Geochemistry

CATHERINECHAUVEL(ISTERRE &IPGP)&BRUNODHUIME (GEOSCIENCESMONTPELLIER)SEMINARBYEMILIEBRUAND (LMV,CLERMONT-

FERRAND)

Program

Lecture1:ToolsandconceptsLecture2:MantlegeochemistryLecture3:MantlegeochemistryLecture4:FormationandevolutionofthecontinentalcrustLecture5: FormationandevolutionofthecontinentalcrustSeminarEmilieBruandLecture6:Formationandevolutionofthecontinentalcrust

O

Si

Mg

FeCa Al

Na, K, other

Bulk Silicate Earth (BSE)

Fe

Si?Ni

other

Core

First order differentiation of the Earth

O

SiMg

Fe

Ca AlNi other

Bulk Earth

Continentalcrustextraction

O

Si

Mg

FeCa Al

Na, K, other

BSE (or "primitive mantle")overall composition: lherzolite

O

Si

FeMg

Ca

AlNa K other

Continental crustoverall composition: andesite

Composition depends on fraction of mantle fromwhich crust extracted

?Residual mantle

Traceelementgeochemistry

Whatisatraceelement?◦ Elementthatdoesnotconstitutethemineralphase◦ Entermineralphasesassubstitutiontoamajorelement

◦ Itspresencedoesnotaffectthephysicalandchemicalpropertiesoftheminerals

Howdotheybehaveduringmagmaticprocesses?◦ Partialmelting,fractionalcrystallizationandmixing

OnEarth,almostallelementsaretraceelements

Periodictableviewbygeochemists

Goldschmidtclassification

RareEarthElements

RareEarthElementsinchondrite“Orgueil”

RareEarthpatterns

Howtraceelementsenterminerals?

PartitioncoefficientKDi=chemicalelement

a&b=2differentphases

C=concentration(ppm)

Convention:Solide/liquid

DistributioncoefficientDWhenthesolidincludesseveralmineralphases,Dcorrespondstotheweightedaverageofthevariousphasesandtheirproportions.

IncompatiblevscompatibleelementsIfKD>1,thetraceelementis“compatible”IfKD<1,thetraceelementis“incompatible”IfKD <<<1,thetraceelementis“veryincompatible”

Partitioncoefficientandionicradius

Example for olivine and some trace elements

Inpractice

KD forREEinthemostcommonmantleminerals

Whichmineralsmeltfirst?

Meltingequations(Shaw,1970)Equilibriummelting:theliquidisinequilibriumwiththeentireresidualsolid

With:

F:degreeofmelting,

D&D0:distributioncoeff inthesolid

P:distributioncoeff forthephasesenteringtheliquid

modal

non modal

Meltingequations(Shaw,1970)Fractionalmelting:theliquidisinequilibriumwiththeinstantaneoussolid

Modal

Non modal

Meltingequations(Shaw,1970)

Ifliquidsarepooled,thefractionalmeltingequationsbecome:

Comparisonoftheeffects

Effect ofpartialmelting onREEpatterns

Albarede (2003)

Because LREEaremoreincompatiblethan HREEinmost mantle phases,melts arerelatively enriched inLREEwhile residues aredepleted.

Continentalcrust - depleted mantle

Crust data:TaylorandMcLennan (1985);Primitivemantle (BSE)data:Hofmann(1988)

continentalcrust

residual after extractionfrom whole mantle

residual afterextractionfromupper mantle

Increasing crustalconcentrations

FractionalcrystallizationDuringcrystallization,theelementspartitionbetweenmineralsandresidualliquidfollowing:

With F=(1-X)

Meltingvsfractionation

Mixing massbalanceWhen two componentsA&Bmix:

&

therefore

with

M mass, C concentration & Elt element

Mixingequations(Langmuiretal,1978)

Generalformula:Ax+Bxy+Cy+D=0with◦ x=P/b,y=Q/a◦ A=a2b1y1-a1b2y1◦ B=a1b2-a2b1◦ C=a2b1x1-a1b2x2◦ D=a1b2x2y1-a2b1x1y2◦ R=a1b2/a2b1

Ingeneral,mixingarraysarehyperbola!!!

