Abstract
Pyroclastic deposits of the Holocene Igwisi Hills kimberlite volcanoes, Tanzania, preserve unequivocal evidence for rapid, syn-eruptive agglutination. The unusual pyroclasts are composed of ash-sized particles agglutinated to each other by thin necks. The textures suggest the magma was disrupted into droplets during ascent. Collisions between particles occurred within a volcanic plume and on deposition within the conduit to form a weakly agglutinated, porous pyroclastic deposit. Theoretical considerations indicate that agglutination occurred over short timescales. Agglutinated clasts were entrained into weak volcanic plumes and deposited around the craters. Our results support the notion that agglutination can occur during kimberlite eruptions, and that some coherent, dense rocks in ancient kimberlite pipes interpreted as intrusive rocks could instead represent agglutinated pyroclastic rocks. Differentiating between agglutinated pyroclastic rocks and effusive or intrusive rocks in kimberlite pipes is important because of the potential effects that pyroclastic processes might have on diamond concentrations in deposits.
Highlights
The ascent and eruptive behaviour of ultrabasic and low silica magmas, such as kimberlites and carbonatites, remain relatively poorly constrained in comparison to more commonly erupted calc-alkaline magmas [see Carracedo Sánchez et al 2015; Church and Jones 1994; Dawson et al 1990; Russell et al 2012; Russell et al 2019; Sparks 2013; Sparks et al 2006]
The age of most kimberlite bodies (>30 Ma) means understanding of kimberlite eruptions has been deduced mostly through analysis of volcanic rocks preserved within the eroded conduits of ancient monogenetic volcanoes [Cas et al 2008; Dawson 1971; Field and Scott Smith 1999; Hawthorne 1975; Skinner and Marsh 2004; Sparks et al 2006]; rocks contained within surface edifices and extra-crater deposits
Agglutination, or welding, here defined as the high temperature adhesion of two or more juvenile pyroclasts, is a common process during volcanic eruptions and occurs within proximal pyroclastic fall deposits, in pyroclastic density current deposits, and in pyroclastic deposits contained within volcanic conduits [Quane and Russell 2005; Sparks and Wright 1979]
Summary
The ascent and eruptive behaviour of ultrabasic and low silica magmas, such as kimberlites and carbonatites, remain relatively poorly constrained in comparison to more commonly erupted calc-alkaline magmas [see Carracedo Sánchez et al 2015; Church and Jones 1994; Dawson et al 1990; Russell et al 2012; Russell et al 2019; Sparks 2013; Sparks et al 2006]. Kimberlite magmas, derived from low degrees of partial melting of mantle rocks, have repeatedly reached the Earth’s surface throughout geologic time and their deposits can host economically important quantities of diamonds [Field and Scott Smith 1999; Mitchell 1986]. The age of most kimberlite bodies (>30 Ma) means understanding of kimberlite eruptions has been deduced mostly through analysis of volcanic rocks preserved within the eroded conduits of ancient monogenetic volcanoes [Cas et al 2008; Dawson 1971; Field and Scott Smith 1999; Hawthorne 1975; Skinner and Marsh 2004; Sparks et al 2006]; rocks contained within surface edifices and extra-crater deposits.
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