Kimberlite

Kimberlite is an igneous rock, which

sometimes contains diamonds. It is
named after the town of Kimberley in
South Africa, where the discovery of an
83.5-carat (16.70 g) diamond called the
Star of South Africa in 1869 spawned a
diamond rush and the digging of the
open-pit mine called the Big Hole.
Previously, the term kimberlite has
been applied to olivine lamproites as
Kimberlite II, however this has been in
error.[1]
 Kimberlite
Igneous rock

Kimberlite from the United States

Composition

Forsteritic olivine and carbonate minerals,

with trace amounts of magnesian ilmenite,
chromium pyrope, almandine-pyrope,
chromium diopside, phlogopite, enstatite and
titanium-poor chromite. Sometimes contains
diamonds.
Cross-section of kimberlite from South Africa. The
kimberlite matrix is made up of clay minerals and
carbonates, presented in blue, purple and buff
colours.

Kimberlite occurs in the Earth's crust in

vertical structures known as kimberlite
pipes, as well as igneous dykes.
Kimberlite also occurs as horizontal
sills.[2] Kimberlite pipes are the most
important source of mined diamonds
today. The consensus on kimberlites is
that they are formed deep within the
mantle. Formation occurs at depths
between 150 and 450 kilometres (93
and 280 mi), potentially from
anomalously enriched exotic mantle
compositions, and they are erupted
rapidly and violently, often with
considerable carbon dioxide[3] and
other volatile components. It is this
depth of melting and generation that
makes kimberlites prone to hosting
diamond xenocrysts.

Despite its relative rarity, kimberlite has

attracted attention because it serves as
a carrier of diamonds and garnet
peridotite mantle xenoliths to the
Earth's surface. Its probable derivation
from depths greater than any other
igneous rock type, and the extreme
magma composition that it reflects in
terms of low silica content and high
levels of incompatible trace-element
enrichment, make an understanding of
kimberlite petrogenesis important. In
this regard, the study of kimberlite has
the potential to provide information
about the composition of the deep
mantle and melting processes
occurring at or near the interface
between the cratonic continental
lithosphere and the underlying
convecting asthenospheric mantle.

Morphology and
volcanology

Distribution of diamond deposits. Cratons: CA-

Central African (Kasai), South African (Kalahari),
WA-West African, Alluvials and Bodies: A-
Akwatia/Birim, B-Banankoro, Bf-Buffels River, Cb-
Carnot/Berberati, Cu-Cuango Valley, Do-
Dokolwayo body, F-Finsch body, G-Gope body, J-
Kwaneng body, Ja-Jagersfontein body, k-Koidu
body, Kb-Kimberley bodies, Ko-Koffiefontein body,
L-Letlhakanebody, Le-Letseng body, Li-
Lichtenburg, Lo-Lower Orange River, Lu-Lunda
bodies, M-Mitzic bodies, Mb-Mbuji-Mayi bodies,
Mo-Mouka Ouadda, Mw-Mwadui body, Na-
Namibia and Namaqualand, O-Orapa body, P-
Primier body, R-River Ranch body, T-Tortiya, Ts-
Tshkipa, V-Venetia body, Vo-Vaal/Orange Rivers,
Ye-Yengema

Many kimberlite structures are

emplaced 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 CO2 (lower amounts
of H2O) in the system, which produces
a deep explosive boiling stage that
causes a significant amount of vertical
flaring.[4] 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,
diatreme and hypabyssal rocks.[5][6]

The morphology of kimberlite pipes

and their classical carrot shape is the
result of explosive diatreme volcanism
from very deep mantle-derived sources.
These volcanic explosions produce
vertical columns of rock that rise from
deep magma reservoirs. The
morphology of kimberlite pipes is
varied, but includes a sheeted dyke
complex of tabular, vertically dipping
feeder dykes in the root of the pipe,
which extends down to the mantle.
Within 1.5–2 km (0.93–1.24 mi) of the
surface, the highly pressured magma
explodes upwards and expands to form
a conical to cylindrical diatreme, which
erupts to the surface. The surface
expression is rarely preserved but is
usually similar to a maar volcano.
Kimberlite dikes and sills can be thin
(1–4 meters), while pipes range in
diameter from about 75 meters to 1.5
kilometers.[7]

Two Jurassic kimberlite dikes exist in

Pennsylvania. One, the Gates-Adah
Dike, outcrops on the Monongahela
River on the border of Fayette and
Greene Counties. The other, the
Dixonville-Tanoma Dike in central
Indiana County, does not outcrop at the
surface and was discovered by
miners.[8] Similarly aged kimberlite is
found in several locations in New
York.[9]
Petrology
Both the location and origin of
kimberlitic magmas are subjects of
contention. Their extreme enrichment
and geochemistry have led to a large
amount of speculation about their
origin, with models placing their source
within the sub-continental lithospheric
mantle (SCLM) or even as deep as the
transition zone. The mechanism of
enrichment has also been the topic of
interest with models including partial
melting, assimilation of subducted
sediment or derivation from a primary
magma source.

