Metamorphic Reactions

Today Updates: Lecture outline: Review on Wednesday Final next Monday, 10.00 am Lecture outline: Finish up Reaction types Example for metapelites

Chemical Equilibrium in Metamorphism “Mineral assemblage” is used by some as a synonym for paragenesis, conventionally assuming equilibrium for the term Impossible to prove that a mineral assemblage now at the Earth’s surface represents thermodynamic (chemical) equilibrium at prior elevated metamorphic conditions Indirect textural and chemical support for such a conclusion is discussed in the text In short, it is typically easy to recognize non-equilibrium minerals (retrograde rims, reaction textures, etc.) We shall assume equilibrium mineral assemblages in the following discussion (will ignore retrograde…) http://www.earth.ox.ac.uk/~sem/SEM+Probe_pics/fras_gt_mini.gif

Types of Metamorphic Reactions Phase transformations Exsolution Solid-solid net transfer Devolitalization Ion-exchange In the field recognized as isograd: First occurrence of new mineral P-T-X dependent Given X, estimate P-T Reactions are always responsible for introducing or consuming mineral phases during metamorphism The classic notion of an isograd as the first appearance of a new mineral phase as one progresses up metamorphic grade is quite useful in the field, because a worker need only be able to recognize new minerals in a hand specimen If we understand the reactions that produce minerals, the physical conditions under which reactions occur, and what controls them, we can use this knowledge to better understand metamorphic processes If we have good experimental and theoretical data on minerals and reactions, we can locate a reaction in P-T-X space and constrain the conditions under which a particular metamorphic rock formed We will review the various types of metamorphic reactions, and discuss what controls them

Phase Transformations: Al2SiO5 Isochemical Only f(P,T) Common in Al rich rocks (metapelites) Rough P-T indicator: Andalusite suggests low P Kyanite = high P Sillimanite = high P and T Figure 26-15. The P-T phase diagram for the system Al2SiO5 showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite. Calculated using the program TWQ (Berman, 1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. We used the presence of andalusite to indicate low-pressure metamorphic conditions, and by referring to Fig. 21-9 we can effectively limit the pressure of andalusite-bearing rocks to values below about 0.38 GPa The presence of two coexisting polymorphs in a single rock has often been taken to indicate that the metamorphic peak corresponded to equilibrium conditions along the univariant boundary curve separating the pair If an independent estimate of either pressure or temperature is available, the other parameter may then be estimated from the location of the equilibrium curve Thus if kyanite and sillimanite were to be observed together, for example, and the pressure were estimated via geobarometry to be 0.5 GPa, then the temperature of equilibration could be determined from Fig. 21-9 to be approximately 560oC If all three Al2SiO5 polymorphs were to be found in stable coexistence, the assemblage would indicate conditions at the invariant point (ca. 500oC and 3.8 GPa)

Exsolution Figure 6-16. T-X phase diagram of the system albite-orthoclase at 0.2 GPa H2O pressure. After Bowen and Tuttle (1950). J. Geology, 58, 489-511. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Solid-Solid Net-Transfer Reactions Figure 27-1. Temperature-pressure phase diagram for the reaction: Albite = Jadeite + Quartz calculated using the program TWQ of Berman (1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Net-Transfer Reactions and Volatiles

Devolatilization Reactions Other volatiles besides H2O-CO2 systems Example: KAl2Si3AlO10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O Ms Qtz Kfs Sill W Dependence: P, T Partial pressure : Not all “pores” filled => Pfluid < Plithostatic Other volatiles in fluid: Plith = PH2O + PCO2 +…

Devolatilization Reactions Fig. 26-2 with the equilibrium curve contoured for various values of pH2O Figure 26-2. P-T phase diagram for the reaction Ms + Qtz = Kfs + Al2SiO5 + H2O showing the shift in equilibrium conditions as pH2O varies (assuming ideal H2O-CO2 mixing). Calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Ion Exchange Mg2+ Fe2+

