1 / 37

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

amish
Download Presentation

Trace Elements - Definitions

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. 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)

  2. 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

  3. 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

  4. Trace Element Associations From W.M. White, 2001

  5. 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

  6. 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

  7. 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

  8. 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

  9. Trace element substitutions

  10. The (Lanthanide) Rare Earth Elements

  11. 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)

  12. Rare Earth Element Ionic Radii NB that 1 pm = 10-6 microns = 10-12 meters

  13. 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

  14. Partition Coefficients for REEs

  15. Partition Coefficients for REE in Melts Amphibole-Melt Dbulk = X1D1 + X2D2 + X3D3 + … + XnDn

  16. Chondrite Normalized REE patterns • By “normalizing” (dividing by abundances in chondrites), the “sawtooth” pattern can be removed.

  17. Trace Element Fractionation During Partial Melting From: http://www.geo.cornell.edu/geology/classes/geo302

  18. 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

  19. 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

  20. 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

  21. 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%

  22. Stable Oxygen Isotopes d18O‰ = [(Rsample - Rstandard)/Rstandard] x 1000

  23. 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

  24. 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

  25. Half-life and exponential decay Linear decay: Eventually get to zero! Exponential decay: Never get to zero!

  26. Rate Law for Radioactive Decay Pt = Po exp - (to –t) 1st order rate law

  27. Rb/Sr Age Dating Equation

  28. Rb/Sr Isochron Systematics M3 M1 M2

  29. 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.

  30. Clean Lab - Chemical Preparation http://www.es.ucsc.edu/images/clean_lab_c.jpg

  31. Thermal Ionization Mass Spectrometer From: http://www.es.ucsc.edu/images/vgms_c.jpg

  32. Schematic of Sector MS

  33. Zircon Laser Ablation Pit

  34. Mantle-Basalt Compatibility Rb> Sr Th> Pb U> Pb Nd>Sm Hf>Lu Parent->Daughter Degree of compatibility

  35. 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

  36. Sr Isotope Evolution on Earth 87Sr/86Sr)0 Time before present (Ga) 87Sr/86Sr)0 Time before present (Ga)

  37. Sr and Nd Isotope Correlations:The Mantle Array 147Sm->143Nd (small->big) 87Rb->87Sr (big->small)

More Related