The document summarizes the rapid ascent of kimberlite magma from deep in the mantle to the surface. It discusses that kimberlite magma forms at depths of 200-250 km and 6-8 GPa pressure. Kimberlite magma travels extremely fast to the surface within an hour or a day. Its speed is facilitated by volatile gases like CO2 and H2O providing buoyancy. When the magma reaches the surface, gas release causes a depressurization wave that implodes the dyke walls and fragments the magma, forming the characteristic diatreme structure and depositing pyroclastic rocks. The entire eruption process is complete within about an hour due to the extreme cooling of mag

1. A Seminar
On
“WHY KIMBERLITE MAGMA COMES SO FAST?”
Submitted by-
Miss. VISHAKHA NATHANI
M.Sc. Geology
Post Graduate Department of Geology
Rashtrasant Tukadoji Maharaj Nagpur University
Nagpur-440001
2016-2017

3. CONTENT
• INTRODUCTION TO KIMBERLITE ROCK
• KIMBERLITE MAGMA: Background and Characteristics
• TRAVEL TIME OF KIMBERLITE MAGMA
• FACTORS AFFECTING IT’S SPEED
• ASCENT OF KIMBERLITE MAGMA
• SUMMARY
• REFERENCES

4. INTRODUCTION
• It is a rare volcanic rock. Host rock of diamonds and originated at the
base of the subcontinental lithospheric mantle.
Petrologic standpoint
• Depths = ~200-250 km & Pressure = 6-8 Gpa.
Geophysical standpoint
• Diatremes are funnel-shaped breccia pipes, extend to max. 2,500m in
depth, and are thought to form by ‘‘hydrovolcanic fragmentation and
wall rock collapse and grade at depth into dykes.’’
Volcanologist standpoint

5. KIMBERLITE MAGMA
Background and characteristics
Crater Facies : it includes pyroclastic
rocks and epiclastic rocks.
Diatreme facies: carrot shaped bodies
terminates at root zone. Composition
includes tuffisitic kimberlite breccias,
containing pelletal lapilli. Olivine,
garnet etc. in the matrix of diopside
and serpentine.
Hypabyssal facies: These are rocks
formed by the crystallization of
volatile-rich kimberlitic magma and
exhibit igneous textures and effects of
magmatic differentiation. They
contain kimberlitic breccias.

6. TRAVEL TIME OF KIMBERLITE MAGMA
Formed at the depths of ~200-250 km and the pressure of at least
6- 8GPa.
Kimberlite magmas travel very fast.
It comes to the surface within an hour to a day.
This rapid ascent from deep mantle is a point of interest.
So, lets understand what really happens deep down the earth
surface.

7. FACTORS AFFECTING IT’S SPEED
Volatile gases: CO2 and water vapour
provides buoyancy.
Pelletal lapilli: —well-rounded clasts consisting
of an inner ‘seed’ particle with a complex rim,
thought to represent quenched juvenile melt.
Other factors: chemical composition,
temperature, density, viscosity and buoyancy
of primary kimberlite melts.

8. ASCENT OF KIMBERLITE MAGMA
Sequence of events in the generation, ascent and eruption of kimberlitic magmas and
diatreme formation. (L. Wilson & J. W. Head An integrated model of kimberlite ascent and
eruption. Nature 447, 53–57 (2007).)

9. ASCENT OF KIMBERLITE MAGMA
Stage 1:
a) It involves dyke tip propagation
out of a deep source region and
CO2 fluid segregation.
CO2 (wt.)% = 20
CO2 (vol.)% = 75
b) Pressure would be buffered at
~2GPa by the release of 90% of
the available CO2. (Fig. a).
Formation of foam layer beneath
the tip cavity (Fig. b).
Why ?

10. ASCENT OF KIMBERLITE MAGMA
Stage 2:
This stage involves dyke ascent and wall
fracturing.
Due to decrease in pressure the magma will
cool adiabatically from ~1,650K to ~1,450K.
The pressure decreases from 2Gpa to 70MPa
i.e. around 99.5% decrease. And again it will
cause temperature decrease to ~1,100K.
Magma rises with the speed of 30 to 50m/s.
Pressure gradient is 60kPa/m.

