Magmatic Processes 


A. Partial melting

-To generate a magma, melt must form, then separate from solids. Requires a certain amount of melt.

-Mechanics : first liquids form at points where 2 or more grains meet - discrete, adsorbed (surface tension); liquid droplets connect to form a network of melt, reducing surface tension; separation also depends on viscosity

B. All processes that change liquid composition = differentiation process

II. Differentiation

A. Fractional crystallization

-F.C. of olivine also occurs by: filter pressing; flow segregation (phenocrysts concentrate away from walls); buoyant rise of liquid away from crystals

B. Volatile transport

-Presence of a vapor phase can also partition elements

-Commonly happens in a late stage of crystallization: example: pegmatite

-Incompatible elements concentrated in vapor > ore deposits, gemstones

C. Immiscible liquids

-Two liquids separate - like solvus behavior of alkali feldspars
- low-T immiscibility field in system Fa-Si-Lc > silicic melt separating from a tholeiitic basaltic one; sulfide-rich melt from silicate magma > ore deposits; carbonate melt from alkalic magma > carbonatite + nephelinite group of lavas

III. Other processes

A. Magma mixing - involves two magmas of different compositions

-Depending on density and viscosity, magmas may mix efficiently to form a homogenous intermediate magma, or may remain separate

B. Assimilation - incorporation of part of wall rock into magma

-Heat needed to melt assimilant comes from heat of fusion (given off as crystals form)

C. Soret effect (thermal diffusion)

-In material subjected to a temperature gradient, heavier elements concentrate at cooler end, lighter at hotter end

-Problem: heat dissipates faster than chemical elements diffuse, so may not happen much

D. Convective effects

-Circulation due to density differences: hotter expands > less dense > rises ; gravity settling of heavy crystals > less dense > remaining liquid rises

E. Combinations - probably more common than these end-member processes discussed

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