chapter 9: trace elements

Chapter 9: Trace ElementsChapter 9: Trace Elements

Note small magnitude Note small magnitude of major element of major element changes. However changes. However (next slide) …(next slide) …

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 From Winter (2001) An Introduction to Igneous and Metamorphic Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Petrology. Prentice Hall.

Please bring to the Field Trip on Saturday a color copy of

http://www.kean.edu/~csmart/Petrology/Lectures/Field%20Trip%201%20Maps.pptx




Transition ElementsTransition Elements

Transition elements often vary by > 103 wrt SiO2. Very useful since so sensitive to distribution & fractionation

Figure 9-1.Figure 9-1. Harker Diagram for Crater Lake. From data Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.to Igneous and Metamorphic Petrology. Prentice Hall.

Zr, a transition elementIncompatible HFSESee slide 43

Goldschmidt’s rules Goldschmidt’s rules (simplistic, but useful)(simplistic, but useful)1.1. 2 ions with the same valence and radius 2 ions with the same valence and radius

should exchange easily and enter a solid should exchange easily and enter a solid solution in amounts equal to their overall solution in amounts equal to their overall proportionsproportions

How does RbHow does Rb++ behave? As K behave? As K++, , concentrated in K-spars, micas, and evolved concentrated in K-spars, micas, and evolved meltsmelts

NiNi++++ behaves as Mg behaves as Mg++++, concentrates in Olivine, concentrates in OlivineTAKE OUT YOUR COORDINATION NUMBERS AND IONIC RADII CHART

Rb+ [6] 1.57 Å follows K+ [6] 1.46 Å & conc. in K-Spar, mica, & late melt.Ni++ [6] 0.77 Å follows Mg++ [6] 0.8 Å & conc in Mg-Olivine “Forsterite”

Goldschmidt’s rulesGoldschmidt’s rules

2. If 2 ions have a similar radius and the same valence: 2. If 2 ions have a similar radius and the same valence: the smaller ion is preferentially incorporated into the the smaller ion is preferentially incorporated into the solid over the liquidsolid 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 From Winter (2001) An Introduction to (2001) An Introduction to Igneous and Metamorphic Igneous and Metamorphic Petrology. Prentice Hall.Petrology. Prentice Hall.

Smaller ion preferentially -> solid (Mg++ is smaller than Fe++ so more Mg++ in high temp Olivine than in melt)

Bounces off the rim less

3. If 2 ions have a similar radius, but different If 2 ions have a similar radius, but different valence: the ion with the higher charge is valence: the ion with the higher charge is preferentially incorporated into the solid preferentially incorporated into the solid over the liquidover the liquid

Example Ti+4 [6] r=0.69 AngstromOther ions this size Fe +3 r=0.68 AMn +3 [6] r=.70 ATi+4 is always preferred in solids over liquids. Example Cr+3 [6] r=0.76 Other ions this size Ni +2 [6] r=0.77 A Fe +2 [6] r=.77Cr+3 is always preferred in solids over liquids.

1 Ångström = 1.0 × 10-10 meters

Rutile TiO2

versusHematite Fe2O3

Chemical FractionationChemical Fractionation The uneven distribution of an ion between The uneven distribution of an ion between

two competing (equilibrium) phasestwo competing (equilibrium) phases Example: the ratio of CaExample: the ratio of Ca++++/Na/Na++ is always is always

greater in plagioclase crystals than in the greater in plagioclase crystals than in the coexisting melt. Cacoexisting melt. Ca++++ higher valance than higher valance than NaNa++ , goes into Plag xtals. first. , goes into Plag xtals. first.

If melting Plag, Na+ goes to melt first.If melting Plag, Na+ goes to melt first. Example: the ratio of MgExample: the ratio of Mg++++/Fe/Fe++++ is always is always

greater in Olivine than in the coexisting greater in Olivine than in the coexisting melt. melt.

Anorthite higher T than AlbiteForsterite higher T than Fayalite

If the reaction between solid and liquid phases is a If the reaction between solid and liquid phases is a phase change of some component iphase change of some component i

ii (liquid)(liquid) ii (solid)(solid)

A distribution constant KD is

Where Xi is the mole fraction mole fraction of component i in some phase.KD the ratio of solubility of component i in these two phases.

Recall that we used the lever principle to estimate this ratio from our phase diagrams

As long as the concentrations of the components are dilute, call KD the partition coefficient

Where C is the concentration of a trace element in the phase indicated, solid and liquid, in ppm or wt%Table 9.1 (three slides below this) gives partition coefficients for

commonly used trace elements in minerals precipitating from basaltic or andesitic melts.

