Trace Elements - Definitions

Transcription

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 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 Al 3+. Ionic Radii Magnesium (Mg 2+ ): 65 pm Calcium (Ca 2+ ): 99 pm Strontium (Sr 2+ ): 118 pm Rubidium (Rb + ): 152 pm Values depend on Coordination Number 1 pm = m 1 Å = 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 1) Low Field Strength (LFS) Large Ion Lithophile (LIL) 2) High Field Strength (HFS) REE s 3) Platinum Group Elements NB 1 Å = 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 H Li Na He Be B C N O F Ne Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

11 Chondrite vs. Solar Chemistry

12 REE Characteristics

13 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)

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

15 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

16 crystal D melt = Partition Coefficients for REEs (concentration in mineral) (concentration in melt)

17 Partition Coefficients for REE in Melts Amphibole-Melt D bulk = X 1 D 1 + X 2 D 2 + X 3 D X n D n

18 Trace Element Fractionation

19 Low Degree Partial Melts - REE Fractionation

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

21 Trace Element Fractionation During Partial Melting From:

22 Differentiation of the Earth Continental Crust La Lu Rb>Sr Nd>Sm La>Lu Rb>Sr Nd>Sm La>Lu Mantle (After partial melt extraction) Rb<Sr Nd<Sm La<Lu La Lu 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:

23 Uses of Isotopes in Geochemistry 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

24 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, 18 O / 16 O, C, S, N (light) and Fe, Ag (heavy) Radiogenic Isotopes: U/Pb, Rb/Sr, Lu/Hf, K/Ar

25 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 87 Rb- 87 Sr (half-life 48 Ga) 147 Sm- 143 Nd (half-life 106 Ga) 238 U- 206 Pb (half-life 4.5 Ga) 235 U- 207 Pb (half-life 0.7 Ga) 232 Th- 208 Pb (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. 87 Rb β 87 Sr

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

27 Rate Law for Radioactive Decay P t = P o exp -λ (t o t) 1st order rate law Where P t! quantity of the parent isotope (i.e. 87 Rb) at time t; P o! quantity of the parent isotope at some earlier time t o, when the isotopic system was closed to any additional isotopic exchange; "! is the characteristic decay constant for the system of interest, which is related to the half-life, t 1/2, by the equation below: " = ln 2 / t 1/2 t 1/2! is defined as the half-life, which is the amount of time required for 1/2 of the original parent to decay and is a constant.

28 Rb/Sr Age Dating Equation 87 Rb t = 87 -! (to t) Rb o e (Assume that t = 0, for the present) 87 Rb o + 87 Sr o = 87 Rb t + 87 Sr t (Conservation of Mass, with 87 Sr o as the initial concentration and 87 Sr t as the concentration today) 87 Sr t - 87 Sr o = 87 Rb t (e! to 1)! # " 87 Sr$ 86 & Sr% t =! # " 87 Sr$ 86 & Sr% o +! # " 87 Rb 86 Sr $ & % t (e 't (1) y = b + x)m

29 Rb/Sr Isochron Systematics M 1 M 2 M 3

30 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. 87 Sr and 86 Sr). 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.

31 Clean Lab - Chemical Preparation

32 Thermal Ionization Mass Spectrometer From:

33 Schematic of Sector MS

34 Zircon Laser Ablation Pit

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

36 Radiogenic Isotope Ratios & Crust-Mantle Evolution Continental Crust Rb>Sr Nd>Sm high 87 Sr/ 86 Sr low 143 Nd/ 144 Nd La Lu Melt same 87 Sr/ 86 Sr and 143 Nd/ 144 Nd as mantle Mantle (After partial melt extraction) Rb<Sr Nd<Sm low 87 Sr/ 86 Sr La high 143 Nd/ 144 Nd Lu Eventually, parent-daughter ratios are reflected in radiogenic isotope ratios. From:

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

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