Ch 11 diversification

Diversification of MagmasDiversification of Magmas
Thanks to John WinterThanks to John Winter

Magmatic DifferentiationMagmatic Differentiation
Any process by which a magma is able to diversify and
produce a magma or rock of different composition.
1. Creates a compositional difference in one or more phases as elements
partition themselves in response to a change in an intensive variable,
such as pressure, temperature, or composition. This will determine the
trend of the differentiation process
2. Preserves the chemical difference by segregating (or fractionating)
the chemically distinct portions so that they may either form a rock, or
continue to evolve as separate systems. The effectiveness of the
fractionation process determines extent of differentiation proceeds
along a particular trend.

Most common: differentiation involving theMost common: differentiation involving the physicalphysical
separationseparation ofof phasesphases in multi-phase systemsin multi-phase systems
TheThe effectivenesseffectiveness depends upondepends upon contrastscontrasts in physicalin physical
propertiesproperties such as density, viscosity, diffusivity, andsuch as density, viscosity, diffusivity, and
size/shapesize/shape
TheThe energyenergy is usuallyis usually thermalthermal oror gravitationalgravitational
TheThe phasesphases that are fractionated in magmatic systems can bethat are fractionated in magmatic systems can be
either liquid-solid, liquid-liquid, or liquid-vaporeither liquid-solid, liquid-liquid, or liquid-vapor

Differentiation processes can be outlined as follows:
I. Closed-system processes
a) Crystal-melt fractionation
1) Gravitational segregation
2) Flowage segregation
3) Filter pressing
4) Convective melt fractionation
b) Physical separation of immiscible melts
c) Melt-fluid separation
II. Open-system processes
a) Assimilation of an initially solid contaminant
b) Mixing of two or more contrasting magmas

Crystal melt fractionationCrystal melt fractionation
• The compositions of igneous minerals are much simpler
than those of natural liquids in which they form; they
consist of a relatively small number of essential
constituents, most of which have proportionately greater
concentrations in crystal than in liquid.
• Because of these differences, elements that have high
concentration in crystal are depleted from the liquid in
which it grows, while those that are excluded become
concentrated in the diminishing volume of residual liquid.

• If crystals and liquid are separated before the magma
solidifies completely, the remaining liquid can continue to
crystallize to form a rock with a chemical composition quite
different from that of the original liquid.
• This, in essence, is the basic principle of crystal
fractionation: separation of crystals from a magma
depletes those components having greater concentrations
in the crystals than the liquid and enriches the remaining
elements that are left in the reduced volume of liquid.

• Consider Hawaiian lava lake crystallization.
• As the crystals grow in its cooling margins, they extract certain
components of the magma while leaving others to accumulate in the
remaining liquid, and in this way the liquid immediately adjacent to
the crystals takes on a composition different from that of the hotter,
still-liquid interior.
• These differences result in a compositional gradients in the liquid,
and elements removed by the crystals diffuse to feed continued
growth, while rejected ones diffuse in opposite direction.

• The slower diffusion of these components and more rapid
growth of the crystals, greater will be the compositional
difference between the liquid next to the crystal faces.
• Rapid crystallization usually inhibits efficient fractional
crystallization, so that even though the liquid changes
composition, it is not effectively segregated and
differentiation is restricted to the interstitial liquid.
• Were the interstitial liquid removed to crystallize a separate
rock, the process of differentiation would be complete.

• Crystallization can produce large scale compositional differences only under
conditions of efficient crystal-liquid fractionation; diffusion alone is too slow
to do this unless the phases are mechanically segregated.
• If the crystals sink, or float, or are otherwise separated from the liquid as they
grow, they come in contact with fresh, undepleted from which they can extract
the elements essential to their growth.
• In this way they may remove these components from a much larger mass
of magma than would be possible by diffusion alone.
• The same effect can result from a flow of liquid over static crystals growing
on the walls of magma chamber or volcanic conduit.
• Regardless of which is moving, crystals or liquid, the efficiency of
fractionation is greatly enhanced by any relative motion of the 2 phases.

