Trace Elements - Definitions

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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 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

4 More Definitions

5 Trace element substitutions

6 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

7 Basalt Types - Trace Elements

8 The (Lanthanide) Rare Earth Elements

9 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)

10 REE Characteristics

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

12 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

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

14 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

15 Trace Element Fractionation

16 Low Degree Partial Melts - REE Fractionation

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

19 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, Hf/Lu, K/Ar

Radioactive decay and radiogenic Isotopes Radiogenic isotope

They can help infer the chemical evolution of the Earth.

Sm- 143 Nd (half-life 106 Ga) 238 U- 206 Pb (half-life 4.

7 Ga) 232 Th- 208 Pb (half-life 14 Ga) Extinct radionuclides

20 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.

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

22 Rate Law for Radioactive Decay P t = P o exp - (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; λ is the characteristic decay constant for the system of interest, which is related to the half-life, t 1/2, by the equation below: λ = 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.

23 Rb/Sr Age Dating Equation 87 Rb t = 87 -λ (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 λ to 1)! # " 87 Sr$ 86 & Sr% t =! # " 87 Sr$ 86 & Sr% o +! # " 87 Rb 86 Sr $ & % t (e 't (1) y = b + x)m

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

25 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:

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

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

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

29 Terrestrial Basalt Generation Summary MORBs are derived from the partial melting of a previously depleted upper mantle under largely anhydrous conditions at relatively shallow depths. True primary mantle melts are rare, although the most primitive alkali basalts are thought to represent the best samples of direct mantle melts. The trace element and isotopic ratio differences among N- MORB (normal), E-MORB (enriched), IAB, and OIB indicate that the Earth s upper mantle has long-lived and physically distinct source regions. Ancient komatiites (>2.5 Ga) indicate that the Earth s upper mantle was hotter in the Archean, but already depleted of continental crustal components.

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

Lecture 38. Igneous geochemistry. Read White Chapter 7 if you haven t already

Lecture 38. Igneous geochemistry. Read White Chapter 7 if you haven t already Lecture 38 Igneous geochemistry Read White Chapter 7 if you haven t already Today. Magma mixing/afc 2. Spot light on using the Rare Earth Elements (REE) to constrain mantle sources and conditions of petrogenesis