11. ASCENT OF KIMBERLITE MAGMA
Stage 3:
Next, the dyke tip breaks the surface,
vents CO2 gas and implodes the
walls.
The sharp decrease in pressure
caused by the gas venting will
fracture and implode the walls of the
upper part of the dyke.

12. ASCENT OF KIMBERLITE MAGMA
Stage 4:
• The depressurization wave
initiated
• Propagates down through the
layer of magmatic foam.
• Speed : 50 m/s
• expanding the bubbles and
disrupting the foam into
magma droplets and released
gas (Fig. e).

13. ASCENT OF KIMBERLITE MAGMA
Stage 4:
• The wave continues into the
bubble free magma at the
speed of 800m/s
• more CO2 is released
• Forms additional foam which
also expands and is disrupted
(Fig. f).
• Cause temperature decrease.
During expansion and disruption, surface
tension will leads to the formation of
pelletal lapilli.

14. What are Pelletal lapilli?
• Formed when fluid melts intrude
into earlier volcaniclastic infill near
root zone.
• Intensive degassing produces a gas
jet in which locally scavenged
particles are simultaneously
fluidised and coated by a spray of
low-viscosity melt.
• Fluidised spray granulation takes
place.
Pelletal
lapilli:
Photographs of pelletal lapilli
from southern African
kimberlites and a synthetic
analogue.

15. ASCENT OF KIMBERLITE MAGMA
Gas expansion creates
upward fluidization wave
and accelerates chilled
pyroclasts.
This gas expansion accelerates
the gas into shattered country
rock, that is the major cause of
formation of the diatreme
structure.
The vent clogging reduces the
pyroclastic escape route,
increasing the pressure, which
causes (Cyclic waves generation)
‘ringing’.

16. Pressure variation
instabiility in gas
exsolution
Sorting and settling of
large blocks of country
rocks occurs.
Catastrophic adiabatic
chilling of magma at
depth leads to ceasing
of diatreme

17. ASCENT OF KIMBERLITE MAGMA

18. CONCLUSION
The termination of the
eruption probably within at
most a few tens of minutes,
is a direct consequence of
the extreme cooling of
magma during the large
pressure reductions that
occur on venting to the
atmosphere.
The very rapid pressure and
temperature fluctuations lead to
the formation of a diverse suite
of rock types.

19. RECENT STUDIES
• Carbonate–silicate liquid immiscibility in the
mantle propels kimberlite magma ascent.
• Kimberlite ascent by assimilation-fuelled
buoyancy.

20. SUMMARY
• Diatremes are carrot-shaped bodies forming the upper parts of
very deep magmatic intrusions of kimberlite rock.
• These unusual, enigmatic and complex features are famous as the
source of diamonds.
• Dyke initiation in a deep CO2-rich source region in the mantle leads
to rapid propagation of the dyke tip, below which CO2 fluid collects,
with a zone of magmatic foam beneath.
• When the tip breaks the surface of the ground, gas release causes
a depressurization wave to travel into the magma.
• This wave implodes the dyke walls, fragments the magma, and
creates a ‘ringing’ fluidization wave. Together, these processes form
the Diatreme.
• Catastrophic magma chilling seals the dyke. No precursor to the
eruption is felt at the surface and the processes are complete in
about an hour.

21. REFERENCES
• V. S. Kamenetsky & G. M. Yaxley Carbonate–silicate liquid
immiscibility in the mantle propels kimberlite magma ascent
(2015).
• T.M. Gernon, R.J. Brown, M.A. Tait & T.K. Hincks The origin of
pelletal lapilli in explosive kimberlite eruptions Nature (2012).
• J. K. Russell, L. A. Porritt, Y. Lavallee & D. B. Dingwell (2012)
Kimberlite ascent by assimilation-fuelled buoyancy. Nature 481,
352–356.
• L. Wilson & J. W. Head An integrated model of kimberlite ascent
and eruption. Nature 447, 53–57 (2007).
• www.portal.gsi.gov.in

22. Thank you!