Transcript Trace Elements

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)
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
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+.
Ionic Radii
Magnesium (Mg2+): 65 pm
Calcium (Ca2+): 99 pm
Strontium (Sr2+): 118 pm
Rubidium (Rb+): 152 pm
Values depend on Coordination Number
1 pm = 10-12 m
1 Å = 10-10 m
1 pm = 10-2 Å
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 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
Trace element substitutions
The (Lanthanide) Rare Earth Elements
H
He
Li Be
B
C
N
O
F
Na Mg
Al
Si
P
S
Cl Ar
K
Ca
Sc
Ti
V
Cr Mn
Fe Co
Ni
Cu Zn
Ga Ge As Se
Rb
Sr
Y
Zr
Nb Mo Tc
Ru Rh
Pd
Ag
Cd
In
Sn
Sb
Cs
Ba La Hf
Pt Au
Hg
Tl
Pb
Bi Po
Fr
Ra
Ta
W
Re Os
Ir
Te
Ne
Br Kr
I
Xe
At
Rn
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
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 forbitals; 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
crystal
Dmelt

(concentration in mineral)
(concentration in melt)
Partition Coefficients for REE in Melts
Amphibole-Melt
Dbulk = X1D1 + X2D2 + X3D3 + … + XnDn
Chondrite Normalized REE patterns
• By “normalizing” (dividing by abundances in chondrites),
the “sawtooth” pattern can be removed.
Trace Element Fractionation
During Partial Melting
La
Lu
Ni
La
Rb
Sr
Re gion of
Partial Melting
Nd
Sm
M elting Re sidue
Co
La
Lu
From: http://www.geo.cornell.edu/geology/classes/geo302
Differentiation of the Earth
Rb>Sr
Nd>Sm
La>Lu
Continental Crust
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: 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 Isotopes
d18O‰ = [(Rsample - Rstandard)/Rstandard] x 1000
Three stable
isotopes of O
found in nature:
16O
= 99.756%
17O = 0.039%
18O = 0.205%
Stable Oxygen Isotopes
d18O‰ = [(Rsample - Rstandard)/Rstandard] x 1000
Isotope Exchange Reactions
2Si16O2 + Fe318O4 = 2Si18O2 + Fe316O4
qtz
mt
qtz
mt
This reaction is temperature dependent and therefore
can be used to formulate a geothermometer
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
•
•
•
•
•
b–
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.
87Rb
87Sr
Half-life and exponential decay
Exponential decay:
Never get to zero!
Linear decay:
Eventually get to zero!
Rate Law for Radioactive Decay
Pt = Po exp
- (to –t)
1st order rate law
Whe re
Pt  quan tity of the parent isotope (i.e. 87Rb) at tim e t;
Po  quan tity of the parent isotope at some earlier time to, when the
isotopic system was closed to any additi ona l is otopic exch ange ;
  is the cha racteristic decay constant for the sys tem of interest, which
is related to the h alf-li fe, t1/2, by th e equa tion below:
ln 2 / t1/2
t1/2  is defined as the half-life, which is the amount of tim e requir ed for 1/2 of the
origin al parent to decay and is a constant.
Rb/Sr Age Dating Equation
Rb t = 87Rb o e -to – t)
(Assume that t = 0, for the p resent)
87
87
Rb o + 87Sro = 87Rb t + 87Srt
(Conserva tion of Mass, with 87Sro as the initi al
conc entration and 87Srt as the con centration today)
Srt - 87Sro = 87Rb t (e to – 1)
87
87Sr  87Sr  87 Rb t
86   86    86  (e  1)
 Sr t  Sr o  Sr t
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>
Parent->Daughter
Sr
Th>
Pb
U> Pb
Nd>Sm
Hf>Lu
Degree of compatibility
Radiogenic Isotope Ratios & Crust-Mantle Evolution
Continental Crust
Rb>Sr high 87Sr/86Sr
Nd>Sm low 143Nd/ 144Nd
Melt
Mantle
(After partial
melt extraction)
Rb<Sr
La
Lu
same 87Sr/86Sr and
143Nd/144Nd as mantle
low 87Sr/86Sr
La
Nd<Sm
Lu
high 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
87Sr/86Sr)
0
Time before present (Ga)
87Sr/86Sr)
0
Time before present (Ga)
Sr and Nd Isotope Correlations:
The Mantle Array
147Sm->143Nd
(small->big)
87Rb->87Sr
(big->small)