Origin and Differentiation of Magma
see Hefferan & O'Brien (2010) sections 8.1-8.4
Origin of Magma
partial melting: If rock is completely melted, the melt composition would be the same
as the elemental composition of the rock. However, the Earth's interior is not hot
enough to completely melt rock in the mantle and crust today. In certain tectonic
environments in the interior, rock can undergo anatexis, partial melting in which the
lower melting temperature components in the rock melt, leaving behind the more
refractory, higher melting temperature components.
Partial melting is subliquidus melting - melting at temperatures above the solidus
but below the liquidus.
In equilibrium melting, the melt remains in contact with the un-melted material (the
restite) and ions can diffuse in and out of the crystals to maintain chemical
equilibrium between the melt and the solid as melting proceeds and the degree of
melting proceeds.
in fractional or disequilibrium melting, early-formed melts move away from the parent
material/restite so that later melts are compositionally distinct.
The Earth's interior is very hot and temperature increases with depth, but so too does
pressure. And as pressure increases, so does the temperature required for melting.
So despite the fact the temperatures deep in the mantle exceed the melting
temperature of rock at the Earth’s surface, the high temperatures are still below the
melting temperature for mantle rocks except in certain tectonic environments.
We know that the crust and mantle are solid from the fact that shear waves from
earthquakes travel through the entire crust and mantle (shear wave, S waves, don’t
travel through liquids).
What could allow rock in the Earth's interior to melt?
raise its temperature:
Small amounts of magma can be produced around high temperature mafic intrusions.
Rock can also be heated along major shear zones, e.g., in continental collisions
to produce small volumes of low-temperature, felsic magma.
decrease its pressure – decompression melting:
Where mantle rock is rising beneath midocean ridges, continental rifts, and hotspots
the reduction in pressure brings rock near its melting temperature, resulting in partial
melting of the ultramafic mantle rock to produce mafic magma.
add volatiles – flux melting:
In subduction zones, at depths between ~80-125 km, sinking ocean crust undergoes
metamorphic dewatering. Hydrous minerals like amphiboles and serpentine
(phyllosilicate) metamorphose into denser, non-hydrous minerals thereby releasing
water. This water permeates the mantle rock above the subducting slab. The water
depresses the solidus temperature allowing partial melting of ultramafic mantle
peridotite to produce mafic magma.
partial melting (anatexis), melting of the lowest melting temperature components,
produces magma that is less mafic than the parent rock
partial melting of ultramafic mantle peridotite produces mafic magma
removes the lowest melting temperature components
e.g., melts Fe-rich olivine and Na-rich plagioclase
leaves behind the highest melting temp components
e.g., Mg-rich olivine and Ca-rich plagioclase
the melt is enriched in SiO2, Na, & K
the residual solid (restite) is enriched in Mg & Ca
Minor & Trace Elements
incompatible elements
elements that don’t fit well into common crystal lattice sites
(too big or wrong charge)
prefer to go into a melt during partial melting
- concentrated in the resulting magma
- depleted in the residual rock left behind
incompatible elements include:
Large Ion Lithopile (LIL) elements (large for their charge)
Cs, Ba, Rb, Sr, U, Pb K, Zr, Th, Ta
Light Rare Earth elements (LREE)
REEs La through Gd
compatible elements
other elements are very stably bonded in silicate structures
they are highly immobile and remain in the restite, depleted in the magma
compatible elements include:
High Field Strength (HFS) elements (small for their charge)
Ti, Ni, Cr, V, Zr, Hf, Nb, Ta, Y
Heavy Rare Earth Elements (HREE)
small percent melts result in magma greatly enriched in incompatible elements
large percent melts result in less distinction between partial melt and parent
How do we know if an element is enriched or depleted in a magma or restite?
Compare the concentration of the element in the rock in question to that found in
chondritic meteorites.
Chondritic meteorites are believed to have the non-volatile composition of the solar
nebula, and the bulk silicate composition of Earth before differentiation of the crust
by partial melting of mantle rocks
Petrologists and geochemists studying crustal rocks crystalized from mantle-derived
magmas describe the mantle parent rocks for those magmas as either having been
depleted mantle rock or enriched or primitive mantle rock.
Modification of Primary (mafic) Magma
Bowen (1928) noticed that in the Palisade Sill intrusion heavy olivine crystals
seemed to have sunk to the bottom
while lighter plagioclase were more concentrated near top
fractional crystallization (Bowen believed)
separation of early formed crystals causes magma composition to change
become more felsic and change elemental balances
(it does, but he also believed that fractional crystallization could produce all of the
range of mafic through felsic magmas from a parent mafic magma produced in the
mantle which is now not believed to be true)
Bowen’s reaction series - order of crystallization
the most ionic minerals crystallize at highest temperature
olivine, then pyroxenes, then amphiboles, then biotite
same time as Ca-rich plag, then intermediate plag, then sodium rich plag
kspar, +/- muscovite, quartz crystallize at lowest temps
Continuous Reaction Series
Ca plag. to Na plag. - solid solution series - all the same structure
composition progressively changes as magma comp. becomes more sodic
Discontinuous Reaction Series
Olivine-Pyroxene-Amphibole-Biotite - each is a different xtal structure
under equilibrium xtalization olivine changes to pyroxene
pyroxene to amphibole, amphibole to biotite as magma becomes more silicic
a reaction rim occurs if non-equilibrium
pyroxene surrounding olivine
or zoned feldspar - Na-rich surrounding Ca-rich plagioclase
mechanisms of fractional crystallization:
gravitational separation:
early-formed heavy crystals sink to bottom of magma chamber
marginal accretion:
early-forming minerals form on cool wall of magma chamber
ilter pressing
crystals left behind as migrating magma squeezes through fractures
assimilation of country rocks:
magma chemistry changes as a result of reaction with
country rocks or melting of lower melting point country rock into the magma
magma mingling: sometimes different batches of independently differentiated
magmas will mingle as they rise into the crust producing a hybrid or they
may form a zonation of different chemical variations if not well mixed
xenoliths: foreign rock within an igneous rock body are evidence for assimilation and
magma mingling