The nature of erupting kimberlite melts

The compositions of kimberlite magma and associated melts during eruption and hypabyssal intrusion are elusive due to contamination by xenolithic material and strong alteration, notably by serpentinisation. Textures and mineralogical attributes of kimberlite dykes and sills indicate that some groundmass minerals, including igneous calcite, are replaced in the process of serpentinisation. Large volume changes during serpentinisation occur in chemically open systems, producing an increase in SiO2 and MgO and decrease in CaO and CO2 in bulk rock chemistry. Reconstructed compositions of kimberlite melt components, taking into account alteration, are shifted towards compositions transitional to carbonatite, indicating overestimation of SiO2 and MgO contents in previous published estimates of kimberlite melt compositions. Further evidence that these compositions are problematic come from experimental investigations of three putative kimberlite melt compositions. Experiments at 100 and 200 MPa at temperatures of 1100 °C to 1275 °C with excess CO2, H2O and CO2–H2O mixtures show that previously proposed compositions cannot be pure melts under eruptive conditions. Their SiO2 and MgO-rich characteristics lead to high liquidus temperatures (> 1350 °C) inconsistent with estimated kimberlite magma temperatures (< 1200 °C). The experimental residual melts are depleted in MgO and have high CaO and SiO2 contents, characteristics that are not consistent with kimberlite groundmass mineralogy that indicate significantly lower SiO2 in the natural systems. Serpentinisation, causing changes in the relative proportions of CaO, MgO and SiO2, can explain these results. Recognising the effects of serpentinisation, combined with petrographic observations, suggest that kimberlite melts have low SiO2, SiO2/MgO < 1 and evolve by crystallization towards increasingly silica-poor carbonate-rich residual melts during eruption of intrusion.