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Earth Science 203: Igneous

Charlie Lucas English

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Igneous
- Elemental Diffusion
  - When temperature is low and viscosity high the extreme cooling occurs so glassy textures are created
    - small ions with low charges diffuse more rapidly (polymerized complexes slow)
- Magma formation
  - Basalts
    - The are partial melt products of primitive rock
  - Ways and where magma forms
    - Lithospheric thinning via extension
      - Melting by astenosphere upwelling and decompression
        P reduction with little T change
        Additional heat not required
        20% partial melting
    - Elevated geotherm (mantle plume)
      - Thick lithosphere (no thinning)
      - convection upwelling from deep mantle (High T)
      - Smaller degree of partial melthing- hindered by thick lithospheric barrier
    - Dehydration of altered oceanic crust
      - Subduction of slabs with hydrous alteration minerals
      - minerals dehydrate to provide fluids that lower the solidus of peridotite
      - melting in the mantle wedge above slab
      - Cooling in mantle wedge geotherms due to cold slab
  - Adding water to the mantle
    - Water greatly lowers the solidus, especially at high P
      - Geotherm intersects water-saturated solidus (melt production)
  - Heating of amphibole bearing peridotite
    - - Requirments for melting
        Free water
        T/P conditions sufficeint to melt water-saturated (above the solidus)
      - Oceanic Geotherm:
        a) At solidus but no melting due to lack of free water
        b) amp melts, releases water (melting)
        c) phlogopite in oceanic melts
      - Continental Geotherm
        d) Has not hit solidus so no melting
        e) Has hit solidus so melting if water present
      - The cold crust in subduction requires a large amount of water for melting
Phase Diagrams/ Magma differentiation
- Causes of Magma diversity
  - Fractional Crystallization
  - Assimilation of crustal mineral
  - Partial melting
  - Source variation
- Crystallization behaviours of melts
  - Cooling melts crystallize from liquid to sollid over a range of temperatures
  - Over the temperature range a number of minerals crystallize, this number increases as temperature does
  - Minerals that involve solid solution change composition as cooling progresses
  - Melt composition also changes (fractional crystallization)
- Phase diagrams
  - - Extra Knowledge
      - Liquidus- Line separating field of liquid from liquid + crystal
      - Solidus- The line separating the field of all solid from liquid + crystals
      - Eutectic point- Point where the max number of allowable phases are in equilibrium. Temperature must remain constant till one of the phases disappears. invariant punts
      - Peritectic point: Where a reaction takes place between a previously precipitated phase and the liquid to produce a new solid phase. Temperature is constant until reaction is complete. Invariant
      - Intermediate Compound: Composition intermediate between two other phases
      - Congruent melting: a solid phase melts to a liquid with same composition as the solid
      - Incongruent melting: Solid melts to a liquid with a different composition and produces a solid of different composition
    - Finding the composition of the melt
      - % MELT= (X-y/x-y)*100
      - % crystals= (X-x/x-y)*100 (Level rule)
    - Parts of the diagram
      - m- liquid with composition of X at a certain temperature above the liquidus
      - m-x cooling path: at x An crystals start to form
      - x-E cooling path: melt moves along liquidus curve (depleted in An as An crystals form)
      - E: An and Di form with melt. Heat lost in the system equal to heat released by crystallization. Stagnation until all melt has crystallized
      - Below E all crystal- Ratio is now equal to original
  - - Solid solution: As you shift the point of temperature around the composition of the melt and crystal begin to change
    - Anything in the liquidus is 100% the melt
    - Any point in the solid is 100% the solid crystal
  - - Equilibrium crystallization: slow cooling: At any moment the crystal mass is unifrom. Final product is homogenous crystals with composition of first form
  - - Fractional crystallization: (rapid cooling). Rapid cooling and new crystal growth prevents reaction of melt with crystal core (zoned).
- Major and Trace elements
  - Magmatic differentiation
    - Suite of magmas related to each other in space and time and linked by some process that causes composition cahnge
      - 1. Distinct melting events from distinct source
      - 2. Various degrees of partial meting from same source
      - 3. Crystal fractionation
        In all cases (Minus eutectic composition) crystallization results in a change of composition of the liquid
        If crystals are removed different magma compositions can be generated from initial parent liquid
        If solid solution minerals are removed then it can produce liquid compositions that would no otherwise have been attained by normal crystallization of parent liquid
      - 4. Mixing of 2+ magmas
        When two or more magmas with different chemical compositions come in contact. Can produce intermediate memebers
        Contrasting magma compositions
        Temperature
        Basaltic would chill (crystallize)
        Rhyolitic could re melt/ heat up
        Density- different densities would inhibit mixing
        Viscosity- requires vigorous stirring
        Evidence for mixing
        Mingling textures- marble cake
        Disequilibrium mineral assemblages- requires stirring beyond point of mingling
        Reversed zoning- new T, P, X environment
        Glass inclusions
        Chemical evidence:rocks resulting from mixtures will lie along a straight line
      - 5. Assimilation/ contamination of magmas by crystal rocks
      - 6. Liquid immiscibility
  - Trace Elements
    - Incompatible
      - Favour melt instead of preference to co-exciting crystals
      - Large ionic radius/ charge
      - influenced by crystal lattice structure
    - Compatible
      - Incorporated into one or more crystallizing melts
    - Partition Coefficient
      - affinity that an element has for a particular crystalline mineral
      - Incompatible: k<1
      - Compatible: K>1
      - K controlled by the host mineral
    - LILE: Large Ion Lithophile Elements
      - ionic radius too large to be accommodated in most minerals, dissolve in aqueous solutions
    - HFSE: High field strength elemets
      - High charge./ radius creates intense electrostatic field (unstable in silicate crystals. Resist dissolution, less mobile
- Crystal Settling
  - Low viscosity, crystal poor magmas
  - High density minerals
  - Burial by later settling crystals (effectively removed from liquid
  - Crystal Mush
    - High crystallinity or silicic melts
    - Liquid extracted from solid residuum by compaction and filter pressing
    - Micro settling: micro rearrangements of crystal framework expulses melts
  - Inward crystallization
    - The magma body is hot and the country rock cold
    - Magma crystallizes from wall inwards
    - Thickening crystallization rind removes crystals from melt equilibrium
Crustal Assimilation and Contamination
- Heating of country rock is limiting
- Assimilation of country rock requires enough heat to raise it to solidus and then change it to liquid
- Only heat source is magma
- Heating of country means cooling of magma
- Hot country rock requires less energy
Xenoliths and Xenocrysts
- Evidence of contamination by country rock
- Complete assimilation would leave no textural features
- Isotope evidence for crustal contamination