Metamorphic Reactions

Metamorphic ReactionsMetamorphic Reactions

TodayTodayUpdates:Updates:Review on WednesdayReview on Wednesday

Final next Monday, 10.00 amFinal next Monday, 10.00 am

Lecture outline:Lecture outline:- Finish upFinish up- Reaction typesReaction types- Example for metapelitesExample for metapelites

Chemical Equilibrium in Chemical Equilibrium in MetamorphismMetamorphism

http://www.earth.ox.ac.uk/~sem/SEM+Probe_pics/fras_gt_mini.gifhttp://www.earth.ox.ac.uk/~sem/SEM+Probe_pics/fras_gt_mini.gif

Types of Metamorphic ReactionsTypes of Metamorphic Reactions

Types:Types: Phase transformationsPhase transformations Exsolution Exsolution Solid-solid net transferSolid-solid net transfer DevolitalizationDevolitalization Ion-exchangeIon-exchange

In the field recognized as isograd:In the field recognized as isograd: First occurrence of new mineralFirst occurrence of new mineral P-T-X dependentP-T-X dependent Given X, estimate P-TGiven X, estimate P-T

Phase Transformations: AlPhase Transformations: Al22SiOSiO55

IsochemicalIsochemical Only f(P,T)Only f(P,T) Common in Al rich Common in Al rich

rocks (metapelites)rocks (metapelites) Rough P-T Rough P-T

indicator:indicator: Andalusite Andalusite

suggests low Psuggests low P Kyanite = high PKyanite = high P Sillimanite = high Sillimanite = high

P and TP and T

ExsolutionExsolution

Solid-Solid Net-Transfer ReactionsSolid-Solid Net-Transfer Reactions

Net-Transfer Reactions and VolatilesNet-Transfer Reactions and Volatiles

Devolatilization ReactionsDevolatilization Reactions Other volatiles besides HOther volatiles besides H22O-COO-CO22 systems systems Example:Example:

KAlKAl22SiSi33AlOAlO1010(OH)(OH)22 + SiO + SiO22 = KAlSi = KAlSi33OO88 + Al + Al22SiOSiO55 + H + H22OO MsMs Qtz Qtz KfsKfs Sill Sill W W

Dependence:Dependence: P, TP, T Partial pressure :Partial pressure :

Not all “pores” filled Not all “pores” filled => P=> Pfluid fluid < P< Plithostaticlithostatic

Other volatiles in fluid: Other volatiles in fluid: PPlith lith = P= PH2O H2O + P+ PCO2 CO2 +…+…

Devolatilization ReactionsDevolatilization Reactions

Ion ExchangeIon Exchange

Mg2+ Fe2+Mg2+ Fe2+

Pseudomorphs and (Dis)ContinuousPseudomorphs and (Dis)ContinuousReactionsReactions

Discontinuous reaction: geotherm crosses reaction curveDiscontinuous reaction: geotherm crosses reaction curve at 1 P & T, reaction takes place at those constant P & Tat 1 P & T, reaction takes place at those constant P & TContinuous reaction: reaction occurs over range in P & TContinuous reaction: reaction occurs over range in P & T similar to melting in solid solutionsimilar to melting in solid solution

Pseudomorphs: small change in Pseudomorphs: small change in entropy: need fair bit of energy from entropy: need fair bit of energy from heat (T): reaction over small T rangeheat (T): reaction over small T range

A(K)FM A(K)FM DiagramDiagram

Project from a phase that is Project from a phase that is present in the mineral present in the mineral assemblages to be studied assemblages to be studied to make a triangular plot to make a triangular plot (AFM)(AFM)

Figure 24-18. AKFM Projection from Mu. After Thompson (1957). Am. Min. 22, 842-858.

ProjectionsProjections Why we ignored SiOWhy we ignored SiO22 in the ACF and AKF diagrams: in the ACF and AKF diagrams:

assumed present in all rocksassumed present in all rocks AFM: A = Al2O3 - (3)K2OAFM: A = Al2O3 - (3)K2O Subtraction for pseudo-components:Subtraction for pseudo-components:

A = AlA = Al22OO33 + Fe + Fe22OO33 - Na - Na22O - KO - K22O (AKF)O (AKF) Na, K typically combined with Al in FspNa, K typically combined with Al in Fsp In the ACF diagram only interested in AlIn the ACF diagram only interested in Al22OO33 outside Fsp outside Fsp Since the ratio of AlSince the ratio of Al22OO33 to Na to Na22O or KO or K22O in feldspars is 1:1, O in feldspars is 1:1,

we subtract from Alwe subtract from Al22OO33 an amount equivalent to Na an amount equivalent to Na22O and O and

KK22O in the same 1:1 ratioO in the same 1:1 ratio

Plotting in A(K)FM DiagramPlotting in A(K)FM DiagramBiotite (from Ms):Biotite (from Ms):

KMgKMg22FeSiFeSi33AlOAlO1010(OH)(OH)22

A = Al2O3-3K2OA = Al2O3-3K2O

= 0.5 - 3 (0.5) =  - 1= 0.5 - 3 (0.5) =  - 1

F = FeO = 1 F = FeO = 1

M = MgO = 2M = MgO = 2

To normalize we multiply To normalize we multiply each by 1.0/(2 + 1 - 1) = each by 1.0/(2 + 1 - 1) = 1.0/2 = 0.51.0/2 = 0.5

Thus Thus A = -0.5A = -0.5

F = 0.5F = 0.5

M = 1M = 1

MgO/FeO+MgO=.67MgO/FeO+MgO=.67

Medium P-T path for MetapelitesMedium P-T path for Metapelites

Upper Biotite zoneUpper Biotite zone

Garnet zoneGarnet zone

Kyanite zoneKyanite zone

Sillimanite zoneSillimanite zone

Granulite faciesGranulite facies