如题所述
Ma Hongwen
1 Basic Principle
Gibbs Free Energy Function
1.1 Re in volatile-free systems(p/T):Fe2+-Mg exchange
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At equilibium,then
Δμ=0=ΔG+RTlnK
ΔG=-RTlnK
Equilibrium constant
K=aFs(opx)·aDi(cpx)/aEn(opx)·aHd(cpx)
For multicomponent systems
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Partition coefficient
Kd=xFs(opx)·xDi(cpx)/xEn(opx)·xHd(cpx)
Gibbs free energy change for a reaction
ΔG=(ΔGT)p=1+(ΔGp)T=T=ΔH-TΔS+ΔVp(p >> 1atm)
lnK=ΔS/R-ΔH/RT-ΔVp/RT
Basic equation for geothermobarometry.
Example:Fe2+-Mg exchange in grt-lherzolite(Brey et al.,1990)
1.2 Re in volatile-bearing systems(
6Fe2SiO4(olv)+O2(g)=3Fe2Si2O6(opx)+2Fe3O4(spn)(lherzolite)
2FeO(liq)+1/2O2(g)=Fe2O3(liq)(magma)
6FeS(po)+2O2(g)=3FeS2(py)+Fe3O4(mgt)(sulfide oxd)
KAlSi3O8(Or)+Fe3O4(Mgt)+H2O(g)=KFe3AlSi3O10(OH)2(Ann)+1/2O2(g)(granite)
KAlSiO4(liq)+1/2SiO2(liq)+3/2Mg2SiO4(liq)+H2O(g)=KMg3AlSi3O10(OH)2(Phl)(kimberlite)
FeO(silicate liq)+1/2S2(g)=FeS(sulfide liq)+1/2O2(g)(immiscibil-ity)
2 Geothermobarometry⇒Lithospheric Mapping
Example:Cenozoic thermal and redox state of the lithosphere in E.China
Data processing:MAFICS·FOR in software ofthermodynamics in crystalline petrology(Ma Hongwen,1999)
3 Phase equilibria⇒Modeling of magmatic processes
Example:Chemical mass transfer in magmatic processes(Ghiorso et al.,1980,1983,1995)
3.1 Basic thermodynamic relations
Re: Fe2SiO4(sol)=Fe2SiO4(liq)
Molar Gibbs free energy of silicate liquid
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Molar Gibbs free energy of solid components
ΔG0=ΔH0-TΔS0+CpdT-T(Cp/T)dT+[V0+aV(T-298)]p-(βV/2)p2
3.2 Crystallizing reactions:solid=liquid
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Continue
3.3 Solid-liquid-vapor
database:systems in SiO2-Al2O3-Fe2O3-FeO MgO-CaO-Na2O K2O-(H2O)
3.4 Thermodynamic parameters
(1)Thermodynzmic properties for the end member solid components(Berman,1988)
(2)Activity/composition models for end member solid components(cf.Ghiorso et al.,1995)
(3)Thermodynamic properties for the liquid components(cf.Ghiorso et al.,1995)
(4)Regular solution type interaction parameters(wij)(Ghiorso et al.,1995)
3.5 Software:MELTS,in ANSI C,UNIX workstation via FTP internet code
fondue.geology.washington.edu
(1)Revising/updating the parameters for the liquid
(2)Modeling equilibrium or fractional crystallization of specified bulk compositions
(3)Modeling crystallization paths under
constrained
total entropy
total volume
any combination of the above constraints
(4)Modeling assimilation or magma mixing
3.6 Example:mantle partial melting modeling(Hirschmann et al.,1998)
Sample:peridotitemm3,mg#=0.90
p=1.0GPa,
Experiments:Baker&Stolper(1994)
(1)Residual phases and mineral modes
Remarks:
phase-out
cpx F=18%
opx F=65%
spn F=93%
F<18% lherzolite/cpx-bearing lherzolite
F>18% harzbergite
F>45% impossible for natural melting
⇒ dunite:possibly cumulate
wehrlite:cumulate(not residue)
(2)Melting temperature
Remarks:
T(calc)>T(exp)~100℃
—anhydrous/hydrous,
(3)Melt compositions
Remarks:excellent agreement
Al2O31%~2% SiO23%~4% lower;CaO+MgO 2%~3% higher
—hydrous,
4 Modeling of immiscibility in silicate liquids(Ma Hongwen et al.,1998)
Immiscibility in silicate liquids,predicted by criterion:
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Compositions and amounts of 2L,calculated from oxide partition coefficients between Si-and Fe-rich melts:
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T(K),P(GPa),Xi,mole fraction of oxide i,
Ho,Si,and Fe,homogeneous,Si-,and Fe-rich melts,
Aito Di,aito di,constants.
Uncertainties:SiO2,Al2O3,and FeO,3.0%~4.0mol%
other oxides,<1.0mol%
predicted 2L amounts,~1.0mol%
Mgt-Apt ore-forming processes can be simulated.
4.1 INTRODUCTION
Greig(1927),system FeO-SiO2,2nd liq in pure SiO2liq.
Roedder(1951),immiscibility in K2O-FeO-Al2O3-SiO2.
Experiments,effects of T,p,fO2,xion immiscibility(Visser et al.,1979a,1979b,1979c;Philpotts,1982;Philpotts etal.,1983;Hess etal.,1982;Naslund,1983;Freestone et al.,1983)
Models:
Currie(1972),GFe+GSi<GHopredicting immiscibility
Ghiorso et al.(1983),regular solution model,deviated significantly from the experiments(Philpotts,1979).
