Reaction to form opx and cpx in migmatite SW Sweden. Image: Darrell Henry

Transcription

1 Metamorphic Reactions (Chapter 26) Reaction to form opx and cpx in migmatite SW Sweden. Image: Darrell Henry

2 Metamorphic Reactions Isogradic Reactions If we treat isograds as reactions we can then: Understand what physical variables may affect location of a particular isograd Estimate P-T-X conditions that an isograd represents

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5 Isograd Temperature Constraints Isogradic reactions at 4 kbar calibrated against Spear et al. (1999) petrogenetic grid Garnet/Staurolite Zone (545 C) Grt + Chl = St + Bt + H 2 O Staurolite/Lower Sillimanite Zone (580 C) St + Chl = Bt + Sil + H 2 O Lower /Upper Sillimanite Zone (620 C) St = Grt + Bt + Sil + H 2 O Upper Sillimanite/Sillimanite Kfs Zone (660 C) Ms = Kfs + Sil + H 2 O

6 Surface-fit fit of near-isobaric biotite data Biotite Ti saturation surface - 4 kbar Ti (apfu) Mg/(Mg+Fe) Temperature re (C) N i b i ( 4 6kb )Ti t ti f Near-isobaric (~4-6 kbar) Ti saturation surface for biotite in aluminous, graphitic metapelites Ti (apfu)

7 1. Phase Transformations Isochemical phase transformations - the simplest type e.g. Polymorphs of SiO 2 or Al 2 SiO 5 or graphite-diamond or calcite-aragonite aragonite The transformations depend on temperature and pressure only

8 1. Phase Transformations Small ΔS for most polymorphic transformations: Results in small ΔG between two polymorphs, even several tens of degrees from equilibrium boundary little driving force for reaction to proceed, and crystals of one polymorph may remain as metastable relics in stability field of another Coexisting polymorphs may represent non-equilibrium states, such as overstepped equilibrium curves or polymetamorphic overprints

9 2. Exsolution T-X phase diagram of the system albite-orthoclase at 02GPaHO pressure.

10 3. Solid-Solid Net-Transfer Reactions Involve solids only Differ from polymorphic transformations in that they involve solids of differing composition, o and thus matter must diffuse from one site to another in order for reaction to proceed Examples: NaAlSi 2 O 6 + SiO 2 = NaAlSi 3 O 8 Jd Qtz Ab MgSiO 3 +C CaAl AlSi 2 2 O 8 = CMSiO CaMgSi 2 6 +AlSiO 2 5 En An Di And 4(M (Mg,Fe Mg,Fe)SiO 3 +C CaAl AlSi 2 2 O 8 = Opx Plag (Mg,Fe Mg,Fe) 3 Al 2 Si 3 O 12 + Ca(Mg,Fe Mg,Fe)Si 2O 6 + SiO 2 Gnt Cpx Qtz

11 3. Solid-Solid Net-Transfer Reactions When solid-solution is limited, solid-solid nettransfer reactions are discontinuous reactions Discontinuous reactions tend to run to completion at single T (at particular P) There is thus an abrupt (discontinuous) change from reactant assemblage to product assemblage at reaction isograd

12 4. Devolatilization Reactions most common metamorphic reactions Concentrate on H 2 O-CO 2 systems, but principles involved may be applied to any reaction involving volatiles Reactions are dependent not only upon T and P, but also the partial pressure of volatile species Example: location on a P-T phase diagram of dehydration reaction: KAl 2 Si 3 AlO 10 (OH) 2 + SiO 2 = KAlSi 3 O 8 + Al 2 SiO 5 + H 2 O Ms + Qtz = Kfs + Sill + W depends upon partial pressure of H 2 O (p H2O ). This dependence d is easily demonstrated dby applying Le Châtelier s principle to reaction at equilibrium.

