 | Magmatic Processes |
See http://plate-tectonic.narod.ru/petrographyigneouslinks.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html
http://ijolite.geology.uiuc.edu/08SprgClass/geo436/436%20lectures/L14-Processes.html
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