Gravity settlingGravity settling
 In a static body of melt, denser crystals might sink whereas less
dense ones might float.
 However, except for the hottest mafic melts and largest crystals,
the plastic yield strength of melts may preclude much movement
of isolated crystals.
 The common texture known as cumulate texture, in which
mutually touching phenocrysts are embedded in an interstitial
matrix is a result.

Filter Pressing (compaction)Filter Pressing (compaction)
Even if the liquid between growing crystals evolves
without equilibrating effectively with the main reservoir
of magma, it may be separated before crystallization is
complete simply by squeezing it out, much as water is
squeezed out of sponge.
Processes of this kind have been observed in Hawaiian
lava lakes, where plumes of volatile rich residual liquid
becomes buoyant and rises to form diapiric pipes.

Flow segregationFlow segregation
Segregation can also occur when magmas containing
suspended crystals flow along the walls of a dike or a
convecting pluton.
The velocity gradient of a viscous fluid is steepest at its
margins and declines to a nearly uniform velocity near the
center.
The shearing produced by a gradient of this kind results in a
weak force, usually referred to as grain-dispersive pressure.
This, grain-dispersive pressure pushes crystals and other solid
particles into the interior of the flowing magma away from
conduit walls where there are strong velocity gradients.

Physical separation of immiscible meltsPhysical separation of immiscible melts
3 types of magmatic systems are known to have compositional ranges
in which 2 immiscible liquids may separate under geologically
reasonable conditions.
1.Sulfide liquids may separate from mafic silicate magmas, even at
low concentrations of sulfur (100ppm)
2.Highly alkaline magmas rich in CO2 can split into 2 immiscible
fractions, one rich in alkalis and silica and another rich in CO3
3.Very Fe-rich tholeiitic magmas may form 2 separate liquids, a
felsic one rich in SiO2 and a mafic one rich in Fe.

Concentrations of only a few hundred parts per million of S are
sufficient to saturate basaltic melts.
Greater concentrations result in separation of a sulfide melt that is
chiefly Fe and S with minor Cu, Ni, and O that can ultimately
crystallize to pyrrhotite, chalcopyrite, and magnetite.
The densities of 2 coexisting immiscible liquids may be sufficiently
different for them to separate gravitationally.
Economically important massive sulfide deposits in large, layered
mafic complexes have formed by separation and accumulation of
immiscible sulfide melts.

A liquid immiscibility gap occurs in highly alkaline magmas that are rich in CO2.
These liquids separate into two fractions, one enriched in silica and alkalis, and
the other in carbonate. These give rise to the nephelinite-carbonatite association.
Fe-Tholeiitic basalts get fractionated to the silicic liquids and since they have
much lower density, they have strong tendency to rise and collect at the top of
magma chamber.
Much depends, however, on the stage of crystallization at which immiscibility
develops.
Often the magma has crystallized to a viscous mush of entangled crystals, so that
the felsic liquid, when it finally forms, does not separate as continuous mass but
remains as small, dispersed droplets locked in the interstices between early-
crystallizing minerals.

Fluid-Melt separationFluid-Melt separation
Chemical differentiation can also be accomplished when a separate vapor
phase coexists with a magma and liquid-vapor fractionation takes place.
A vapor phase may be introduced in any of the 3 ways
1. A fluid may be released by heating of hydrated or carbonated wall rocks.
2. 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 is released.
1. This vapor phase rises and concentrates on top of magma chamber and in some
cases might permeate into roof rocks.
2. This process usually involves water-rich fluid, and it produces variety of
hydrothermal effects. Eg. Fenitization above nepheline-carbonatite intrusions
due to alkali rich fluids.

3. Late stage fractionation.
1. Most early-formed igneous minerals are anhydrous, so their
segregation from a hydrous melt enriches the melt in H2O and
other volatile phases.
2. Eventually the magma reaches the saturation point, and a
hydrous vapor phase is produced.
3. This is called retrograde or resurgent “boiling”.

The liquids tends to get concentrated in incompatible elements, LIL
and other elements.
Additionally the vapor phase my contain high concentrations of
such phases as H2O, CO2, S, Cl, F, B and P.
The volatile release and concentration associated with pluton rise or
resurgent boiling may momentarily increase the pressure at the top
of the intrusion and fracture the roof rocks in some shallow
intrusions.
Both the vapor phase and some of the late silicate melt are likely to
escape along a network of these fractures as dikes of various sizes.