Isotopegeochemistry

Atomicnucleus=neutron(N)

+protons(Z)

N+Z=atomic mass

Structureofatoms

Atom=nucleus+electrons

ByBdushaw - Ownwork,CCBY-SA4.0,https://commons.wikimedia.org/w/index.php?curid=61302797

Thestabilityvalley

Isotopes

IsobarsIsotones

Atomic nucleus= neutron (N) + protons (Z)N+Z= atomic mass

Examples

Hydrogen

Z = 1N = 0 => 1H (stable)N = 1=> 2H (stable)N = 2 => 3H (radioactive)

Oxygen

Z = 8N = 8 => 16O (stable)N = 9 => 17O (stable)N = 10 => 18O (stable)

Isotopes

§ Producedbydecayofaradioactiveisotope

§ Threetypesofdecay:b-, a andb+

Radiogenicisotopes

ByBdushaw - Ownwork,CCBY-SA4.0,https://commons.wikimedia.org/w/index.php?curid=61302797

b-

b+

T:half-livel:decay constantN:radioactivenuclideD:daughter isotope

T=ln2/l

N=N0.e-lt

Radioactivedecay

Numberofatomsoftheradioactivenuclide

Time

Number of newly formed radiogenic atoms: D = N0-N = Nelt-N = N(elt-1)

Number of radiogenic atoms present: D = D0 + N(elt-1)

Example: 87Rb -> 87Sr

87Sr = 87Sr0 + 87Rb (elt-1)

87Sr/ 86Sr = (87Sr/ 86Sr)0 + 87Rb/ 86Sr (elt-1)

Anisotopicratioiscontrolledbytheparent/daughterratioANDtime

Generalequation

Decaymode

l Half-life Daughter Ratio

40K b+,e.c.,b-

5.543*10-10a-1 1.28*109 a 40Ar,40Ca 40Ar/36Ar

87Rb b- 1.42*10-11a-1 48*109 a 87Sr 87Sr/86Sr

138La b- 2.67*10-12a-1 259*109 a 138Ce 138Ce/142Ce138Ce/136Ce

147Sm a 6.54*10-12a-1 106*109 a 143Nd 143Nd/144Nd

176Lu b- 1.87*10-11a-1 36*109 a 176Hf 176Hf/177Hf

187Re b- 1.64*10-11a-1 42*109 a 187Os 187Os/186Os

232Th a 4.946*10-11a-1 14*109 a 208Pb,4He 208Pb/204Pb3He/4He

235U a 9.849*10-10a-1 0.707*109 a 207Pb,4He 207Pb/204Pb3He/4He

238U a 1.551*10-10a-1 4.47*109 a 206Pb,4He 206Pb/204Pb3He/4He

Isochron:

t1

t0

t2

87Sr/8

6 Sr

87Rb/86Sr

sample 3sample 2sample 1

y-intercept (initial ratio)

Slope (dependson age)

87Sr/86Sr = (87Sr/86Sr)o + (87Rb/86Sr) (elt - 1)

Radiogenicisotopes,earthreservoirsandtimecontinentalcrust

residual after extractionfrom whole mantle

residual afterextractionfromupper mantle

Rb,Sr,Sm,Nd,LuandHfareallincompatiblebut:DRb<DSr, DSm>DNd and DLu>DHf

BABI(BasalticAchondriteBestInitial)=estimate ofsolar systeminitial87Sr/86Sr

Srisotopic evolution ofContinentalCrust andMantle

White(2013)

Overtime,thecrusthasdevelopedaradiogenicsignaturerelativetoBSEwhiletheresidualmantlehasdevelopedanunradiogenicsignature.Exact87Sr/86Srvaluesofeachreservoirdependoncrustalextractionhistory.

Since DRb<DSr

Rb/Srcrust >Rb/SrBSE>Rb/Srresidualmantle(?)

