Abstract
Kimberlite volcanism involves the emplacement of olivine-rich volcaniclastic deposits into volcanic vents or pipes. Kimberlite deposits are typically pervasively serpentinised as a result of the reaction of olivine and water within a temperature range of 130–400 °C or less. We present a model for the influx of ground water into hot kimberlite deposits coupled with progressive cooling and serpentisation. Large-pressure gradients cause influx and heating of water within the pipe with horizontal convergent flow in the host rock and along pipe margins, and upward flow within the pipe centre. Complete serpentisation is predicted for wide ranges of permeability of the host rocks and kimberlite deposits. For typical pipe dimensions, cooling times are centuries to a few millennia. Excess volume of serpentine results in filling of pore spaces, eventually inhibiting fluid flow. Fresh olivine is preserved in lithofacies with initial low porosity, and at the base of the pipe where deeper-level host rocks have low permeability, and the pipe is narrower leading to faster cooling. These predictions are consistent with fresh olivine and serpentine distribution in the Diavik A418 kimberlite pipe, (NWT, Canada) and with features of kimberlites of the Yakutian province in Russia affected by influx of ground water brines. Fast reactions and increases in the volume of solid products compared to the reactants result in self-sealing and low water–rock ratios (estimated at <0.2). Such low water–rock ratios result in only small changes in stable isotope compositions; for example, δO18 is predicted only to change slightly from mantle values. The model supports alteration of kimberlites predominantly by interactions with external non-magmatic fluids.
Highlights
Many volcaniclastic kimberlite deposits are strongly altered with the original igneous minerals typically altered to hydrous minerals and carbonate
The pressure distributions inside the pipe and in the host rock are close to hydrostatic, but, because the pipe is filled with hot gas connected to the atmosphere, pore pressure inside the pipe is initially close to atmospheric (Fig. 4a)
Our model shows that the timescales of cooling of a kimberlite pipe, from emplacement temperatures through the serpentinisation temperature window to Te, and the timescale of infiltration of external water, which drives the serpentinisation reaction, are comparable
Summary
Many volcaniclastic kimberlite deposits are strongly altered with the original igneous minerals typically altered to hydrous minerals and carbonate. Others (Stripp et al 2006; Hayman et al 2009; Buse et al 2011; Porritt et al 2012; Giuliani et al 2014) have argued that alteration is the result of circulation by external water through hot olivine-rich pyroclastic deposits. This controversy is addressed here through numerical modelling of post-emplacement external water circulation through kimberlite pipes. The model couples fluid flow, heat transfer, and serpentinisation reactions with the evolution of porosity and permeability in the pipe’s
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