chapter 9: trace elements note magnitude of major element changes figure 8.2. harker variation...
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Chapter 9: Trace ElementsChapter 9: Trace Elements
Note magnitude of major element changes
Figure 8.2. Harker variation diagram for 310 analyzed volcanic rocks from Crater Lake (Mt. Mazama), Oregon Cascades. Data compiled by Rick Conrey (personal communication). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Chapter 9: Trace ElementsChapter 9: Trace Elements
Now note magnitude of trace element changes
Figure 9.1. Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Element DistributionElement DistributionGoldschmidt’s rules (simplistic, but useful)
1. 2 ions with the same valence and radius should exchange easily and enter a solid solution in amounts equal to their overall proportions
How does Rb behave? Ni?
Goldschmidt’s rules
2. If 2 ions have a similar radius and the same valence: the smaller ion is preferentially incorporated into the solid over the liquid
Fig. 6.10. Isobaric T-X phase diagram at atmospheric pressure After Bowen and Shairer (1932), Amer. J. Sci. 5th Ser., 24, 177-213. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
3. If 2 ions have a similar radius, but different valence: the ion with the higher charge is preferentially incorporated into the solid over the liquid
Chemical FractionationChemical Fractionation
The uneven distribution of an ion between two competing (equilibrium) phases
Exchange equilibrium of a component i between two phases (solid and liquid)
i (liquid) = i (solid)
eq. 9.2 K = =
K = equilibrium constant
a solid
a liquidi
i
X solid
X liquidi
i
i
i
Trace element concentrations are in the Henry’s Law region of concentration, so their activity varies in direct relation to their concentration in the system
Thus if XNi in the system doubles the XNi in all phases will double This does not mean that XNi in all phases
is the same, since trace elements do fractionate. Rather the XNi within each phase will vary in proportion to the system concentration
incompatible elements are concentrated in the melt
(KD or D) « 1
compatible elements are concentrated in the solid
KD or D » 1
For dilute solutions can substitute D for KD:
D =
Where CS = the concentration of some element in the solid phase
CS
CL
Incompatible elements commonly two subgroups Smaller, highly charged
high field strength (HFS) elements (REE, Th, U, Ce, Pb4+, Zr, Hf, Ti, Nb, Ta)
Low field strength large ion lithophile (LIL) elements (K, Rb, Cs, Ba, Pb2+, Sr, Eu2+) are more mobile, particularly if a fluid phase is involved
Compatibility depends on minerals and melts involved.
Which are incompatible? Why?
Table 9-1. Partition Coefficients (CS/CL) for Some Commonly Used Trace
Elements in Basaltic and Andesitic Rocks
Olivine Opx Cpx Garnet Plag Amph MagnetiteRb 0.01 0.022 0.031 0.042 0.071 0.29 Sr 0.014 0.04 0.06 0.012 1.83 0.46 Ba 0.01 0.013 0.026 0.023 0.23 0.42 Ni 14.0 5.0 7.0 0.955 0.01 6.8 29.Cr 0.7 10.0 34.0 1.345 0.01 2.0 7.4La 0.007 0.03 0.056 0.001 0.148 0.544 2.Ce 0.006 0.02 0.092 0.007 0.082 0.843 2.Nd 0.006 0.03 0.23 0.026 0.055 1.34 2.Sm 0.007 0.05 0.445 0.102 0.039 1.804 1.Eu 0.007 0.05 0.474 0.243 0.1/1.5* 1.557 1.Dy 0.013 0.15 0.582 3.17 0.023 2.024 1.Er 0.026 0.23 0.583 6.56 0.02 1.74 1.5Yb 0.049 0.34 0.542 11.5 0.023 1.642 1.4Lu 0.045 0.42 0.506 11.9 0.019 1.563
Data from Rollinson (1993). * Eu3+/Eu2+ Italics are estimated
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For a rock, determine the bulk distribution coefficient D for an element by calculating the contribution for each mineral
eq. 9.4: Di = WA Di
WA = weight % of mineral A in the rock
Di = partition coefficient of element i in mineral A
