Download What is kimberlite?

What is kimberlite?
􀂊 Ultramafic, incompatible-element and volatile rich igneous
rock formed by melting and entrainment of deep mantle
material
􀂊 Occur in volcanic and sub-volcanic settings
􀂊 Complex, hybrid rocks → wide range of characteristics
􀂊 Heterogeneous – “magma” includes the following
components:
􀂊 solids - mantle-derived
􀂊 silicate melt - phenocrysts, groundmass
􀂊 volatiles - H2O / CO2 → segregations, deuteric alteration,
eruption
􀂊 Complicated by:
􀂊 contamination with crustal material during emplacement /
eruption and;
􀂊 considerable textural variation associated with different
intrusion / eruption / reworking processes
􀂊 weathering
• Commonly, they exhibit a distinctive
inequigranular texture resulting from the
presence of macrocrysts (and in some instances
megacrysts) set in a fine grained matrix.
• The megacryst/macrocryst assemblage consists
of rounded anhedral crystals of magnesian
ilmenite, Cr-poor titanian pyrope, olivine, Cr-poor
clinopyroxene, phlogopite, enstatite and Ti-poor
chromite. Olivine is the dominant member of the
macrocryst assemblage.
Morphology of Kimberlite
•
Based on studies of numerous
kimberlite deposits, geologists have
divided kimberlites into 3 distinct units
based on their morphology and
petrology. These units are:
1) Crater Facies Kimberlite
2) Diatreme Facies Kimberlite
3) Hypabyssal Facies Kimberlite
1) Crater Facies Kimberlite:
The surface morphology of an unweathered
kimberlite is characterised by a crater, up
to 2 kilometers in diameter, whose floor
may be several hundred meters below
ground level.
• he crater is generally deepest in the middle.
Around the crater is a tuff ring which is relatively
small, generally less than 30 meters, when
compared to the diameter of the crater. Two
main categories of rocks are found in crater
facies kimberlite:
a.)pyroclastic, those deposited by eruptive forces;
b.)epiclastic, which are rocks reworked by water.
2) Diatreme Facies Kimberlite
• Kimberlite diatremes are 1-2 kilometer deep,
generally carrot-shaped bodies which are
circular to elliptical at surface and taper with
depth. The dip contact with the host rocks is
usually 80-85 degrees.
• The zone is characterized by fragmented
volcaniclastic kimberlitic material and xenoliths
plucked from various levels in the Earths crust
during the kimberlites journey to surface.
• Pelletal lapilli - appear to have formed by the
rapid crystallization of a volatile poor magma
containing phenocrysts. They are characterised
by a crystal nucleus surrounded by
microphenocrysts which align themselves
tangentially to the central crystal.
• Nucleated autoliths - similar to pelletal
lapilli but lacking microphenocryst
orientation. Kernel grain usually country
rock. Magmatic nucleation about a
nucleating center.
• Matrix composed almost entirely of finegrained diopside, serpentine and
phlogopite.
3) Hypabyssal Facies Kimberlite
• These rocks are formed by the
crystallization of hot, volatile-rich kimberlite
magma. Generally, they lack
fragmentation features and appear
igneous.
i. Calcite-serpentine segregations in matrix.
ii. Globular segregations of kimberlite in a carbonate-rich matrix.
iii. Rock fragments have been metamorphosed or exhibit concentric zoning.
iv. Inequigranular texture creates a pseudoporphyritic texture.
Facts: about Kimberlite
• It is named after the town of Kimberley in
South Africa, where the discovery of an
83.5-carat (16.7 g) diamond in 1871
spawned a diamond rush, eventually
creating the Big Hole.
• Kimberlite occurs in the Earth's crust in
vertical structures known as kimberlite
pipes.