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 (similar to 30-80 MPa). In this study, we perform experiments on some putative primary kimberlite magma compositions to assess the amount of CO(2) and H(2)O retained if these compositions represent magma as it enters the root zone. At the conditions investigated (100-200 MPa and 1,275-1,100 degrees 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 (similar to 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).