Transfer of subcratonic carbon into kimberlites and rare earth carbonatites

Abstract

Carbon and volatile components involved in the genesis of kimberlites and carbonatites rise from a mantle reservoir below the asthenosphere. Vapors include the components C-H-O-S-K, in molecular form dependent on the oxygen fugacity, a parameter that varies as a function of depth in ways not yet fully understood. Kimberlites are generated where upward percolating reduced volatile components cross the solidus for peridotite-C-H-O. The depleted, refractory base of the lithosphere, 200-150 km deep, is a collecting site for kimberlite magma at temperatures above its solidus; this layer has been intermittently invaded by small bodies of carbonated kimberlite, through billions of years; most of these aborted and gave off vapors enriching the lower lithosphere by metasomatism, but some reached the surface, through vapor-enhanced crack propagation. Nephelinites and associated carbonatites require upward movement of solid mantle as a plume. Thinning of the lithosphere above a mantle plume, beneath a rift, results in magma trapped in the asthenosphere-lithosphere boundary layer rising with the isotherms, without crossing the solidus until the magma reaches the depth interval 90-65 km, where the solidus for peridotite-CO_2-H_2O becomes subhorizontal, with low dP/dT, pressure independent, and forming a ledge or phase equilibrium barrier. At this level, magma chambers form, and crystallization is accompanied by evolution of vapors, enhancing crack propagation and the eruption of nephelinitic magmas that differentiate to carbonatites. The release of vapors at this level generates another metasomatic layer, al depths known to contain metasomatic RE-titanates. These metasomes may be the source of the REE in carbonatites. Liquidus studies in the system CaCO_3-Ca(OH)_2-La(OH)_3 at 1 kbar demonstrate that residual carbonatite magmas may contain more than 20 weight percent La(OH)_3 , as long as the REE were not removed at earlier stages of differentiation by other minerals.