The genesis of kimberlites and some low-SiO_2, high-alkali magmas

Abstract

There are four levels within the upper mantle where critical changes occur in the physical processes that control the chemistry and migration of these magmas. The first two (2) and (3), are the depths where the solidus for peridotite-C-H-O is intersected by the geotherm, limiting the depth interval within which the magmas can be generated. The third critical level, (1), is the depth of the asthenosphere-lithosphere boundary, where the mantle flow regime changes from convective (ductile) to static (brinle). The fourth critical level, (4), is the narrow depth interval within which the solidus for peridotite-CO_2-H_2O changes slope, and becomes subhorizontal, with low dP/dT. The depth of level (4) differs according to different experimental investigators. Levels (2), (3) and (4) are different for lherzolites and harzburgites. The depths of levels (2), (3) and (1) vary as a function of geotherm and local thermal history, and levels (2) and (3) vary as a function of oxygen fugacity. Consider a mantle plume diverging laterally at the base of the lithosphere. Sparse volatile components (C-H-O-S-K) entrained in the plume will generate melt at level (2), near 275 km. As the plume diverges laterally below depth (1), 200 km, the melt becomes concentrated in the lithosphere. This is followed by thinning of the lithosphere. The evolution of vapour from lateral magma chambers promotes the propagation of cracks through the lithosphere, and the eruption of kimberlite magmas. Magmas in the heated lithosphere above the plume may percolate upwards or promote the uprise of diapirs, and intersect the solidus at level (4) in the range of 100-75 km. Vapours will be evolved causing metasomatism in the overlying mantle, and causing intermittent crack propagation releasing magmas through the lithosphere. A variety of alkalic magma compositions may be generated at level (4), depending sensitively upon conditions.