Scientists from the University of Southampton have discovered a previously¹ unrecognised volcanic² process, similar to one that is used in chocolate manufacturing, which gives important new insights into the dynamics³ of volcanic eruptions⁴. The scientists investigated how a process called 'fluidised（液化） spray granulation（粗糙） ' can occur during kimberlite（金伯利岩） eruptions to produce well-rounded particles containing fragments from Earth's mantle⁶, most notably⁷ diamonds. This physical process is similar to the gas injection and spraying process used to form smooth coatings on confectionary（糖果） , and layered and delayed-release coatings in the manufacture of pharmaceuticals⁹ 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 diverging¹¹（发散的） pipes or 'diatremes', which can be several hundred metres wide and several kilometres deep. A conspicuous¹² 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 rim¹⁰, thought to represent quenched¹³（淬火的） magma.

These pelletal lapilli form by spray granulation when kimberlite magma intrudes¹⁴ into earlier volcaniclastic infill close to the diatreme root zone. Intensive degassing produces a gas jet in which the seed particles are simultaneously¹⁵ 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 eruption⁵ dynamics and constraints¹⁶ on vent¹⁸ conditions, notably gas velocity¹⁹."

"The ability to tightly constrain¹⁷ gas velocities²⁰ 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 assessments²¹.

Dr Gernon, who is based at the National Oceanography Centre at Southampton's waterfront campus, 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, pharmaceutical⁸ and food scientists who use this process routinely. The scale and complexity²² of this granulation process is unique, as it has not previously been recognised in natural systems."