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Metamorphic Reactions. Today. Updates: Review on Wednesday Final next Monday , 10.00 am Lecture outline: Finish up Reaction types Example for metapelites. Chemical Equilibrium in Metamorphism. http:// www.earth.ox.ac.uk/~sem/SEM+Probe_pics/fras_gt_mini.gif. - PowerPoint PPT Presentation
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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