A
A
Example: hypothetical garnet lherzolite = 60% olivine, 25% orthopyroxene, 10% clinopyroxene, and 5% garnet (all by weight), using the data in Table 9.1, is:
DEr = (0.6 · 0.026) + (0.25 · 0.23) + (0.10 · 0.583) + (0.05 · 4.7) = 0.366
Table 9-1. Partition Coefficients (CS/CL) for Some Commonly Used Trace
Elements in Basaltic and Andesitic Rocks
Olivine Opx Cpx Garnet Plag Amph MagnetiteRb 0.010 0.022 0.031 0.042 0.071 0.29 Sr 0.014 0.040 0.060 0.012 1.830 0.46 Ba 0.010 0.013 0.026 0.023 0.23 0.42 Ni 14 5 7 0.955 0.01 6.8 29Cr 0.70 10 34 1.345 0.01 2.00 7.4La 0.007 0.03 0.056 0.001 0.148 0.544 2Ce 0.006 0.02 0.092 0.007 0.082 0.843 2Nd 0.006 0.03 0.230 0.026 0.055 1.340 2Sm 0.007 0.05 0.445 0.102 0.039 1.804 1Eu 0.007 0.05 0.474 0.243 0.1/1.5* 1.557 1Dy 0.013 0.15 0.582 1.940 0.023 2.024 1Er 0.026 0.23 0.583 4.700 0.020 1.740 1.5Yb 0.049 0.34 0.542 6.167 0.023 1.642 1.4Lu 0.045 0.42 0.506 6.950 0.019 1.563Data from Rollinson (1993). * Eu3+/Eu2+ Italics are estimated
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Trace elements strongly partitioned into a single mineral
Ni - olivine in Table 9.1 = 14
Figure 9.1a. Ni Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Incompatible trace elements concentrate liquid
Reflect the proportion of liquid at a given state of crystallization or melting
Figure 9.1b. Zr Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Trace Element BehaviorTrace Element Behavior The concentration of a major element in a
phase is usually buffered by the system, so that it varies little in a phase as the system composition changes
At a given T we could vary Xbulk from 35 70 % Mg/Fe without changing the composition of the melt or the olivine
Trace element concentrations are in the Henry’s Law region of concentration, so their activity varies in direct relation to their concentration in the system
Trace element concentrations are in the Henry’s Law region of concentration, so their activity varies in direct relation to their concentration in the system
Thus if XNi in the system doubles the XNi in all phases will double
Trace element concentrations are in the Henry’s Law region of concentration, so their activity varies in direct relation to their concentration in the system
Thus if XNi in the system doubles the XNi in all phases will double
Because of this, the ratios of trace elements are often superior to the concentration of a single element in identifying the role of a specific mineral
K/Rb often used the importance of amphibole in a source rock K & Rb behave very similarly, so K/Rb should be ~ constant If amphibole, almost all K and Rb reside in it Amphibole has a D of about 1.0 for K and 0.3 for Rb
Table 9-1. Partition Coefficients (CS/CL) for Some Commonly Used Trace
Elements in Basaltic and Andesitic Rocks
Olivine Opx Cpx Garnet Plag Amph MagnetiteRb 0.010 0.022 0.031 0.042 0.071 0.29 Sr 0.014 0.040 0.060 0.012 1.830 0.46 Ba 0.010 0.013 0.026 0.023 0.23 0.42 Ni 14 5 7 0.955 0.01 6.8 29Cr 0.70 10 34 1.345 0.01 2.00 7.4La 0.007 0.03 0.056 0.001 0.148 0.544 2Ce 0.006 0.02 0.092 0.007 0.082 0.843 2Nd 0.006 0.03 0.230 0.026 0.055 1.340 2Sm 0.007 0.05 0.445 0.102 0.039 1.804 1Eu 0.007 0.05 0.474 0.243 0.1/1.5* 1.557 1Dy 0.013 0.15 0.582 1.940 0.023 2.024 1Er 0.026 0.23 0.583 4.700 0.020 1.740 1.5Yb 0.049 0.34 0.542 6.167 0.023 1.642 1.4Lu 0.045 0.42 0.506 6.950 0.019 1.563Data from Rollinson (1993). * Eu3+/Eu2+ Italics are estimated
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Sr and Ba (also incompatible elements) Sr is excluded from most common minerals
except plagioclase Ba similarly excluded except in alkali feldspar
Table 9-1. Partition Coefficients (CS/CL) for Some Commonly Used Trace
