Geo Chemistary Isotope

8/6/2019 Geo Chemistary Isotope

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Fractionation

A reaction or process which selects for

one of the stable isotopes of a particular

element If the process selects for the heavier

isotope, the reaction product is heavy, the

reactant remaining is light

Isotope fractionation occurs for isotopic

exchange reactions and mass-dependent

differences in the rates of chemical

reactions and physical processes

a

bba

a

b (!} HHEln103


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Temperature effects on

fractionation The fractionation factors, E, are affected

by T (recall that this affects EA) and

defined empirically:

T

hen,

As T increases, ( decreases at high T(

goes to zero

BT

Aab

v!

2

6

3 10ln10 EWhere A and B are constants

determined for particular reactions

and T is temp. in Kelvins

a

bbaa

b (!} HHEln103


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Equilibrium vs. Kinetic fractionation

Fractionation is a

reaction, but one in

which the free energy

differences are on theorder of 1000x smaller

than other types of

chemical reactions

Just like other chemical

reactions, we candescribe the proportion

of reactants and

products as an

equilibrium or as akinetic function


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FRACTIONATION DURING

PHYSICAL PROCESSES

Mass differences also give rise to

fractionation during physical processes

(diffusion, evaporation, freezing, etc.).

Fractionation during physical process is aresult of differences in the velocities of

isotopic molecules of the same compound.

Consider molecules in a gas. All molecules

have the same average kinetic energy, which

is a function of temperature.

2

21 mvE

kinetic !


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Because the kinetic energy for heavy and light

isotopes is the same, we can write:

In the case of

12

C

16

O and

13

C

16

O we have:

Regardless of the temperature, the velocity of12C16O is 1.0177 times that of13C16O, so thelighter molecule will diffuse faster and

evaporate faster.

L

H

H

L

m

m

v

v!

0177.1994915.27

99827.28!!

H

L

v

v


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Equilibrium Fractionation

For an exchange reaction:

C16O2 + H218O C18O2 + H2

16O

Write the equilibrium:

Where activity coefficients effectively cancel

out

For isotope reactions, K is always small,

usually 1.0xx (this K is 1.047 for example)

)()(

)()(

2

182

1

2

16

2

162

1

2

18

OHOC

OHOCK!


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WHY IS K DIFFERENTFROM

1.0?

Because 18O forms a stronger covalent bond

with C than does 16O.

The vibrational energy of a molecule is given by

the equations:

H

O

H

RhE lvibrationa 21!

m

k

TR

2

1! kxF !

Thus, the frequency of vibration depends

on the mass of the atoms, so the energy

of a molecule depends on its mass.


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The heavy isotope forms a lower energy

bond; it does not vibrate as violently.

Therefore, it forms a stronger bond in

the compound. The Rule of Bigeleisen (1965) - The

heavy isotope goes preferentially into

the compound with the strongest bonds.


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Equilibrium Fractionation II

For a mass-dependent reaction:

Ca2+ + C18O32- CaC18O3

Ca2+ + C16O32- CaC16O3

Measure H18O in calcite (H18Occ) and water

(H18Osw)

Assumes 18O/16O between H2O and CO32- at

some equilibrium

T C = 16.998 - 4.52 (H18Occ - H18Osw) + 0.028 (H

18Occ-H18Osw)

2


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Empirical Relationship between Temp. &

Oxygen Isotope Ratios in Carbonates

At lower temperatures, calcite

crystallization tends to incorporate a

relativelylarger proportion of18O

because the energylevel(vibration)of ions containing this heavier isotope

decreases by a greater amount than ions

containing 16O.

As temperatures drop, the energylevel

of18O declines progressively by anamount that this disproportionately

greater than that of the lighter16O.


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Distillation

2 varieties, Batch and Rayleigh distillationdependent on if the products stay in contact

and re-equilibrate with the reactants

Batch Distillation:Hf=Hi (1 F) 10

3lnECO2-Rock

where the isotope of the rock (Hi) depends on

its initial value (Hf) and the fractionation factor Rayleigh Distillation

Hf - Hi =103(F(E 1) 1)


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RAYLEIGH DISTILLATION

Isotopic fractionation that occurs duringcondensation in a moist air mass can be

described by Rayleigh Distillation. The equation

governing this process is:

where Rv = isotope ratio of remaining vapor, Rv =

isotope ratio in initial vapor, = the fraction of

vapor remaining and a = the isotopic

fractionation factor

1! EfRR ovv


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Effect of Rayleigh

distillation on the

H18

O value of watervapor remaining in

the air mass and of

meteoric precipitation

falling from it at a

constant temperatureof25C.