The silicate melt commonly crystallizes to form a mixture of quartz
and feldspar. It is typically found in small dikes with a sugar-like
texture, which is informally called aplite.
The vapor phase is typically concentrated as dikes or pods in or
adjacent to the parental granitic pluton, where it crystallizes to form
a characteristic pegmatite.
The size of crystals in pegmatites is impressive, such as spodumene,
microcline or mica crystals 6 to 10m across.
Most pegmatites are “simple”, essentially very coarse granites.
Others are more complex, with a tremendous concentration of
incompatible elements and a highly varied mineralogy.

Volatile release raises liquidus
temperature → porphyritic texture
May increase P - fracture the roof rocks
Vapor and melt escape along fractures as
dikes
Silicate melt → quartz and feldspar
→ small dikes of aplite
Vapor phase → dikes or pods of
pegmatite

Miarolitic pods or cavities are smaller fluid
segregations trapped in plutonic host.
When finally exposed at the surface, they are coarse
mineral clusters (usually a few cms across), the
centers of which are typically hollow voids from
which the fluid subsequently escaped.
The hollow cavities have euhedral crystals (of the
same minerals comprising the pluton) that extend
inward, where they grew into fluid, inimpeded by
other minerals.

Concentrate incompatible elements
Complex: varied mineralogy
May display concentric zonation
Figure 11.6 Sections of three zoned fluid-phase deposits (not at the same scale). a. Miarolitic pod in granite (several cm across). b.
Asymmetric zoned pegmatite dike with aplitic base (several tens of cm across). c. Asymmetric zoned pegmatite with granitoid outer
portion (several meters across). From Jahns and Burnham (1969). Econ. Geol., 64, 843-864.

8 cm tourmaline crystals8 cm tourmaline crystals
from pegmatitefrom pegmatite
5 mm gold from a5 mm gold from a
hydrothermal deposithydrothermal deposit

Boundary layer, In situ crystallizationBoundary layer, In situ crystallization
and compositional convectionand compositional convection
Magmatic differentiation involves partitioning elements between two
phases (one a liquid and the other a solid, liquid, or vapor) and the
subsequent differentiation that results when those phases are physically
separated.
Recent studies of magma chambers have shown that many are
diversified in ways not adequately explained by the classical
mechanisms of crystal settling.
This has led several researchers to reevaluate historical ideas of
magmatic differentiation, and to propose alternative methods in which
diversification takes place by in situ (in place) crystallization and
compositionally induced convective processes within an initially
stationary liquid or liquid-solid boundary layer.

Hildreth (1979) proposed that, the magma near the vertical contacts
became enriched in H2O from the wall-rocks.
This water-enriched boundary layer, although cooler, was less dense
then the interior magma, and it rose under the influence of gravity to
concentrate at the top of magma chamber.
This resulted, he proposed, in a growing density-stabilized boundary
layer cap that inhibited convection in the top portion of the magma
chamber (although convection is likely to continue in the main
portion of the chamber below the cap).

Although the cap rock was relatively stationary, there were initial
gradients in temperature and H2O content, with the most H2O-rich,
low density liquids increasing upward.
The H2O gradient, Hildreth reasoned, should affect the structure of the
melt and the degree of polymerization.
Higher H2O content decreases the polymerization.
He postulated that the resulting compositional gradients, combined
with the temperature gradient, induced further diffusional mass
transfer within the cap, resulting in vertical compositional gradients in
the other components.
There may also have been an exchange of matter with the walls and
roof, as well as with the convecting lower chamber.

The result, according to Hildreth (1979),
was a compositionally stratified uppermost
magma chamber that developed the
stratification much faster than the rates
attainable by diffusion alone.

Illustrates a possible mechanism for the development of
compositional stratification along the walls and top of a magma
chamber that may work even in relatively viscous silicic chambers.
Because the magma cools from the margins inward, thermal
gradients occur in the marginal areas where the magma is in
contact with cooler wall and cap rocks (although the cap rock may
be heated more by convection).
If equilibrium is maintained, the magma will crystallize to a
progressively increasing extent toward the cooler walls: a crystal-
liquid “mush zone” or “solidifiction front”.