Different units

Trace element activity varies in direct relation to their concentration in the system.

Thus if for Nickel 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

For example: suppose C(Ni) = 20 ppm in a systemC(Ni) in olivine may be 100 ppmC(Ni) in plagioclase may be 1 ppmC(Ni) in liquid may be 10 ppm

Double C(Ni) in system to 40 ppm: Ol -> 200 ppm, Plag -> 2 ppm and liquid -> 20 ppm

Incompatible elements concentrate in Incompatible elements concentrate in the melt the melt

KKDD « 1 much less than 1 « 1 much less than 1Lesson: incompatibles don’t stick on crystal faces Lesson: incompatibles don’t stick on crystal faces

unless the Temps are low. Melt a xtal, unless the Temps are low. Melt a xtal, incompatibles go to the melt.incompatibles go to the melt.

Compatible elements concentrate in Compatible elements concentrate in the solid the solid

KKDD» 1 much more than 1» 1 much more than 1

Compatibility depends on minerals and melts involved. Compatibility depends on minerals and melts involved. Which are incompatible? KWhich are incompatible? KDD<< 1 , in liquid melt<< 1 , in liquid melt

I marked some extreme examples

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

eq. 9-4: Deq. 9-4: Dii = = W WAA D Dii

WWAA = weight % of mineral A in the rock = weight % of mineral A in the rock

DDii = partition coefficient of element i in = partition coefficient of element i in

mineral Amineral A

AA

AA

Garnet LherzoliteGarnet Lherzolite

Mantle Xenolith (Garnet Lherzolite)Kimberley, SOUTH AFRICAPhotographed by Tony Peterson

Example: hypothetical garnet lherzolite = 60% olivine, 25% Example: hypothetical garnet lherzolite = 60% olivine, 25% orthopyroxene, 10% clinopyroxene, and 5% garnet (all by orthopyroxene, 10% clinopyroxene, and 5% garnet (all by weightweight), ), using the data in Table 9-1, the bulk distribution coefficient D for Er is:using the data in Table 9-1, the bulk distribution coefficient D for Er is:

DDErEr = (0.6 · 0.026) + (0.25 · 0.23) + (0.10 · 0.583) + (0.05 · 4.7) = 0.366 = (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

Rare

Eart

h E

lem

ents

http://earthref.org/KDD/e:68/

Er = Erbium



Coef. D for Ni in Olivine in Tb 9-1 = 14>>1Coef. D for Ni in Olivine in Tb 9-1 = 14>>1

Figure 9-1a.Figure 9-1a. Ni Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Ni Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Nippm

We mentioned earlier Transition elements often vary by > 103 wrt SiO2. Very useful since so sensitive to distribution & fractionation

The abrupt drop in Ni below 55% SiO2 indicates fractionation of Olivine there. At SiO2 > 55% another mineral or process is removing Ni

Incompatible trace elements D<< 1 concentrate Incompatible trace elements D<< 1 concentrate in the in the liquid until the melt cools, which is when it has more liquid until the melt cools, which is when it has more silica. So Trace Elements reflect the proportion of silica. So Trace Elements reflect the proportion of liquid at a given state of crystallization or meltingliquid at a given state of crystallization or melting

Figure 9-1b.Figure 9-1b. Zr Harker Diagram for Crater Lake. From data compiled by Rick Conrey. Zr Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Zrppm

Zr, a transition elementIncompatible HFSESee slide 43

Trace element concentrations are in Trace element concentrations are in the Henry’s Law* region of the Henry’s Law* region of concentration, so their activity concentration, so their activity varies in direct relation to their varies in direct relation to their concentration in the systemconcentration in the system

Thus if XThus if XNiNi in the system doubles the in the system doubles the XXNiNi in all phases will double in all phases will double

Because of this, the Because of this, the ratiosratios of trace of trace elements are often superior to the elements are often superior to the concentration of a single element concentration of a single element in identifying the role of a specific in identifying the role of a specific mineralmineral

* "At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid."

