The story of diamonds in the earth by Dr Jock Robey

1. THE STORY OF DIAMONDS
IN THE EARTH
Jock Robey
• The big bang – 13.6 billion years ago
• Carbon- element 6 on the periodic table is a
common element in the universe:
carbonaceous meteorites, carbonados
• Earth as we know it stabilized at 4.5 billion
• Oldest dated zircons pushing back to 4by
• Some of oldest rocks in RSA - Barberton

2. EARTH STRUCTURE
• Essentially 3 layers
– Crust Rigid and brittle
– Mantle
• Upper mantle
– Lithosphere Rigid, brittle, non-convecting, cold
– Aesthenosphere Plastic, convecting, hot
• Lower mantle Plastic, convecting >670km
– Core
• Outer core Liquid, cooling down
• Inner core Solid Fe Ni

3. Mass
(%) <-1.7-><------29.3--------><----- 68.3-------->0.7
Neutrinos and Geology
0-40 km. Crust
40-410. Upper mantle
410—670. Transition zone
670-2890. Lower mantle
From Chris Hatton

4. WHY IS THE UPPER MANTLE
IMPORTANT
• This is where Diamonds are formed
• This is where Kimberlite magmas are
formed
• Sampling of mantle by kimberlites produces
mantle minerals that are used as indicators
during diamond exploration programs – the
so-called kimberlite indicator minerals

5. Diamonds are the natural high pressure form of
the element carbon and its classic crystal growth
form is an octahedron
DIAMONDS, the MANTLE and KIMBERLITES
presentation by jock robey
1 carat = 200 mg
5 carats = 1 gram
Largest diamond – the Cullinan weighed 3024 cara

6. Old Archaean crust
Convecting asthenosphere
Subducting
slab
Cold
lithosphere
Model for South Africa – Kaapvaal Sub Continental Lithopshere Mantle
SW NE

7. Kimberlite volcanoes were first discovered in Kimberley in 1869 at Du Toitspan.
Though these are small volcanoes they are thought to be very violent. The kimberlite
volcanoes of Kimberley erupted 90 million years ago. Note there is no modern
kimberlite volcano.
the volcanic
catapult

8. The first kimberlite pipes/volcanoes to be discovered in the world
Du Toitspan Bultfontein

9. Mantle rocks are picked up by the kimberlite from between 150 and 220km and
brought to the surface – these are the rocks in which diamonds actually grows
Peridotite mantle
xenoliths from
Kimberley
photo JJG

10. garnet
Cr- diopsideorthopyroxene
olivine
Photo Thomas Stachel

11. diamond inclusion in a purple peridotite garnet – photo JJG from
Menzies PhD at UCT; in one garnet saw max 32 d’s - Newlands

12. eclogites from ROVIC; photo JVR; hammer is ~0.75 for
scale

13. orange garnets (Cr <2%) and green omphacitic (Na rich) clinopyroxene of eclogites – photo JJG

14. Ds in an eclogite xenolith from orapa mine, debswana: a typical rich ~1ct mine equates to ~
<0.25ppm, way by far the lowest resource mined by man, this ecl runs at 1000ct – photo JJG

15. Research/career opportunities
•Leading research groups coll. with de beers:
•Diamonds – Thomas Stachel at Univ. of
Alberta, Edmonton + Jeff Harris – Univ.
of Glasgow + Rchardson at UCT
•Upper mantle petrology –
•Gerhard Brey – Univ. Frankfurt
•Bill Griffin and Sue Orielly –
GEMOC, McQuarrie Univ. Sydney
•Within De Beers Exploration have 1 diamond
specialist and 3 mantle mineral chemistry
specialists. These advise exploration programs
from early to advanced on these topics
Diamonds from Kimberley are 3000
million years old
garnet inclusions in diamond are dated to infer a diamond age photo SHR

16. Kimberley Mine – the Big Hole – photo Dirk Coetzee

17. The ultimate dyke, from Bellsbank: ~30% olivine xenocrysts, minor
garnet, flow zoned and multiple intrusions

18. Column - vertical
Wind - sorting
Surface – maar crater +
pyroclastic fall deposits
What do kimberlite eruptions look like at surface?
don’t know – no modern activity
Ukinrek volcano

19. Kimberley
reconstruction at
~90my ago; we
believe there has
been ~850m
erosion at
Kimberley from
~90my to
present; release
of diamonds into
fluvial (Vaal and
Orange rivers)
and marine
(West Coast)
systems

20. crater formed by
violent kimberlite
eruption,
surrounded by low
tuff ring and
possible internal
tuff cone?;
reconstruction of a
typical Orapa and
probably Kimberley
Grp 1 style
kimberlite volcano
at ~90my