Igneous - Magmatic Differentiation Flashcards
General processes required in differentiation
Demands two essential processes - Usually creating a variation which is then isolated and preserved
- Creates a compositional difference in one or more phases in response to changes in P, T or composition. Determines the trend of the differentiation process
- Preserves the chemical difference by segregating (or separating) the chemically distinct portions, so that they form a rock or evolve as separate systems. Determines the extent of differentiation along a trend
Most commonly differentiation involves the physical separation of phases in multi-phase systems
The effectiveness depends upon contrasts in physical properties such as density, viscosity, diffusivity, and size/shape
The phases that are fractionated in magmatic systems can be either liquid-solid, liquid-liquid, or liquid-vapor
The energy is usually thermal or gravitational
Melting and incompatible elements
In general, if enriched in incompatible elements = smaller degree of melting
Schematic processes in differentiation
Closed system:
- Melt separation during partial melting
- Fractional crystallisation - segregation of crystals
- Liquid immiscibility - physical separation of melts
- Degassing - melt-fluid separation
Open system:
- Mixing
- Assimilation
Eutectic partial melting
Effects of removing liquid at various stages melting
- Eutectic systems
First melt always = eutectic composition – compositions constant
Major element composition of eutectic melt is constant until one of the source mineral phases is consumed
Once a phase is consumed, the next increment of melt will be different composition and T
Melt separation
• Separation of a partially melted liquid from the solid residue requires a critical melt %.
• Sufficient melt must be produced for it to
Form a continuous, interconnected film between grain boundaries
Have enough interior volume that it is not adsorbed to the crystal surfaces
• The ability to form an interconnected film depends upon the dihedral angle () a property of the melt
• the ratio of solid-solid interfacial energy to solid liquid
• 2cos /2 = γs-s/γs-l
• Mafic systems < 50o very small melt fractions should be extractable
• Rhyolitic melts may have higher angles ( = 45-60o) so harder to extract
• Angle greater than 60 degrees, melt gets trapped at grain corners
• Lower than 60 degrees = interconnected
The critical melt fraction
rheological critical melt percentage, RCMP):
• % melt at which a crystal dominated, high viscosity granular framework gives way to a melt-dominated lower viscosity suspension
• Theoretical system of spheres: RCMP = 26% melt
• Irregular shapes and variable sizes: RCMP may vary between 30-50% for static situations involving granitic compositions.
Other controls on RCPM:
• Gravitational effects (buoyant liquid rise and escapes)
• Filter pressing, or compaction, of crystal mush
• Shear - the RCMP drops considerably
• RCMP varies with
Temperature
Viscosity
Composition
• Melt mantle and produce silicic fraction it is mobile at small melt fraction
• Much melting needed for mobility of granites from melting crust
Crystal gravitational settling
The differential motion of crystals and liquid under the influence of gravity due to differences in densities
Dense crystals settle and rain to the floor of the chamber
Forms cumulate layers
Cumulate texture: Mutually touching phenocrysts with interstitial crystallised residual melt
Settle by stokes law
Olivine settling
Olivine crystals settle
form layers composed almost entirely of olivine
• these layers are ultrabasic in composition
• but don’t represent the composition of the magma
• Then Plagioclase crystals settle out -they may get separated because of their lower density and form plagioclase-rich cumulate layers
BUT
Such efficient crystal segregation is rare
• olivine and plagioclase crystals often stay together
• gabbro usually has cumulus olivine and plagioclase with intercumulus pyroxene
Problems with stokes law
Stokes Law oversimplified
1. Crystals are not spherical
2. Only basaltic magmas very near their liquidus temperatures behave as Newtonian fluids
Once even these begin to crystallize they develop a significant yield strength, that must be overcome before any motion is possible.
Yield strength considerably higher for cooler and more silicic liquids
Gravity settling only viable in a mafic magma within a few degrees of the liquidus?
Two other mechanisms that facilitate the separation of crystals and liquid:
- Compaction
- Flow segregation – chambers with convection currents
The motion of the magma past the stationary walls of the country rock creates shear in the viscous liquid.
Magma must flow around phenocrysts grain dispersive pressure, forcing the grains apart and away from the contact
Liquid Immiscibility:
• Under certain p-t conditions liquid melt will become immiscible
• Very rare
Examples:
• Late silica-rich immiscible droplets in Fe-rich tholeiitic basalts
• Sulfide-silicate immiscibility (massive sulfide deposits and basalts)
• Carbonatite-nephelinite systems
Degassing: melt-vapour separation
- Vapour released by heating of hydrated or carbonated wall rocks
- As a volatile-bearing (but undersaturated) magma rises and pressure is reduced, the magma may eventually become saturated in the vapor, and a free vapor phase will be released – degassing
Volcanic Degassing: Volatile solubilities
• Solubilities dependent on temp, composition and pressure.
• CO2 has a lower solubility than H2O at any pressure.
• As pressure decreases the magma can contain less and less dissolved CO2 and H2O and so on saturation it will exsolve as a gas
• Can’t get as much CO2 (not as soluble) into a melt as H2O
Open vs closed system degassing
Open system degassing – gas is removed and is not in equilibrium with the melt.
Closed system degassing – gas remains in eqilibrium with the melt – CO2 comes out but has nowhere to go
Magma explosivity
- Because CO2 has lower solubility in melt than H2O, CO2 will be first to exsolve.
- Degassing is one of main controls on explosivity of eruptions – controls volatile content
- Volatiles dissolved in magma at high pressures/depths.
- As magma rises – gas is exsolved – to form bubbles.
- Bubbles rise and grow with ↓ pressure.
- When magma can no longer accommodate bubble growth magma fragments.
- Boyle’s gas law – ↑ bubble volume as pressure
Volatile release affects on temp
raises liquidus temperature
porphyritic texture
- May increase P - fracture the roof rocks
- Vapour and melt escape along fractures as dikes
Silicate melt quartz and feldspar
small dikes of aplite
Vapour phase dikes or pods of pegmatite
