The origin of pelletal lapilli in explosive kimberlite eruptions

• Published in: Nature communications 2012-05, Vol.3 (1), p.832-832, Article 832
• Main Authors: Gernon, T.M., Brown, R.J., Tait, M.A., Hincks, T.K.
• Format: Article
• Language: English
• Subjects: 704/2151/213; 704/2151/431; 704/2151/598; Humanities and Social Sciences; multidisciplinary; Science; Science (multidisciplinary)
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms1842

Kimberlites are volatile-rich magmas from mantle depths of ≥150 km and are the primary source of diamonds. Kimberlite volcanism involves the formation of diverging pipes or diatremes, which are the locus of high-intensity explosive eruptions. A conspicuous and previously enigmatic feature of diatreme fills are 'pelletal lapilli'—well-rounded clasts consisting of an inner 'seed' particle with a complex rim, thought to represent quenched juvenile melt. Here we show that these coincide with a transition from magmatic to pyroclastic behaviour, thus offering fundamental insights into eruption dynamics and constraints on vent conditions. We propose that pelletal lapilli are formed when fluid melts intrude into earlier volcaniclastic infill close to the diatreme root zone. Intensive degassing produces a gas jet in which locally scavenged 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. Kimberlites are volatile-rich magmas that form diverging pipes containing pelletal lapilli - well rounded clasts that consist of an inner seed particle. Gernon et al . suggest that pelletal lapilli are formed when fluid volatile-rich melts intrude into earlier volcaniclastic infill close to the diatreme root zone.