Presentation on theme: "Trace Elements - Definitions"— Presentation transcript:
1 Trace Elements - Definitions Elements that are not stoichiometric constituents in phases in the system of interestFor example, IG/MET systems would have different “trace elements” than aqueous systemsDo not affect chemical or physical properties of the system as a whole to any significant extentElements 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 CaFrom 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 coreChalcophile: elements with an affinity for a liquid sulphide phase; depleted in BSE and are also likely partitioned in the coreLithophile: elements with an affinity for silicate phases, concentrated in the Earth’s mantle and crustAtmophile: 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 ionicGeochemical behavior of lithophile trace elements is governed by how easily they substitute for other ions in crystal latticesThis substitution depends primarily by two factors:Ionic radiusIonic charge
6 Effect of Ionic Radius and Charge Magnesium (Mg2+): 65 pmCalcium (Ca2+): 99 pmStrontium (Sr2+): 118 pmRubidium (Rb+): 152 pmIonic RadiiThe 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 Number1 pm = m1 Å = m1 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) moremobilityLow Field Strength (LFS)Large Ion Lithophile (LIL)2) High Field Strength (HFS)REE’s3) Platinum Group ElementsNB 1 Å = meters = 100 pm
8 More DefinitionsElements 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 incompatibleExamples: K, Rb, Sr, Ba, rare earth elements (REE), Ta, Hf, U, PbElements readily accommodated in lattice sites of mantle minerals remain in solid phases during melting.Such elements are termed compatibleExamples: Ni, Cr, Co, Os
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 = 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
15 Partition Coefficients for REE in Melts Amphibole-MeltDbulk = 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:
18 Differentiation of the Earth Melts extracted from the mantle rise to the crust, carrying with them their “enrichment” in incompatible elementsContinental crust becomes “incompatible element enriched”Mantle becomes “incompatible element depleted”From:
19 Uses of Isotopes in Petrology Processes of magma generation and evolution - source region fingerprintingTemperature of crystallizationThermal historyAbsolute age determination - geochronologyIndicators 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 1000Three stableisotopes of Ofound in nature:16O = %17O = %18O = %
23 Isotope Exchange Reactions 2Si16O2 + Fe318O4 = 2Si18O2 + Fe316O4qtz mt qtz mtThis reaction is temperature dependent and thereforecan 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 schemes87Rb-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–87Rb87Sr
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
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 spectrometrySIMS: secondary ionization mass spectrometry - bombard target with heavy ions or use a laserMC-ICP-MS: multicollector-inductively coupled plasma-msSample Preparation: TIMS requires doing chemical separation using chromatographic columns.