diamonds-where to look
Where to look for diamonds
The search for diamonds has determined
that most are derived from kimberlite pipes in the oldest, nuclear
portions
of the continents, where the basement rocks are older than 1.5 billion
years.
The oldest parts of continents are called cratons, and can be divided into two terranes: Archean-age archons, which are older than 2,500 million years, and Proterozoic-age protons, which are 1,600 -- 2,500 million years old. The distribution of these terranes is shown on the map.
Kimberlite pipes occur in many parts
of the continental crust, but most diamond-rich ones are found in
archons.
This fact suggests that most diamonds were formed and stored deep below
the cratons, in the area shown in the lower figure, and were later
transported
to the surface by kimberlite and lamproite magmas that extracted them
and
other samples from the mantle.
Kimberlite and Lamprolite
The complex volcanic magmas that solidify into kimberlite and lamproite are not the source of diamonds, only the elevators that bring them with other minerals and mantle rocks to Earth's surface. Although rising from much greater depths than other magmas,
these pipes and volcanic cones are relatively
small and rare, but they erupt in extraordinary supersonic explosions.
Kimberlite and lamproite are similar
mixtures of rock material. Their important constituents include
fragments
of rock from Earth's mantle, large crystals, and the crystallized magma
that glues the mixture together. The magmas are very rich in magnesium
and volatile compounds such as water and carbon dioxide. As the
volatiles
dissolved in the magma change to gas near Earth's surface, explosive
eruptions
create the characteristic carrot- or bowl-shaped pipes.
Kimberlite magma rises through Earth's crust in networks of cracks or dikes. The pipes only form near Earth's surface. This cross-section of a kimberlite pipe shows the carrot-shaped profile produced by explosive eruption. The root zone starts in fissures, where gases are released from the rising magma and drive the eruption; they blow out the fragment-laden kimberlite to form the volcano's tuff ring and fill the pipe.
Depth measurements show the level of
erosion for various kimberlite pipes in South Africa. Adapted from
Hawthorne
(1975).
These drawings illustrate the formation
and filling of the typical champagne-glass shape of a lamproite pipe.
The
initial stage of the eruption, powered by gases either from the
lamproite
magma or from boiling ground water, corrodes the hosting rock to form
the
champagne-glass shape (top). The eruption then produces particles of
ash,
lapilli, and pumice that partially fill the crater and form a tuff ring
(middle). Finally, the crater fills with a lava pond from the degassed
lamproite magma (bottom). Adapted from a sketch by Barbara Scott-Smith