K/Rb often used to measure the importance of K/Rb often used to measure the importance of amphiboleamphibole in a source rock in a source rock– K & Rb behave very similarly, so K/Rb should be ~ K & Rb behave very similarly, so K/Rb should be ~

constantconstant– If amphibole important, almost all K and Rb reside If amphibole important, almost all K and Rb reside

in itin it– Amphibole has a D of about 1.0 for K and 0.3 for Amphibole has a D of about 1.0 for K and 0.3 for

RbRbTable 9-1. Partition Coefficients (CS/CL) for Some Commonly Used Trace



Rare E

arth

Ele

ments

Sr and Ba (also incompatible Sr and Ba (also incompatible elements)elements)

Sr is excluded from most common Sr is excluded from most common minerals except plagioclase minerals except plagioclase

Ba similarly excluded except in alkali Ba similarly excluded except in alkali feldspar (Sanidine, Orthoclase, feldspar (Sanidine, Orthoclase, Microcline)Microcline)

ratio Ba/Sr increases w plagioclase, ratio Ba/Sr increases w plagioclase, decreases when orthoclase xtals decreases when orthoclase xtals presentpresent

Table 9-1. Partition Coefficients (CS/CL) for Some Commonly Used Trace



Rare E

arth

Ele

ments

Compatible example: Compatible example: Ni strongly fractionated (goes to solids) from Ni strongly fractionated (goes to solids) from

olivines to pyroxenesolivines to pyroxenes Cr and Sc Cr and Sc pyroxenes > olivine pyroxenes > olivine Ni/Cr or Ni/Sc can distinguish the effects of Ni/Cr or Ni/Sc can distinguish the effects of

olivine and Augite in a partial melt or a suite of olivine and Augite in a partial melt or a suite of rocks produced by fractional crystallizationrocks produced by fractional crystallization

Sc Scandium element 21

Models of Magma EvolutionModels of Magma Evolution Batch MeltingBatch Melting

The melt remains in equilibrium with the solid until The melt remains in equilibrium with the solid until at some point it floats upward, separating from the at some point it floats upward, separating from the solidssolids

Models of Magma Models of Magma EvolutionEvolution Batch MeltingBatch Melting (Shaw) (Shaw)

eq. 9-5eq. 9-5

CCLL = trace element concentration in the = trace element concentration in the liquidliquid

CCOO = trace element concentration in the = trace element concentration in the original rock before melting beganoriginal rock before melting began

F = wt fraction of melt F = wt fraction of melt producedproduced = = melt/(melt + rock)melt/(melt + rock)

DDii = bulk distribution coefficient for = bulk distribution coefficient for element i element i

CCCC

11DDii(1(1 F)F) FF

LL

OO

Batch MeltingBatch Melting

A plot of CA plot of CLL/C/COO vs. F for vs. F for various values of Dvarious values of Dii using eq. 9-5using eq. 9-5For DFor Dii = 1.0 there is = 1.0 there is (by (by

definition) definition) no no fractionation,fractionation,

concentration of element i concentration of element i

the same in solid and the same in solid and liquidliquid

Figure 9-2.Figure 9-2. Variation in the relative concentration of a Variation in the relative concentration of a trace element in a liquid vs. source rock as a fiunction trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9-5) for of D and the fraction melted, using equation (9-5) for equilibrium batch melting. From Winter (2001) An equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Prentice Hall.

UnlikelyF values

low D = incompatiblelow F little partial melting

no fractionation

– Especially true for low % Especially true for low % melting (low F) and Dmelting (low F) and Dii<< 1.0 << 1.0 (=(=incompatibleincompatible element)element)

Figure 9-2.Figure 9-2. Variation in the relative concentration of a Variation in the relative concentration of a trace element in a liquid vs. source rock as a function trace element in a liquid vs. source rock as a function of D and the fraction melted, using equation (9-5) for of D and the fraction melted, using equation (9-5) for equilibrium batch melting. From Winter (2001) An equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Prentice Hall.

The concentration of the trace element in The concentration of the trace element in the liquid varies more as Dthe liquid varies more as Di i deviates from deviates from 1. 1.

Example: D near 0.001, and F near 0.02, CL/Co varies from 20 to 100

Highly Highly incompatibleincompatible elementselements– Greatly Greatly

concentrated in the concentrated in the initial small fraction initial small fraction of melt produced by of melt produced by partial meltingpartial melting

– Subsequently Subsequently diluted as F diluted as F increasesincreasesFigure 9-2.Figure 9-2. Variation in the relative concentration of a Variation in the relative concentration of a

trace element in a liquid vs. source rock as a fiunction trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9-5) for of D and the fraction melted, using equation (9-5) for equilibrium batch melting. From Winter (2001) An equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Prentice Hall.