Elements in Basaltic and Andesitic Rocks
Olivine Opx Cpx Garnet Plag Amph MagnetiteRb 0.010 0.022 0.031 0.042 0.071 0.29 Sr 0.014 0.040 0.060 0.012 1.830 0.46 Ba 0.010 0.013 0.026 0.023 0.23 0.42 Ni 14 5 7 0.955 0.01 6.8 29Cr 0.70 10 34 1.345 0.01 2.00 7.4La 0.007 0.03 0.056 0.001 0.148 0.544 2Ce 0.006 0.02 0.092 0.007 0.082 0.843 2Nd 0.006 0.03 0.230 0.026 0.055 1.340 2Sm 0.007 0.05 0.445 0.102 0.039 1.804 1Eu 0.007 0.05 0.474 0.243 0.1/1.5* 1.557 1Dy 0.013 0.15 0.582 1.940 0.023 2.024 1Er 0.026 0.23 0.583 4.700 0.020 1.740 1.5Yb 0.049 0.34 0.542 6.167 0.023 1.642 1.4Lu 0.045 0.42 0.506 6.950 0.019 1.563Data from Rollinson (1993). * Eu3+/Eu2+ Italics are estimated
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Compatible example: Ni strongly fractionated olivine > pyroxene Cr and Sc pyroxenes » olivine Ni/Cr or Ni/Sc can distinguish the effects of olivine
and augite in a partial melt or a suite of rocks produced by fractional crystallization
Models of Magma EvolutionModels of Magma Evolution Batch Melting
The melt remains resident until at some point it is released and moves upward
Equilibrium melting process with variable % melting
Models of Magma EvolutionModels of Magma Evolution Batch Melting
eq. 9.5
CL = trace element concentration in the liquid
CO = trace element concentration in the original rock before melting began
F = wt fraction of melt produced = melt/(melt + rock)
CC
1Di(1 F) F
L
O
Batch Melting
A plot of CL/CO vs. F for various values of Di using eq. 9.5
Di = 1.0
Figure 9.2. Variation in the relative concentration of a trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9.5) for equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Di » 1.0 (compatible element)
Very low concentration in melt
Especially for low % melting (low F)
Figure 9.2. Variation in the relative concentration of a trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9.5) for equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Highly incompatible elements Greatly concentrated in
the initial small fraction of melt produced by partial melting
Subsequently diluted as F increases
Figure 9.2. Variation in the relative concentration of a trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9.5) for equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
As F 1 the concentration of every trace element in the liquid = the source rock (CL/CO 1)
As F 1
CL/CO 1
CC
1Di (1 F) F
L
O
Figure 9.2. Variation in the relative concentration of a trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9.5) for equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
As F 0 CL/CO 1/Di
If we know CL of a magma derived by a small degree of batch melting, and we know Di we can estimate the concentration of that element in the source region (CO)
CC
1Di (1 F) F
L
O
Figure 9.2. Variation in the relative concentration of a trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9.5) for equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
For very incompatible elements as Di 0
equation 9.5 reduces to:
eq. 9.7
C
C
1
FL
O
CC
1Di (1 F) F
L
O
If we know the concentration of a very incompatible element in both a magma and the source rock, we can determine the fraction of partial melt produced
Worked Example of Batch Melting: Rb and SrBasalt with the mode:
1. Convert to weight % minerals (Wol Wcpx etc.)
Table 9.2. Conversion from mode to
weight percent
Mineral Mode Density Wt prop Wt%
ol 15 3.6 54 0.18
cpx 33 3.4 112.2 0.37
plag 51 2.7 137.7 0.45
Sum 303.9 1.00
Worked Example of Batch Melting: Rb and Sr
Table 9.2. Conversion from mode to
weight percent
Mineral Mode Density Wt prop Wt%
ol 15 3.6 54 0.18
cpx 33 3.4 112.2 0.37
plag 51 2.7 137.7 0.45
Sum 303.9 1.00
Basalt with the mode:
