CONSEQUENCES OF RECYCLED CARBON IN CARBONATITES

DANIEL S. BARKER
Department of Geological Sciences, C1100, The University of Texas at Austin, Austin, Texas 78712, U.S.A.


Abstract

Formerly, carbonatites were thought to be results of the mantle's purging itself of "juvenile" carbon. However, evidence has been accumulating that carbon from the crust is recycled deep into the mantle by subduction. This non-juvenile carbon may nourish carbonatite magmas. The process by which carbon in the mantle may eventually be incorporated into carbonatite in the crust is probably a circuitous one, involving transport in a variety of phases before accumulation in diverse sources of carbonatite magma. With time, these sources have become more varied and more radiogenic. During its migration, carbonate liquid acts as an efficient sequestering agent in the mantle, gathering a distinctive retinue of trace elements, especially Sr, Nb, Ba, the light rare-earth elements, Pb, Th, and U. Elements that are strongly depleted in carbonatites are Si, Al, Mg, Cr, and Ni. Partition coefficients for carbonate liquid/minerals yield calculated enrichments that do not agree with those observed unless there are repeated episodes of interaction between carbonate liquid and progressively enriched mantle. Carbonate-rich liquid can only survive its upward passage if the rock through which it flows has already lost most of its capacity to react with carbonate liquid. Such "path clearing" requires the aborted ascent of precursor batches of carbonatite magma. Ultramafic xenoliths from many localities have trace-element and isotopic enrichment and depletion patterns attributed to metasomatic reactions of lithospheric mantle with carbonate-rich liquids. These carbonate-metasomatized xenoliths indicate repeated invasion of the lithosphere by carbon-bearing liquids or fluids. This in turn implies that the rarity of carbonatitic magma in the upper crust is caused by barriers to its ascent, not by a shortage of source material.