chapter 9: trace elements note magnitude of major element changes figure 8.2. harker variation...

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



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


Thus if XNi in the system doubles the XNi in all phases will double


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




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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




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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)


Highly incompatible elements Greatly concentrated in

the initial small fraction of melt produced by partial melting

Subsequently diluted as F increases


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


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


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

te

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


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


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?


Evaporation seawater water vapor (clouds) Light isotope enriched in vapor > liquid Pretty efficient, since mass = 1/8 total mass


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


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


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.


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.

chapter 9: trace elements note magnitude of major element changes figure 8.2. harker variation...

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