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Trace Elements - Definitions. Elements that are not stoichiometric constituents in phases in the system of interest For example, IG/MET systems would have different “trace elements” than aqueous systems

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trace elements definitions
Trace Elements - Definitions
  • Elements that are not stoichiometric constituents in phases in the system of interest
    • For example, IG/MET systems would have different “trace elements” than aqueous systems
  • Do not affect chemical or physical properties of the system as a whole to any significant extent
  • Elements that obey Henry’s Law (i.e. has ideal solution behavior at very high dilution)

Graphical Representation of Elemental Abundance

In Bulk Silicate Earth (BSE)

Six elements make up 99.1% of BSE ->

The Big Six: O, Si, Al, Mg, Fe, and Ca

From W. M. White, 2001

goldschmidt s geochemical associations 1922
Goldschmidt’s Geochemical Associations (1922)
  • Siderophile: elements with an affinity for a liquid metallic phase (usually iron), e.g. Earth’s core
  • Chalcophile: elements with an affinity for a liquid sulphide phase; depleted in BSE and are also likely partitioned in the core
  • Lithophile: elements with an affinity for silicate phases, concentrated in the Earth’s mantle and crust
  • Atmophile: elements that are extremely volatile and concentrated in the Earth’s hydrosphere and atmosphere
trace element associations
Trace Element Associations

From W.M. White, 2001

trace element geochemistry
Trace Element Geochemistry
  • Electronic structure of lithophile elements is such that they can be modeled as approximately as hard spheres; bonding is primarily ionic
  • Geochemical behavior of lithophile trace elements is governed by how easily they substitute for other ions in crystal lattices
  • This substitution depends primarily by two factors:
    • Ionic radius
    • Ionic charge
effect of ionic radius and charge

Ionic Radii

Magnesium (Mg2+): 65 pm

Calcium (Ca2+): 99 pm

Strontium (Sr2+): 118 pm

Rubidium (Rb+): 152 pm

Effect of Ionic Radius and Charge
  • The greater the difference in charge or radius between the ion normally in the site and the ion being substituted, the more difficult the substitution.
  • Lattice sites available are principally those of Mg, Fe, and Ca, all of which have charge of 2+.
  • Some rare earths can substitute for Al3+.

Values depend on Coordination Number

1 pm = 10-12 m

1 Å = 10-10 m

1 pm = 10-2 Å

classification of based on radii and charge
Classification of Based on Radii and Charge

Ionic Potential - charge/radius -

rough index for mobility

(solubility)in aqueous solutions:

<3 (low) & >12 (high) more


Low Field Strength (LFS)

Large Ion Lithophile (LIL)

  • 2) High Field Strength (HFS)
  • REE’s

3) Platinum Group Elements

NB 1 Å = 10-10 meters = 100 pm

more definitions
More Definitions
  • Elements whose charge or size differs significantly from that of available lattice sites in mantle minerals will tend to partition (i.e. preferentially enter) into the melt phase during melting.
    • Such elements are termed incompatible
    • Examples: K, Rb, Sr, Ba, rare earth elements (REE), Ta, Hf, U, Pb
  • Elements readily accommodated in lattice sites of mantle minerals remain in solid phases during melting.
    • Such elements are termed compatible
    • Examples: Ni, Cr, Co, Os
rare earth element behavior
Rare Earth Element Behavior
  • The lanthanide rare earths all have similar outer electron orbit configurations and an ionic charge of +3 (except Ce and Eu under certain conditions, which can be +4 and +2 respectively)
  • Ionic radius shrinks steadily from La (the lightest rare earth) to Lu (the heaviest rare earth); filling f-orbitals; called the “Lanthanide Contraction”
  • As a consequence, geochemical behavior varies smoothly from highly incompatible (La) to slightly incompatible (Lu)
rare earth element ionic radii
Rare Earth Element Ionic Radii

NB that 1 pm = 10-6 microns = 10-12 meters

rare earth abundances in chondrites
Rare Earth Abundances in Chondrites
  • “Sawtooth” pattern of cosmic abundance reflects:
    • (1) the way the elements were created (greater abundances of lighter elements)
    • (2) greater stability of nuclei with even atomic numbers
partition coefficients for ree in melts
Partition Coefficients for REE in Melts