Complications:

1) Re-evaporation

2) Temperaturedependency ofE


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Using isotopes to get information on

physical and chemical processes Fractionation is due to some reaction,

different isotopes can have different

fractionation for the same reaction, anddifferent reactions have differentfractionations, as well as being different atdifferent temperatures and pressures

Use this to understand physical-chemicalprocesses, mass transfer, temperaturechanges, and other things


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Volatilization

calcite + quartz = wollastonite + carbon dioxideCaCO

3+ SiO

2= CaSiO

3+ CO

2

As the CO2

is produced, it is likely to be expelled


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Other volatilization reaction examples


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ISOTOPE FRACTIONATION IN THE

HYDROSPHERE

Evaporation of surface water in equatorial regions

causes formation of air masses with H2O vapor

depleted in 18O and D compared to seawater.

This moist air is forced into more northerly, cooler airin the northern hemisphere, where water

condenses, and this condensate is enriched in 18O

and D compared to the remaining vapor.

The relationship between the isotopic composition of

liquid and vapor is:

331818 1010 !v

l

vl HEH


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Assuming that H18Ov = -13.1 and Evl(O) =

1.0092 at 25C, then

and assuming HDv = -94.8 and Evl(H) = 1.074

at 25C, then

These equations give the isotopic composition

of the first bit of precipitation. As 18O and D

are removed from the vapor, the remaining

vapor becomes more and more depleted.

Thus, H18O and HD values become increasingly

negative with increasing geographic latititude

(and altitude.

0003318 0.410101.130092.1 !!lH

00033

8.2710108.94074.1 !!lDH


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Map of North

America

showing

contours of theapproximate

average HD

values of

meteoric surface

waters.


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Because both H and O occur together in water, H18O and HD

are highlycorrelated, yielding the meteoric waterline

(MWL

): HD} 8

H

18O

+10


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Deviation from MWL

Any additional fractionation process which

affects O and D differently, or one to the

exclusion of the other will skew a water

away from the MWL plot

These effects include:

Elevation effects - (HD -8/1000m, -4/C)

Temperature (E different!)

Evapotranspiration and steam loss

Water/rock interaction (little H in most rocks)


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Kinetic Fractionation

lighter isotopes form weaker bonds incompounds, so they are more easily

broken and hence react faster. Thus, in

reactions governed by kinetics, the lightisotopes are concentrated in the products.

Again, isotope reactions can be exchange

reactions or mass-dependent chemical orphysical reactions kinetic factors may affect

any of these!


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Kinetic fractionation I

SO4

2- reduction

SO42- + CH4 + 2 H

+ H2S + CO2 + 2 H2O

This reaction is chemically slow at low T,

bacteria utilize this for E in anoxic settings Isotope fractionation of S in sulfide generated

by microbes from this process generates

some of the biggest fractionations in the

environment (-120 for S)

THEN we need to think about exchange

reactions with H2S orFeS(aq) as it may

continue to interact with other S species


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S isotopes and microbes

The fractionation of H2S formed frombacterial sulfate reduction (BSR) is affected

by several processes:

Recycling and physical differentiation yields

excessively depleted H2S

Open systems H2S loss removes34S

Limited sulfate governed by Rayleigh process,

enriching34

S Different organisms and different organic

substrates yield very different experimental H34S

Ends up as a poor indicator of BSR vs. TSR


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IronIsotopesEarths Oceans 3 Ga had no

oxygen and lots ofFe2+,cyanobacteria evolved,

produced O2 which oxidized

the iron to form BIFs in time

the Fe2+ was more depleted

and the oceans were stratified,

then later become oxic as theyare today

This interpretation is largely based on ironisotopes in iron oxides and sulfide minerals

deposited at those times (Rouxel et al., 2005)

No one has yet bothered tomeasure how iron isotopes

change when iron sulfide

minerals precipitate thats

where we come in


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Mass-independent fractionation

Mass effects for 3 stable isotopes(such as 18O, 17O, and 16O) shouldhave a mass-dependent relationshipbetween each for any process

Deviation from this is mass-

independent and thought to beindicative of a nuclear process(radiogenic, nucleosynthetic,spallation) as opposed to a physico-chemical process

Found mainly associated withatmospheric chemistry, effect can bepreserved as many geochemicalreactions in water and rock are mass-dependent


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S-isotopic evidence of Archaen

atmosphere

Farquar et al., 2001; Mojzsis et al., 2003found MIF signal in S isotopes (32S, 33S,

34S) preserved in archaen pyritesprecipitated before 2.45 Ga

Interpreted to be signal from the photolysisof SO2 in that atmosphere the reaction

occurs at 190-220nm light, indicating lowO2 and O3 (which very effficiently absorbthat wavelength)

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