Find a small early partial melt, concentrated incompatibles there

Incompatibles go to liquid

As F As F 1, everything is 1, everything is melted, so the melted, so the concentration of every concentration of every trace element in the trace element in the liquid => the source liquid => the source rock (Crock (CLL/C/COO 1) 1)

As F As F 1 1

CCLL/C/COO 1 1

CC

1Di (1 F) F

L

O


Di the bulk distribution coefficient

As F As F 0 0 CCLL/C/COO 1/D 1/Dii

If we know the If we know the concentration of a trace concentration of a trace element in a magma Celement in a magma CLL , , derived by a small degree derived by a small degree of batch melting, and we of batch melting, and we know Dknow Dii we can estimate we can estimate the concentration of that the concentration of that element in the element in the sourcesource region (Cregion (COO). This tells us ). This tells us which area was the source.which area was the source.

CC

1Di (1 F) F

L

O


Recall then:


For very incompatible elements as DFor very incompatible elements as D ii 0 0

equation 9-5 equation 9-5 reduces reduces to:to:

eq. 9-7eq. 9-7

C

C

1

FL

O

CC

1Di (1 F) F

L

O

If we know the concentration of a very If we know the concentration of a very incompatible element in both a magma and the incompatible element in both a magma and the source rock, we can determine the fraction of source rock, we can determine the fraction of partial melt producedpartial melt produced


Worked Example of Batch Melting: Worked Example of Batch Melting: Rb and Rb and SrSrBasalt with the “mode” = Volume% [cmBasalt with the “mode” = Volume% [cm33]:]:

1. Convert to weight % minerals (W1. Convert to weight % minerals (Wolol W Wcpxcpx etc.) etc.)

Done, see rightmost column above.Done, see rightmost column above.

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

15 cm cm33 x 3.6g/ cm cm33 = 54g

54g/303.9g ~ 0.18

so WWol ol ~ 0.18 ~ 0.18

Worked Example of Batch Worked Example of Batch Melting: Melting: Rb and SrRb 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

A Basalt (ol, cpx, plag) with the mode:A Basalt (ol, cpx, plag) with the mode:

1. Convert to weight % minerals (W1. Convert to weight % minerals (Wolol W Wcpxcpx W Wplagplag) ) (previous slide)(previous slide)

2.2. Use equation eq. 9-4: D Use equation eq. 9-4: Dii = = W WAA D Dii

and the table of D (Table 9.1) values for Rb and Sr in each and the table of D (Table 9.1) values for Rb and Sr in each mineral to calculate the bulk distribution coefficients: mineral to calculate the bulk distribution coefficients:

DDRbRb = 0.045 (Rb incompatible) and D = 0.045 (Rb incompatible) and DSrSr = 0.848 (close to 1) = 0.848 (close to 1)

DRb = 0.01 x 0.18 + 0.031 x 0.37 + 0.071 x 0.45 = 0.045

3.3. Use the batch melting equation Use the batch melting equation

to calculate Cto calculate CLL/C/COO for various values of F for various values of F

From Winter (2010) 2From Winter (2010) 2ndnd edition An Introduction to Igneous and edition An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Metamorphic Petrology. Prentice Hall.

Similarly for F = 0.05 and SrCL/Co = 1.18

Then the ratioRb/Sr = 10.78/1.18 = 9.12

Continue with remaining F

For F =0.05 and for Rb: CL/Co = 1/ ((0.045(1-0.05)) +0.05) = 10.78

4.4. Plot C Plot CLL/C/COO vs. F for each element vs. F for each element

Figure 9-3.Figure 9-3. Change in the concentration Change in the concentration of Rb and Sr in the melt derived by of Rb and Sr in the melt derived by progressive batch melting of a basaltic progressive batch melting of a basaltic rock consisting of plagioclase, augite, rock consisting of plagioclase, augite, and olivine. From Winter (2001) An and olivine. From Winter (2001) An Introduction to Igneous and Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Metamorphic Petrology. Prentice Hall.

Rubidium incompatiblestrongly partitions i.e. strongly concentrated in the early small melt

Sr doesn’t change much, so Rb/Sr is approximately the Rb value

DiscussionDiscussion

Any ratio of Incompatible to Compatible Any ratio of Incompatible to Compatible elements will be sensitive to the degree elements will be sensitive to the degree of partial melting in initial stagesof partial melting in initial stages

Our experience with Ternary systems Our experience with Ternary systems tells us it is unrealistic to expect a ratio tells us it is unrealistic to expect a ratio of the minerals in the solid residues to of the minerals in the solid residues to remain constant.remain constant.

What should we do?What should we do?

