Mineral reactions and equilibria

1. Paragenesis: mineral assemblages that are in equilibrium Paragenesis found through experimental studies. Criteria for evaluating equilibrium: Absence of known incompatibles: hematite and graphite, ol and Qtz All phases are in mutual contact No evidence of replacement Absence of domains showing deformation next to strain free grains Grain shapes indicative of minimum surface area No zoned grains Mineral reactions and equilibria Metamorphic mineral reactions • Solid-solid reactions • Solid-fluid reactions (redox reactions, metasomatic reactions) • Discontinuous reactions: Univariant • Polymorphic transition (calcite-aragonite) • Net transfer (heterogeneous reactions) movement of matter between phases • Contineous reaction: continuously adjusting equilibrium: solid solution

2. Example: Quartz-coesite, graphite-diamond, calcite-aragonite Independent of the bulk composition Al2SiO5-system: kyanite-sillimanite-andalusite V(J/bar) S(J/K) H(kJ) Andalusite 5.146 91.6 -2589.66 Sillimanite 4.984 95.08 -2586.37 Kyanite 4.408 82.86 -2593.70 Polymorphic transitions Difficulty achieving phase change. Al-diffusion is slow. Sillimanite nucleation on muscovite grains: fibrolite

3. Al2SiO5 V(J/bar) S(J/K) H(kJ) Andalusite 5.146 91.6 -2589.66 Sillimanite 4.984 95.08 -2586.37 Kyanite 4.408 82.86 -2593.70 Kyanite: smallest molar volume: high P phase Sillimanite: largest entropy: high T phase kyaniteandalusite Clapeyron equation: Difficult to obtain equilibrium.

4. Solid-solid reactions Reactions: stability fields in P-T space with reactions delineating the fields Pure solid phases: Ca3Al2Si3O12+ SiO2CaAl2Si2O8+2CaSiO3. High T (greater entropy and molar volume) on right V(J/bar) S(J/K) H(kJ) G(kJ) Anorthite 10.08 199.3 -4234 -4008 Grossular 12.53 260 -6640 -6279 Wollastonite 3.99 81.7 -1635 -1549 Quartz 2.27 41.5 -911 -856 Coesite 2.06 38.5 -908 -853 Allows determination of the slope on P-T diagram At equilibrium G=0. G=H-TS. Or, At equilibrium G=0 and at Tref it simplifies to: Peq at 250C=-8858.6 bar

5. Solid-solid reactions cont’d Stability field of a phase, or phase assemblage, shrinks in the presence of an additional reacting phase. Stick with te reaction: Ca3Al2Si3O12+ SiO2CaAl2Si2O8+2CaSiO3. Example: 15kb-8000C. If kyanite present there will be no gross and qtz. Boundary line: univariant equilibrium: four phases, three components (CaO-SiO2-Al2O3). Or anorthite or wollasonite can exist at that P and T, but not together Reaction results in switching tielines

6. Olivine and plag not stable Basalt-granulite-eclogite 6. 2Mg2SiO42MgSiO3+MgO and CaAl2Si2O8+2MgOCaMgSi2O6+MgAl2O4 S(2Fo+An)=387.5J/K; S(2En+Di+Sp)=364.0J/K V(2Fo+An)=188.09cm3; V(2En+Di+Sp)=168.42cm3 Next reaction: 7. 3Mg2SiO4+4CaAl2Si2O82MgSiO3.MgAl(AlSi)O6 +2CaMgSi2O6.CaAl(AlSi)O6+SiO2 At higher P Al in tetrahedral site: Tschermak component 8. NaAlSi3O8NaAlSi2O6+SiO2 9. CaAl2Si2O8+2(Mg,Fe)SiO3 Ca(Mg,Fe)2Al2Si3O12+SiO2 10.3CaAl2Si2O8Ca3Al2Si3O12+2Al2SiO5+SiO2 Reaction 9: Plag-Opx tieline disappears, replaced by Cpx-Gt Gt