The gradient in the extent of crystallization will produce a
corresponding gradient in the liquid composition.
The liquid will be highly evolved at the margin where crystallization
is nearly complete, and unevolved in the purely liquid interior.
This boundary layer crystallization process, in which the crystals
remain in situ, results in gravitational instabilities, because the
evolved liquids are less dense than the crystals and the liquid in the
interior of the chamber.
As a result the marginal liquids rise toward the top, and are replaced
by liquid moving toward the boundary layer from the interior.

Langmuir Model
Thermal gradient at wall
and cap → variation in %
crystallized
Compositional convection
→ evolved magmas from
boundary layer to cap (or
mix into interior)
Formation of boundary layers along the
walls and top of a magma chamber.
From Winter (2001) An Introduction to
Igneous and Metamorphic Petrology.
Prentice Hall

This process is called compositional convection, convective rise of
low density material, not because it is hot and expanded, but because
it loses elements, such as Fe, Ca, and Mg to the crystals as it solidifies.
The convecting liquid rises to the cap area, where it spreads laterally
and stagnates, as Hildreth (1979) proposed.
The stagnant stratification at the cap is thus not a result of diffusion
generated by an initial H2O gradient only, as Hildreth (1979) proposed,
but is inherited from fractional crystallization along the walls, because
the most evolved liquids are the least dense, and occupy the highest
portions of the cap boundary layer.

Open system Differentiation:Open system Differentiation:
Magma MixingMagma Mixing
 Just as they assimilate rocks, magmas can also assimilate other
liquids by mixing with them. In some respects it is opposite of
fractionation.
 If two or more dissimilar parent magmas blend together, a
hybrid daughter magma compositionally intermediate between
them is produced.
 Magmas can be derived from different sources, such as basaltic
magma from the upper mantle and silicic magma from the deep
continental crust, or they may have had a common parent
magma but followed different evolutionary tracks, such as the
contrasting magmas in a compositionally zoned chamber.

Magma Mixing-The ProcessMagma Mixing-The Process
 Initially, dissimilar magmas are physically mingled.
 If solidification occurs soon afterward, the composite rock has layers,
lenses, pillow-shaped blobs, or more irregularly shaped bodies in a
dissimilar matrix.
 Rocks formed by mingling of magmas retaining their contrasting identity
are evident on scales ranging from a thin section to large outcrops.
 After mingling, magmas may become mixed on an atomic scale by
diffusion, if sufficient time and thermal energy are available, forming an
essentially homogeneous melt.
 Homogenization and equilibration of crystals from the two batches of
magma take a longer time.

 The dynamics of magma mixing depends on the contrasting magma
properties, such as the temperature, composition, density, volatile
content, and viscosity, as well as the location and the turbulence with
which one magma injects into the chamber containing the other.
 Gas-charged or pressure-induced fountaining may mix magmas
considerably, whereas quiescent injection of dense magmas may
pond at the bottom of a silicic magma chamber and result in
stratification with little mixing at all.
 Mixing is most efficient when magmas of similar physical properties
are turbulently intermingled.

 The density differences of strongly contrasting liquids inhibit mixing
and, instead, cause them to become gravitationally stratified with
dense liquids remaining near the floor and light ones rising to collect
under the roof.
 Any new magma must seek a level at which it is gravitationally
stable, and unless convection causes wholesale overturn and mixing,
the new and old magmas may have very little interaction.
 Because chemical diffusion is so slow, thorough mixing requires
strong, turbulent stirring, in order that different liquids may be
mechanically associated on an intimate scale.

 Two magmas can commonly be seen as comingled swirls of
contrasting colors on the hand sample or outcrop scale, or even as
intimate mixtures of contrasting glass in thin section.
 As mentioned above, because basalt is initially at a higher
temperature than silicic magma, their comingling would tend to
chill the basalt and superheat the other.
 Basaltic magmas entering granitic chambers commonly form
pillow-like structures with curved boundaries and glassy quenched
marginal textures that accumulate at the bottom of the chamber.

