The origin of pelletal lapilli in explosive kimberliteeruptions

摘要

Kimberlites are volatile-rich magmas from mantle depths of ≥150km and are the primary sourceof diamonds. Kimberlite volcanism involves the formation of diverging pipes or diatremes,which are the locus of high-intensity explosive eruptions. A conspicuous and previouslyenigmatic feature of diatreme fills are ‘pelletal lapilli’—well-rounded clasts consisting of aninner ‘seed’ particle with a complex rim, thought to represent quenched juvenile melt. Herewe show that these coincide with a transition from magmatic to pyroclastic behaviour, thusoffering fundamental insights into eruption dynamics and constraints on vent conditions.We propose that pelletal lapilli are formed when fluid melts intrude into earlier volcaniclasticinfill close to the diatreme root zone. Intensive degassing produces a gas jet in which locallyscavenged particles are simultaneously fluidised and coated by a spray of low-viscosity melt.A similar origin may apply to pelletal lapilli in other alkaline volcanic rocks, including carbonatites,kamafugites and melilitites.