7. Basalt-granulite-eclogite cont’d

8. Contineous reactions Solid solutions make many reactions contineous: Plagioclase=jadeitic cpx+quartz albite=Jadeite+ quartz Jadeite-diopside solid solution Na-Al=Ca-Mg and Fe3+=Al. Exact substitution hard to define Lower jadeite fraction moves reaction to lower P G=GJd+GQtz-GAb=0. Which is for the pure phases. However, for solid solutions: Remember: G=RTlnKeq. Activities must be evaluated

9. Contineous reactions cont’d Fe-Mg Solid solutions: In pellitic systems: The Mg/Fe ratio between two minerals is constant over large X-range Distribution coeficient:KD is constant for given P,T. For garnet-biotite: XMg=Mg/(Mg+Fe2+) always molar

10. In Fe-endmembers: 17. Fe-chlorite+quartz+muscovite=almandine+annite+water Five components (Fe, Si, Al, Ca, H-oxide), six phases: univariant Add MgOdivariant Continuous reaction: 18. Chlorite+quartz+muscovite=garnet+biotite+ water Contains two exchange reactios: 19. Fe3Al2Si3O12+KMg3AlSi3O10(OH)2=Mg3Al2Si3O12+KFe3AlSi3O10(OH)2, and 20. Fe3Al2Si3O12+Mg5AlSi3AlO10(OH)8=Mg3Al2Si3O12+Fe5AlSi3AlO10(OH)8. KD for Fe/Mg for garnet biotite is strongly dependent on T, excellent thermometer Fe-Mg exchange cont’d Heating of chlorite, quartz, muscovite rock. At lower T Fe-rich minerals are formed, with increasing T more Mg-rich

11. Fe-Mg exchange cont’d Result: two contrasting paths of crystallization: Equilibrium, every phase is homogeneous. Reaction complete when muscovite is consumed. Fractional crystallization: Slow rates of diffusion in garnetzoned garnets XMg increases outwards: prograde zonation, XMg decreases outwards retrograde zonation.

12. Three types of crustal fluids: Hydrosphere Metamorphic Magmatic Sediment up to 30% pore aaspace/water, most released during burial With increasing to structurally bound water (5-10wt%) can be driven of:dehydration Prograde sequence in pellitic rocks: clay mineralschloritesmicasanhydrous silicates Mineral-fluid reactions

13. Entropy-volume relations Solid-fluid reactions Volatile phase has larger partial molar volume then when bound in a hydrous silicate. Separate volatile phase always at the high T-side: positive T needs to be combined with positive S for G=-ST to be negative. V is positive so positive slope on P-T diagram at low P. Effect of increasing P larger on volatile phase: results in steepening univariant line. At high P fluid can be compressed enough so V<0 Stability field shrinks in the presence of other reactable phases: Pure calcite: CaCO3=CaO+CO2, at >12000C. 23. CaCO3+SiO2=CaSiO3+CO2 at 6000C

14. Pvolatile<Ptotal Mixed fluids Either mixed fluids of not enough volatile phase present. If Pfluid=PH2O+PCO2, then aH2O<1. The composition of the fluid is an explicit variable in mineral equilibria, in addition to P and T. 23. CaCO3+SiO2=CaSiO3+CO2 If PCO2<Pfluid, XCO2<1, then reaction 23 is forced to the right: Rule: where the partial pressure of a reacting volatile phase is <Pfluid the stability field of the phase that contains the volatile shrinks to lower T. Fluid is mobile and can escape. Rule: if there is insufficient volatile to complete a reaction, the unreacted excess solid remains stable with the volatile-bearing product assemblage. 24. VS change in volume of solids Dependence shown in fig b.

