Trace Elements - Definitions

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)