declines with time, i.e., the
  clock starts ticking.

3

• Decay reaction 14C ? 14N ?- energy
• T1/2 ? 5730 a
• A activity, as measured by a scintillation
  counter.
• A present-day activity A0 initial activity
  (usually assumed to be equal to 14C activity in
  atmosphere.
• This method is used to date C-bearing materials
  such as charcoal, wood, peat, and CaCO3 in
  fossils and sediment.

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• After 7 half-lives, the activity decays to
  immeasurably low values. Thus, the limit for 14C
  dates is 35,000-45,000 a.
• OTHER COMPLICATIONS
• 1) Variations in 14C production rate
• - fluctuations in cosmic-ray flux
• - changes in magnetic field
• 2) Variations in 14C content due to chemical and
  physical fractionation
• 3) Dilution of 14C by low-14C CO2 from fossil
  fuel burning.

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10Be, 26Al

• Both of these isotopes are formed by spallation
  reactions between cosmic rays and O and N in
  atmosphere.
• Both isotopes are removed by rain and snow.
• Upon entering oceans or lakes, the isotopes are
  scavenged by adsorption onto sediment particles
  and carried to the bottom.
• After deposition and removal from contact with
  cosmic rays, their concentrations decrease owing
  to decay.
• 10Be T1/2 1.5 Ma 26Al T1/2 0.716 Ma

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• Similar processes occur during the formation of
  ice sheets in Greenland and Antarctica, and in
  successive lava flow.
• Decay occurs according to the following two
  reactions
• With knowledge of the thickness of the sequence
  (sediment column, ice sheet, lava layers) and t,
  the rate of accumulation of sediment/ice/lava can
  be calculated.

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THE K-Ar METHOD

• Based on the decay reaction
• with a half-life
• T1/2 11.9 B.Y.
• The pertinent geochronometry equation is
• The factor ?e/? is the ratio of decay by 40K ?
  40Ar to total decay of 40K, which also includes
  40K ? 40Ca.
• It is generally assumed that 40Ar0 ? 0, because
  Ar does not usually become incorporated in
  minerals at the time of formation.

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• Why not use the decay 40K ? 40Ca as a
  geochronometer?
• 40Ca is the most common Ca isotope, and Ca
  concentrations are quite high in most rocks. The
  amount of 40Ca that forms in a rock due to decay
  of 40K is relatively small compared to the amount
  of 40Ca already present at time t 0. We cannot
  analyze the small additional amount of radiogenic
  40Ca accurately enough.
• The K-Ar method has been widely used to date
  K-bearing minerals, e.g., K-feldspar, muscovite,
  biotite and hornblende. It is used less
  frequently now because of the ease of loss of
  radiogenic Ar.

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BLOCKING TEMPERATURE

• The K-Ar method actually dates the time at which
  the mineral cooled sufficiently so that
  radiogenic 40Ar cannot diffuse out of the
  crystals.
• Blocking temperature - the temperature at which
  the mineral becomes closed with respect to Ar
  loss.
• Thus, the date obtained with the K-Ar method will
  generally be less than the true age, unless the
  rocks being dated cooled very rapidly.
• Blocking temperatures are different for different
  minerals. We can use this fact to calculate rates
  of uplift.

10
Measuring Isotopes

• While different, isotopes of the same element
  exist in certain fractions corresponding to their
  natural abundance (adjusted by fractionation)
• We measure isotopes as a ratio of the isotope vs.
  a standard material (per mille )

Where Ra is the ratio of heavy/light isotope and
a is the fractionation factor

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Fractionation

• A reaction or process which selects for one of
  the stable isotopes of a particular element
• If the process selects for the heavier isotope,
  the reaction product is heavy, the reactant
  remaining is light
• Isotope fractionation occurs for isotopic
  exchange reactions and mass-dependent differences
  in the rates of chemical reactions and physical
  processes

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Equilibrium vs. Kinetic fractionation

• Fractionaction is a reaction, but one in which
  the free energy differences are on the order of
  1000x smaller than other types of chemical
  reactions
• Just like other chemical reactions, we can
  describe the proportion of reactants and products
  as an equilibrium or as a kinetic function

13
Temperature effects on fractionation

• The fractionation factors, a, are affected by T
  (recall that this affects EA) and defined
  empirically
• Then,
• As T increases, D decreases at high T D goes to
  zero

Where A and B are constants determined for
particular reactions and T is temp. in Kelvins
14
FRACTIONATION DURING PHYSICAL PROCESSES

• Mass differences also give rise to fractionation
  during physical processes (diffusion,
  evaporation, freezing, etc.).
• Fractionation during physical process is a result
  of differences in the velocities of isotopic
  molecules of the same compound.
• Consider molecules in a gas. All molecules have
  the same average kinetic energy, which is a
  function of temperature.

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• Because the kinetic energy for heavy and light
  isotopes is the same, we can write
• In the case of 12C16O and 13C16O we have
• Regardless of the temperature, the velocity of
  12C16O is 1.0177 times that of 13C16O, so the
  lighter molecule will diffuse faster and
  evaporate faster.