1. Convert to weight % minerals (Wol Wcpx etc.)
2. Use equation eq. 9.4: Di = WA Di
and the table of D values for Rb and Sr in each mineral to calculate the bulk distribution coefficients: DRb = 0.045 and DSr = 0.848
Table 9.3 . Batch Fractionation Model for Rb and Sr
CL/CO = 1/(D(1-F)+F)
DRb DSr
F 0.045 0.848 Rb/Sr0.05 9.35 1.14 8.190.1 6.49 1.13 5.730.15 4.98 1.12 4.430.2 4.03 1.12 3.610.3 2.92 1.10 2.660.4 2.29 1.08 2.110.5 1.89 1.07 1.760.6 1.60 1.05 1.520.7 1.39 1.04 1.340.8 1.23 1.03 1.200.9 1.10 1.01 1.09
3. Use the batch melting equation
(9.5) to calculate CL/CO for various values of F
From Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
CC
1Di(1 F) F
L
O
4. Plot CL/CO vs. F for each element
Figure 9.3. Change in the concentration of Rb and Sr in the melt derived by progressive batch melting of a basaltic rock consisting of plagioclase, augite, and olivine. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Incremental Batch MeltingIncremental Batch Melting
Calculate batch melting for successive batches (same equation)
Must recalculate Di as solids change as minerals are selectively melted (computer)
Fractional CrystallizationFractional Crystallization1. Crystals remain in equilibrium with each
melt increment
Rayleigh fractionationRayleigh fractionation The other extreme: separation of each
crystal as it formed = perfectly continuous fractional crystallization in a magma chamber
Rayleigh fractionation
The other extreme: separation of each crystal as it formed = perfectly continuous fractional crystallization in a magma chamber Concentration of some element in the residual
liquid, CL is modeled by the Rayleigh equation:
eq. 9.8 CL/CO = F (D -1) Rayleigh Fractionation
Other models are used to analyze Mixing of magmas Wall-rock assimilation Zone refining Combinations of processes
The Rare Earth Elements (REE)The Rare Earth Elements (REE)
Contrasts and similarities in the D values:
All are incompatible
Also Note:
HREE are less incompatible
Especially in garnet
Eu can 2+ which conc. in plagioclase
Table 9-1. Partition Coefficients (CS/CL) for Some Commonly Used Trace Elements in Basaltic and Andesitic Rocks
Olivine Opx Cpx Garnet Plag Amph MagnetiteRb 0.01 0.022 0.031 0.042 0.071 0.29 Sr 0.014 0.04 0.06 0.012 1.83 0.46 Ba 0.01 0.013 0.026 0.023 0.23 0.42 Ni 14.0 5.0 7.0 0.955 0.01 6.8 29.Cr 0.7 10.0 34.0 1.345 0.01 2.0 7.4La 0.007 0.03 0.056 0.001 0.148 0.544 2.Ce 0.006 0.02 0.092 0.007 0.082 0.843 2.Nd 0.006 0.03 0.23 0.026 0.055 1.34 2.Sm 0.007 0.05 0.445 0.102 0.039 1.804 1.Eu 0.007 0.05 0.474 0.243 0.1/1.5* 1.557 1.Dy 0.013 0.15 0.582 3.17 0.023 2.024 1.Er 0.026 0.23 0.583 6.56 0.02 1.74 1.5Yb 0.049 0.34 0.542 11.5 0.023 1.642 1.4Lu 0.045 0.42 0.506 11.9 0.019 1.563Data from Rollinson (1993). * Eu3+/Eu2+ Italics are estimated
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REE DiagramsREE DiagramsPlots of concentration as the ordinate (y-axis)
against increasing atomic number Degree of compatibility increases from left to right
across the diagram (“lanthanide contraction”)
Con
cent
rati
on
La Ce Nd Sm Eu Tb Er Dy Yb Lu
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
10
11
0 10 20 30 40 50 60 70 80 90 100
Atomic Number (Z)
Lo
g (
Ab
un
da
nce
in
CI
Ch
on
dri
tic
Me
teo
rite
) HHe
Li
Be
B
C
N
O
F
Sc
Fe
Ni
Ne MgSi
SCa
Ar
Ti
PbPtSn Ba
VK
NaAlP
Cl
ThU
Eliminate Oddo-Harkins effect and make y-scale more functional by normalizing to a standard
estimates of primordial mantle REE chondrite meteorite concentrations
What would an REE diagram look like for an analysis of a chondrite
meteorite?
