Kimberlite volcanoes, the primary source of diamonds, contain pelletal lapilli—enigmatic magma-coated clasts. These are generated deep in the volcanic vent by a granulation process analogous to that commonly used in coating chocolates, drugs and fertilizers. Credit: University of Southampton

Scientists
from the University of Southampton have discovered a previously
unrecognised volcanic process, similar to one that is used in chocolate
manufacturing, which gives important new insights into the dynamics of
volcanic eruptions.

The
scientists investigated how a process called ‘fluidised spray
granulation’ can occur during kimberlite eruptions to produce
well-rounded particles containing fragments from the Earth’s mantle,
most notably diamonds. This physical process is similar to the gas
injection and spraying process used to form smooth coatings on
confectionary, and layered and delayed-release coatings in the
manufacture of pharmaceuticals and fertilisers.

Kimberlite
volcanoes are the primary source of diamonds on Earth, and are formed
by gas-rich magmas from mantle depths of over 150 km. Kimberlite
volcanism involves high-intensity explosive eruptions, forming diverging
pipes or “diatremes”, which can be several hundred metres wide and
several kilometres deep. A conspicuous and previously mysterious feature
of these pipes are “pelletal lapilli”—well-rounded magma coated
fragments of rock consisting of an inner ‘seed’ particle with a complex
rim, thought to represent quenched magma.

These
pelletal lapilli form by spray granulation when kimberlite magma
intrudes into earlier volcaniclastic infill close to the diatreme root
zone. Intensive degassing produces a gas jet in which the seed particles
are simultaneously fluidised and coated by a spray of low-viscosity
melt.

In
kimberlites, the occurrence of pelletal lapilli is linked to diamond
grade (carats per tonne), size and quality, and therefore has economic
as well as academic significance.

Dr.
Thomas Gernon, Lecturer in Earth Science at the University of
Southampton, says: “The origin of pelletal lapilli is important for
understanding how magmatic pyroclasts are transported to the surface
during explosive eruptions, offering fundamental new insights into
eruption dynamics and constraints on vent conditions, notably gas
velocity.”

This is a diamond in kimberlite rock, Jwaneng Diamond Mine, Botswana. Credit: Professor Tom Gernon

“The
ability to tightly constrain gas velocities is significant, as it
enables estimation of the maximum diamond size transported in the flow.
Gas fluidisation and magma-coating processes are also likely to affect
the diamond surface properties.”

Dr
Gernon and colleagues studied two of the world’s largest diamond mines
in South Africa and Lesotho. In the Letseng pipe in Lesotho, pelletal
lapilli have been found in association with concentrations of large
diamonds (up to 215 carat), which individually can fetch up to tens of
millions of pounds. Knowledge of flow dynamics will inform models of
mineral transport, and ultimately could improve resource assessments.

Dr
Gernon says: “This multidisciplinary research, incorporating Earth
sciences, chemical and mechanical engineering, provides evidence for
fluidised granulation in natural systems which will be of considerable
interest to engineers and chemical, pharmaceutical and food scientists
who use this process routinely. The scale and complexity of this
granulation process is unique, as it has not previously been recognised
in natural systems.”

The origin of pelletal lapilli in explosive kimberlite eruptions

Source: University of Southampton