This chapter deals with the evolution and differentiation of magma as it rises upward due to density contrast with the surrounding rocks. It will be discussed about the processes that cause the chemical differentiation of the magma.
2. Chapter Six: Magmatic Processes
and Chemical Evolution of
Magmas
6.1. Magmatic evolution
6.2. Parental, primitive, primary, evolved and
derivative magma
6.3. Major mechanisms and vehicles of
magmatic differentiation
3. Magma compositions
• Magma is completely or partly molten material, which on cooling
solidifies to form an igneous rock.
• Magma derived from the mantle is rich in magnesium and iron (mafic
composition).
• Magma derived from the continental crust is rich in silica and
aluminium.
• Oxygen is the most abundant element and represents about 50% by
weight of the total mass of magmas.
• Si, Al, Fe, Mg, Ca, Na, K, Ti and P are present at concentration levels
ranging from less than 1% to more than 25 to 30%. These are called
as major elements.
• Due to the dominance of oxygen the composition of magmas and
magmatic rocks are expressed as weight percent of the oxides.
• Silica (SiO2) is the most abundant major oxide followed by Al2O3,
Fe2O3, FeO, MgO, CaO, Na2O, K2O, TiO2 and P2O5.
4. • Most magma consist of three parts liquid, solid and gaseous component.
• The liquid portion is called melt and contain mobile ions of those elements
commonly found in the earths crust.
• Melt is made up of mostly ions of silicon and oxygen along with lesser
amounts of aluminium, potassium, calcium, sodium, iron and magnesium.
• The solid components in magma are silicate minerals that have crystallized
from the melts.
• The most common volatiles found in magma are water vapour (H2O), carbon
dioxide (CO2) and sulphur dioxide (SO2), which are confined by the
immense pressure exerted by the overlying rocks.
• Mechanisms of magmatic differentiation may be visualized by crystal-liquid
fractionation, liquid immiscibility, vapour transport and thermo-gravitational
diffusion. These mechanisms involve separation of
1. crystals from liquid,
2. liquid from liquid,
3. liquid (and crystals) from vapour and
4. ions from other ions.
5. primary, primitive, Parental, evolved and
derivative magmas
• Primary Magma: is unmodified magmas.
– is the "first melt" produced by partial melting within the mantle and which has
not yet undergone any differentiation.
– evolve into a parental magma by differentiation
• Primitive magma: is the magma that underwent minimal
differentiation
• Parental magma: is least differentiated magma in a series leading to
evolved rocks.
– is capable of producing all rocks belonging to an igneous rock series by
differentiation i.e., igneous rocks that are genetically related (co-magmatic).
6. For a melt to qualify for the definition of primary magma, it must
fulfil the following conditions:
(1) have a higher liquidus T compared to its differentiation products,
(2) be richer in minerals removed by fractional crystallization compared
to its differentiation products,
(3) have a composition in equilibrium with the mantle phases from which
it was produced by partial melting at high pressure.
• All primary magmas must have >10% MgO wt%
• Primary magmas need not to contain inclusions.
• Because of the uncertainties involved in establishing what is a primary
magma, petrologists often attempt to decide which rock in a co-
magmatic suite under investigation solidified from a primary magma,
one whose composition was least unmodified by differentiation
processes after leaving its source.
• Such magma tends to have the highest Mg number and concentrations
of Ni and Cr.
7. Major mechanisms and vehicles of magmatic
differentiation
• If magmas are related to each other by some processes, that process
would have to be one that causes magma composition to change.
• Any process that causes magma composition to change is called
magmatic differentiation.
• Over the years, various process have been suggested to explain the
variation of magma compositions observed within small regions.
Among the processes are:
– Distinct melting events from distinct sources.
– Various degrees of partial melting from the same source.
– Crystal fractionation.
– Mixing of two or more magmas.
– Assimilation/contamination of magmas by crustal rocks.
– Liquid Immiscibility
8. • Mechanisms of magmatic differentiation may be visualized by crystal-liquid
fractionation, liquid immiscibility, vapour transport and thermo-gravitational
diffusion. These mechanisms involve separation of
1. crystals from liquid,
2. liquid from liquid,
3. liquid (and crystals) from vapour and
4. ions from other ions.
9. Magma evolution
• The source regions of magmas are upper mantle and lower or
intermediate continental crust.
• Once the magma is formed, it has the tendency to rise towards surface
due to their low density compared to the surrounding rocks.
• During their way to surface, magmas may undergo compositional
modifications that generate series of evolved magmas.
