On the Welding of Pyroclasts from Very Low-Viscosity Magmas: Examples from Kimberlite Volcanoes

Abstract

Lithofacies in kimberlite pipes in southern Africa exhibit features consistent with welded pyroclastic rocks. These include flared conduit-filling geometries, abundant lithic clasts, lithic clast layers, subhorizontal clast fabrics, gradational contacts with volcaniclastic rocks and sintered and coalesced globular ash, and lapilli and melt-coated particles. The welding dynamics of kimberlite pyroclasts differ from those of glassy, vesiculated pyroclasts in silicic volcanic systems. Low melt viscosity (∼0.1 Pa s) results in the efficient separation of volatiles and melt and the breakup of magma into nonvesicular spherical droplets. The glassy state is difficult to form in silica-deficient magmas because the crystallization kinetics are fast in low-viscosity melts. Three pyroclastic lithofacies are recognized that primarily relate to the state of the initial melt phase in the pyroclasts on deposition: (1) densely welded kimberlite forms where the initial melt phase in pyroclasts remains as pure melt, coalescing almost immediately to form a degassed homogeneous melt that subsequently crystallizes and may be texturally indistinguishable from igneous kimberlite; (2) incipiently welded kimberlite forms where the melt phase in pyroclasts is mostly crystalline with some residual melt (pyroclasts are resistant to deformation but are sticky and can sinter together); and (3) nonwelded kimberlite forms where the melt phase in pyroclasts is fully crystalline on deposition. Transitions between these pyroclast conditions may be abrupt. Gradations to nonwelded deposits and overlaps in their textural features in several kimberlite pipes suggest that welded rocks may be deposited from large-scale fluidized systems. The presence of crystals and lithic clasts may inhibit compaction deformation-welding textures by the formation of hard-particle percolation networks. Similar rocks in other kimberlite pipes may turn out to be welded rocks. The processes outlined here may be generally applicable to other low-viscosity magmas.