Incremental Batch MeltingIncremental Batch Melting Calculate batch melting for Calculate batch melting for

successive batches (same successive batches (same equation)equation)

Must recalculate DMust recalculate Dii as solids as solids change and minerals are change and minerals are selectively melted selectively melted

Figure 9-2 and 9-3 (earlier slides) Figure 9-2 and 9-3 (earlier slides) show that the model we just used show that the model we just used is most sensitive to Dis most sensitive to Dii at low at low values of F, so it is most important values of F, so it is most important to use small increments in that to use small increments in that area. Once F > 0.4, batch melts area. Once F > 0.4, batch melts vary less and are less likely vary less and are less likely anyway.anyway.

Crystallization ExtremesCrystallization ExtremesEither: Crystals remain in equilibrium with Either: Crystals remain in equilibrium with

each melt incrementeach melt increment

OR The other extreme: separation of each OR The other extreme: separation of each crystal as it formed = perfectly continuous crystal as it formed = perfectly continuous fractional crystallization in a magma fractional crystallization in a magma chamberchamber

Rayleigh fractional crystallizationRayleigh fractional crystallization The second extreme: separation of The second extreme: separation of

each crystal as it formed = each crystal as it formed = perfectly continuous fractional perfectly continuous fractional crystallization in a magma crystallization in a magma chamberchamber

Crystals accumulate on floor of Crystals accumulate on floor of magma chamber, isolating formed magma chamber, isolating formed crystals crystals – Concentration of some element in the Concentration of some element in the residualresidual liquid, C liquid, CLL is modeled by the is modeled by the Rayleigh equation:Rayleigh equation:

eq. 9-8eq. 9-8 CCLL/C/COO = F = F (D(Di i -1) -1)

CCoo conc. of element in original melt conc. of element in original melt

F fraction of melt remaining after xtals F fraction of melt remaining after xtals removedremoved

Other models are used to analyzeOther models are used to analyze Mixing of magmasMixing of magmas Wall-rock assimilationWall-rock assimilation Zone refiningZone refining Combinations of processes Combinations of processes

End of Part 1

The Rare Earth Elements (REE)

Begins Part 2

REEsREEs

Group IIIA, +3 oxidation state, ionic radius Group IIIA, +3 oxidation state, ionic radius decreases with increasing atomic number decreases with increasing atomic number (the “Lanthanide contraction”) so right side (the “Lanthanide contraction”) so right side smaller ions, fit more readily, more smaller ions, fit more readily, more compatible.compatible.

bigger smaller

Density Groups of REEsDensity Groups of REEs LREE = light rare earth elements (Sc, La, Ce, Pr, Nd, LREE = light rare earth elements (Sc, La, Ce, Pr, Nd,

Pm, Sm, Eu, and Gd; also known as the cerium group)Pm, Sm, Eu, and Gd; also known as the cerium group) HREE = heavy rare earth elements (Y, Tb, Dy, Ho, Er, HREE = heavy rare earth elements (Y, Tb, Dy, Ho, Er,

Tm, Yb, and Lu; also known as the yttrium group)Tm, Yb, and Lu; also known as the yttrium group)

The densities of the LREEs (as pure elements) range The densities of the LREEs (as pure elements) range from 2.989 (scandium) to 7.9 g/cc (gadolinium), while from 2.989 (scandium) to 7.9 g/cc (gadolinium), while those of the HREEs are from 8.2 to 9.8, except for those of the HREEs are from 8.2 to 9.8, except for yttrium (4.47) and ytterbium (between 6.9 and 7) The yttrium (4.47) and ytterbium (between 6.9 and 7) The latter are groups with the HREE due to similar latter are groups with the HREE due to similar geological behavior.geological behavior.

Oxygen FugacityOxygen Fugacity Fugacity is the partial pressure of a gas phase which takes into account that Fugacity is the partial pressure of a gas phase which takes into account that

the gas is able to chemically react with other components in the system. A the gas is able to chemically react with other components in the system. A measure of departure from Ideal Gas behavior.measure of departure from Ideal Gas behavior.

Oxygen fugacity is the measure of the availability of oxygen (within a given Oxygen fugacity is the measure of the availability of oxygen (within a given system) to partake in chemical reactions. It represents the chemical potential system) to partake in chemical reactions. It represents the chemical potential of oxygen. It is also a way to quantify the redox state of a given system.of oxygen. It is also a way to quantify the redox state of a given system.

This is very important in terms of determining the stable mineral present in a This is very important in terms of determining the stable mineral present in a given geological system.given geological system.

More oxidizing conditions, in igneous or hydrothermal systems for example, More oxidizing conditions, in igneous or hydrothermal systems for example, favor the crystallization (and stability) of the more oxidized mineral phases.favor the crystallization (and stability) of the more oxidized mineral phases.

