Experimental boundaries for the origin and evolution of carbonatites

Abstract

An integrated model is presented for the generation of crustal carbonatites in rift environments from mantle carbon, both primordial and recycled through subduction. The carbon is involved in the generation of melts in the asthenosphere and the melts are then processed in the lithosphere associated with continental rifting. Parental nephelinitic melts from about 75 km depth then yield the carbon in the form of carbonatite at shallower depths in the upper mantle, or within the crust. Selected experimental phase equilibrium data are reviewed to test parts of the model. Similar tests can be made for carbonatites formed in different tectonic environments. Experiments with lherzolite (synthetic and natural) confirm that with CO_2 and H_2O at depths greater than about 75 km, near-solidus liquids correspond to Ca-Mg carbonatites, enriched in alkalis; the prospect that primary carbonatites could be erupted from the mantle justifies a search for them, but volume relationships favor an origin by differentiation for most occurrences. A wide miscibility gap between silicate and carbonate liquids with variable Ca/Na and Ca/Mg extends from crust to mantle pressures, but primitive CO_2-bearing nephelinites do not yield immiscible carbonatite magmas at 75 km depth; immiscibility should occur commonly at shallower depths, certainly in the crust. Fractional crystallization of nepheline-normative magmas in the crust can yield the series of rocks occurring in some alkalic complexes with carbonatites. Carbonatite magmas differentiate: crystallization paths confirm the prospect of fractional crystallization of dolomite and calcite and the coprecipitation of calcite and bastnaesite from magmas.

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