0.00
2.00
4.00
6.00
8.00
10.00
56 58 60 62 64 66 68 70 72
sam
ple
/ch
on
dri
te
L La Ce Nd Sm Eu Tb Er Yb Lu
?
Divide each element in analysis by the concentration in a chondrite standard
0.00
2.00
4.00
6.00
8.00
10.00
56 58 60 62 64 66 68 70 72
sam
ple
/ch
on
dri
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L La Ce Nd Sm Eu Tb Er Yb Lu
REE diagrams using batch melting model of a garnet lherzolite for various values of F:
Figure 9.4. Rare Earth concentrations (normalized to chondrite) for melts produced at various values of F via melting of a hypothetical garnet lherzolite using the batch melting model (equation 9.5). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Europium anomaly when plagioclase is a fractionating phenocryst
or a residual solid in source
Figure 9.5. REE diagram for 10% batch melting of a hypothetical lherzolite with 20% plagioclase, resulting in a pronounced negative Europium anomaly. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Normalized Multielement (Spider) Diagrams
An extension of the normalized REE technique to a broader spectrum of elements
Fig. 9.6. Spider diagram for an alkaline basalt from Gough Island, southern Atlantic. After Sun and MacDonough (1989). In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp. 313-345.
Chondrite-normalized spider diagrams are commonly organized by (the author’s estimate) of increasing incompatibility L R
Different estimates different ordering (poor standardization)
MORB-normalized Spider MORB-normalized Spider Separates LIL and HFS
Figure 9.7. Ocean island basalt plotted on a mid-ocean ridge basalt (MORB) normalized spider diagram of the type used by Pearce (1983). Data from Sun and McDonough (1989). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Application of Trace Elements to Igneous Systems
1. Use like major elements on variation diagrams to document FX, assimilation, etc. in a suite of rocks More sensitive larger variations as process
continues
Figure 9.1a. Ni Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
2. Identification of the source rock or a particular mineral involved in either partial melting or fractional crystallization processes
Table 9-1. Partition Coefficients (CS/CL) for Some Commonly Used Trace Elements in Basaltic and Andesitic Rocks
Olivine Opx Cpx Garnet Plag Amph MagnetiteRb 0.01 0.022 0.031 0.042 0.071 0.29 Sr 0.014 0.04 0.06 0.012 1.83 0.46 Ba 0.01 0.013 0.026 0.023 0.23 0.42 Ni 14.0 5.0 7.0 0.955 0.01 6.8 29.Cr 0.7 10.0 34.0 1.345 0.01 2.0 7.4La 0.007 0.03 0.056 0.001 0.148 0.544 2.Ce 0.006 0.02 0.092 0.007 0.082 0.843 2.Nd 0.006 0.03 0.23 0.026 0.055 1.34 2.Sm 0.007 0.05 0.445 0.102 0.039 1.804 1.Eu 0.007 0.05 0.474 0.243 0.1/1.5* 1.557 1.Dy 0.013 0.15 0.582 3.17 0.023 2.024 1.Er 0.026 0.23 0.583 6.56 0.02 1.74 1.5Yb 0.049 0.34 0.542 11.5 0.023 1.642 1.4Lu 0.045 0.42 0.506 11.9 0.019 1.563Data from Rollinson (1993). * Eu3+/Eu2+ Italics are estimated
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Garnet concentrates the HREE and fractionates among them
Thus if garnet is in equilibrium with the partial melt (a residual phase in the source left behind) expect a steep (-) slope in REE and HREE
Shallow (< 40 km) partial melting of the mantle will have plagioclase in the resuduum and a Eu anomaly will result
0.00
2.00
4.00
6.00
8.00
10.00
56 58 60 62 64 66 68 70 72
sa
mp
le/c
ho
nd
rite
La Ce Nd Sm Eu Tb Er Yb Lu
67% Ol 17% Opx 17% Cpx
0.00
2.00
4.00
6.00
8.00
10.00
56 58 60 62 64 66 68 70 72
sa
mp
le/c
ho
nd
rite
La Ce Nd Sm Eu Tb Er Yb Lu
57% Ol 14% Opx 14% Cpx 14% Grt
Garnet and Plagioclase effect on HREE
0.00
2.00
4.00
6.00
8.00
10.00
sam
ple
/ch
on
dri
te
60% Ol 15% Opx 15% Cpx 10%Plag
La Ce Nd Sm Eu Tb Er Yb Lu
Figure 9.3. Change in the concentration of Rb and Sr in the melt derived by progressive batch melting of a basaltic rock consisting of plagioclase, augite, and olivine. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Element Use as a Petrogenetic Indicator
Ni, Co, Cr Highly compatible elements. Ni and Co are concentrated in olivine, and Cr in spinel and clinopyroxene. High
concentrations indicate a mantle source, limited fractionation, or crystal accumulation.