• The processes responsible for magma evolution are
1) fractional crystallization
2) mixing and
3) assimilation of wall rocks.
10. 1. Fractional crystallization
• refers to any process that separates the early formed minerals of magma
from the remaining liquid.
• Early formed minerals are separated from the parent magma due to
density contrast between solid and liquid.
• This causes a change in the composition of the residual liquid.
• occurs when magmas stops either within the crust or at the interface between
mantle and crust.
• Early formed crystals may be separated from the residual melt by
processes such as
1. settling of early formed crystals to the bottom of a magma chamber
2. growth of crystals on the walls of a magma chamber and
3. filtering out of large, early formed crystals as the melt migrates
through narrow fractures.
11.
12. Gravitational crystal settling
• Gravitational separation of crystals from liquid normally involves sinking of the
crystals by virtue of their greater density. Occasionally, however, floating may be
the process if the density of the crystals is less than that of the liquid.
• Settling rates are a function of the crystal size, the viscousity of the liquid, and
the density contrast b/n the liquid and the crystal.
13. • If the crystals have a lower density in the magma, they
will tend to float or rise upward through the magma.
• Again the first layer that accumulates at the top of the
magma body will initially be in contact with the liquid,
but as more crystals float to the top and accumulate, the
earlier formed layers will be effectively removed from
contact with the liquid.
14. • Inward Crystallization - Because a magma body is hot and the
country rock which surrounds it is expected to be much cooler, heat
will move outward away from the magma.
• Thus, the walls of the magma body will be coolest, and
crystallization would be expected to take place first in this cooler
portion of the magma near the walls.
• The magma would then be expected to crystallize from the walls
inward. Just like in the example above, the first layer of crystals
precipitated will still be in contact with the liquid, but will
eventually become buried by later crystals and effectively be
removed from contact with the liquid.
15. • Filter pressing - this mechanism has been proposed as a way to
separate a liquid from a crystal-liquid mush. In such a situation
where there is a high concentration of crystals the liquid could be
forced out of the spaces between crystals by some kind of tectonic
squeezing that moves the liquid into a fracture or other free space,
leaving the crystals behind.
• It would be kind of like squeezing the water out of a sponge. This
mechanism is difficult to envision taking place in nature because (1)
unlike a sponge the matrix of crystals is brittle and will not deform
easily to squeeze the liquid out, and (2) the fractures required for the
liquid to move into are generally formed by extensional forces and
the mechanism to get the liquid into the fractures involves
compressional forces.
16. • The compositional modifications that take place in magmas during
fractional crystallization depend on the composition of magmas and
on that of the separating minerals.
• As a rule it can be stated that the ratio of the concentrations of a given
element between minerals and magmas (i.e., the partition coefficient)
is
– higher than unity, the concentration of that element decreases in
the residual melt during fractional crystallization.
– lower than unity the element concentration increases in the melt
with fractional crystallization.
– Equal to unity equally abundant in both magma and crystals its
abundance does not change with fractional crystallization.
17. • In fractional crystallization olivine is always the first formed mineral
that crystallizes from any type of basaltic magma at moderate or low
pressure.
• Olivine is generally joined by diopsidic clinopyroxene and anorthitic
plagioclase.
• Successively the crystallizing mineral phases are different from the
various types of magmas.
• As example let us consider the calc alkaline and potassic alkaline
basaltic magmas.
• The crystallization sequences of calc alkaline magmas at low pressure
are given by Bowen Reaction Series.
18. • Olivine crystallizes earlier followed by anorthitic plagioclase and
clinopyroxen.
• Increasingly Na-plagioclase, amphibole, biotite, alkali feldspars
and quartz crystallize during the intermediate and late stages of
evolution.
• This liquid composition during fractional crystallization becomes
increasingly rich in silica and alkalis, and depleted in FeO, MgO
and CaO and form evolving calc alkaline magmas.
• The final stage of
the fractional
crystallization gives
acid liquids similar
to those formed by
crustal melting.
19. • The overall process produces a series of magmas which range from
basalt, andesite to dacite and rhyolite.
• Potassium alkaline magmas, here also olivine, plagioclase and clino-
pyroxene separate during the early stages of evolution, but these are
followed by leucite and K-feldspar.
• As a result, the K-alkaline basalts or magmas initially become enriched
in alkalis and silica; and depleted in CaO and MgO.
• The separation of alkali feldspars and leucite produces depletion in silica
in the residual liquids.
• In fact these minerals have high SiO2 than the magmas from which they
crystallize.
