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)