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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