More available oxygen in the system = more oxidizing More available oxygen in the system = more oxidizing System precipitates components with more oxygen (i.e. System precipitates components with more oxygen (i.e.

Hematite FeHematite Fe22OO33 (ratio 3/2 = 1.5) would crystallize, rather than Magnetite (ratio 3/2 = 1.5) would crystallize, rather than Magnetite FeFe33OO44 (ratio 4/3 = 1.333). (ratio 4/3 = 1.333).

Incompatible elementsIncompatible elements An element unsuitable in size and/or charge to the cation sites of the rock forming An element unsuitable in size and/or charge to the cation sites of the rock forming

minerals. The partition coefficient for them between rock-forming minerals and melt minerals. The partition coefficient for them between rock-forming minerals and melt is much smaller than 1.is much smaller than 1.

During the partial melting of the Earth's mantle and crust, elements that have During the partial melting of the Earth's mantle and crust, elements that have difficulty in entering cation sites of the basaltic minerals (Olivine, Clinopyroxenes, Ca-difficulty in entering cation sites of the basaltic minerals (Olivine, Clinopyroxenes, Ca-rich Plagioclases) are concentrated in the liquid phase of the magma. They are rich Plagioclases) are concentrated in the liquid phase of the magma. They are destined for later minerals and glasses. For example, Potassium K enters the K-spars: destined for later minerals and glasses. For example, Potassium K enters the K-spars: Sanidine, Orthoclase, Microcline very late in fractionation.Sanidine, Orthoclase, Microcline very late in fractionation.

Two groups of incompatible elements are known by acronyms. One group includes Two groups of incompatible elements are known by acronyms. One group includes elements having large ionic radius, (called LILE, or large-ion lithophile elements. This elements having large ionic radius, (called LILE, or large-ion lithophile elements. This LILE group includes potassium, rubidium, cesium, strontium, and barium. LILE group includes potassium, rubidium, cesium, strontium, and barium.

The other group includes elements of large ionic valences (or high charges), such as The other group includes elements of large ionic valences (or high charges), such as zirconium zirconium ZrZr+4+4 , niobium, hafnium, rare earth elements (REE), thorium, uranium and , niobium, hafnium, rare earth elements (REE), thorium, uranium and tantalum (called HFSE, or high field strength elements). These can occur in tantalum (called HFSE, or high field strength elements). These can occur in pegmatites, for example the alkaline pegmatites we will see at Cranberry Lake.pegmatites, for example the alkaline pegmatites we will see at Cranberry Lake.

Europium Europium

Note: at low Oxygen fugacity, Europium can have Note: at low Oxygen fugacity, Europium can have a +2 valence, and Eua +2 valence, and Eu+2 +2 can be more abundant can be more abundant than Euthan Eu+3+3

Eu+2 is a Large Ion Lithophile (LIL)

Eu+3 is a high field strength (HFS) element.

LILs include K, Rb, Cs, Ba, Pb+2 , Sr and Eu+2

LIL’s are low field strength, e.g. Potassium’s ion is K+, and are generally more mobile if a fluid is adjacent.

The term LILE is The term LILE is restricted to restricted to lithophile trace lithophile trace elements having elements having a large a large ionic ionic radius to radius to charge ratio; charge ratio; that have ionic that have ionic radii greater radii greater than those of than those of CaCa2+ 2+ and Naand Na1+1+, , some of the some of the largest cations largest cations common to rock common to rock forming forming minerals. By this minerals. By this definition, LILEs definition, LILEs are K, Rb, Cs, Sr, are K, Rb, Cs, Sr, Ba, Pb and EuBa, Pb and Eu+2 +2

-3

-2

-1

0

1

2

3

4

5

6

7

8

9

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

Oddo-Harkins Rule: atoms with even atomic numbers are more stable, and thus more abundant, than their odd-numbered neighbors:elements with odd atomic numbers have one unpaired proton and are more likely to capture another

Europium anomalyEuropium anomaly A negative dip is evidence liquid was in A negative dip is evidence liquid was in

equilibrium with now-absent plagioclase*, equilibrium with now-absent plagioclase*, which captured Euwhich captured Eu+2+2

Figure 9-5.Figure 9-5. REE diagram for 10% REE diagram for 10% batch melting of a hypothetical batch melting of a hypothetical lherzolite with 20% plagioclase, lherzolite with 20% plagioclase, resulting in a pronounced negative resulting in a pronounced negative Europium anomaly. From Winter Europium anomaly. From Winter (2001) An Introduction to Igneous (2001) An Introduction to Igneous and Metamorphic Petrology. and Metamorphic Petrology. Prentice Hall.Prentice Hall.

