Igneous petrology notes pdf carbonatites

Sedimentogy

1. Mantle Plumes, Hotspots, Mantle
metasomatism & Mantle heterogeneity
Lecture M.Sc Tech (I year) 2008-2009
Dr. N.V.CHALAPATHI RAO
READER
DEPARTMENT OF GEOLOGY
BANARAS HINDU UNIVERSITY
VARANASI-221005 (U.P)
[email protected]
http://www.nvcraobravehost.com
http://www.bhu.ac.in/Geology/STAFF/NVC%20Rao.htm

2. Global Tectonics or Plate Tectonics

3. Introduction
The majority of Earth's volcanic activity
occurs at plate boundaries. 
However, when we look at a map of
volcanic activity we find that there
are volcanoes centered within
plates, the existence of which
cannot be explained by convergent
or divergent plate movement. 
The Hawaiian Islands are a classic
example of this phenomenon. 

4. Jason Morgan (Concept of mantle plume theory)
The original
Plume
Concept
Jason Morgan
(1972)

5. Mantle Plumes & Hot-spots
Intraplate
Hotspots
Core
Mantle
M
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tle
Plume
P
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P
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6. Definitions
•Long, nearly vertical columns of hot,
upwelling solids ascend from deep
within the mantle.  These columns of
hot rising material are called mantle
plumes.
•The volcanically active surface
expressions of mantle plumes are
called hotspots.

7. What is a mantle plume?
Morgan (1971) originally proposed that
“plumes”, which he described as hot
upwellings of relatively primordial
material, rise from the deep mantle and
feed surface “hot spots”.
Such plumes rise because of thermal
buoyancy, and must originate at a thermal
boundary layer. The only such layer
known to exist in the deep mantle is the
core-mantle boundary (D"), and thus
“Morgan-type” plumes are generally
assumed to rise from this layer.

8. Some concepts about plumes
•Plumes consist of a large head followed by a narrower tail
•Plume heads should flatten to form a disk 2000 to 2500 km
in diameter at the top of their ascent
•Plume tails should have a diameter of 100 to 200 km in the
upper mantle
•Plumes must originate from a hot boundary layer,
probably the core-mantle boundary
•Plume heads should remain hot for at least 300 Myr
•Flood volcanism should be preceded by domal uplift
•The temperature excess of the plume head is predicted to
be appreciably hotter at the centre of the head than at the
margin
•Picritic magmas should be produced early during melting
of a plume head, be most abundant near the centre of the
head and less abundant towards the margin.

9. Characteristics of Hot spots
1.Zones of high heat flow, volcanism and
earthquakes far from plate boundaries - hotspots;
2.Hotspots do not drift with plates, they are almost
stationary;
3.Basalt erupted from hotspot volcanoes differs in
composition from basalt erupted at divergent
plate boundaries;
4.Hotspots are associated with large topographic
swells; and
5.Tomographic images reveal a narrow column of
mantle beneath hotspots.

10. Global Distribution of Hotspots
Pacific
plate

11. How Hot spots work?

12. Hawaii Hotspot- Example -1
H
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Line Islands in the Pacific Ocean
P
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P
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D
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Y
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Youngest Island
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13. Hawaii Hotspot- Example -2

14. Hawaii Hotspot- Example -3
Fixed
Hotspot
(Mantle
Plume)
Line Island
Formation
M
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D
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15. Consequences of Mantle Plumes

16. Continental break-up due to mantle plume

17. Mantle plume & rifting

18. Shona
Cape Verde
St Helena
Tristan
Discovery
Bouvet
Shona
Inferred mantle plumes
Mesozoic break-up of Gondwana
Is the Mesozoic
an anomalous period
of Earth history?

19. Deccan Traps

20. • Deccan Traps – Geological Map

21. Deccan Traps- Plume related?
•The widely accepted mantle plume model (e.g.,
Morgan, 1981; Richards et al., 1989; Campbell &
Griffiths, 1990) postulates that
•(i) the currently active Réunion Island, in the Indian
Ocean, is fed by the narrow “tail” of a mantle plume that
rises from the core-mantle boundary,
•(ii) the Deccan continental flood basalt (CFB) province
of India originated from the “head” of the same plume
during its early eruptive phase near the end of the
Cretaceous, and
•(iii) the Lakshadweep-Chagos Ridge, an important
linear volcanic ridge in the Indian Ocean, is a product of
this plume

22. Deccan Traps – Reunion Hot spot related?

23. Characteristics of hotspots & mantle
plumes-1
•Are responsible for intraplate volcanoes and volcanic features Away
from the plate boundaries volcanoes and lava flows can be explained by
the presence of mantle plumes.
• Form chains of islands A volcano (seamount) develops over a mantle
plume, but as the plate moves the surface expression, the volcano, also
moves while the mantle plume below remains stationary.  Volcanoes
continue to be formed, and continue to move.  The farther away the
volcano is from the hotspot, the older it is.
•Mantle plumes start off as large bulbous 'blobs' rising to the surface
but eventually develop into long, narrow, vertical streaks Once the
bulbous head has drained material continuous towards the surface in a
long, narrow, vertical streak.  The effect is similar to that seen in a lava
lamp.
•The upwelling material causes the overlying lithosphere to arch
upwards Pressure and heating of the overlying material causes it to
arch upwards.  The extension causes tension resulting in faulting and
small, shallow earthquakes.
• Plumes have an average diameter of 100's of km and rise at rates of
2 m / year 
•They rise under continents and oceans

24. Characteristics of hotspots & mantle
plumes-2
•They occur in the center of plates and along some mid-
oceanic ridges Iceland is an example of where a mantle
plume rises from beneath a mid-oceanic ridge.
•Mantle plumes originate deep within the earth While
geologists agree that mantle plumes originate from deep
within the earth, they disagree as to exactly how deep.  Some
geologists believe that they originate at a depth of 700 km, at
the boundary of the upper and lower mantle.  Other geologists
contend that they form much deeper, 2700 km below the
surface at the mantle-core boundary. 
•Compared to divergent and convergent plate boundaries
only small amounts of magma is erupted 
•Plumes are responsible for 10% of Earth's total heat loss
Earth is cooling down.  Most of the heat is lost at plate
boundaries.  But plumes do their part to release heat from
below the surface.