Basalt pillows
accumulating at the bottom
of a in granitic magma
chamber, Vinalhaven
Island, Maine
Comingled basalt-Rhyolite
Mt. McLoughlin, Oregon
Figure 11.8 From Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall

 Some magma mixing occurrences may appear like
immiscible liquids, but the mixed magmas are not in
equlibrium, so disequilibrium assemblages (such as
plagioclase or pyroxenes of radically different
compositions, or juxtaposed olivine and quartz, and
corroded and partially resorbed phenocrysts) are
common.
 For similar magmas, the mixing may be more
extensive, resulting in homogeneous mixtures, but
the evidence for such mixtures is also obscured.

 2 mechanisms have been proposed for magma mixing
1. If hot, dense mafic magma lies beneath a cooler, lighter one and begins
to crystallize and differentiate, the resulting compositional change may
lower its density until it is lighter than the overlying liquid and is able to
rise and mix with it turbulently.
1. Such a process has been proposed, but physical plausibility is still a matter of
debate.
2. More general process may be associated with the crystallization on the
walls of certain shallow type of intrusions.
1. Crystallization produces a differentiated liquid that is less dense and a buoyant
boundary layer may rise along the walls and accumulate under the roof.

2. More general process may be associated with the
crystallization on the walls of certain shallow type of
intrusions.
1. If the flow of the rising layer is turbulent, it may entrain magma
of the interior and, by mixing with the undifferentiated liquid,
produce a range of intermediate compositions.
2. This process, unlike the previous one, does not involve 2
independently derived magmas, but rather back-mixing of a
derivative liquid with its parent.

AssimilationAssimilation
 Assimilation is the incorporation and digestion of foreign,
usually solid, material by a magma producing a contaminated
magma, which is also hybrid, like mixed magmas.
 In the past it was widely believed that many igneous rocks, and
even suites of igneous rocks, evolved as a result of assimilation
of continental crustal rocks by primary magmas.
 Such views should not surprise one because no body of magma
is ever completely independent of wall rocks that contain it.

 The main reason why assimilation is not a major process in
the evolution of magmatic rocks is that there is an enormous
discrepancy between the specific heat capacities of silicate
magmas and the heats of solution of silicate country rocks
(1:350)
 As pointed out by Bowen (1928), the country rock must first
be heated to the melting point, and then at least partially
melted in order to be assimilated, and this heat must be
supplied by the magma itself.
 Let's evaluate the process, using the some estimates.

 The specific heat (the energy required to raise the temperature
of 1g of rock to 10
C) of 1 J/g °C and heat of fusion (the energy
required to melt 1g of rock at the solidus) of 400 J /g.
 In order to bring 1g of granitic country rock from an ambient
temperature of 200°C to a hypothetical melting point at
800°C would require 600°C • 1 J/g °C = 600 J (per gram).
 To melt a gram would require a further 400 J.
 The magma would have thus to expend 1000 J of energy to
heat, melt, and assimilate one gram of country rock.

 If the magma is at its liquidus temperature (as most magmas are), the only energy it can
supply is its own heat of crystallization, so 2.5 g of magma would have to crystallize in
order to melt and assimilate 1 g of wall rock.
 It is thus theoretically possible for a magma to assimilate 40% of its weight in country
rock, and more if the country rock is initially closer to its liquidus temperature.
 There is, however, no evidence that magmas normally contain significant amounts of
superheat; as magmas themselves are generated through partial or complete melting of
solid source rocks at temperatures not far above liquidus temperatures.
 In some bodies of magma, however, convection currents may supply enough heat
for the local assimilation of country rocks, particularly at contacts.

 The portion of the magma that crystallizes in order to supply the heat for
assimilation occurs at the cool walls where this heat energy is consumed.
 Here it likely forms a barrier to inhibit further exchange with the wall
rocks (unless turbulent flow or gravitational effects continuously sweep
the contact clean, as proposed for some situations by Huppert and
Sparks, 1985).
 Assimilated components must then diffuse through the marginal barrier.
 As diffusion of heat is much faster than diffusion of mass, formation of
such a boundary should inhibit chemical exchange in most cases, and the
magma would solidify before appreciable assimilation has occurred.

