The origin of kimberlite

Abstract

A new diapiric model for kimberlite genesis takes into account recent interpretations of peridotite-CO_2-H_2O melting relationships. A minor thermal perturbation at depth might trigger release of reduced vapors with major components C-H-O. Where these volatile components cross the estimated solidus boundary near 260 km, partial melting occurs, the density inversion causes diapiric uprise along adiabats, and the partially melted diapirs begin to crystallize at 100 to 80-km depth, where they reach a temperature maximum (thermal barrier) on the solidus. The released vapor enhances the prospects for crack propagation through overlying lithosphere in tension, and this could produce an initial channel to the surface. Magma separation could then occur from progressively greater depths, releasing CO_2-under-saturated kimberlitic magma for independent uprise through the established conduit, quite unaffected by the thermal barrier on the solidus of peridotite-CO_2-H_2O. Cooler diapirs would cross the solidus at somewhat greater depth, solidifying to phlogopite-dolomite-peridotite with the release of aqueous solutions. These solutions are likely candidates for the mantle metasomatism commonly considered to be a precursor for the generation of kimberlites and other alkalic magmas. According to this model the metasomatism is a consequence of kimberlite magmatism rather than its precursory cause.

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