* It floated away

Application of Trace Elements to Igneous Systems

1. Use like major elements on variation diagrams to 1. Use like major elements on variation diagrams to document fractionation, assimilation, etc. in a document fractionation, assimilation, etc. in a suite of rockssuite of rocks TE more sensitive TE more sensitive larger variations as larger variations as

process continuesprocess continues

Figure 9-1a.Figure 9-1a. Ni Harker Diagram for Ni Harker Diagram for Crater Lake. From data compiled by Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Rick Conrey. From Winter (2001) An Introduction to Igneous and Introduction to Igneous and Metamorphic Petrology. Prentice Metamorphic Petrology. Prentice Hall.Hall.

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

Example: can use REE to distinguish between high Example: can use REE to distinguish between high pressure and low pressure sources of a mantle-pressure and low pressure sources of a mantle-derived magmaderived magma

In the deep continental crust, and at depths over In the deep continental crust, and at depths over about 100 km in the mantle, garnet and about 100 km in the mantle, garnet and clinopyroxene are important phases, which remain clinopyroxene are important phases, which remain as residual solids during the generation of up to as residual solids during the generation of up to 15-20% partial melting15-20% partial melting

Application of Trace Elements to Igneous Systems

Table 9-1. Partition Coefficients for some commonly used trace elements in basaltic and andesitic rocks Bulk D calculation

Olivine Opx Cpx Garnet Plag Amph

Rb 0.006 0.02 0.04 0.001 0.1 0.3

Sr 0.01 0.01 0.14 0.001 1.8 0.57

Ba 0.006 0.12 0.07 0.002 0.23 0.31

Ni 14 5 2.6 0.4 0.01 3

Cr 2.1 10 8.4 0.17 10 1.6

La 0.007 0.02 0.08 0.05 0.14 0.27

Ce 0.009 0.02 0.34 0.05 0.14 0.34

Nd 0.009 0.05 0.6 0.07 0.08 0.19

Sm 0.009 0.05 0.9 0.06 0.08 0.91

Eu 0.008 0.05 0.9 0.9 0.1/1.5* 1.01

Tb 0.01 0.05 1 5.6 0.03 1.4

Er 0.013 0.31 1 18 0.08 0.48

Yb 0.014 0.34 0.2 30 0.07 0.97

Lu 0.016 0.11 0.82 35 0.08 0.89

data from Henderson (1982) * Eu3+/Eu2+ Italics are estimated

Rare

Eart

h E

lem

ents

GarnetGarnet concentrates the HREE and fractionates among them concentrates the HREE and fractionates among them

Thus if Garnet is in equilibrium with the partial melt (a residual Thus if Garnet is in equilibrium with the partial melt (a residual phase in the source left behind) expect a concentration of Tb, Er, phase in the source left behind) expect a concentration of Tb, Er, Yb, and Lu in the GarnetYb, and Lu in the Garnet

Shallow (< 40 Shallow (< 40 km) partial km) partial melting of the melting of the mantle will have mantle will have plagioclase in plagioclase in the residuum the residuum and a Eu and a Eu anomaly will anomaly will resultresult

Table 9-6 A brief summary of some particularly useful trace elements in igneous petrology

Element Use as a petrogenetic indicator

Ni, Co, Cr Highly compatible elements. Ni (and Co) are concentrated in olivine, and Cr in spinel andclinopyroxene. High concentrations indicate a mantle source.

V, Ti Both show strong fractionation into Fe-Ti oxides (ilmenite or titanomagnetite). If they behavedifferently, 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 mayreplace Ti in sphene or rutile).

Ba, Rb Incompatible element that substitutes for K in K-feldspar, micas, or hornblende. Rb substitutesless readily in hornblende than K-spar and micas, such that the K/Ba ratio may distinguish thesephases.

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, butas an incompatible at higher pressure where plagioclase is no longer stable.

REE Garnet accommodates the HREE more than the LREE, and orthopyroxene and hornblende doso to a lesser degree. Sphene and plagioclase accommodates more LREE. Eu2+

is stronglypartitioned into plagioclase.

Y Commonly incompatible (like HREE). Strongly partitioned into garnet and amphibole. Spheneand apatite also concentrate Y, so the presence of these as accessories could have asignificant effect.

Table 9-6.Table 9-6. After Green (1980). Tectonophys., After Green (1980). Tectonophys., 6363, , 367-385. From Winter (2001) An Introduction to 367-385. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Igneous and Metamorphic Petrology. Prentice Hall.

