The volatile content of hypabyssal kimberlite magmas: some constraints from experiments on natural rock compositions

Kimberlites are volatile rich magmas that ascend from deep in the mantle at high velocities, then as they reach a ‘root zone’ at 1–3 km in depth they either discharge explosively through to the surface or stall to form dykes and sills. Understanding this eruptive behaviour is difficult due to a lack of data on volatile solubility, particularly at conditions where the magmas enter the ‘root zone’ (∼30–80 MPa). In this study, we perform experiments on some putative primary kimberlite magma compositions to assess the amount of CO₂ and H₂O retained if these compositions represent magma as it enters the root zone. At the conditions investigated (100–200 MPa and 1,275–1,100°C) the results suggest that none of these particular kimberlite compositions reproduce a magma that can retain the observed high volatile content when intruded at these pressures (∼4–8 km). In our experiments, the low volatile retention is due to a combination of factors including a high proportion of solid phases, none of which are volatile-bearing, and inadequate volatile solubilities in the subordinate amounts of melt present. Modelled solubilities also suggest that the dissolved volatile contents remain too low even at super-liquidus temperatures (i.e. 100% melt). For water, the higher values observed in natural rocks can be explained by the addition of H₂O associated with ubiquitous post-emplacement serpentinization. The high CO₂ contents in hypabyssal rocks are unlikely to be related to alteration. We suggest that most kimberlites originally had lower SiO₂ contents and as such may have been ‘transitional’ between silicate and carbonate melts. This results in both higher CO₂ solubilities and lower liquidus temperatures. For such compositions, it is possible that both CO₂ and water solubility may first decrease and then increase as magmas decompress and crystallize. Such unusual behaviour can help explain why kimberlite magmas can be very explosive or form shallow hypabyssal intrusions.