Zr, Hf Very incompatible elements that do not substitute into major silicate phases (although they may replace Ti in titanite or
rutile). High concentrations imply an enriched source or extensive liquid evolution.
Nb, Ta High field-strength elements that partition into Ti-rich phases (titanite, Ti-amphibole, Fe-Ti oxides. Typically low
concentrations in subduction-related melts.
Ru, Rh, Pd,
Re, Os, Ir,
Pd
Platinum group elements (PGEs) are siderophile and used mostly to study melting and crystallization in mafic-ultramafic
systems in which PGEs are typically hosted by sulfides. The Re/Os isotopic system is controlled by initial PGE
differentiation and is applied to mantle evolution and mafic melt processes.
Sc Concentrates in pyroxenes and may be used as an indicator of pyroxene fractionation.
Sr Substitutes for Ca in plagioclase (but not in pyroxene), and, to a lesser extent, for K in K-feldspar. Behaves as a compatible
element at low pressure where plagioclase forms early, but as an incompatible element at higher pressure where
plagioclase is no longer stable.
REE Myriad uses in modeling source characteristics and liquid evolution. Garnet accommodates the HREE more than the
LREE, and orthopyroxene and hornblende do so to a lesser degree. Titanite and plagioclase accommodates more LREE.
Eu2+ is strongly partitioned into plagioclase.
Y Commonly incompatible. Strongly partitioned into garnet and amphibole. Titanite and apatite also concentrate Y, so the
presence of these as accessories could have a significant effect.
Table 9.6 A Brief Summary of Some Particularly Useful Trace Elements in Igneous Petrology
Trace elements as a tool to determine Trace elements as a tool to determine paleotectonic environmentpaleotectonic environment
Useful for rocks in mobile belts that are no longer recognizably in their original setting
Can trace elements be discriminators of igneous environment?
Approach is empirical on modern occurrences Concentrate on elements that are immobile
during low/medium grade metamorphism
Figure 9.8 Examples of discrimination diagrams used to infer tectonic setting of ancient (meta)volcanics. (a) after Pearce and Cann (1973), (b) after Pearce (1982), Coish et al. (1986). Reprinted by permission of the American Journal of Science, (c) after Mullen (1983) Copyright © with permission from Elsevier Science, (d) and (e) after Vermeesch (2005) © AGU with permission.
IsotopesIsotopes
Same Z, different A (variable # of neutrons)
General notation for a nuclide: 614 C
IsotopesIsotopes
Same Z, different A (variable # of neutrons)
General notation for a nuclide: 614 C
As n varies different isotopes of an element
12C 13C 14C
Stable IsotopesStable Isotopes
Stable: last ~ forever Chemical fractionation is impossible Mass fractionation is the only type possible
Example: Oxygen IsotopesExample: Oxygen Isotopes
Concentrations expressed by reference to a standard
International standard for O isotopes = standard mean ocean water (SMOW)
16O 99.756% of natural oxygen17O 0.039% “18O 0.205% “
18O and 16O are the commonly used isotopes and their ratio is expressed as :
18O/16O) = eq
result expressed in per mille (‰)
( O/ O) ( O/ O)
( O/ O)x1000
18 16sample
18 16SMOW
18 16SMOW
What is of SMOW??
What is for meteoric water?
What is for meteoric water?
Evaporation seawater water vapor (clouds) Light isotope enriched in vapor > liquid Pretty efficient, since mass = 1/8 total mass
What is for meteoric water?