Pseudomorphs and (Dis)Continuous Reactions Discontinuous reaction: geotherm crosses reaction curve at 1 P & T, reaction takes place at those constant P & T Continuous reaction: reaction occurs over range in P & T similar to melting in solid solution Pseudomorphs: small change in entropy: need fair bit of energy from heat (T): reaction over small T range Often by carefully observing the textures one can distinguish partial replacement and metastable coexistence from true stable equilibrium grain boundaries Hietanen (1956) reported all three Al2SiO5 polymorphs in northern Idaho, and proposed that metamorphic events in the area occurred near the invariant point More likely that kyanite is partially replaced by sillimanite during a prograde event near the kyanite-sillimanite boundary, and that andalusite replaces kyanite during a later event at lower pressure

A(K)FM Diagram Project from a phase that is present in the mineral assemblages to be studied to make a triangular plot (AFM) Since muscovite is the most widespread K-rich phase in metapelites, he decided to project from muscovite (Mu) to the AFM base as shown Projecting from muscovite can lead to a strange looking AFM projections Note that Mu is still rather K-poor, and only mineral phases in the volume A-F-M-Mu in Fig. 24-18 will be projected to points within the AFM face of the AKFM tetrahedron Biotite is outside this volume, and projecting it from Mu causes it to plot as a band (of variable Fe/Mg) outside the AFM triangle Figure 24-18. AKFM Projection from Mu. After Thompson (1957). Am. Min. 22, 842-858.

Projections Why we ignored SiO2 in the ACF and AKF diagrams: assumed present in all rocks AFM: A = Al2O3 - (3)K2O Subtraction for pseudo-components: A = Al2O3 + Fe2O3 - Na2O - K2O (AKF) Na, K typically combined with Al in Fsp In the ACF diagram only interested in Al2O3 outside Fsp Since the ratio of Al2O3 to Na2O or K2O in feldspars is 1:1, we subtract from Al2O3 an amount equivalent to Na2O and K2O in the same 1:1 ratio

Plotting in A(K)FM Diagram Biotite (from Ms): KMg2FeSi3AlO10(OH)2 A = Al2O3-3K2O = 0.5 - 3 (0.5) =  - 1 F = FeO = 1 M = MgO = 2 To normalize we multiply each by 1.0/(2 + 1 - 1) = 1.0/2 = 0.5 Thus A = -0.5 F = 0.5 M = 1 MgO/FeO+MgO=.67 To plot the point, we extend a line from A at a constant M/F ratio. Since M/(F+M) = 0.66 We next extend a vertical line from A a distance equal to half the distance from A to the F-M base, but we extend this distance beyond the base, because A is negative Thus it extends to A/(A+F+M) = - 0.5 and not 0.5 Where a horizontal line at this value of A intersects the first line of constant F:M is the location of our biotite on the AFM diagram The broad biotite field in Fig. 24-19 is due to Al-Al substitution for (Fe-Mg)-Si in biotite K-feldspar, when projected from muscovite, projects away from the AFM diagram In order to include Kfs in the AFM diagram it is assigned a position at negative infinity

Medium P-T path for Metapelites Figure 28-5. AFM projection for the biotite zone, greenschist facies, above the chloritoid isograd. The compositional ranges of common pelites and granitoids are shaded. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Upper Biotite zone Figure 28-6. AFM projection for the upper biotite zone, greenschist facies. Although garnet is stable, it is limited to unusually Fe-rich compositions, and does not occur in natural pelites (shaded). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Garnet zone Figure 28-7. AFM projection for the garnet zone, transitional to the amphibolite facies, showing the tie-line flip associated with reaction (28-8) (compare to Figure 28-6) which introduces garnet into the more Fe-rich types of common (shaded) pelites. After Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Kyanite zone Figure 28-14. AFM projection for the kyanite zone, amphibolite facies, showing the tie-line flip associated with reaction (28-15) which introduces kyanite into many low-Al common pelites (shaded). After Carmichael (1970) J. Petrol., 11, 147-181. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Sillimanite zone Figure 28-15. AFM projection above the sillimanite and “staurolite-out” isograds, sillimanite zone, upper amphibolite facies. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Granulite facies Figure 28-16. AFM diagram (projected from K-feldspar) above the cordierite-in isograds, granulite facies. Cordierite forms first by reaction (29-14), and then the dashed Sil-Bt tie-line is lost and the Grt-Crd tie-line forms as a result of reaction (28-17). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.