Diamonds have long been considered a source of wealth due to their rarity, marketing,
and commonplace usage in high end jewelry.
As a result, miners and local businessmen have looked far and wide for rock deposits
which contain these highly pressurized carbon gemstones.
One such highly rare rock type which diamonds are typically found within is known as kimberlite.
Unlike the vast majority of other types of lava, as kimberlite can be both intrusive
and extrusive, it is not mainlly composed of silica or silicon dioxide.
Whereas for example rhyolite generally has more than 69% by weight silica, kimberlite
tends to have less than 30% silica.
As a result, kimberlite much like komatiite and carbonatite does not fit on a typical
TAS diagram.
Diamond bearing kimberlites generally form in areas of the planet which have a section
of stable crust which has remained largely unaltered for more than 500 million years.
Over this time, elemental carbon deep in the crust undergoes significant heat and pressure
which converts it to another form of carbon, diamond.
A carbon dioxide and gas rich intrusion might then eventually occur, sending molten rock
towards the surface at speeds close to 70 miles or 112 kilometers per hour.
Along the way, the kimberlite magma might intrude into an area with diamonds in a craton,
carrying with it some of the diamonds as it moves upwards in the crust.
Eventually, this magma reaches the surface and a highly explosive eruption occurs, depositing
diamonds on the surface and at depth below the newly created volcanic crater.
To understand just how rare kimberlite is, let me put it into context.
Until very recently, only a single kimberlite pipe had been found and dated to originating
within the last 30 million years; an approximately 25 million year old several mile wide pipe
which does not contain any diamonds in Buell Park, Arizona.
This is why it was all the more surprising when a far younger kimberlite deposit was
discovered in Tanzania.
This deposit formed just before the end of the last ice age, and encompasses what is
known as the Igwisi Hills volcanic complex.
The Igwisi volcano is located in west central Tanzania where it is 102 kilometers west of
the city of Tabora.
This volcano is located in surprisingly close proximity to one of the main sections of the
east African Rift Zone where the crust is spreading apart to the east and west, and
this is quite important.
Although the exact reason why this occurs is unclear, kimberlites seem to be generated
in a surprising abundance during certain large scale breakups of a continent such as Pangea.
Although the young kimberlite deposit here is seemingly an isolated occurrence, its existence
suggests that more similar features might form in the future.
The Igwisi Hills volcano consists of 3 closely spaced features known as tuff cones.
They formed when repeated explosive eruptions over a short duration built up ash around
an active crater, forming a low prominence feature which might look like a cinder cone
but is not one.
This ash and pyroclastic material has largely buried sections of the original active craters
which in the case of the northeast cone was initially 370 meters or 1,214 feet wide.
It is unclear whether the three tuff cones formed at the same time or erupted as 3 separate
events.
However, due to their occurrence along a northeast line of activity, it suggests that an underlying
fault line or fissure helped defined the site of the 3 eruptions.
While the exact date of when these cones formed is unclear, we are pretty sure when the northernmost
cone formed; 10,450 BCE, during the human stone age.
This volcanic complex is quite weird because the expected volume of material at distance
is not present to suggest a highly explosive eruption like would be expected from a kimberlite
pipe.
After the mildly explosive eruption or eruptions ended at this complex, it produced three low
viscosity but short distance lava flows.
Sadly, none of these kimberlites contain any diamonds.
As a final note, I want to add that some scientists do not consider the rock which erupted at
Igwisi Hills to be kimberlite.
The composition of the supposed kimberlites at this volcano are slightly different than
traditional kimberlites.
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