Spatial and temporal evolution of kimberlite magma at A154N, Diavik, Northwest Territories, Canada

Abstract

The A154N kimberlite pipe is part of the Diavik kimberlite cluster in the Northwest Territories, Canada. The pipe is filled by deposits that record a sequence of eruptive and intrusive events for which we have established relative times and modes of emplacement. As such, this sequence of kimberlite deposits provides a unique opportunity to explore the connections between magma properties, pipe or conduit geometry and eruption style. Deposits within the A154N pipe represent four discrete volcanic events including: (1) an array of pre-cursor dykes of coherent kimberlite that intrude the adjoining country rock (CK 1); (2) pyroclastic deposits recording the explosive eruption of A154N (PK 1); (3) late dykes of coherent kimberlite intruding the main infill of the pipe (CK 2); and (4) a sequence of water-laid pyroclastic deposits that sourced from another kimberlite volcano (PK 2). We use observations on polished slabs and thin sections to characterize the magma that erupted to produce each deposit. Image analysis techniques are used to quantify olivine content (abundance and size distributions) and abundances of vesicles within deposits and within individual juvenile pyroclasts. Our analysis shows that during the eruption cycle (CK 1 to PK 1 to CK 2), olivine content increased and volatile content decreased. Lastly, we estimate physical properties for the magmas that produced each deposit. The magma properties are combined with 3-D models for the geometry of the conduit at the time of eruption to constrain the mass flux and duration of each eruptive event. Steady-state assumptions for velocity and magma flux yield total eruption durations of minutes to hours. The diversity of textures and compositions recorded in pyroclastic kimberlite (PK 1) and dykes of coherent kimberlite (CK 1 and CK 2) are a possible manifestation of separated three-phase flow involving kimberlite melt and crystals, and an ever-increasing proportion of exsolved CO 2–H 2O fluid. Specifically, we suggest that the early dykes (CK 1), pyroclastic kimberlite (PK 1), and late-stage dykes (CK 2) are representative of three separate regimes of kimberlite magma created by the ascent of a vigorously degassing magma. We envisage this being composed of: (a) the gas-charged front of the magma created by decoupling the more buoyant gas phase from the silicate melt; (b) a transient, gas-rich body of magma in which later-stage exsolved fluids/gases are essentially coupled to the melt; and (c) a fluid-depleted tail of magma which remains buoyant within the lithosphere but ascends more slowly than the fluid and exsolving gas phases.