Historically, kimberlites have been

classified into two distinct varieties,
termed "basaltic" and "micaceous"
based primarily on petrographic
observations.[10] This was later revised
by C. B. Smith, who renamed these
divisions "group I" and "group II" based
on the isotopic affinities of these rocks
using the Nd, Sr and Pb systems.[11]
Roger Mitchell later proposed that
these group I and II kimberlites display
such distinct differences, that they may
not be as closely related as once
thought. He showed that group II
kimberlites show closer affinities to
lamproites than they do to group I
kimberlites. Hence, he reclassified
group II kimberlites as orangeites to
prevent confusion.[12]

Group I kimberlites

Group-I kimberlites are of CO2-rich

ultramafic potassic igneous rocks
dominated by primary forsteritic olivine
and carbonate minerals, with a trace-
mineral assemblage of magnesian
ilmenite, chromium pyrope, almandine-
pyrope, chromium diopside (in some
cases subcalcic), phlogopite, enstatite
and of Ti-poor chromite. Group I
kimberlites exhibit a distinctive
inequigranular texture caused by
macrocrystic (0.5–10 mm or 0.020–
0.394 in) to megacrystic (10–200 mm
or 0.39–7.87 in) phenocrysts of olivine,
pyrope, chromian diopside, magnesian
ilmenite, and phlogopite, in a fine- to
medium-grained groundmass.

The groundmass mineralogy, which

more closely resembles a true
composition of the igneous rock, is
dominated by carbonate and
significant amounts of forsteritic
olivine, with lesser amounts of pyrope
garnet, Cr-diopside, magnesian
ilmenite, and spinel.

Olivine lamproites

Olivine lamproites were previously

called group II kimberlite or orangeite in
response to the mistaken belief that
they only occurred in South Africa.
Their occurrence and petrology,
however, are identical globally and
should not be erroneously referred to
as kimberlite.[13] Olivine lamproites are
ultrapotassic, peralkaline rocks rich in
volatiles (dominantly H2O). The
distinctive characteristic of olivine
lamproites is phlogopite macrocrysts
and microphenocrysts, together with
groundmass micas that vary in
composition from phlogopite to
"tetraferriphlogopite" (anomalously Al-
poor phlogopite requiring Fe to enter
the tetrahedral site). Resorbed olivine
macrocrysts and euhedral primary
crystals of groundmass olivine are
common but not essential
constituents.
Characteristic primary phases in the
groundmass include zoned pyroxenes
(cores of diopside rimmed by Ti-
aegirine), spinel-group minerals
(magnesian chromite to titaniferous
magnetite), Sr- and REE-rich perovskite,
Sr-rich apatite, REE-rich phosphates
(monazite, daqingshanite), potassian
barian hollandite group minerals, Nb-
bearing rutile and Mn-bearing ilmenite.

Kimberlitic indicator minerals

Kimberlites are peculiar igneous rocks

because they contain a variety of
mineral species with chemical
compositions that indicate they formed
under high pressure and temperature
within the mantle. These minerals, such
as chromium diopside (a pyroxene),
chromium spinels, magnesian ilmenite,
and pyrope garnets rich in chromium,
are generally absent from most other
igneous rocks, making them
particularly useful as indicators for
kimberlites.

These indicator minerals are generally

sought in stream sediments in modern
alluvial material. Their presence may
indicate the presence of a kimberlite
within the erosional watershed that
produced the alluvium.

Geochemistry
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The geochemistry of Kimberlites is

defined by the following parameters:

ultramafic, MgO >12% and generally

>15%;
ultrapotassic, molar K2O/Al2O3 >3;
near-primitive Ni (>400 ppm), Cr
 (>1000 ppm), Co (>150 ppm);
REE-enrichment;[14]
moderate to high large-ion lithophile
element (LILE)[15] enrichment, ΣLILE
= >1,000 ppm;
high H2O and CO2.

Economic importance
Kimberlites are the most important
source of primary diamonds. Many
kimberlite pipes also produce rich
alluvial or eluvial diamond placer
deposits. About 6,400 kimberlite pipes
have been discovered in the world, of
those about 900 have been classified
as diamondiferous, and of those just
over 30 have been economic enough to
diamond mine.[16]

The deposits occurring at Kimberley,

South Africa, were the first recognized
and the source of the name. The
Kimberley diamonds were originally
found in weathered kimberlite, which
was colored yellow by limonite, and so
was called "yellow ground". Deeper
workings encountered less altered
rock, serpentinized kimberlite, which
miners call "blue ground".
See also Mir Mine and Udachnaya pipe,
both in the Sakha Republic, Siberia.

The blue and yellow ground were both

prolific producers of diamonds. After
the yellow ground had been exhausted,
miners in the late 19th century
accidentally cut into the blue ground
and found gem-quality diamonds in
quantity. The economic importance of
the time was such that, with a flood of
diamonds being found, the miners
undercut each other's prices and
eventually decreased the diamonds'
value down to cost in a short time.[17]