 The manner in which igneous materials are assimilated will be
considered by examining what happens during the assimilation
during the assimilation of igneous rock-forming minerals.
 When such minerals are assimilated, the process may be considered
to operate in one of 3 ways, depending on whether the mineral is
1. A phase the magma may have crystallized earlier but is no longer
precipitating (earlier in reaction series)
2. A phase the magma is currently precipitating
3. A phase that magma may precipitate at a late stage in cooling
history.

 In the first example, the magma is unable to melt the mineral, and
thus reacts with it;
For eg. If pyroxene be the precipitating phase in basaltic magma, it can be
considered to be supersaturated with respect to olivine.
This magma is therefore not capable of dissolving or melting olivine.
Now, if such a mineral or mineral assemblage with relative position higher up
in the reaction series, is incorporated in the magma then equilibrium has to be
established by chemical exchange process.
In case of olivine, the mineral will tend to come down to a lower position in
the series without being melted.
It will change in composition and finally can be converted to pyroxene, the
then precipitating phase in the reacting magma.

 In the second example, the mineral is stable in the magma, and
the assimilation process increases the volume of that mineral
without altering the composition of the liquid part of the magma
 In third example, there is normally reactive solution of the
mineral by the magma.
However, melting of incorporated solid phases are sometimes partial
and restricted along the margins of foreign materials.
Resorption of phenocrystal minerals by partial dissolution is observed in
some hybrid rocks.

 The assimilation of xenoliths of sedimentary, or metamorphic, origin
is often complicated, because these materials may contain phases that
do not belong in the reaction series for; and sandstones and
limestones have extreme chemical compositions as compared to
normal magmatic rocks.
 When quartz-rich xenoliths are engulfed in sub alkalic basaltic lava,
the earliest stages in the assimilation process consist of the formation
of rims of glass that “penetrate along grain boundaries and
fractures, and as the process of solution advances, the grains of
quartz are reduced to rounded and embayed remnants surrounded
by pale brown glass and corona of augite”

Fractionation by Partial MeltingFractionation by Partial Melting
 The processes of melting are in many ways the reverse of
solidification; the compositions of liquids vary as melting
advances, much as they do with crystallization.
 If a melt is extracted at an early stage of melting, it will be richer
in low-temperature components, whereas more advanced melting
produces liquids with larger proportions of the more refractory
components.
 Thus a series of liquids can be produced that resembles the
compositions of liquids evolving by crystal fractionation, but their
order of appearance in the magmatic sequence would, of course,
be reversed.

1. Fractional melting:
In this case the melt phase formed is continuously removed from
the solid residue so that continuity of its equilibration with the
source rocks is not feasible.
For such a mechanism, the bulk composition of the system,
cannot remain constant and it continues to change.
In this process, the melt separates rather rapidly accompanied by
small degrees of melting.
The great variability of melt composition produced by this
process may be an important clue to magma diversification.

2. Equilibrium melting:
In such a case, the generated partial melt phase continually reacts and
equilibrates with the solid residue until the magma segregates.
The bulk composition of the magma remains the same till magma
segregation.
3. Zone melting:
 Harris (1957) proposed that some magmas are generated deep within the mantle,
and as they rise, the crystals at the top of such bodies of magma will be more
soluble than those at the base because there is drop in pressure from bottom to top
of the magma body.
If the composition of the magma is kept uniform by convection, then the magma
will rise by solution-stoping with solid phases in the roof going into solution and
equivalent amounts of material crystallizing at the base of the magma body.

3. Zone melting:
 Heat flow through heat of crystallization will further favor melting of
country rocks thus making way for magma.
 This is essentially a combined assimilative and fractionation process.
 This process will operate at depths where country rocks are at their
liquidus temperatures, and there will be little loss of heat because
heat of solution of crystals at the top is balanced by the heat of
crystallization of minerals at the base of the magma.
 The moving magma front will progressively evolve through change
in composition and will particularly collect incompatible elements
(K, Rb, Cs, Ba, Pb, Zr, Th, U, Nb, P, C, H and Cl) selectively from
enclosing rocks though the initial magma might have been somewhat