Evaporation seawater water vapor (clouds) Light isotope enriched in vapor > liquid Pretty efficient, since mass = 1/8 total mass
=
therefore <
thus clouds is (-)
( O/ O) ( O/ O)
( O/ O)x1000
18 16vapor
18 16SMOW
18 16SMOW
( O/ O)18 16Vapor ( O/ O)18 16
SMOW
Figure 9.9. Relationship between d(18O/16O) and mean annual temperature for meteoric precipitation, after Dansgaard (1964). Tellus, 16, 436-468.
O and H isotopes - juvenile vs. meteoric vs. brine water
18O for mantle rocks surface-reworked sediments: evaluate contamination of mantle-derived magmas by crustal sediments
Stable isotopes useful in assessing relative contribution of various reservoirs, each with a distinctive isotopic signature
Radioactive IsotopesRadioactive Isotopes
Unstable isotopes decay to other nuclides The rate of decay is constant, and not
affected by P, T, X… Parent nuclide = radioactive nuclide that
decays Daughter nuclide(s) are the radiogenic
atomic products
Isotopic variations between rocks, etc. due to:1. Mass fractionation (as for stable isotopes)
Only effective for light isotopes: H He C O S
Isotopic variations between rocks, etc. due to:1. Mass fractionation (as for stable isotopes)
2. Daughters produced in varying proportions resulting from previous event of chemical fractionation
40K 40Ar by radioactive decay
Basalt rhyolite by FX (a chemical fractionation process)
Rhyolite has more K than basalt
40K more 40Ar over time in rhyolite than in basalt
40Ar/39Ar ratio will be different in each
Isotopic variations between rocks, etc. due to:1. Mass fractionation (as for stable isotopes)
2. Daughters produced in varying proportions resulting from previous event of chemical fractionation
3. TimeThe longer 40K 40Ar decay takes place, the greater
the difference between the basalt and rhyolite will be
Radioactive DecayRadioactive Decay
The Law of Radioactive Decay
eq. 9.11 dN
dtN or
dN
dt= N
# pa
rent
ato
ms
time
1
½
¼
D = Net - N = N(et -1) eq 9.14
age of a sample (t) if we know: D the amount of the daughter nuclide produced
N the amount of the original parent nuclide remaining
the decay constant for the system in question
The K-Ar SystemThe K-Ar System40K either 40Ca or 40Ar
40Ca is common. Cannot distinguish radiogenic 40Ca from non-radiogenic 40Ca
40Ar is an inert gas which can be trapped in
many solid phases as it forms in them
The appropriate decay equation is:
eq 9.16 40Ar = 40Aro + 40K(e-t -1)
Where e = 0.581 x 10-10 a-1 (proton capture)
and = 5.543 x 10-10 a-1 (whole process)
e
Blocking temperatures for various minerals differ
40Ar-39Ar technique grew from this discovery
Sr-Rb System Sr-Rb System
87Rb 87Sr + a beta particle ( = 1.42 x 10-11 a-1)
Rb behaves like K micas and alkali feldspar
Sr behaves like Ca plagioclase and apatite (but not clinopyroxene)
88Sr : 87Sr : 86Sr : 84Sr ave. sample = 10 : 0.7 : 1 : 0.07
86Sr is a stable isotope, and not created by breakdown of any other parent
Isochron Technique
Requires 3 or more cogenetic samples with a range of Rb/Sr
Could be:
• 3 cogenetic rocks derived from a single source by partial melting, FX, etc.
Figure 9.3. Change in the concentration of Rb and Sr in the melt derived by progressive batch melting of a basaltic rock consisting of plagioclase, augite, and olivine. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Isochron Technique
Requires 3 or more cogenetic samples with a range of Rb/Sr
Could be:
• 3 cogenetic rocks derived from a single source by partial melting, FX, etc.
