Magmatic differentiation in Igneous Petrology

1. IGNEOUS & METAMORPHIC
PETROLOGY
MAGMATIC DIFFERENTIATION
Submitted by,
BARKAVE B
M.Sc. Applied Geology
Periyar University, Salem

2. INTRODUCTION
The defining characteristic of Igneous Rocks is that at one time they were
molten and part of magmas or lavas.
A Magma is a body of molten siliceous substance which contains water vapour
and volatile substances that occurs below the surface of the earth. When magma rises along a
deep fault and pours out on the earth's surface, it is termed Lava. This molten material then
cooled to form a variety of intrusive and extrusive igneous rocks.
When a magma cools, chemical reactions occur that create a series of
different minerals. The process of differentiation of magma occurs.

3. Magmatic
Differentiation
Magmatic differentiation is a complex
process whereby a single melt can
produce a wide variety of different
igneous rocks.
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.

4. Cont.
The process of magmatic differentiation or
geochemical evolution of magmas beneath the
volcanic arcs results in the formation of
relatively high-silica andesitic or rhyolitic
magmas that solidify as low-density rocks.

5. MECHANISMS OF MAGMA DIVERSIFICATION
(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 or contamination of magmas by crustal rocks
 Liquid Immiscibility

6. Cont.
The process of differentiation occurs along two branches:
Discontinuous and Continuous.
These theories were first proven in the laboratory by N. L. Bowen
in the early 1900s and are also known as Bowen's reaction series.
The progression in the series explains why the first lavas from a
volcanic vent are rich in iron, magnesium, and calcium, are low in quartz, and are dark
green to black and why the later lavas are lighter colored and contain more quartz.

8. DISCONTINUOUS BRANCH
The minerals that form in the discontinuous branch are
all ferromagnesian—that is, they contain high percentages of iron and magnesium,
which impart a dark green to black color.
The branch is called discontinuous because the minerals form at
discrete temperatures and not continuously during cooling.
The first mineral to crystallize is olivine, followed by pyroxene,
amphibole, and biotite.

9. CONTINUOUS BRANCH
The Continuous branch is made up of the plagioclase feldspars. The
calcium or sodium ratio in this mineral type changes continuously as the magma cools.
The first feldspars to form contain the highest amounts of calcium;
subsequent feldspars have progressively less calcium and more sodium.
These minerals tend to be pink, tan, brown, or whitish.
Any magma left over after all these reactions have been completed
crystallizes at the lowest temperature as quartz

10. CONCLUSION
Differentiation is the process by which magmas evolve to give rise to a
variety of magmas and rock types (that have different compositions). Therefore, certain
physical processes are required to cause the chemical diversification of a magma.
Most melts develop in the lower crust or in the asthenosphere (upper
mantle), so, such melts have a primitive mafic or basaltic composition, whereas melts
developing in the upper crust have a higher initial silica content.

11. THANK YOU!!
By,
Barkave Balusamy