Nd isotopic evolutionofCrust andMantle

Since DSm>DNd

Sm/Nd crust<Sm/Nd BSE<Sm/Nd mantle

Asaresult,residualmantleNdbecomesradiogenicwhilecrustalNdbecomesrelativelyunradiogenicovertime.

chondritic Sm/Nd

CHUR=chondriticuniformreservoir

(?)

TheopposingbehaviorofNd andSr isotopesisduetothefacttheparentismoreincompatiblethanthedaughterinonecase(Rb/Sr)andlessincompatibleintheother(Sm/Nd)

eNd and87Sr/86Srare

anti-correlatedamong main

Earthreservoirs

White(2013)

(?)

235U➜ ...➜ 207Pbt1/2=0.70Gy

238U➜ ...➜ 206Pbt1/2=4.47Gy

232Th➜ ...➜ 208Pbt1/2=14.0Gy

235U,238Uand232Thdecay toPbthrough aseries ofshort-lived radioactivedaughterisotopes.

Forlong-term processes (e.g.evolution ofmantle reservoirs),can beapproximated asadirectdecay from theparent(UorTh)toPb.

Forrelatively rapid processes (e.g.magmageneration)intermediatedaughters can be very useful forconstraining rates

ThePbcase

Several intermediate daughters with half-lives appropriate forstudying magmatism (234U,230Th,226Ra)

238UDecay chain

235U ➜ ... ➜ 207Pbt1/2= 0.70 Gy

238U ➜ ... ➜ 206Pbt1/2= 4.47 Gy

232Th ➜ ... ➜ 208Pbt1/2= 14.0 Gy

Pbhas4isotopes204Pb,206Pb,207Pb,208Pb

207Pb/204Pb=(207Pb/204Pb)0 +235U/204Pb(el235T-1)

206Pb/204Pb=(206Pb/204Pb)0 +238U/204Pb(el238T-1)

208Pb/204Pb=(208Pb/204Pb)0 +232Th/204Pb(el232T-1)

TheseequationsareneverusedbecauseU(andPb)ismobile

Pb-Pbisochron

14.2

14.4

14.6

14.8

15.0

15.2

15.4

15.6

15.8

16.0

12 13 14 15 16 17 18 19 20

206Pb/204Pb

207Pb/204Pb

t=0t=1 Gyt=2 Gy

µ=8µ=9

µ=10

µ = 238U/204Pb

µ =238U/204Pb

238U/235U=137.88

235U/204Pb=µ/137.88

207Pb/204Pb- 207Pb/204Pbi206Pb/204Pb- 206Pb/204Pbi

(el235t - 1)

137.88(el238t - 1)=

Holmes-Houtermansmodel(1946)

Estimate oftheage oftheEarth 60years ago with asinglesample!!

Oceansediments(whichaveragelargevolumesofmaterial)plotontheisochron definedbymeteorites.

Patterson(1956)

Geochron

LeadParadox

(1) Almost allaccessibleEarth reservoirs plotononeside oftheGeochronwhen BSEis supposed tobe onthegeochron.Where is the"missing"unradiogenic Pb?Core?Lower crust?Subducted slabs?Still notresolved.

(2) TheMORBsourceis depleted inincompatibleelements butliesontheenriched side ofthegeochron!Puzzling andstill notresolved.

White(2013)

References

§ Langmuiretal.,EPSL1978,Ageneralmixingequationwithapplicationstoicelandic basalts

§White,W.M.,2013.Geochemistry.JohnWiley&Sons,Ltd.,Chichester,UK,660ppp.

§ ZouH.,2007.Quantitativegeochemistry,ImperialCollege Press

§ Rollinson H.Usinggeochemicaldata:evaluation,presentation,interpretation,LongmanScientific&Technicalgroup,1993

§ Albarède F.,IntroductiontoGeochemicalModeling,CambridgeUniversityPress,1995

References

§ Albarède F.,Geochemistry,anintroduction,CambridgeUniversityPress,2003

§ Shaw,D.M.,Traceelementfractionationduringanatexis,GCA,34,pp237-243,1970

§ Gast P.W.Traceelementfractionationandtheoriginoftholeiiticandalkalinemagmatypes,GCA,32,pp1057-1086,1968