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Equilibrium Fractionation

• For an exchange reaction
• ½ C16O2 H218O ? ½ C18O2 H216O
• Write the equilibrium
• Where activity coefficients effectively cancel
  out
• For isotope reactions, K is always small, usually
  1.0xx (this K is 1.047 for example)

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WHY IS K DIFFERENT FROM 1.0?

• Because 18O forms a stronger covalent bond with C
  than does 16O.
• The vibrational energy of a molecule is given by
  the equations

Thus, the frequency of vibration depends on the
mass of the atoms, so the energy of a molecule
depends on its mass.
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• The heavy isotope forms a lower energy bond it
  does not vibrate as violently. Therefore, it
  forms a stronger bond in the compound.
• The Rule of Bigeleisen (1965) - The heavy isotope
  goes preferentially into the compound with the
  strongest bonds.

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Equilibrium Fractionation II

• For a mass-dependent reaction
• Ca2 C18O32- ? CaC18O3
• Ca2 C16O32- ? CaC16O3
• Measure d18O in calcite (d18Occ) and water
  (d18Osw)
• Assumes 18O/16O between H2O and CO32- at some
  equilibrium

T ºC 16.998 - 4.52 (d18Occ - d18Osw) 0.028
(d18Occ-d18Osw)2
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Empirical Relationship between Temp. Oxygen
Isotope Ratios in Carbonates
At lower temperatures, calcite crystallization
tends to incorporate a relatively larger
proportion of 18O because the energy level
(vibration) of ions containing this heavier
isotope decreases by a greater amount than
ions containing 16O. As temperatures drop, the
energy level of 18O declines progressively by
an amount that this disproportionately greater
than that of the lighter 16O.
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RAYLEIGH DISTILLATION

• Isotopic fractionation that occurs during
  condensation in a moist air mass can be described
  by Rayleigh Distillation. The equation governing
  this process is
• where Rv isotope ratio of remaining vapor, Rv
  isotope ratio in initial vapor, the
  fraction of vapor remaining and

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Effect of Rayleigh distillation on the ?18O value
of water vapor remaining in the air mass and of
meteoric precipitation falling from it at a
constant temperature of 25C. Complications 1)
Re-evaporation 2) Temperature dependency of ?
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ISOTOPE FRACTIONATION IN THE HYDROSPHERE

• Evaporation of surface water in equatorial
  regions causes formation of air masses with H2O
  vapor depleted in 18O and D compared to seawater.
• This moist air is forced into more northerly,
  cooler air in the northern hemisphere, where
  water condenses, and this condensate is enriched
  in 18O and D compared to the remaining vapor.
• The relationship between the isotopic composition
  of liquid and vapor is

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• Assuming that ?18Ov -13.1 and ?vl(O) 1.0092
  at 25C, then
• and assuming ?Dv -94.8 and ?vl(H) 1.074 at
  25C, then
• These equations give the isotopic composition of
  the first bit of precipitation. As 18O and D are
  removed from the vapor, the remaining vapor
  becomes more and more depleted.
• Thus, ?18O and ?D values become increasingly
  negative with increasing geographic latititude
  (and altitude.

25
Map of North America showing contours of the
approximate average ?D values of meteoric surface
waters.
26
Because both H and O occur together in water,
?18O and ?D are highly correlated, yielding the
meteoric water line (MWL) ?D ? 8?18O 10
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Deviation from MWL

• Any additional fractionation process which
  affects O and D differently, or one to the
  exclusion of the other will skew a water away
  from the MWL plot
• These effects include
• Elevation effects - (dD -8/1000m, -4/ºC)
• Temperature (a different!)
• Evapotranspiration and steam loss
• Water/rock interaction (little H in most rocks)

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Kinetic Fractionation

• lighter isotopes form weaker bonds in compounds,
  so they are more easily broken and hence react
  faster. Thus, in reactions governed by kinetics,
  the light isotopes are concentrated in the
  products.
• Again, isotope reactions can be exchange
  reactions or mass-dependent chemical or physical
  reactions kinetic factors may affect any of
  these!

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Kinetic fractionation I SO42- reduction

• SO42- CH4 2 H ? H2S CO2 2 H2O
• This reaction is chemically slow at low T,
  bacteria utilize this for E in anoxic settings
• Isotope fractionation of S in sulfide generated
  by microbes from this process generates some of
  the biggest fractionations in the environment
  (-120 for S)
• THEN we need to think about exchange reactions
  with H2S or FeS(aq) as it may continue to
  interact with other S species

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S isotopes and microbes

• The fractionation of H2S formed from bacterial
  sulfate reduction (BSR) is affected by several
  processes
• Recycling and physical differentiation yields
  excessively depleted H2S
• Open systems H2S loss removes 34S
• Limited sulfate governed by Rayleigh process,
  enriching 34S
• Different organisms and different organic
  substrates yield very different experimental d34S
• Ends up as a poor indicator of BSR vs. TSR