• So potassium alkaline magmas never evolve towards acidic composition
and resulting magmatic series consist of K-alkaline basalts, phonolitic
tephrites (gabbroic composition contain augite, labrodorite, leucite and
nepheline) and phonolite (syenite clan, contain alkali feldspars,
feldspathoids and mafic minerals).
• Na-alkaline magmas also follow similar evolution path, even though
nepheline (felspathiod group) and Na- feldspar (albite) are the main
crystallizing minerals in these magmas in magmatic process.
20.
21.
22.
23. • Because the composition of the crust is generally different from the
composition of magmas which must pass through the crust to reach
the surface, there is always the possibility that reactions between the
crust and the magma could take place.
• If crustal rocks are picked up, incorporated into the magma, and
dissolved to become part of the magma, we say that the crustal rocks
have been assimilated by the magma. If the magma absorbs part of
the rock through which it passes we say that the magma has become
contaminated by the crust. Either of these process would produce a
change in the chemical composition of the magma unless the
material being added has the same chemical composition as the
magma.
24.
25. Magma Mixing
• If two or more magmas with different chemical compositions come in contact
with one another beneath the surface of the Earth, then it is possible that they
could mix with each other to produce compositions intermediate between the end
members.
– If the compositions of the magmas are greatly different (i.e. basalt and
rhyolite), there are several factors that would tend to inhibit mixing.
• Temperature contrast - basaltic and rhyolitic magmas have very different
temperatures. If they come in contact with one another the basaltic magma would
tend to cool or even crystallize and the rhyolitic magma would tend to heat up and
begin to dissolve any crystals that it had precipitated.
• Density Contrast- basaltic magmas have densities on the order of 2600 to 2700
kg/m3, whereas rhyolitic magmas have densities of 2300 to 2500 kg/m3. This
contrast in density would mean that the lighter rhyolitic magmas would tend to
float on the heavier basaltic magma and inhibit mixing.
• Viscosity Contrast- basaltic magmas and rhyolitic magmas would have very
different viscosities. Thus, some kind of vigorous stirring would be necessary to
get the magmas to mix.
26.
27. Liquid Immiscibility
• Liquid immiscibility is where liquids do not mix with each other. We are
all familiar with this phenomenon in the case of oil and water/vinegar in
salad dressing.
• We have also discussed immiscibility in solids, for example in the alkali
feldspar system. Just like in the alkali feldspar system, immiscibility is
temperature dependent. For example, in a two component system if there
is a field of immiscibility it would appear as in the diagram shown here.
• Cooling of a liquid with a composition of 25%B & 75%A would
eventually result in the liquid separating into two different
compositions. With further cooling one liquid would become more
enriched in A and the other more enriched in B.
• Eventually both liquids would reach a temperature where crystals of A
would start to form. Note that both liquids would be in equilibrium with
crystals of A at the same temperature. Further cooling would result in the
disappearance of the A-rich liquid.
28. • Major elements: usually greater than 1%
SiO2 Al2O3 FeO* MgO CaO Na2O K2O H2O
• Minor elements: usually 0.1 - 1%
TiO2 MnO P2O5 CO2
• Trace elements: usually < 0.1%
everything else
Element Wt % Oxide Atom %
O 60.8
Si 59.3 21.2
Al 15.3 6.4
Fe 7.5 2.2
Ca 6.9 2.6
Mg 4.5 2.4
Na 2.8 1.9
Abundance of the elements
in the Earth’s crust
Behaviour of major-and trace-elements
30. Variation Diagrams
• How do we display chemical data in a meaningful way?
• The compositional variations of magmatic series formed by fractional
crystallization can be shown by plotting the concentration of major oxides
against silica for the various rocks having different degrees of evolution.
• These diagrams are called as variation diagrams or Harker diagrams.
• The change in slope of the trends indicates variations of the mineral assemblages that
separate from the magma.
• Note, for instance, how shift from olivine to olivine +clinopyroxene +plagioclase
fractionation produces a change in the slope of MgO vs SiO2 (B).
31. • From the above diagram it can be noticed that the trend of the plots is
in curved.
– This shape is typical of fractional crystallization and is related to variations of
mineral assemblages that separate from the magma in ongoing evolution.
• This causes mediations in the rate of increase or decrease of the
elements, whose variations are reflected in the curved nature.
• For example at the beginning of the fractional crystallization of
basaltic magma, there is a steep decrease of MgO due to the
separation of abundant olivine.
• When olivine stops crystallization or it is joined by other minerals,
then the decrease in MgO becomes less steep.
