magmas best, ch. 8. constitution of magmas hot molten rock t = 700 - 1200 degrees c composed of ions...
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MagmasMagmas
Best, Ch. 8
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Constitution of MagmasConstitution of Magmas
• Hot molten rock
• T = 700 - 1200 degrees C
• Composed of ions or complexes
• Phase– Homogeneous– Separable part of the system– With an interface
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CompositionComposition
• Most components – Low vapor pressure
– Designated by mole fraction (Xi)
• Volatile components– Mainly exist as a gas
– Designated by vapor pressure (pi)
• Fluid pressure = sum of partial pressures
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Gas lawGas law
PV = nRT
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Atomic Structure of MagmaAtomic Structure of Magma
• Quenched to form a glass
• Si & Al are polymerized with O
• Forming networks of Si-O chains
• Short-range structural order
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Structural ModelStructural Model
• Network formers– Si, Al
• Network modifiers– Ca, Mg, etc
• Dissolved water has a strong effect
H2O + O-2 = 2(OH)-
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Magma GenerationMagma Generation
• Magmas form at perturbations in P,T,X
• Convergent plates
• Divergent plates
• Peridotite mantle source
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Source RegionsSource Regions
• Must originate in the mantle or crust
• At Hawaii 60 km deep
• Only 1 to 3% melt in peridotite
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MeltingMelting
• Heat of fusion– About 300 times the rock’s specific heat– Melting of rock consumes much heat
• Mechanisms for melting– Temperature increase by mass transfer– Decompression– Changes in composition reducing melting
point
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Temperature IncreaseTemperature Increase
• Mechanical deformation
– Friction generates heat
• Mass transfer of rock
– Descending oceanic lithosphere
– Basaltic underplating of continental crust
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DecompressionDecompression
• Upwelling mantle
– Beneath oceanic or continental rift
• Adiabatic system
– Pressure causes all temperature
change
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Changes in CompositionChanges in Composition
• Increase in water pressure
• Lowers the solidus
• Subduction zones
– Peridotite wedge
– Over subducting oceanic
crust
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Magma From Solid RockMagma From Solid Rock
• Basalt & peridotite systems
• Granite systems
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Basalt & PeridotiteBasalt & Peridotite
• Equilibrium fusion– Solid and liquid remain in equilibrium– Continuous but limited composition range
• Fractional fusion– Liquid is immediately removed from host
rock– Melts are both oversaturated &
undersaturated with respect to Si
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Influence of PressureInfluence of Pressure
• Pressure strongly influences the cotectic
• Partial melts of mantle peridotite are basalts
• At higher pressures partial melts are more
silica deficient
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Role of CORole of CO22
• Polymerizes melt
• Contracts olivine field
• Favors silica-poor alkali melts
• Repeated melting episodes favors
incompatible element enrichment
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Role of HRole of H22OO
• Depolymerizes melt & stabilizes olivine
• Partial melts more silica rich
• Favors tholeiitic basalts
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Mantle-derived Primary MeltsMantle-derived Primary Melts
• Wide range of melt compositions possible• Fractional crystallization vs. Partial melting• Primary melt
– Segregated from peridotite source rock– First crystallized minerals similar to mantle
source zone• Derivative melt
– Modified after leaving the source region
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Granitic SystemsGranitic Systems
• Impossible to generate granites by partial
melting of mantle peridotite or subducted
oceanic floor basalt
• Their origin is related to older sialic crust
• Granites concentrated along old subduction
zones
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Water SaturationWater Saturation
• Saturated granite melts have 10 to 15% H2O
• Natural granite melts have about 4% H2O
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Water UndersaturationWater Undersaturation
• Common granite mineral assemblage
– Biotite, K-spar, Fe-Ti oxide
½ O2 + biotite = K-spar + Fe3O4 + H2O
• Excess water drives this reaction to the left
• Hence, most granites are not water saturated
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Origin of GranitesOrigin of Granites
• Partial Melting of lower crust
• Source in mica-amphibolites
• Contain 1-2% H2O
• Lowest T melts are K-rich granite
• Higher T, deeper melts are Ca-rich granodiorite
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Subduction Zone MagmaSubduction Zone Magma
• Subducted slab
– Mafic primary melts
• Peridotite mantle wedge
– Mafic primary melts
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Dehydration Beneath OrogenDehydration Beneath Orogen
• Large amount of water in oceanic slab– Water in pore space– Water in alteration minerals
• Heating dehydrates the slab
• Liberated water promotes partial melting of peridotite
• Composition is Si-saturated tholeiite
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Magma DiversificationMagma Diversification
• Magmatic differentiation
• Gravitational settling
• Liquid immiscibility
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Crystal-liquid FractionationCrystal-liquid Fractionation
• Regular pattern of compositional variation
• Variation of MgO is a good measure of
olivine fractionation
• Computer mixing programs can be used
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Magma MixingMagma Mixing
• Two different magmas may blend to
produce a hybrid
• Common with calc-alkali magma
• Blended magmas should have linear
composition with the parents