Trace Elements - Definitions Elements that are not stoichiometric constituents in phases in the...
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Transcript of Trace Elements - Definitions Elements that are not stoichiometric constituents in phases in the...
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)
From W. M. White, 2001
Graphical Representation of Elemental AbundanceIn Bulk Silicate Earth (BSE)
Six elements make up 99.1% of BSE ->
The Big Six: O, Si, Al, Mg, Fe, and Ca
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
Trace Element Associations
From W.M. White, 2001
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
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+.
Magnesium (MgMagnesium (Mg2+2+): 65 pm): 65 pm
Calcium (CaCalcium (Ca2+2+): 99 pm): 99 pm
Strontium (SrStrontium (Sr2+2+): 118 pm): 118 pm
Rubidium (RbRubidium (Rb++): 152 pm): 152 pm
Ionic RadiiIonic Radii
1 pm = 10-12 m1 Å = 10-10 m1 pm = 10-2 Å
Values depend on Coordination Number
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 = 10-10 meters = 100 pm
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 incompatibleincompatible– Examples: K, Rb, Sr, Ba, rare earth elements (REE), Ta, Examples: K, Rb, Sr, Ba, rare earth elements (REE), Ta,
Hf, U, PbHf, U, Pb
• Elements readily accommodated in lattice sites of mantle minerals remain in solid phases during melting.– Such elements are termed compatiblecompatible– Examples: Ni, Cr, Co, Os
Trace element substitutions
The (Lanthanide) Rare Earth Elements
Rb
Ce Pr Pm Eu Gd Tb Dy Ho Er Tm Yb Lu
Pa Np Pu Am Cm Bk Cf Es Fm Md No Lr
Nd Sm
Th U
H
Li
Na Mg
Be B C N O F Ne
He
Al Si P S Cl Ar
K Ca Ga Ge As Se Br Kr
XeY Zr Nb Mo Tc Ru Pd Ag CdRh
TaHfLa
Ac
In Sn ITeSb
RnPt Au Tl Bi Po AtHgOsW Re IrCs
Fr
Sc Ti V Cr Mn Fe Co Ni Cu Zn
Ba
Ra
Sr
Pb
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)
Rare Earth Element Ionic Radii
NB that 1 pm = 10-6 microns = 10-12 meters
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
Partition Coefficients for REEs
Dmeltcrystal =
(concentration in mineral)(concentration in melt)
Partition Coefficients for REE in Melts
Dbulk = X1D1 + X2D2 + X3D3 + … + XnDn
Amphibole-Melt
Chondrite Normalized REE patterns
• By “normalizing” (dividing by abundances in chondrites), the “sawtooth” pattern can be removed.
Trace Element Fractionation During Partial Melting
LaNd Rb
Melting ResidueLaLu
LaLuNi
CoSmSrRegion ofPartial Melting
From: http://www.geo.cornell.edu/geology/classes/geo302
Differentiation of the Earth
Mantle
Continental Crust
Rb>SrNd>Sm
La Lu
La>Lu
La LuRb<SrNd<SmLa<Lu
Rb>SrNd>SmLa>Lu
(After partialmelt extraction)
• 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
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
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
Stable Oxygen Isotopes18O‰ = [(Rsample - Rstandard)/Rstandard] x 1000
Three stableisotopes of Ofound in nature:
16O = 99.756%17O = 0.039%18O = 0.205%
Stable Oxygen Isotopes18O‰ = [(Rsample - Rstandard)/Rstandard] x 1000
Isotope Exchange Reactions
2Si16O2 + Fe318O4 = 2Si18O2 + Fe3
16O4
qtz mt qtz mt
This reaction is temperature dependent and thereforecan be used to formulate a geothermometer
8787RbRb
––
8787SrSr
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.
Half-life and exponential decay
Exponential decay:Never get to zero!
Linear decay:Eventually get to zero!
Rate Law for Radioactive Decay
WherePt ≡ quantity of the parent isotope (i.e. 87Rb) at tim et;Po ≡ quantity of the parent isotope a t some earlie r tim eto, whentheisotopic system was closed t o any additional isotopic exchange;λ ≡ i s the characteristic decay constant for the system of interes ,t whichis relat edto t he hal -f life, t1/2, by the equation below:
λ = l 2n / t1/2
t1/2 ≡ i sdefined as the half-life, whi chis the amount of time required fo r 1/2 of theoriginal parent to decay and i s aconstant.
Pt = Po exp -λ (to –t)
1st order rate law
Rb/Sr Age Dating Equation 87Rbt = 87Rbo e -λ (to – )t
(Assum etha t t = 0, fo r t he present)
87Rbo + 87Sro =
87Rb t+ 87Srt
(Conservation of Mass, with 87Sro ast heinitialconcentration and87Srt ast he concentration today)
87Srt - 87Sro = 87Rbt (e λ to – 1)
€
87Sr86Sr
⎛ ⎝ ⎜
⎞ ⎠ ⎟t
=87Sr86Sr
⎛ ⎝ ⎜
⎞ ⎠ ⎟o
+87Rb86Sr
⎛ ⎝ ⎜
⎞ ⎠ ⎟t
(eλt −1)
y = b+ x⋅m
Rb/Sr Isochron Systematics
M1 M2 M3
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.
Clean Lab - Chemical Preparation
http://www.es.ucsc.edu/images/clean_lab_c.jpg
Thermal Ionization Mass Spectrometer
From: http://www.es.ucsc.edu/images/vgms_c.jpg
Schematic of Sector MS
Zircon Laser Ablation Pit
Mantle-Basalt Compatibility
Rb> SrTh> Pb
U> PbNd>SmHf>Lu
Degree of compatibility
Parent->Daughter
Radiogenic Isotope Ratios & Crust-Mantle Evolution
Mantle
Continental CrustLa Lu
La Lu
Rb<Sr
Nd<Sm
Rb>SrNd>Sm
(After partialmelt extraction)
low 87Sr/86Sr
high 143Nd/144Nd
same 87Sr/86Sr and143Nd/144Nd as mantle
Melt
high 87Sr/86Srlow 143Nd/144Nd
Eventually, parent-daughter ratios are reflected in radiogenic isotope ratios.
From: http://www.geo.cornell.edu/geology/classes/geo302
Sr Isotope Evolution on Earth
Time before present (Ga)
Time before present (Ga)
87Sr/86Sr)0
87Sr/86Sr)0
Sr and Nd Isotope Correlations:The Mantle Array
147Sm->143Nd(small->big)
87Rb->87Sr (big->small)