Figure 9-8.Figure 9-8. (a) (a) after Pearce and Cann (1973), after Pearce and Cann (1973), Earth Planet, Sci. Lett., Earth Planet, Sci. Lett., 1919, 290-300, 290-300. . (b)(b) after Pearce (1982) after Pearce (1982) in in Thorpe (ed.), Andesites: Orogenic andesites and related rocks. Wiley. Chichester. pp. 525-548Thorpe (ed.), Andesites: Orogenic andesites and related rocks. Wiley. Chichester. pp. 525-548 , Coish et al. (1986), , Coish et al. (1986), Amer. J. Sci., Amer. J. Sci., 286286, 1-28, 1-28.. (c)(c) after Mullen (1983), after Mullen (1983), Earth Planet. Sci. Lett., Earth Planet. Sci. Lett., 6262, 53-62., 53-62.

Petrology Field Trip to Bemco Mining District, a side trip

Ores from weathered Sulfide deposits• Mineral deposits

containing sulfide minerals, e.g. copper sulfides that are subjected to weathering, can go into solution and trickle down to the reducing conditions below the water table, where native metals or rich concentrations of ores are precipitated.

Black Smokers, hydrothermal circulationsGossan Intensely oxidized, weathered or decomposed rock, usually the upper and exposed part of an ore

deposit or mineral vein. In the classic gossan or iron cap all that remains is iron oxides and quartz often in the

form of boxworks, quartz lined cavities retaining the shape of the dissolved ore minerals.

Solubility in waterThe Solubility Rules

1. Salts containing Group I elements are soluble (Li+, Na+, K+, Cs+, Rb+). Exceptions to this rule are rare. Salts containing the ammonium ion (NH4+) are also

soluble. 2. Salts containing nitrate ion (NO3

-) are generally soluble. 3. Salts containing Cl -, Br -, I - are generally soluble. Important exceptions to this rule are halide salts of Ag+, Pb2+, and (Hg2)2+. Thus, AgCl, PbBr2, and Hg2Cl2 are all insoluble. 4. Most silver salts are insoluble. AgNO3 and Ag(C2H3O2) are common soluble salts of silver; virtually anything else is insoluble.

5. Most sulfate salts are soluble, for example FeSO4 is soluble. Important exceptions to this rule include BaSO4, PbSO4, Ag2SO4 and SrSO4 . 6. Most hydroxide salts are only slightly soluble. Hydroxide salts of Group I elements are soluble. Hydroxide salts of Group II elements (Ca, Sr, and Ba) are slightly soluble. Hydroxide salts of transition metals and Al3+ are insoluble. Thus, Fe(OH)3, Al(OH)3, Co(OH)2 are not soluble.

7. Most sulfides of transition metals are highly insoluble. Thus, CuS, FeS, FeS2, ZnS, Ag2S are all insoluble. Arsenic, antimony, bismuth, and lead sulfides are also insoluble. 8. Carbonates are frequently insoluble. Group II carbonates (Ca, Sr, and Ba) are insoluble. Some other insoluble carbonates include FeCO3 and PbCO3. 9. Chromates are frequently insoluble. Examples: PbCrO4, BaCrO4 10. Phosphates are frequently insoluble. Examples: Ca3(PO4)2, Ag3PO4 11. Fluorides are frequently insoluble. Examples: BaF2, MgF2 PbF2.

Putting insoluble sulfides into solution

• Oxidizing Zone above the water table • Sulfide minerals, for example ferrous and

copper sulfides, are subject to weathering.

• Sulfide minerals are oxidized near the surface and produce sulfuric acid. For example:

• FeS2 (s) + 7O + H2O →FeSO4 (aq) + H2SO4

Reaction and Trickling Down

• Iron sulfate reacts with sulfides, they go into solution as sulfates, acid rainwater then carries, for example copper, as copper sulfate, down to the water table.

• CuS(s) + Fe2(SO4)3 (aq) →2FeSO4 (aq) + S(s) + CuSO4 (aq)

• The net result is that dissolved copper sulfide trickles down from the oxidizing upper portion of the deposit to that portion at and just below the water table.

Reducing Zone below the water table

• Below the water table, where additional sulfide minerals remain solid and unoxidized (e.g. Pyrite FeS2), any iron sulfide grains present will react with the copper sulfate solution, putting iron into solution and precipitating copper.

• FeS2 (s) + CuSO4 (aq) → FeSO4 (aq) + Cu(s) + 2S(s)

Hydrothermal Deposit, Bemco MineHydrothermal Deposit, Bemco Mine

chapter 9: trace elements

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liquid i solid

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smaller ion

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