ORIGIN OF GROUP 1A KIMBERLITES: FLUID-SATURATED MELTING EXPERIMENTS AT 45-55 KBAR

Abstract

The composition and amounts of fluids strongly affect phase relations near the liquidus of Group 1A kimberlites. Under CO2-saturated conditions olivine disappears from the liquidus at 45 kbar and orthopyroxene, garnet and magnesite crystallize very close to the liquidus at 55 kbar. The addition of water stabilizes olivine to some extent, but garnet, magnesite and orthopyroxene remain near the liquidus. The Mg mole fraction of liquidus silicates is very high (> 0.94 for orthopyroxene), similar to that of strongly depleted garnet harzburgites. Clinopyroxene is absent in near-liquidus assemblages at all conditions investigated. Fluid-saturated kimberlite melts may be in equilibrium with carbonated garnet harzburgite at 60 kbar and with a CO2-dominated fluid. Garnets obtained in the experiments are poor in Cr and rich in Ti compared with garnets from depleted harzburgite xenoliths from kimberlites. The direct melting of a depleted, magnesite-bearing garnet harzburgite is thus not a plausible mechanism of kimberlite formation. Based on our data and experimental results up to 160 kbar by Ringwood et al. [1] on a similar composition, a model is proposed for the formation of Group 1A kimberlites through the interaction of deep-seated melts from the asthenosphere or transition zone with strongly depleted harzburgites at the base of the continental lithosphere. Such interaction results in the enrichment in CO2 in the initial fluid-undersaturated melts up to a saturation level. The separation of CO2-rich fluid triggers kimberlite eruption before complete equilibration with the enclosing peridotite. This model explains many features of kimberlite geology and geochemistry, such as rapid ascent, small volumes and isotopic signatures similar to ocean island basalts.