3. Zone melting:
 Though theoretically it can operate in the crust and the mantle, it
is considered to be more effective in the mantle.
 But if the country rocks are well below their melting point
(subcrustal levels), more minerals would have to crystallize than
melt, and the pluton would quickly solidify.
 Subcrustal zone melting, may be favored by upward migration of
water and consequent drying effect towards the bottom of the
magma body.
 The stoped xenoliths will be partly or fully melted and the magma
gets largely contaminated.
 With this we end the topic Diversification

Effects of removing liquid at various stages of
melting
Eutectic systems
First melt always = eutectic composition
Major element composition of eutectic
melt is constant until one of the source
mineral phases is consumed (trace
elements differ)
Once a phase is consumed, the next
increment of melt will be different X and
T

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
Have enough interior volume that it is not
all of it is adsorbed to the crystal surfaces

The ability to form an interconnected film is dependent upon
the dihedral angle (θ) a property of the melt
Figure 11.1 Illustration of the dihedral angle
(θ) of melt droplets that typically form at
multiple grain junctions. After Hunter (1987)
In I. Parsons (ed.), Origins of Igneous
Layering. Reidel, Dordrecht, pp. 473-504.

Gravity settlingGravity settling
Cool point a → olivine layer at base of pluton
if first olivine sinks
Next get ol+cpx layer
finally get ol+cpx+plag
Cumulate texture:
Mutually touching
phenocrysts with
interstitial crystallized
residual melt
Figure 7-2. After Bowen
(1915), A. J. Sci., and
Morse (1994), Basalts
and Phase Diagrams.
Krieger Publishers.

Figure 11.2 Variation diagram using MgO as the abscissa for lavas associated with the 1959 Kilauea eruption in Hawaii. After
Murata and Richter, 1966 (as modified by Best, 1982)

Stoke’s LawStoke’s Law
V = the settling velocity (cm/sec)
g = the acceleration due to gravity (980 cm/sec2
)
r = the radius of a spherical particle (cm)
ρs = the density of the solid spherical particle (g/cm3
)
ρl = the density of the liquid (g/cm3
)
η = the viscosity of the liquid (1 c/cm sec = 1 poise)
V
2gr ( )
9
2
=
−ρ ρ
η
s l

Olivine in basalt
Olivine (ρs = 3.3 g/cm3
, r = 0.1 cm)
Basaltic liquid (ρl = 2.65 g/cm3
, η = 1000 poise)
V = 2·980·0.12
(3.3-2.65)/9·1000 = 0.0013 cm/sec

Rhyolitic melt
η = 107
poise and ρl = 2.3 g/cm3
hornblende crystal (ρs = 3.2 g/cm3
, r = 0.1 cm)
V = 2 x 10-7
cm/sec, or 6 cm/year
feldspars (ρl = 2.7 g/cm3
)
V = 2 cm/year
= 200 m in the 104
years that a stock might cool
If 0.5 cm in radius (1 cm diameter) settle at 0.65
meters/year, or 6.5 km in 104
year cooling of stock

Stokes’ Law is overly simplified
1. Crystals are not spherical
2. Only basaltic magmas very near their liquidus temperatures
behave as Newtonian fluids

Many silicic magmas approach the ternary eutectic
Either fractional crystallization does take place or they
are minimum (eutectic) melts
Figure 11.3 Position of the H2O-saturated
ternary eutectic in the albite-orthoclase-silica
system at various pressures. The shaded
portion represents the composition of most
granites. Included are the compositions of the
Tuolumne Intrusive Series (Figure 4-32), with
the arrow showing the direction of the trend
from early to late magma batches.
Experimental data from Wyllie et al. (1976).
From Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice
Hall

Polybaric Fractional Crystallization
1. Stability of phases will change (hi-P garnet)

Polybaric Fractional Crystallization
1. Stability of phases changes (hi-P garnet...)
2. Shift of the eutectic point with pressure will
cause the quantity of the liquidus phases to vary

Ol
Low-P
Pyx
Hi-PHigh-P (purple tie-
line) has liq > ol
Low-P (blue tie-line)
has ol > liquid
Expansion of olivine field at low pressure causes
an increase in the quantity of crystallized olivine

Two other mechanisms that facilitate the
separation of crystals and liquid
1. Compaction

Liquid immiscibility in the Fo-SiO2 system
Liquid ImmiscibilityLiquid Immiscibility
Figure 6.12. Isobaric T-X phase diagram
of the system Fo-Silica at 0.1 MPa. After
Bowen and Anderson (1914) and Grieg
(1927). Amer. J. Sci.