• Trace elements behave in a similar way as major elements.
• Those entering readily into the lattices of fractionating minerals
(compatible elements) decrease in abundance in the residual liquids.
• In contrary to this, the elements that prefer to remain in the liquid
phase (incompatible elements) become concentrated in the residual
melts.
32. Bivariate (x-y)
diagrams
Figure 8.2. Harker variation
diagram for 310 analyzed
volcanic rocks from Crater
Lake (Mt. Mazama), Oregon
Cascades. Data compiled by
Rick Conrey (personal
communication).
33. Bivariate (x-y)
diagrams
Figure 8.2. Harker variation
diagram for 310 analyzed
volcanic rocks from Crater
Lake (Mt. Mazama), Oregon
Cascades. Data compiled by
Rick Conrey (personal
communication).
34. Models of Magmatic Evolution
hypothetical set of related volcanics.
Oxide B BA A D RD R
SiO2 50.2 54.3 60.1 64.9 66.2 71.5
TiO2 1.1 0.8 0.7 0.6 0.5 0.3
Al2O3 14.9 15.7 16.1 16.4 15.3 14.1
Fe2O3* 10.4 9.2 6.9 5.1 5.1 2.8
MgO 7.4 3.7 2.8 1.7 0.9 0.5
CaO 10.0 8.2 5.9 3.6 3.5 1.1
Na2O 2.6 3.2 3.8 3.6 3.9 3.4
K2O 1.0 2.1 2.5 2.5 3.1 4.1
LOI 1.9 2.0 1.8 1.6 1.2 1.4
Total 99.5 99.2 100.6 100.0 99.7 99.2
B = basalt, BA = basaltic andesite, A = andesite, D = dacite,
RD = rhyo-dacite, R = rhyolite. Data from Ragland (1989)
Table 8-5 . Chemical analyses (wt. %) of a
35. Harker diagram
– Smooth trends
– Model with 3 assumptions:
1 Rocks are related by FX
2 Trends = liquid line of
descent
3 The basalt is the parent
magma from which the others
are derived
36. Magma Series
Can chemistry be used to distinguish families of
magma types?
Early on it was recognized that some chemical
parameters were very useful in regard to
distinguishing magmatic groups
– Total Alkalis (Na2O + K2O)
– Silica (SiO2) and silica saturation
– Alumina (Al2O3)
37. •Alkali vs. Silica diagram for Hawaiian volcanics:
•Seems to be two distinct groupings: alkaline and subalkaline
Figure 8.11. Total
alkalis vs. silica
diagram for the alkaline
and sub-alkaline rocks
of Hawaii. After
MacDonald (1968).
GSA Memoir 116
38. The Basalt Tetrahedron and the Ne-Ol-Q base
•Alkaline and subalkaline fields are again distinct
Figure 8.12. Left: the basalt tetrahedron (after Yoder and Tilley, 1962). J. Pet., 3, 342-532. Right: the base of the
basalt tetrahedron using cation normative minerals, with the compositions of subalkaline rocks (black) and
alkaline rocks (gray) from Figure 8-11, projected from Cpx. After Irvine and Baragar (1971). Can. J. Earth Sci., 8,
523-548.
39. AFM diagram: can further subdivide the subalkaline
magma series into a tholeiitic and a calc-alkaline series
Figure 8.14. AFM diagram showing
the distinction between selected
tholeiitic rocks from Iceland, the
Mid-Atlantic Ridge, the Columbia
River Basalts, and Hawaii (solid
circles) plus the calc-alkaline rocks
of the Cascade volcanics (open
circles). From Irving and Baragar
(1971). After Irvine and Baragar
(1971). Can. J. Earth Sci., 8, 523-
548.
40.
41. Figure 18.2. Alumina saturation classes based on the molar proportions of Al2O3/(CaO+Na2O+K2O)
(“A/CNK”) after Shand (1927). Common non-quartzo-feldspathic minerals for each type are included. After
Clarke (1992). Granitoid Rocks. Chapman Hall.
42. Characteristic
Series Convergent Divergent Oceanic Continental
Alkaline yes yes yes
Tholeiitic yes yes yes yes
Calc-alkaline yes
Plate Margin Within Plate
A world-wide survey suggests that there may be
some important differences between the three series
After Wilson (1989). Igneous Petrogenesis. Unwin Hyman - Kluwer
43. • Trace elements behave in a similar way as major
elements.
• Those entering readily into the lattices of
fractionating minerals (compatible elements)
decrease in abundance in the residual liquids.
