Chapter 9: Trace Elements

1
Chapter 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.
2
Chapter 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.
3
Element Distribution Goldschmidts 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?
4
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5
Goldschmidts 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.
6
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
7
Chemical Fractionation

• The uneven distribution of an ion between two
  competing (equilibrium) phases

8
Exchange equilibrium of a component i between two
phases (solid and liquid) i (liquid) i
(solid) eq. 9-2 K K
equilibrium constant
? X solid ? X liquid
a solid a liquid
i
i
i
i
i
i
9

• Trace element concentrations are in the Henrys
  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

10

• incompatible elements are concentrated in the
  melt
• (KD or D) 1
• compatible elements are concentrated in the solid
  
• KD or D 1

11

• For dilute solutions can substitute D for KD
• D
• Where CS the concentration of some element in
  the solid phase

12

• 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

13
Compatibility depends on minerals and melts
involved. Which are incompatible? Why?
14

• For a rock, determine the bulk distribution
  coefficient D for an element by calculating the
  contribution for each mineral

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16

• 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.
17

• 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.
18
Trace 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
19
Trace element concentrations are in the Henrys
Law region of concentration, so their activity
varies in direct relation to their concentration
in the system
20
Trace element concentrations are in the Henrys
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
21
Trace element concentrations are in the Henrys
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
22

• 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

23

• Sr and Ba (also incompatible elements)
• Sr is excluded from most common minerals except
  plagioclase
• Ba similarly excluded except in alkali feldspar

24

• Compatible example
• Ni strongly fractionated ? olivine gt 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

25
Models 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

26
Models 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)

27
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.
28

• 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.
29

• 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.
30

• As F ? 1 the concentration of every trace element
  in the liquid the source rock (CL/CO ? 1)
• As F ? 1
• CL/CO ? 1

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.
31
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)
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.
32

• For very incompatible elements as Di ? 0
• equation 9-5 reduces to
• eq. 9-7

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
33
Worked Example of Batch Melting Rb and Sr
Basalt with the mode 1. Convert
to weight minerals (Wol Wcpx etc.)
34
Worked Example of Batch Melting Rb and Sr
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
35
3. Use the batch melting equation (9-5)
to calculate CL/CO for various values of F
From Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
36
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.
37
Incremental Batch Melting

• Calculate batch melting for successive batches
  (same equation)
• Must recalculate Di as solids change as minerals
  are selectively melted (computer)

38
Fractional Crystallization 1. Crystals remain in
equilibrium with each melt increment
39

• Rayleigh fractionation
• The other extreme separation of each crystal
  as it formed perfectly continuous fractional
  crystallization in a magma chamber

40

• 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

41

• Other models are used to analyze
• Mixing of magmas
• Wall-rock assimilation
• Zone refining
• Combinations of processes

42
The Rare Earth Elements (REE)
43
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
44
REE Diagrams

• Plots of concentration as the ordinate (y-axis)
  against increasing atomic number
• Degree of compatibility increases from left to
  right across the diagram

Concentration
La Ce Nd Sm Eu Tb Er Dy Yb Lu
45

• Eliminate Oddo-Harkins effect and make y-scale
  more functional by normalizing to a standard
• estimates of primordial mantle REE
• chondrite meteorite concentrations

46
What would an REE diagram look like for an
analysis of a chondrite meteorite?
47
Divide each element in analysis by the
concentration in a chondrite standard
48
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.
49

• 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.
50
Spider Diagrams
An extension of the normalized REE technique to a
broader spectrum of elements
Chondrite-normalized spider diagrams are commonly
organized by (the authors estimate) of
increasing incompatibility L ? R Different
estimates ? different ordering (poor
standardization)
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.
51
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.
52
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.
53

• 2. Identification of the source rock or a
  particular mineral involved in either partial
  melting or fractional crystallization processes

54
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
55
Garnet and Plagioclase effect on HREE
56
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.
57
Table 9-6 A brief summary of some particularly
useful trace elements in igneous petrology
Use as a petrogenetic indicator
Element
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.
V, Ti
Both show strong fractionation into Fe-Ti oxides
(ilmenite or titanomagnetite). If they behave
differently, Ti probably fractionates into an
accessory phase, such as sphene or rutile.
Zr, Hf
Very incompatible elements that do not substitute
into major silicate phases (although they may
replace Ti in sphene or rutile).
Ba, Rb
Incompatible element that substitutes for K in
K-feldspar, micas, or hornblende. Rb substitutes
less readily in hornblende than K-spar and micas,
such that the K/Ba ratio may distinguish these
phases.
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 at higher pressure where
plagioclase is no longer stable.
REE
Garnet accommodates the HREE more than the LREE,
and orthopyroxene and hornblende do
2
so to a lesser degree. Sphene and plagioclase
accommodates more LREE. Eu
is strongly
partitioned into plagioclase.
Y
Commonly incompatible (like HREE). Strongly
partitioned into garnet and amphibole. Sphene
and apatite also concentrate Y, so the presence
of these as accessories could have a
significant effect.
Table 9-6. After Green (1980). Tectonophys., 63,
367-385. From Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.