The effect of adding
alkalis, alumina, etc. is
to eliminate the solvus
completely
Figure 7.4. Isobaric
diagram illustrating the
cotectic and peritectic
curves in the system
forsterite-anorthite-silica
at 0.1 MPa. After
Anderson (1915) A. J. Sci.,
and Irvine (1975) CIW
Yearb. 74.

Renewed interest when Roedder (1951) discovered a
second immiscibility gap in the iron-rich Fa-Lc-SiO2
system
Figure 11-7. Two immiscibility
gaps in the system fayalite-
leucite-silica (after Roedder,
1979). Yoder (ed.), The
Evolution of the Igneous
Rocks. Princeton University
Press. pp. 15-58. Projected
into the simplified system are
the compositions of natural
immiscible silicate pair
droplets from interstitial Fe-
rich tholeiitic glasses
(Philpotts, 1982). Contrib.
Mineral. Petrol., 80, 201-218.

Tests for immiscible origin ofTests for immiscible origin of
associated rock pairsassociated rock pairs
1. The magmas must be immiscible when
heated experimentally, or they must plot on
the boundaries of a known immiscibility
gap, as in Fig. 11.7

2. Immiscible liquids are in equilibrium with
each other, and thus they must be in
equilibrium with the same minerals
Tests for immiscible origin ofTests for immiscible origin of
associated rock pairsassociated rock pairs

Walker and DeLong (1982) subjected two basalts to
thermal gradients of nearly 50o
C/mm (!)
Found that:
Samples reached a steady
state in a few days
Heavier elements → cooler
end and the lighter → hot
end
The chemical concentration
is similar to that expected
from fractional
crystallization
Figure 7.4. After Walker, D.
C. and S. E. DeLong
(1982). Contrib. Mineral.
Petrol., 79, 231-240.

Detecting and assessing assimilation
Isotopes are generally the best
Continental crust becomes progressively enriched
in 87
Sr/86
Sr and depleted in 143
Nd/144
Nd
Figure 9-13.
Estimated Rb and Sr
isotopic evolution of
the Earth’s upper
mantle, assuming a
large-scale melting
event producing
granitic-type
continental rocks at
3.0 Ga b.p After
Wilson (1989).
Igneous
Petrogenesis. Unwin
Hyman/Kluwer.

9-21 238
U → 234
U → 206
Pb (λ = 1.5512 x 10-10
a-1
)
9-22 235
U → 207
Pb (λ = 9.8485 x 10-10
a-1
)
9-23 232
Th → 208
Pb (λ = 4.9475 x 10-11
a-1
)
U-Th-Pb system as an indicator of continental
contamination is particularly useful
All are incompatible LIL elements, so they
concentrate strongly into the continental crust
Detecting and assessing assimilationDetecting and assessing assimilation

Mixed ProcessesMixed Processes
May be more than coincidence: two
processes may operate in conjunction
(cooperation?)
AFC: FX supplies the necessary heat
for assimilation
Fractional crystallization + recharge of
more primitive magma

Tectonic-Igneous AssociationsTectonic-Igneous Associations
Associations on a larger scale than the
petrogenetic provinces
An attempt to address global patterns
of igneous activity by grouping
provinces based upon similarities in
occurrence and genesis

Mid-Ocean Ridge VolcanismMid-Ocean Ridge Volcanism
Ocean Intra-plate (Island) volcanismOcean Intra-plate (Island) volcanism
Continental Plateau BasaltsContinental Plateau Basalts
Subduction-related volcanism and plutonismSubduction-related volcanism and plutonism
Island ArcsIsland Arcs
Continental ArcsContinental Arcs
Granites (not a true T-I Association)Granites (not a true T-I Association)
Mostly alkaline igneous processes of stableMostly alkaline igneous processes of stable
craton interiorscraton interiors
Anorthosite MassifsAnorthosite Massifs
Tectonic-Igneous AssociationsTectonic-Igneous Associations

Ch 11 diversification

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