Origin of Carbonatites: Evidence from Phase Equilibrium Studies

Abstract

An integrated model is developed for the generation of crustal carbonatites from deep-mantle carbon, some probably derived through subduction. After generation of silicate melts in the asthenosphere, the melts are then processed in the lithosphere associated with rifting, and parental nephelinitic melts from about 75 km depth then yield the carbon in the form of carbonatite at depths between 75 km and the surface. Experimental phase equilibrium data are reviewed covering: (a) melting temperature for Ca-Mg carbonates to 30 Kb (100 km depth) and the effect of H_2O and alkalis in lowering liquidus temperatures down to those appropriate for the precipitation of calcite and dolomite in magmatic carbonatites; (b) conditions for the precipitation with calcite of accessory minerals pyrochlore, apatite, sulphide, bastnaesite, barite, and fluorite; (c) the relationships between silicate and carbonatite melts, with respect to crystal fractionation, the syntexis hypotheses, and silicate-carbonate liquid immiscibility; (d) the effect of CO_2 in generating Ca-Na carbonatite melts from peridotite and other rocks at temperatures lower than normal rock melting temperatures; (e) the effect of CO_2 in the generation of Ca-Mg carbonatite melts from peridotite at depths greater than 75 km; (f) the generation of kimberlite-like magmas in the asthenosphere and lower lithosphere; (g) the uprise of those magmas with thinning of the lithosphere beneath a rift, and the formation of magma chambers at about 75 km depth; (h) the eruption of parental nephelinitic or melilititic magmas from this level; and (i) the formation of carbonatites by fractionation or immiscibility from these parents, at 75 km, or within the crust. The real prospect that primary carbonatites could be erupted from the mantle justifies a search for them, but the high ratio of silicate: carbonatite in most alkalic complexes argues against this origin.

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