15. Other mixed fluid reactions Generalized mixed volatile reaction A=B+iH2O+jCO2 Solid-solid reaction: grossular+quartz=wollastonite+anorthite, no volatiles in products or reactants, equi independent fluid composition Decarbonation reaction equilibrium T highest when XCO2=1. Dehydration reaction, as 2. Reaction releasing CO2 and H2O: remolite+3calcite+2 quartz=5diopside+H2O+3CO2. Univariant reaction at highest T when H2O and CO2 present in abundance of the reaction stoichiometry CO2 as reactant and H2O as product: 2zoisite+CO2=3anorthite+calcite+H2O H2O as reactant, CO2 as product: 6dolomite+8quartz+2H2O=talc+6calcite+6CO2.

16. Buffered devolatilization Closed system: heating : system has to follow the univariant curve untill one of the phases s depleted or Tmax is reached. At Tmax reaction progresses until one of the phase is exhausted Open system: I: fluid of same composition fluxes through the system as rock is heated reaches univariant line: 3 components, 4 (solid phases), isobaric, no freedom. System stays at 525C until a phase is exhausted. II: Fluid with XH2O=1 fluxes through the system. At XCO2=0.3 system is halted till one phase is exhausted. Fluid composition drives the devolatization .

17. Fluid flow in continental crust Evidence: Dischare from hydrothermal fields (Salton Sea) Veins in metamorphic terrains Stable isotope studies Fluid-induced mineral reactions in shear zones: transform plagioclase+Al-rich pyroxenes+garnet to omphacite, garnet, kyanite, clinozoisite, phengite, amphibole and quartz Hydration reactions in contact aureoles

18. Contact aureole

19. Mechanics of fluid flow Porosity: amount of pore space Permeability: amount of interconnected pore space In most metamorphic rocks dihedral angle >600, fluid will “pool” at multiple grain corners Reaction enhanced permeability: products that are devolatilized have smaller volume then their volatile-bearing counterparts. 3% reduction for muscovite+quartz=sillimanite +K-feldspar+H2O 33% reduction for calcite+quartz=wollastonite+CO2. Fractures causes: Intercrystalline mismatchesduring heating Non-hydrstatic pressure Fluid overpressure Healed microcracks • Driving force for fluid flow • Flow of fluid through porous medium quantified by Darcy’s Law: • is viscosity (10-4 Pas for H2O-CO2 fluids), VD is volume flux: volume of flow per unit area per unit time. Geothermal gradients >200C/km density of water decreases with depth

20. Metasomatism Process where distance from source to sink is larger than grain scale. But is a local process, often associated with contact metamorphism Significant change in bulk composition. Three unknowns: Character of protolith Gains and losses of elements Change in volume. Al2O3, TiO2 and HFSE assumed to be immobile Rock layers along strike can divulge protolith Solubility of components vary greatly, dependent on P,T, X Greater Cl concentrations enhances solubility, especially alkalies Silicates least soluble, chlorides, sulfates, carbonates higher solubility Ion-exchange reactions: AX + B+ = BX + A+. solid fluid solid fluid

21. Ion exchange reactions Common: alkali exchange reactions: 28. KAl2AlSi3O10(OH)2+6SiO2+2K+=3KAlSi3O8+2H+ muscovite fluid K-fsp fluid 29. KAl2AlSi3O10(OH)2+2H+=3Al2SiO5+3SiO2+3H2O+2K+ muscovite fluid fluid Pure phases activity=1 for 29: High H+ stabilizes alumino silicates over feldspars High H+ in granitic system results in bleaching: removal Of Na, K, Ca, Mg and Fe. In initial stages: sericitization white micas and clay minerals stable (expense of feldspar) 31. 2KAlSi3O8+2H++H2O=Al2Si2O5(OH)4+4SiO2+2K+ Biotite, amphibole and pyroxene replaced by epidote and aluminous chlorite. Continued alteration:feldspar and micas replaced by pyrophyllite, andalusite. High S activity”: pyrite High F activity: topaz, High B activity: tourmaline

