Kimberlite ash: Fact or fiction

Abstract

Kimberlite pipes represent the conduits and vents of eroded volcanoes and are commonly filled with both coherent and volcaniclastic kimberlite including pyroclastic deposits, resulting from explosive volcanic eruptions. The properties of pyroclastic deposits, including grain size and grain shape distributions of pyroclasts, provide insight into the intensity and style of explosive volcanic eruptions. Pyroclastic kimberlite deposits generally comprise abundant coarse (>5mm) crystals and juvenile and country rock lapilli within a matrix of serpentine and other minerals interpreted to be late-stage alteration products. A consequence of the late-stage alteration processes affecting most kimberlite is that there is substantial textural ambiguity in the interpretation of the ash-sized pyroclast fraction. For example, vitric ash is never directly observed. It is presently unknown whether vitric ash is: (i) simply not produced, (ii) completely removed from the system prior to deposition, or (iii) incorporated into the deposits and then subsequently overprinted by alteration. The presence or absence of fine ash within these deposits greatly affects pyroclast size distributions and, therefore, has substantial implications on the nature and energetics of magma fragmentation and eruption processes. Our analysis suggests that explosive kimberlite eruptions can be expected to commonly produce fine ash. Deposits that preserve ash-sized mineral grains, lithic fragments, and coarse ash-sized juvenile clasts provide direct evidence for the intensity and style of kimberlite eruption. Our calculations of glass transition temperatures and melt relaxation timescales relative to rates of cooling and crystallization of pyroclasts show that vitric kimberlitic ash can only be produced by high intensity eruptions with extremely rapid cooling.