13 4. Devolatilization Reactions Suppose H 2 O is withdrawn from system on H 2 O- saturated equilibrium curve: p H2O <P lithostatic According to Le Châtelier s Principle, removing H 2 Oat equilibrium will be compensated by reaction running to right, thereby producing more water Effectively stabilizes right side of reaction at expense of left side As H 2 O is reduced Kfs + Sill + H 2 O field expands at expense of Mu + Qtz field, and reaction curve shifts toward lower T

14 4. Devolatilization Reactions p H2O can become less than P Lith by either of two ways P fluid < P Lith by drying out the rock and reducing the fluid content t P fluid = P Lith, but H 2 O in fluid can become diluted by adding another fluid T-X H2O phase diagram for the reaction Ms + Qtz = Kfs + Sil + H 2 O at 0.5 GPa assuming ideal H 2 O-CO 2 mixing, component, such as CO 2 or some other volatile phase or dissolved salts (i.e. reduces the activity it of H 2 O)

15 4. Devolatilization Reactions They have a maximum T at pure H 2 O end T of reaction can thus be practically any T below maximum. Thus fluid composition should be constrained!

16 4. Devolatilization Reactions Decarbonation reactions may be treated in an identical fashion

17 4. Devolatilization Reactions 5 types of devolatilization reactions, each with a unique general shape on a T-X diagram Type 3: T max at X CO2 determined by the stoichiometric ratio of CO 2 /H 2 O produced For example: Ca 2 Mg 5 Si 8 O 22 (OH) 2 +3C 3CaCOCO 3 +2SiO 2 = 5CaMgSi 2 O 6 + 3CO 2 + H 2 O

18 5. Continuous Reactions Two reasons for variation of garnet appearance: 1. Contrasting composition that garnet reaction is different Example: in some rocks garnet may be created by general reaction: Chl + Ms + Qtz = Grt + Bt + H 2 O Geologic map of hypothetical field area in which metamorphosed pelitic sediments strike directly up metamorphic grade. Note distinctly tl different location of appearance of garnet in each of lihologies. Whereas in more Fe-rich and K- poor pelites, garnet might be generated by reaction involving chloritoid: Chl + Cld + Qtz = Grt + H 2 O

19 5. Continuous Reactions Two reasons for variation of garnet appearance: 2. Reaction on which isograd is based is same in each unit, but it is a continuous reaction, and its location is sensitive to composition of solutions (either solid of fluid) involved Geologic map of hypothetical field area in which metamorphosed pelitic sediments strike directly up metamorphic grade. Note distinctly tl different location of appearance of garnet in each of lihologies. The offsets this creates in an isograd are usually more subtle than for reason #1, but in some cases they can be substantial

20 5. Continuous Reactions Continuous reactions occur when reactants and products coexist over a T interval Chl + Ms + Qtz = Grt + Bt + H 2 O Chlorite, muscovite, quartz, biotite, i and garnet can occur together in same rock over interval of metamorphic grade above garnet-in isograd Composition of solid solution phases vary across interval, and proportions of minerals change until one of the reactants disappears with increasing grade i.e. is bulk-composition dependent

21 6. Exchange Reactions Reciprocal exchange of components between 2 or more minerals MgSiO 3 + CaFeSi 2 O 6 = FeSiO 3 + CaMgSi 2 O 6 Enstatite + Hedenbergitte = Ferrosilite + Diopside Expressed as pure end-members, but really involves Mg-Fe (or other) exchange between intermediate solutions Basis for many geothermobarometers

22 7. Redox Reactions Involves a change in oxidation state of an element e.g.: 6 Fe 2 O 3 = 4 Fe 3 O 4 + O 2 2F Fe 3 O 4 +3SiO 2 =3F Fe 2 SiO 4 +O 2 At any particular pressure these become oxygen buffers

23 8. Reactions involving dissolved species Minerals plus ions neutral molecules dissolved in a fluid One example is hydrolysis: 2 KAlSi 3 O H + + H 2 O = Al 2 Si 2 O 5 (OH) 4 + SiO K + Kfs + aq. species = kaolinite + aq. species

24 Reactions and Chemographics A + B = C + D What reaction is possible between A-B-C-D? This is called a tie-line flip, and results in new groupings in the next metamorphic zone

25 Petrogenetic grids P-T diagrams for multicomponent systems that show set of reactions, generally for specific rock bulk composition