22. Isocon diagrams Provide a frame of reference: What is removed, added, diluted and concentrated? Plot concentration in metasomatized material versus probable protolith. Use TiO2, Al2O3, MnO and Zr as immobile elements Iso con connects these elements of equal geochemical concentration

23. Redox equilibria Oxygen fugacity. In the presence of free water, at 2kbar 3000C 10-11bar, 10000C 10-3barr In crust near NNO (nickel-nickel oxide) or QFM (quartz-fayalite-magnetite) At the surface hematite. Generalized reaction: Fe2+-rich hydrous silicate+O2=Mg-rich hydrous silicate+Fe3+-oxides Carbon can buffer O: carbonate-graphite-methane Sulfur can buffer O: sulfate-sulfide Although metamorphic rocks tend to keep the oxidation state of protolith Also, increasing FeO/Fe2O3 with increasing grade. Possible reaction: 30: C+2Fe2+Fe23+O4+KAlSi3O10(OH)2+3SiO2= magnetite muscovite KAlFe62+Al2Si6O10(OH)2+Fe32+Al2Si3O12+CO2 annite almandine

24. Kinetics General principle: reaction cannot proceed unless there is a finite amount of disequilibrium Reaction affinity: S(T-Teq) where (T-Teq) is called overstepping. Large S requires less overstepping. Dehydration reactions often require just a few degrees of overstepping Solid-solid tens of degrees (Al-polymorphs) Sometimes metastable intermediates speed up the reaction Example: At 1 bar 8200C talc decomposes to anthophyllite which subsequently breaks down to enstatite-quartz-H2O. Three processes need for metamorphic eaction: Breakdown of unstable phase Transport of ions Growth of new phase The slowest rate determines the overall reaction. Reactive surface area high- higher reaction rate.

25. Role of fluids in kinetics Al2SiO5 polymorph transitions under dry conditions extremely slow In most rocks andalusite and sillimanite occur in different domains Common texture:fibrolitic sillimanite intergrown with muscovite. Domain I sillimanite XX Domain II Kyanite breakdown In general terms (Carmichael): given a choice between a number of possible reaction paths, a natural metamorphic reaction will proceed to completion by means of that path for which the activation energy is lowest, provided that overstepping of the equilibrium T is too small to activate other paths.

26. Isograds: mapable zones of stability fields and their boundaries. Simplest isograd: polymorph independent of bulk composition Petrological implications Increasing T garnet more Mg-rich garnet+chlorite+muscovite= staurolite+biotite+quartz+water

27. Calibration with experiments Thermobarometers: Fe-Ti-oxides: magnetite-ilmenite (fO2 and T) magmatic and metamorphic Ternary feldspars and pyroxenes high grade metamophic-magmatic Jadeite endmember in cpx coexisting with plag and qtz For amphibolite facies rocks: coexisting garnet+biotite+plagioclase+Al2SiO5+quartz For pure endmembers: Garnet-biotite Fe-Mg exchange 19. Fe3Al2Si3O12+KMg3AlSi3O10(OH)2=Mg3Al2Si3O12+KFe3AlSi3O10(OH)2 almandine phlogopite pyrope annite Small V makes it insensitive to P and large S/V=dP/dT 37. 3CaAl2Si2O8=Ca3Al2Si3O12+2Al2SiO5+SiO2 GASP anorthite grossular Heterogeneous reaction large V, small S/V=dP/dT insensitive to T. At equilibrium: RTlnKeq=-(H-TS)+V(P-Pref) and In ideal solutions activities can be cast in terms of mole fraction: P-T estimates and

28. P-T estimates cont’d Or, 42. 52,112-19.51T+0.238P=3RTlnKD For 37. 3CaAl2Si2O8=Ca3Al2Si3O12+2Al2SiO5+SiO2 44. -48,357+150.66T+(P-1)(-6.608)+RTlnKeq=0