Dbulk = X1D1 + X2D2 + X3D3 + … + XnDn

chondrite normalized ree patterns
Chondrite Normalized REE patterns
  • By “normalizing” (dividing by abundances in chondrites), the “sawtooth” pattern can be removed.
trace element fractionation during partial melting
Trace Element Fractionation During Partial Melting


differentiation of the earth
Differentiation of the Earth
  • Melts extracted from the mantle rise to the crust, carrying with them their “enrichment” in incompatible elements
    • Continental crust becomes “incompatible element enriched”
    • Mantle becomes “incompatible element depleted”


uses of isotopes in petrology
Uses of Isotopes in Petrology
  • Processes of magma generation and evolution - source region fingerprinting
  • Temperature of crystallization
  • Thermal history
  • Absolute age determination - geochronology
  • Indicators of other geological processes, such as advective migration of aqueous fluids around magmatic intrusions
isotopic systems and definitions
Isotopic Systems and Definitions
  • Isotopes of an element are atoms whose nuclei contain the same number of protons but different number of neutrons.
  • Two basic types:
    • Stable Isotopes: H/D, 18O /16O, C, S, N (light) and Fe, Ag (heavy)
    • Radiogenic Isotopes: U/Pb, Rb/Sr, Hf/Lu, K/Ar
stable oxygen isotopes
Stable Oxygen Isotopes

d18O‰ = [(Rsample - Rstandard)/Rstandard] x 1000

Three stable

isotopes of O

found in nature:

16O = 99.756%

17O = 0.039%

18O = 0.205%

stable oxygen isotopes1
Stable Oxygen Isotopes

d18O‰ = [(Rsample - Rstandard)/Rstandard] x 1000

isotope exchange reactions
Isotope Exchange Reactions

2Si16O2 + Fe318O4 = 2Si18O2 + Fe316O4

qtz mt qtz mt

This reaction is temperature dependent and therefore

can be used to formulate a geothermometer

radioactive decay and radiogenic isotopes
Radioactive decay and radiogenic Isotopes
  • “Radiogenic” isotope ratios are functions of both time and parent/daughter ratios. They can help infer the chemical evolution of the Earth.
    • Radioactive decay schemes
      • 87Rb-87Sr (half-life 48 Ga)
      • 147Sm-143Nd (half-life 106 Ga)
      • 238U-206Pb (half-life 4.5 Ga)
      • 235U-207Pb (half-life 0.7 Ga)
      • 232Th-208Pb (half-life 14 Ga)
  • “Extinct” radionuclides
    • “Extinct” radionuclides have half-lives too short to survive 4.55 Ga, but were present in the early solar system.




half life and exponential decay
Half-life and exponential decay

Linear decay:

Eventually get to zero!

Exponential decay:

Never get to zero!

rate law for radioactive decay
Rate Law for Radioactive Decay

Pt = Po exp - (to –t)

1st order rate law

instruments and techniques
Instruments and Techniques
  • Mass Spectrometry: measure different abundances of specific nuclides based on atomic mass.
    • Basic technique requires ionization of the atomic species of interest and acceleration through a strong magnetic field to cause separation between closely similar masses (e.g.87Sr and 86Sr). Count individual particles using electronic detectors.
    • TIMS: thermal ionization mass spectrometry
    • SIMS: secondary ionization mass spectrometry - bombard target with heavy ions or use a laser
    • MC-ICP-MS: multicollector-inductively coupled plasma-ms
  • Sample Preparation: TIMS requires doing chemical separation using chromatographic columns.
clean lab chemical preparation
Clean Lab - Chemical Preparation

thermal ionization mass spectrometer
Thermal Ionization Mass Spectrometer


mantle basalt compatibility
Mantle-Basalt Compatibility

Rb> Sr

Th> Pb

U> Pb




Degree of compatibility

radiogenic isotope ratios crust mantle evolution
Radiogenic Isotope Ratios & Crust-Mantle Evolution

Eventually, parent-daughter ratios are reflected in radiogenic isotope ratios.


sr isotope evolution on earth
Sr Isotope Evolution on Earth


Time before present (Ga)


Time before present (Ga)

sr and nd isotope correlations the mantle array
Sr and Nd Isotope Correlations:The Mantle Array



87Rb->87Sr (big->small)