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

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology. A B S T R A C TLithofacies 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.

The importance of the transport system in shaping the growth and form of kimberlite volcanoes

“…These deposits commonly form in proximal areas during vigorous fire-fountaining activity (e.g., Head and Wilson, 1989;Sumner et al, 2005). Although welded volcaniclastic rocks are common in volcanic successions worldwide, welded deposits have only been described for kimberlite deposits comparatively recently (e.g., Brown et al, 2008b;van Straaten et al, 2008).…”

The influence of complex intra- and extra-vent processes on facies characteristics of the Koala Kimberlite, NWT, Canada: volcanology, sedimentology and intrusive processes

“…Hayman et al 2008;Moss et al 2008). The round juvenile clasts retain the unmodified shape of kimberlite pyroclasts, reflecting the low viscosity of the magma and surface tension effects (Dawson 1980;Sparks et al 2006;Brown et al 2008a). None of the components of this facies, therefore, show any evidence of reworking or tractional transport and deposition.…”

“…Sparks et al (2006) considered the coherent kimberlite bodies in the root zones to be welded or agglutinated clastogenic facies rather than former magmatic intrusions. Brown et al (2008a) interpreted the transition zone between the coherent and fragmental textures to represent changes in the welding intensity in the deposits of the Venetia pipes.…”

“…Coherent kimberlite is commonly found at depth, beneath MVK, and a textural gradation between the coherent and pyroclastic deposits is observed in some pipes. The origin of this so-called transitional facies kimberlite (TFK) is also contentious; origins proposed include a 'frozen' magma fragmentation front Skinner and Marsh 2004), welded or agglutinated pyroclastic deposits Brown et al 2008a), or alteration obscuring the contact zone. Kimberlitic pyroclastic flow deposits have recently been identified within non-related kimberlite pipes, in the Diavik A154N pipe (Moss et al 2008) and Orapa South (Gernon et al 2009a), indicating that kimberlite eruptions can generate pyroclastic flows.…”

Geology of the BK9 kimberlite (Damtshaa, Botswana): implications for the formation of dark volcaniclastic kimberlite

“…The DVK variety encompasses a range of kimberlite rock types including many dense rocks historically interpreted as intrusive hypabyssal kimberlite (Clement and Skinner 1985;Mitchell 1986;Field and Scott Smith 1999;Hetman et al 2004), for which subsequent work has established a volcaniclastic origin (e.g. Brown et al 2008a;Nowicki et al 2008;Hayman et al 2008;van Straaten et al 2009). Hypabyssal kimberlite, in view of the controversy surrounding its origin, has recently been referred to by non-genetic terms, such as coherent kimberlite or dark and competent kimberlite (van Straaten et al 2009).…”