Igneous petrology

1. Seminar On
“Kimberlites”

2. Contents
1. Introduction
2. Mineralogy
3. Morphology
4. Petrology
5. Classification
6. Origin
7. Kimberlite emplacement models
8. Indian occurrence
9. Economic importance
10. Conclusion
11. Reference

3. Introduction
• Kimberlites are fascinating rocks derived from small-
volume ultramafic magmas enriched in alkalis and
volatiles(especially, K and CO2)
• This are the potassic ultrabasic rocks.
• It has a porphyritic texture .
• Inequigranular texture consisting of large crystals or rock
fragments (xenoliths) enclosed in a much finer grained
matrix.
• This named after the town of Kimberly in South Africa.
• It was found for the first time in Kimberly provenance.

4. Mineralogy
It is mainly consist olivine, phlogophite, ilmenite, garnet,
pyroxenes and groundmass.
• Olivine- altered to serpentine.
• Phlogophite- replaced by chlorite, vermiculate, magnetite.
• Ilmenite- alter to leucoxene or rimmed by perovskite.
• Garnet- rich in pyrope. It commonly shows kelyphitic rims.
• Groundmass- fine grained, consisting of serpentine &
magnetite, upto 50% primary calcite.
• Pyroxenes- enstatite may show rims or internal lamellae of
clinopyroxene, garnet.

5. Morphology
Kimberlites occur as carrot-shaped, vertical intrusions
termed ‘pipes’. This classic carrot shape is formed due to a
complex intrusive process of kimberlitic magma which
inherits a large proportion of both CO2 and H2O.
The morphology of kimberlite pipes and the classical
carrot shape, is the result of explosive diatreme volcanism
from very deep mantle-derived source.

6. Petrology
Historically, kimberlites have been subdivided into two
distinct varieties termed 'basaltic' and 'micaceous‘(Wagner,
1914). It was re-named by Smith (1983) as Group I and Group
II based on the isotopic affinities of these rocks .Mitchell
(1995) , showed that Group II kimberlites actually show closer
affinities to lamproites than Group I kimberlites.
Group I kimberlites
Group- I kimberlites are of CO2-rich ultramafic
potassic igneous rocks dominated by a primary mineral
assemblage of forsteritic olivine, ilmenite, Cr-pyrope,
phlogopite, enststite. Group-I kimberlites exhibit a distinctive
inequigranular texture.

7. Group II kimberlites
Group II kimberlites are ultrapotassic rocks rich
in volatiles. The distinctive characteristic of orangeites is
phlogopite macrocrysts and microphenocrysts, together
with groundmass micas that vary in composition from
phlogopite to “tetraferriphlogopite” (Fe-rich phlogopite).

8. Classification
Kimberlite classification is based on
the recognition of differing rock facies. These
differing facies are associated with a
particular style of magmatic activity, namely
• Crater facies
• Diatreme facies
• Hypabyssal facies

10. Crater Facies Kimberlites
• The surface morphology of an unweathered kimberlite is
characterized by a crater, upto 2 Km in diameter.
• The crater is generally deepest in the middle.
• Around the crater is a tuff ring which is relatively small
generally less than 30 meter, when compared to crater
diameter.
• Two main categories of rocks are found in crater facies
kimberlites, namely
Pyroclastic and Epiclastic

11. Crater facies kimberlite

12. • Kimberlite diatremes are 1-2 Km deep, generally carrot-
shaped bodies which are circular to elliptical at surface and
taper with depth .
Some Textural Features:
• Country rock fragments- angular.
• Cognate fragments- rounded to angular.
• Pelletal lapilli- rapid crystallization of a volatile poor
magma containing phenocrysts
Diatreme Facies Kimberite

13. Diatreme facies kimberlite

14. Hypabyssal Facies Kimberlites
• These rocks are formed by the crystallization of hot,
volatile rich kimberlite magma. Generally, they lack
fragmentation features and appear igneous.
Some Textural Features:
Calcite- serpentine segregations in matrix.
Globular segregations of kimberlites in a carbonate rich
matrix.
Rock fragments have been metamorphosed or exhibit
concentric zoning.

