Processes describing how a homogeneous magma undergo compositional variations and give rise to a variety of igneous rocks.

1. MAGMATIC DIFFERENTIATION
AND ITS IMPLICATION TO ROCK
FORMATION
Department of Applied Geology
Dr. Harisingh Gour Vishwavidyalaya
Sagar, Madhya Pradesh
Under Guidance of: Abhishek Kumar
Prof. H. Thomas M.Tech IIIrd Sem
Prof. A.K. Singh Reg. No. Y22251018

2. CONTENT
• Introduction
• Why there’s so much variation in Igneous rocks?
• Various processes of magma differentiation
o Partial Melting
o Fractional Crystallization
o Volatile Transport
o Crustal Assimilation/Contamination
o Magma Mixing
o Liquid Immiscibility
o Thermal Diffusion
• References

3. INTRODUCTION
 Magma is the molten natural rock forming silicate material
from which all Igneous rocks are formed.
 Produced by melting of the mantle or the crust in
various tectonic settings,
such as subduction zones,
continental rift zones, mid-
ocean ridges and hotspots.
 There are basically 3
types of magma: basaltic,
andesitic & rhyolitic
forming a variety of
igneous rocks. Source:
https://upload.wikimedia.org/wikipedia/commons/thumb/0/07/Earth_
poster.svg/525px-Earth_poster.svg.png

4. Why there’s so much
variation in Igneous rocks?
 Magmatic differentiation is a process by which a
homogeneous magma produces two or more fractions of
different composition, which ultimately forms diverse rock
types.
 Occurs due to different physical and chemical properties of
the minerals that make up magma.
 The primary cause of change in composition of a magma is
cooling.
 Minerals begin to crystallize at different temperatures
depending on their chemical composition.

5. VARIOUS PROCESSES OF
DIFFERENTIATION
1. Partial Melting
2. Fractional Crystallization
3. Volatile Transport
4. Crustal Assimilation/Contamination
5. Magma Mixing
6. Liquid Immiscibility
7. Thermal Diffusion

6. 1. PARTIAL MELTING
• Separation of a liquid from the partially melted solid
residue is a form of diversification.
• It can produce a variety of melt compositions from a single
source as different minerals in rock melt at different
temperatures.
https://opengeology.org/petrology/wp-
content/uploads/Ch03/yellowstone-cartoon-v3-
1024x451.jpg

7. 2. FRACTIONAL
CRYSTALLIZATION
• Considered to be the dominant mechanism of magma
differentiation.
• Occurs when minerals crystallize and separate from the melt.
• Cumulate rock texture is a result of crystal fractionation and
accumulation.

8. Bowen’s Reaction Series
 Bowen (1915) reproduced the fractional crystallization
process experimentally.
 It is a series of crystallization sequences that occur during
magmatic cooling.
 Illustrates how a primary basaltic magma may give rise to
granite or other igneous rocks.
 The reaction series of Fe-Mg minerals is called as
Discontinuous Series and that of Plagioclase is called as
Continuous Series.
 Both these series converge and merge into a single
series.

10. Mechanisms of crystal
fractionation
A. Crystal Settling/Floating
 Crystals forming from a
magma will have different
densities than the liquid.
 Higher density crystals tend to
sink or settle to the floor of the
magma body e.g., pyroxenes.
 Lower density crystals tend to
float or rise upward through
the magma e.g., tridymite.
Source:
https://www.alexstrekeisen.it/immagini/diagrammi/cryst
als_fraction2020.png

11.  A mechanism to separate a liquid from a crystal-liquid mush.
 When high concentration of crystals the liquid forced out of
the spaces between crystals by some tectonic squeezing
that moves the liquid into a fracture or other free space.
 Very unlikely to happen in nature.
B. Filter pressing
Source: https://o.quizlet.com/.7NpvQcmVCSXM1GtspYpGA.png

12. C. Flow segregation
 Crystals may be segregated
from the liquid when crystal-
rich magmas flow in a laminar
fashion near the walls of
country rock.
 Motion past the walls creates
shear in melt due to velocity
gradient.
 The differential motion forces
the magma to flow around
phenocrysts and thus
concentrate phenocrysts away
from the wall.
Figure showing Increase in size and
concentration of olivine phenocrysts
toward the center of small dikes by
flow differentiation. (Source: Winter)

13. 3. VOLATILE TRANSPORT
 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 by 3 ways:
Fluid released by heating of hydrated or carbonated walls.
When a volatile-bearing magma rises a free vapor phase is
released. This produces a variety of hydrothermal alteration
like alkali metasomatism (fenitization) above nephelinite-
carbonatite bodies.
In late stage crystallization magma get enriched in H2O &
other volatiles and a hydrous vapor phase is produced
(retrograde boiling).
 These vapor phases form Pegmatite & Porphyritic texture.

14. 4. CRUSTAL
ASSIMILATION/CONTAMINATION
 Magma incorporates material from the surrounding rock
into its composition.
 If crustal rocks are picked up, incorporated and dissolved to
become part of the magma, said to be assimilated by the
magma.
 If the magma
absorbs part of the
rock through which it
passes, magma
become
contaminated by the
crust.
 Resorbed contacts
or Xenoliths. Source: https://blogs.helsinki.fi/jsheinon/files/2016/09/cropped-
magmachamber-2.jpg

15. 5. MAGMA MIXING
 Magma can mix with other magmas of different compositions
resulting in hybrid compositions.
 Degree of mixing of these
magmas may be limited by
contrasting properties like
temperature, density, viscosity.
 Two magmas can commonly be
seen as commingled swirls of
contrasting colors.
Illustration of magma mixing
Source:
https://www.frontiersin.org/files/Articles/650014/feart-09-
650014-HTML/image_m/feart-09-650014-g009.jpg
Commingled basaltic
and rhyolitic magma
(source: Winter).

16. 6. LIQUID IMMISCIBILITY
 Laboratory experiments have shown that the Liquid
Immiscibility may play a role in magmatic differentiation.
 A homogenous liquid magma on cooling, split into two
immiscible liquid fractions having different composition.
 For example, a Tholeiitic liquid magma on cooling may split
into two fractions (a) Granitic melt (b) Fe-Mg rich melt.
Tholeiitic Magma
Fe-Mg rich Melt
Granitic Melt
Cooling

17.  Likewise, from mafic silicate magmas, Sulfide liquids may
separate as immiscible fraction.
 Similarly, highly alkaline magmas rich in CO2 may separate
into two liquids, one rich in carbonate, and the other rich in
silica and alkalies.
This process may be responsible for forming the rare
carbonatite magmas.

18. 7. THERMAL DIFFUSION
 Also known as “Soret Effect”.
 It is the compositional variation due to the difference of heat
(thermal gradient) within the magma chamber.
 Walker & DeLong (1982) has demonstrated experimentally
Soret effect on a basaltic magma the heavier elements (Ca,
Fe, Ti, and Mg) concentrated at the cooler end and the
lighter ones (Si, Na, and K) at the hot end of the sample.
 Here, the hot end was more andesitic, whereas the cool
end was a low-silica basalt.

19. REFERENCES
• Winter, John D. (2014). Principles of Igneous and Metamorphic
Petrology 2nd Edition. Pearson Education Limited. Edinburgh.
• https://www2.tulane.edu/~sanelson/eens212/magmadiff.htm
• https://en.wikipedia.org/wiki/Igneous_differentiation

20. Thank You!