• In contrary to this, the elements that prefer to
remain in the liquid phase (incompatible elements)
become concentrated in the residual melts.
Behaviour of trace-elements
44. Element Use as a Petrogenetic Indicator
Ni, Co, Cr Highly compatible elements. Ni and Co are concentrated in olivine, and Cr in spinel and clinopyroxene. High concentrations
indicate a mantle source, limited fractionation, or crystal accumulation.
Zr, Hf Very incompatible elements that do not substitute into major silicate phases (although they may replace Ti in titanite or
rutile). High concentrations imply an enriched source or extensive liquid evolution.
Nb, Ta High field-strength elements that partition into Ti-rich phases (titanite, Ti-amphibole, Fe-Ti oxides. Typically low
concentrations in subduction-related melts.
Ru, Rh, Pd,
Re, Os, Ir,
Pd
Platinum group elements (PGEs) are siderophile and used mostly to study melting and crystallization in mafic-ultramafic
systems in which PGEs are typically hosted by sulfides. The Re/Os isotopic system is controlled by initial PGE
differentiation and is applied to mantle evolution and mafic melt processes.
Sc Concentrates in pyroxenes and may be used as an indicator of pyroxene fractionation.
Sr Substitutes for Ca in plagioclase (but not in pyroxene), and, to a lesser extent, for K in K-feldspar. Behaves as a compatible
element at low pressure where plagioclase forms early, but as an incompatible element at higher pressure where
plagioclase is no longer stable.
REE Myriad uses in modeling source characteristics and liquid evolution. Garnet accommodates the HREE more than the LREE,
and orthopyroxene and hornblende do so to a lesser degree. Titanite and plagioclase accommodates more LREE. Eu2+ is
strongly partitioned into plagioclase.
Y Commonly incompatible. Strongly partitioned into garnet and amphibole. Titanite and apatite also concentrate Y, so the
presence of these as accessories could have a significant effect.
Table 9.6 A Brief Summary of Some Particularly Useful Trace Elements in Igneous Petrology
45. Pegmatites-Hydrothermal solutions
• The contents of volatile components in magma notably water, chlorine
and fluorine increase with fractional crystallization and concentrated
in the residual liquids.
• That is the volatile components have a behaviour similar to
incompatible elements.
• But, most of the igneous minerals do not contain volatile chemical
species such as H2O, CO2, F, Cl and S. (exceptions amphibole, biotite
and apatite).
• At the end of crystallization of magmas, the final residual liquid
consists of a mixture of magmatic melt and fluids (mainly H2O),
which may contain high concentrations of rare elements such as As,B,
Be, Bi, Ce, Cs, La, Li, Mo, Rb, Sb, Sn, Ta, Th, U, W, Y, and Zr.
• If these magmas are not erupted at the surface, they are intruded into
wall rocks along fractures and cool down giving rise to rock type
called pegmatite.
46. • Pegmatites are characterized by very coarse and well formed crystals.
• Pegmatites generally consist of quartz, potash feldspar, albite and
muscovite mica with variable amounts of rare minerals.
• They contain rare minerals such as beryl, topaz, wolframite, allanite,
tourmaline, zircon, apatite, and tantalite-columbite.
• These rare minerals have economic importance, because of
concentration of rare elements and minerals of gem quality.
• Pegmatites are associated with granitic intrusions and represent residual
liquids of strongly crystallized granitic magmas.
• After complete crystallization of pegmatitic magmas, the excess
volatile components that have not been accommodated into the lattice
of the crystallizing minerals form into high-temperature fluids known
as hydrothermal solutions.
• Hydrothermal fluids migrate away from the cooling magmatic bodies
and penetrate into the wall rocks along fractures or mineral grain
boundaries.
47. • Hydrothermal solutions separate not only from pegmatites but also
from other kinds of magmatic rocks such as granites, granodiorites,
and diorites when they crystallize in the intrusive environment.
• Hydrothermal solutions modify their composition, while they move
through the rocks.
• Studies carried out on modern hydrothermal systems and on fluid
inclusions indicate that hydrothermal solutions are generally rich in
Cl, Na, Ca, and K at percentage level with lower concentration of Fe
and Mg.
• They also contain high concentration of Sr, Li, Rb, Zn, Pb and S (in
ppm level).
• Hydrothermal solutions are very important in the formation and
migration of economic ore metals (eg., Cu, Pb, Zn, Sn, Ag, Au etc,).
48. Group Assignments
Write a brief note on Igneous Rocks of
the Ocean Basins and continental rift
zones.
(presentation will be arranged)