25. Evolution of mantle plumes – How they are
formed ?
•Mantle plumes have a limited lifespan, somewhere in the order of
10's of millions of years.  They are born, but eventually die out.  As
one becomes exhausted it is likely that somewhere on the planet
another is developing.
Mantle plumes go through the following stages of development:
1.Heating of material at the mantle-core boundary;
2.Development of a starting plume in the form of a large, bulbous head;
3.The bulbous head rises and continues to be fed through the narrow
pipe of material below it;
4.Plume head encounters strong lithosphere at surface and spreads out;
5.Uplift of the lithosphere above the mantle plume with associated
extension, faulting, and rifting;
6.Decompression melting results in the eruption of lava at the surface
forming flood basalts or ocean plateaus covering very large areas -
initially massive volumes of lava are erupted as the plume head
drains, later the tail sustains a much smaller volume of output;
7.Plume loses thermal energy and dies. 

26. How mantle plume circulation differs from plate
tectonic circulation
•Plate motion convection develops into circulation
cells that cover huge areas.  Hot material rises at
divergent plate boundaries, loses heat as it is
carried along under the lithosphere by plate
movement, and when cool and dense eventually
descends again at convergent plate boundaries.
•In the case of mantle plumes, only thin columns of
material rise from deep within the mantle
transporting with it the internal heat of the earth. 
It is only at very small, isolated locations that
mantle plumes act to circulate heat.

27. Mantle plumes – products of recycling of ocean crust
•Based on the composition, it has been suggested that basalts
related to mantle plumes are derived from mantle material
contaminated by ancient ocean crust. 
• The explanation is that ocean crust subducted at convergent
plate boundaries is metamorphosed and dehydrated, its
density increased, allowing it to make it all the way down to
the core. 
•The crust cannot penetrate the core because the core is much
denser than the crustal material. 
•Heat lost from the core heats the crustal material, making it
less dense and buoyant, and causing it to rise back towards the
surface ... as a mantle plume.

28. Continental rifting and mantle plumes - the
possible connection
•Plumes cause rifting when uplift over
the mantle plume causes tensions
which lead to faulting, rifting, and
tearing.  Eventually magmas erupt and
if rifting continues ocean crust
develops between the two pieces of
continental crust.

29. Plumes, climate change, extinctions, magnetic reversals - another
connection
•The rapid and voluminous outpouring of lava associated with
the formation of flood basalts expels huge amounts of
aerosols and gases (water vapour, carbon dioxide, sulfur
dioxide, etc.) into the atmosphere. 
•These aerosols and gases may increase the greenhouse effect
leading to global warming which in turn, may have lead to
mass extinctions.  Fluctuations in Earth's climate may be
affected by the intensity of mantle plume activity at any
given time.
•Mantle plumes may also change the convective pattern in the
liquid core.  The magnetic field of the earth is generated by
the circulation of the liquid outer core.  If large numbers of
mantle plumes were to develop at any one time and drain
larger amounts of heat from the core the convective pattern
may change, thus leading to magnetic reversals.

30. Seismic tomography provides evidence for
the existence of mantle plumes ….

31. Mantle Heterogeneity & mantle
metasomatism
•Major & trace element geochemistry of alkaline rocks
of mantle origin (e.g. carbonatites, kimberlites,
lamproites) reveal that they are more enriched in
composition than other.
•Alkaline rocks are products of mantle metasomatism.
•The existence of chemical & isotopic anomalies in
Earth’s mantle is well established.
•Mantle metasomatism or mantle enrichment is
believed to be the process responsible for this
phenomenon.
•Direct evidence for the operation of this process
comes from metasomatised garnet peridotite nodules
(xenoliths) from kimberlites.

32. Types of mantle metasomatism
•PATENT or MODAL METASOMATISM:New mineral
growth takes place in veins or matrix of a host
mineral and major and trace element composition is
clearly modified. Textural recrystallization takes
place. Fluids and melts in mantle are responsible for
it (water, Co
2 etc).
•CRYPTIC METASOMATISM: Minor and trace
elements are enriched by substitution in existing
phases without textural changes or mineral growth.

33. Metasomatic Minerals
•Phlogopite, K-richterite, LIL-titanites,
calcite and zircon are found in mantle
rocks such as eclogites, harzburgites,
pyroxenites, websterite etc.
•All these minerals are characterized by
high K,, Ti, LILE or water or Co2
contents.

34. What causes mantle metasomatism?
•Mantle metasomatism is attributed to two
processes:
•(1) Melts or fluids derived by subduction
processes
•(2) volatile and K-rich extremely low viscosity
fluids that leaked continuously from the lower
asthenosphere and accumulated in the
overlying lithosphere.
•The subsequent melting of mantle gives rise
to enriched melts.