58
Trace elements as a tool to determine
paleotectonic 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

59
Figure 9-8. (a) after Pearce and Cann (1973),
Earth Planet, Sci. Lett., 19, 290-300. (b) after
Pearce (1982) in Thorpe (ed.), Andesites
Orogenic andesites and related rocks. Wiley.
Chichester. pp. 525-548, Coish et al. (1986),
Amer. J. Sci., 286, 1-28. (c) after Mullen
(1983), Earth Planet. Sci. Lett., 62, 53-62.
60
Isotopes
Same Z, different A (variable of
neutrons) General notation for a nuclide
61
Isotopes
Same Z, different A (variable of
neutrons) General notation for a nuclide
As n varies ? different isotopes of an
element 12C 13C 14C
62
Stable Isotopes

• Stable last forever
• Chemical fractionation is impossible
• Mass fractionation is the only type possible

63
Example Oxygen Isotopes
16O 99.756 of natural oxygen 17O
0.039 18O 0.205
Concentrations expressed by reference to a
standard International standard for O isotopes
standard mean ocean water (SMOW)
64
18O and 16O are the commonly used isotopes and
their ratio is expressed as d d (18O/16O)
eq 9-10 result expressed in per
mille ()
What is d of SMOW?? What is d for meteoric water?
65

• What is d for meteoric water?
• Evaporation seawater ? water vapor (clouds)
• Light isotope enriched in vapor gt liquid
• Pretty efficient, since D mass 1/8 total mass

66

• What is d for meteoric water?
• Evaporation seawater ? water vapor (clouds)
• Light isotope enriched in vapor gt liquid
• Pretty efficient, since D mass 1/8 total mass
• d
• therefore lt
• thus dclouds is (-)

67
Figure 9-9. Relationship between d(18O/16O) and
mean annual temperature for meteoric
precipitation, after Dansgaard (1964). Tellus,
16, 436-468.
68
Stable isotopes useful in assessing relative
contribution of various reservoirs, each with a
distinctive isotopic signature

• O and H isotopes - juvenile vs. meteoric vs.
  brine water
• d18O for mantle rocks ? surface-reworked
  sediments evaluate contamination of
  mantle-derived magmas by crustal sediments

69
Radioactive 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

70
Isotopic variations between rocks, etc. due
to 1. Mass fractionation (as for stable
isotopes) Only effective for light isotopes H
He C O S
71
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
72

• 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. Time
• The longer 40K ? 40Ar decay takes place, the
  greater
• the difference between the basalt and rhyolite
  will be

73
Radioactive Decay
The Law of Radioactive Decay eq. 9-11
1 ½ ¼
parent atoms
time ?
74
D Nelt - N N(elt -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
l the decay constant for the system in
question
75
The K-Ar System

• 40K ? 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

76
The appropriate decay equation is eq 9-16 40Ar
40Aro 40K(e-lt -1) Where le
0.581 x 10-10 a-1 (electron capture) and
l 5.543 x 10-10 a-1 (whole process)
77

• Blocking temperatures for various minerals differ
  
• 40Ar-39Ar technique grew from this discovery

78
Sr-Rb System

• 87Rb ? 87Sr a beta particle (l 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

79
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.
80
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

81
Recast age equation by dividing through by stable
86Sr 87Sr/86Sr (87Sr/86Sr)o (87Rb/86Sr)(elt
-1) eq 9-17 l 1.4 x 10-11 a-1
For values of lt less than 0.1 elt-1 ? lt Thus
eq. 9-15 for t lt 70 Ga (!!) reduces to eq 9-18
87Sr/86Sr (87Sr/86Sr)o (87Rb/86Sr)lt y
b x m
equation for a line in 87Sr/86Sr vs. 87Rb/86Sr
plot
82
Begin with 3 rocks plotting at a b c at time to
to
a
b
c
83
After some time increment (t0 ?t1) each sample
loses some 87Rb and gains an equivalent amount of
87Sr
84
At time t2 each rock system has evolved ? new
line Again still linear and steeper line
85
Isochron technique produces 2 valuable things 1.
The age of the rocks (from the slope lt) 2.
(87Sr/86Sr)o the initial value of 87Sr/86Sr
Figure 9-9. 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.
86
Figure 9-13. Estimated Rb and Sr isotopic
evolution of the Earths 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.
87
The Sm-Nd System

• Both Sm and Nd are LREE
• Incompatible elements fractionate ? melts
• Nd has lower Z ? larger ? liquids gt does Sm

88

• 147Sm ? 143Nd by alpha decay
• l 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)lt

89
Evolution curve is opposite to Rb - Sr
Figure 9-15. Estimated Nd isotopic evolution of
the Earths upper mantle, assuming a large-scale
melting or enrichment event at 3.0 Ga b.p. After
Wilson (1989). Igneous Petrogenesis. Unwin
Hyman/Kluwer.
90
The U-Pb-Th System

• Very 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 (l 1.5512 x 10-10 a-1)
• 235U ? 207Pb (l 9.8485 x 10-10 a-1)
• 232Th ? 208Pb (l 4.9475 x 10-11 a-1)

91
The U-Pb-Th System
Concordia Simultaneous co-evolution of 206Pb
and 207Pb via 238U ? 234U ? 206Pb 235U ? 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.
92
The U-Pb-Th System
Discordia 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.
93
The U-Pb-Th System
Concordia diagram after 3.5 Ga total evolution
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.