• 3 coexisting minerals with different K/Ca ratios in a single rock
For values of t less than 0.1: et-1 t
Thus eq. 9.15 for t < 70 Ga (!!) reduces to:
eq 9.18 87Sr/86Sr = (87Sr/86Sr)o + (87Rb/86Sr)t
y = b + x m
= equation for a line in 87Sr/86Sr vs. 87Rb/86Sr plot
Recast age equation by dividing through by stable 86Sr
87Sr/86Sr = (87Sr/86Sr)o + (87Rb/86Sr)(et -1) eq 9.17
= 1.4 x 10-11 a-1
a b c to86Sr
87Sr
o( )
86Sr
87Sr
86Sr
87Rb
Begin with 3 rocks plotting at a b c at time to
After some time increment (t0 t1) each sample loses some 87Rb and gains an equivalent amount of 87Sr
a b c
a1b1
c1t1
to
86Sr
87Sr
86Sr
87Rb
86Sr
87Sr
o( )
At time t2 each rock system has evolved new line
Again still linear and steeper line
a b c
a1b1
c1a2
b2
c2t1
to
t2
86Sr
87Sr
86Sr
87Sr
o( )
86Sr
87Rb
Isochron technique produces 2 valuable things:
1. The age of the rocks (from the slope = t)
2. (87Sr/86Sr)o = the initial value of 87Sr/86Sr
Figure 9.12. Rb-Sr isochron for the Eagle Peak Pluton, central Sierra Nevada Batholith, California, USA. Filled circles are whole-rock analyses, open circles are hornblende separates. The regression equation for the data is also given. After Hill et al. (1988). Amer. J. Sci., 288-A, 213-241.
Figure 9.13. Estimated Rb and Sr isotopic evolution of the Earth’s upper mantle, assuming a large-scale melting event producing granitic-type continental rocks at 3.0 Ga b.p After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.
The Sm-Nd System The Sm-Nd System
Both Sm and Nd are LREE Incompatible elements fractionate melts Nd has lower Z larger liquids > does Sm
147Sm 143Nd by alpha decay = 6.54 x 10-13 a-1 (half life 106 Ga)
Decay equation derived by reference to the non-radiogenic 144Nd 143Nd/144Nd = (143Nd/144Nd)o
+ (147Sm/144Nd)t
Evolution curve is opposite to Rb - Sr
Figure 9.15. Estimated Nd isotopic evolution of the Earth’s upper mantle, assuming a large-scale melting or enrichment event at 3.0 Ga b.p. After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.
The U-Pb-Th SystemThe U-Pb-Th SystemVery complex system.
3 radioactive isotopes of U: 234U, 235U, 238U 3 radiogenic isotopes of Pb: 206Pb, 207Pb, and 208Pb
Only 204Pb is strictly non-radiogenic U, Th, and Pb are incompatible elements, &
concentrate in early melts Isotopic composition of Pb in rocks = function of
238U 234U 206Pb ( = 1.5512 x 10-10 a-1) 235U 207Pb ( = 9.8485 x 10-10 a-1) 232Th 208Pb ( = 4.9475 x 10-11 a-1)
The U-Pb-Th SystemThe U-Pb-Th SystemConcordia = Simultaneous co-
evolution of 206Pb and 207Pb via:
238U 234U 206Pb235U 207Pb
Figure 9.16a. Concordia diagram illustrating the Pb isotopic development of a 3.5 Ga old rock with a single episode of Pb loss. After Faure (1986). Principles of Isotope Geology. 2nd, ed. John Wiley & Sons. New York.
The U-Pb-Th SystemThe U-Pb-Th SystemDiscordia = loss of both
206Pb and 207Pb
Figure 9.16a. Concordia diagram illustrating the Pb isotopic development of a 3.5 Ga old rock with a single episode of Pb loss. After Faure (1986). Principles of Isotope Geology. 2nd, ed. John Wiley & Sons. New York.
The U-Pb-Th SystemThe U-Pb-Th SystemConcordia diagram after 3.5 Ga total evolution
Figure 9.16b. Concordia diagram illustrating the Pb isotopic development of a 3.5 Ga old rock with a single episode of Pb loss. After Faure (1986). Principles of Isotope Geology. 2nd, ed. John Wiley & Sons. New York.
The U-Pb-Th SystemThe U-Pb-Th System
Figure 9.17. Concordia diagram for three discordant zircons separated from an Archean gneiss at Morton and Granite Falls, Minnesota. The discordia intersects the concordia at 3.55 Ga, yielding the U-Pb age of the gneiss, and at 1.85 Ga, yielding the U-Pb age of the depletion event. From Faure (1986). Copyright © reprinted by permission of John Wiley & Sons, Inc.