4.2 THERMODYNAMIC MODEL
For an immiscible equilibrium:
ΔG=ΔH0-TΔS0+PΔV0=0 (1)
assuming that ΔCp=0,ΔV0constant,Smixideal.
The total G of a mixture of Ho,Si-and Fe-rich melts:
G=∑nigi=∑niμi(i=1,n) (2)
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In a closed system,mass balance must be maintained,i.e.,
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Equilibrium constant for Eq.(4):
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The Δg of ith component for Eq.(4):
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Ho,Si-,Fe-rich phases are all liquids with various amounts of oxides.Postulating regular solution model:
lnai=lnXi+φi/T+PΔVi/RT(7)
lnγi=[φi+PΔVi/R]/T (8)
where ΔVi,difference between partial molar volume of i in the liquids and that in pure ith com-ponent liquid;φi,compositional dependence of γiin the melt.
The extent of
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assuming φiis a linear function of liquid compositions,
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where Ai,Bi,Ci,corresponding to
When an immiscible reaction is at equilibrium,
(1)
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(2) GSi=GFe (13)
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where
Oxide partition coefficients:
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where ai,bi,ci,corresponding to
ΔVi/R,and∑diXi,compositional de
pendence of γi.
4.3 MODEL LIMITS
Compositions(wt%):SiO244.8~73.3,TiO20~12.7,Al2O32.1~19.0,FeO*0.9~40.9,MgO0~10.3,CaO0~12.3,Na2O,0~6.1,K2O0~11.2,P2O50~13.7;T=960~1550℃,p=1atm~1.5GPa,
4.4 PREDICTION OF IMMISCIBILITY
When immiscibile 2L are at equilibrium,
ΔG=∑ΔgiXi=0
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At beginning of an immiscible reaction,the amount of one of the 2L,say Fe-rich liquid,close to zero,i.e.,b≈0,a≈1,and ln
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Giving criterion of immiscibility:
ΔG=∑ΔgiXi≤0 (19)
For all components,
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239 immiscible experiments,232 correctly predicted
2L composition standard deviations(Fig.1):3.0mol%~4.0mol% for SiO2,Al2O3,and FeO;<1.0mol% for TiO2,MgO,CaO,Na2O,K2O,and P2O5;~1.0mol% for a mounts of both Si-and Fe-rich liquids.
4.5 MODELING OF Mgt-Apt ORE-FORMING PROCESS
Example:Yangyuan mgt apt ore deposit,Hebei province
Host rock:apt 6%~10%;spherulitic bio-pyx-syenite,evidence of immiscibility
globules:Or megacrystals with pyx,bio,mgt
mesostasis:pyx,bio,apt,interstitial Or
modeling:p=0.5GPa,
ΔG=-0.021~-0.103
At~1250℃,Si-/Fe-rich melts,<6mol%/>94mol%;MgO,CaO,K2O in 2L tend to converge;
1200℃,homogeneous T of gl.Inc.(Hou,1990).~1100℃,Si-/Fe-rich melts,~25mol%~75mol%,
consistent with:Si-rich,globules;Fe-rich,mesostasis,resemble syenite/alkaline pyrox-enite
Yangyuan mgt apt deposits,T=1250~1150℃.
Solubility of P2O5in Fe rich melts controlled by T,
<1100℃,unfavorable to mgt-apt ore-forming
>1100℃,highly enriched in Fe-rich liq(pyroxenite)
5 CONCLUDING REMARKS
Phase equilibria-exactly describing chemical mass transfer processes in petrology;
Thermodynamics-a powerful tool for constraining conditions of chemical reactions and genesis for crystalline petrology.
References
马鸿文.1993.硅酸盐岩浆中的硫化物不混溶作用模拟.中国科学(B辑),23(9):986~992
马鸿文,胡颖,袁家铮,方同辉.1998.岩浆不混溶作用模拟:热力学模型与数值方法,地球科学,23(1):41~48
马鸿文.2001.结晶岩热力学概论(第二版).北京:高等教育出版社,297
马鸿文.1999.结晶岩热力学软件.北京:地质出版社,340
Brey G P,Kohler T P.1990.Geothermobarometry in four-phase lherzolite:Ⅱ.New thermobarmeters,and practical assessment of existing thermobarometers.J Petrol 31:1353~1378
Berman R G.1988.Internally-consistent thermodynamic data for minerals in the system Na2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-SiO2-TiO2-H2O-CO2.J Petrol 29:445~522
Ghiorso M S,Sack R O.1995.Chemical mass transfer in magmatic processesⅥ.A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid-solid equilibria in magmatic systems at elevated temperatures and pressures.Contrib Mineral Petrol 119:197~212
Hirschmann M,Ghiorso M S,Wasylenki L E,Asimow P D,Stolper E M.1998.Calculation of peridotite partial melting from thermodynamic models of minerals and melts.Ⅰ.Review of methods and comparison with experiments.J Petrol 39:1091~1115
Ma Hongwen.1994.Modeling of sulfide immiscibility in silicate magmas.Science in China,Series B,37(9):1138~1146
Ma H,Hu Y,Fang T.1999.TWOLIQ:A FORTRAN77 program for simulating immiscibility in silicate liquids.Computer&Geosci,25:151~159
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