15. Hypabyssal facies kimberlite

16. Classification

17. ORIGIN
 Kimberlites appear to originate at depths of 100 to 200 km
in the mantle asthenosphere.
 At a depth of 200 km the pressure is 60,000 times greater
than the surface and the temperature is about 1500°c.
 Kimberlite magmas are rich in carbondioxide and water
which brings the magma quickly and violently to the surface.
 Kimberlites occurs in the earth’s crust in vertical
structures known as kimberlite pipes.

18. Indian Occurrence
 Kimberlite are found in Anantapur, Prakasam and
Mahboob nagar district. The well known Wajrakarur
diamond field in Anantapur district and area about 250 sq
km, out of 12 pipes identified, pipes 2 and 5 are considered as
micaceous kimberlite and the rest are typical kimberlite, these
kimberlites are diamondiferrous. The kimberlite diatreme of
late protrozoic age(840 to 1020 my) are intrusive into the
archean granites and migmatite gneiss.
 Maddur- Mahboob nagar sector, covers an area of about
400 sq km. In kimberlite bodies are reported around Maddur
and some bodies around Kotakonda.

19. Kimberlite Emplacement Models
Mitchell (1986) consider several theories and presents
a more comprehensive critique of each emplacement theory.
Here we will examine three theories :
1. Explosive volcanism theory
2. Magmatic (fluidization) theory
3. Hydrovolcanic theory

20. 1. Explosive Volcanism Theory
This theory involves the pooling of kimberlite
magma at shallow depths and the subsequent build-up of
volatiles. When the pressure within this pocket, termed an
intermediate chamber, is sufficient to overcome the load of
rocks above, an eruption follows. The epicenter of the
eruption was believed to be at the diatreme facies contact.
Through extensive mining it is clear that this
theory is untenable. No intermediate chamber has been
found at depth.

21. 2. Magmatic Theory
This original proponent of this theory was Dawson
(1962, 1971). It was subsequently built upon by
Clement(1982) and is pushed by Field and Scott
Smith(1999).
Kimberlite magma rises from depth with different
pulses building termed as ‘embryonic pipes’. The surface is
not breached and the volatiles do not escape. At some point
the embryonic pipes reach a shallow enough depth (~500
mtr) whereby the pressure of the volatiles is able to
overcome the load of the overlying rocks. As the volatiles
are escaping, a brief period of fluidization ensues.
Fluidization is believed to be short lived as fragments are
commonly angular.

22. From Mitchell 1986

23. 3. Hydrovolcanic Theory
The main proponent of this theory is Lorenz(1999).
Kimberlites magmas rise from depth through narrow ~1m
thick fissures. The kimberlite magma is focused along
structural faults which act as focuses for waters or
resultant brecciation due to volatile exsolution from the
rising kimberlites may act as a focus for water. The
brecciated rock becomes recharged with groundwater.
Another pulse of kimberlite magma follows the same
structural weakness in the rock to surface and again comes
in contact with water producing another explosion.

24. From Mitchell 1986

25. Economic Importance
 Kimberlites are the most important source of primary
diamonds.
 Many kimberlite pipes also produce rich alluvial
diamond placer deposits.
 Only about 1 in 200 kimberlite pipes contain gem
quality diamonds.

26. Conclusion
 Kimberlite magmas are rich in carbondioxide and water
which brings the magma quickly and violently to the
mantle.
 Kimberlite is a gas rich potassic ultramafic igneous rock.
 Australia is currently the world’s largest producer of
diamonds are low quality and used for industrial purposes.
 The crater facies kimberlite is recognized by sedimentary
features.
 The diatreme facies are recognized by pelletal lapilli .
 The hypabyssal facies is commonly recognized by
segregationary texture and the presence of abundant
calcite.

27. Reference
 1995, Loren A Raymond, Petrology, Wm.c.Brown
publisher, volume 1, pp:83,98,117,118.
 1997, P.K.Raman and V.N.Murthy, Geology of Andhra
pradesh, Geological society of India, pp:114 to 121.
1968, Reginald Aldworth Daly, Igneous rocks and depth
of the Earth, Hafner Publishing Company, pp:551-555
www.wikipedia.com/kimberlite.htm
www.geology.net
www.ias.ac.in/currsci/apr25/articles13.htm

29. Occurrence of diamond