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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)
slide2

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

mobility

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

Amphibole-Melt

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

From: http://www.geo.cornell.edu/geology/classes/geo302

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”

From: http://www.geo.cornell.edu/geology/classes/geo302

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.

b–

87Rb

87Sr

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

http://www.es.ucsc.edu/images/clean_lab_c.jpg

thermal ionization mass spectrometer
Thermal Ionization Mass Spectrometer

From: http://www.es.ucsc.edu/images/vgms_c.jpg

mantle basalt compatibility
Mantle-Basalt Compatibility

Rb> Sr

Th> Pb

U> Pb

Nd>Sm

Hf>Lu

Parent->Daughter

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.

From: http://www.geo.cornell.edu/geology/classes/geo302

sr isotope evolution on earth
Sr Isotope Evolution on Earth

87Sr/86Sr)0

Time before present (Ga)

87Sr/86Sr)0

Time before present (Ga)

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

147Sm->143Nd

(small->big)

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