metamorphic reactions
DESCRIPTION
Metamorphic Reactions. Reading: Winter Chapter 26. Univariant Reaction Lines. The grid define stability limits End-member minerals Mineral assemblages More thermodynamic data is needed to construct a useful grid. Isograds. - PowerPoint PPT PresentationTRANSCRIPT
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Metamorphic ReactionsMetamorphic Reactions
Reading: Winter Chapter 26
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Univariant Reaction LinesUnivariant Reaction Lines
• The grid define stability limits
– End-member minerals
– Mineral assemblages
• More thermodynamic data is needed to
construct a useful grid
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IsogradsIsograds
• Mark the intersection of the ground with a Univariant reaction curve
• Example: cordierite reaction to Mg-Fe garnet defines the garnet isograd
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Problems with IsogradsProblems with Isograds
• Most rocks have complex compositions, (Mg,Fe) or (Na,Ca) substitutions
• Many reactions are at least divariant
• Even isograds in simple rocks are “smeared”
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Coupled ReactionsCoupled Reactions
• Involve several minerals
• Appearance or disappearance of key minerals
– Depends on rock compositions
• Presence or absence of quartz is a factor
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Stability of MuscoviteStability of MuscoviteWith quartz Mus decomposes at a lower T
Muscovite + Quartz = K-spar + Corundum + water
KAl2AlSi3O10(OH)2 + SiO2 = KAlSi3O8 + Al2O3 + H2O
Without quartz Mus decomposes at a higher T
Muscovite = K-spar + Sillimanite + water
KAl2AlSi3O10(OH)2 = KAlSi3O8 + Al2SiO5 + H2O
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Reactions in RocksReactions in Rocks
• More complicated than in theory
• Generally the conditions of metamorphism can be estimated
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Regional Calc-silicateRegional Calc-silicate
Sequence of appearance
• Talc (low XCO2)
• Tremolite
• Diopside
• Forsterite
• Wollastonite
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Regional Pelitic RocksRegional Pelitic Rocks
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Biotite ZoneBiotite Zone
• Under medium P-T the following reaction occurs at 400-450°
Chl + Kfs = Bt + Ms ( + Qtz + H2O)
• K-spar is generally consumed before all chlorite is consumed, thus it persists
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ChloritoidChloritoid
• Chloritoid introduction may occur at T > 250º
Chl + Prl = Cld ( + Qtz + H2O)
• Chloritoid goes out at T = 590°
Cld = Grt + St (+Qtz +H2O)
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Garnet ZoneGarnet Zone
• In Fe-rich rocks at ~ 525°
Fe-Chl (+ Qtz) = Alm (+ H2O)
• Under medium P-T in normal pelites the reaction occurs at ~ 610°
Chl (+Ms + Qtz) = Grt + Bt (+ H2O)
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Staurolite zoneStaurolite zone
• Under medium P-T may appear at 570°
Cld + Ky = St + Chl (+Qtz +H2O)
• An alternate reaction occurs at 610°
Grt + Chl = St + Bt (+Qtz +H2O)
• Staurolite goes out at ~700°
St (+Ms +Qtz) = Grt +Bt +Als +H2O
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Kyanite ZoneKyanite Zone
• Under medium P-T at ~630°
St + Chl (+Ms +Qtz) = Ky + Bt (+H2O)
• This reaction is considered the transition to the granulite facies in pelitic rocks
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Sillimanite ZoneSillimanite Zone• A polymorphic transformation occurs at ~ 690°
Ky = Sil
• Sillimanite nucleates as tiny needles on micas
• Muscovite goes out at ~ 790°
Ms + Qtz = Kfs + Sil + H2O
• Cordierite appears at higher temperatures
Phl + Sil (+Qtz) = Mg-Cdr (+Kfs + H2O)
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Melting of PelitesMelting of Pelites
• Assume all the water is due to metamorphic dehydration reactions
• Muscovite decomposition causes melting, provided the pressure is high enough to retain the water
Ms + Ab + Qtz = Al2SiO5 + Kfs + liquid
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MigmatitesMigmatites
• Dehydrated rocks become granulites
• Some high-grade rocks appear “mixed”– Dark schistose layers (melanosome) alternate with– Light-colored igneous-looking layers (leucosome)
• The leucosome has a tonalite composition – (Not that of a minimum melt!)
• They represent high-grade metamorphic rocks in which melting is important
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Metasomatism ModelMetasomatism Model• Obvious in rocks with contrasting mineral
layers
• Related to unequal partitioning of elements between solid phases and fluids
• Model uses ion-exchange reactions
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Potassium MigrationPotassium Migration• K+ is concentrated relative to Na+ in chloride fluids
• In fractured rock under a T gradient K+ moves toward higher temperature regions
• K-spar is replaced by Na-spar in low temperature regions
• This could explain large K-spar megacrysts
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Amphibolite Facies MetasomatismAmphibolite Facies Metasomatism
• Alternating sedimentary layers with more and less
calcite
• Leads to slightly different plagioclase composition
in metamorphic bands
• Ion exchange produces plagioclase and K-spar
bands